Change search
Refine search result
1234567 151 - 200 of 1127
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 151.
    Cataldi, Gianni
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Thébault, Philippe
    Singer, Kelsi
    Ahmed, Engy
    Stockholm University, Faculty of Science, Department of Geological Sciences. Royal Institute of Technology (KTH), Sweden.
    de Vries, Bernard L.
    Stockholm University, Faculty of Science, Department of Astronomy. European Space Research and Technology Centre (ESA/ESTEC), The Netherlands.
    Neubeck, Anna
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Searching for Biosignatures in Exoplanetary Impact Ejecta2017In: Astrobiology, ISSN 1531-1074, E-ISSN 1557-8070, Vol. 17, no 8, p. 721-746Article in journal (Refereed)
    Abstract [en]

    With the number of confirmed rocky exoplanets increasing steadily, their characterization and the search for exoplanetary biospheres are becoming increasingly urgent issues in astrobiology. To date, most efforts have concentrated on the study of exoplanetary atmospheres. Instead, we aim to investigate the possibility of characterizing an exoplanet (in terms of habitability, geology, presence of life, etc.) by studying material ejected from the surface during an impact event. For a number of impact scenarios, we estimate the escaping mass and assess its subsequent collisional evolution in a circumstellar orbit, assuming a Sun-like host star. We calculate the fractional luminosity of the dust as a function of time after the impact event and study its detectability with current and future instrumentation. We consider the possibility to constrain the dust composition, giving information on the geology or the presence of a biosphere. As examples, we investigate whether calcite, silica, or ejected microorganisms could be detected. For a 20km diameter impactor, we find that the dust mass escaping the exoplanet is roughly comparable to the zodiacal dust, depending on the exoplanet's size. The collisional evolution is best modeled by considering two independent dust populations, a spalled population consisting of nonmelted ejecta evolving on timescales of millions of years, and dust recondensed from melt or vapor evolving on much shorter timescales. While the presence of dust can potentially be inferred with current telescopes, studying its composition requires advanced instrumentation not yet available. The direct detection of biological matter turns out to be extremely challenging. Despite considerable difficulties (small dust masses, noise such as exozodiacal dust, etc.), studying dusty material ejected from an exoplanetary surface might become an interesting complement to atmospheric studies in the future.

  • 152. Cave, Ben J.
    et al.
    Barnes, Sarah-Jane
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sack, Patrick J.
    Kuikka, Helena
    Johnson, Sean C.
    Duran, Charley J.
    Multi-stage precipitation and redistribution of gold, and its collection by lead-bismuth and lead immiscible liquids in a reduced-intrusion related gold system (RIRGS); Dublin Gulch, western Canada2019In: Ore Geology Reviews, ISSN 0169-1368, E-ISSN 1872-7360, Vol. 106, p. 28-55Article in journal (Refereed)
    Abstract [en]

    Dublin Gulch reduced intrusion-related gold system (RIRGS), located in the Selwyn Basin area of western Canada, represents one of the best examples of RIRGS mineralization globally and hence can be studied to unravel genesis and evolution of these types of deposits. Based on textural relationships, mineralogy, and trace element mineral chemistry, three auriferous vein stages were identified. The paragenetic sequence for the auriferous vein stages are: 1) Eagle Style (ES), quartz-albite, low sulfide and sulfosalt content ( < 5% vol.), As-Fe-Mo-W-Pb-Bi-Au-Ag veins; 2) Potato Hills Style-1 (PHS-1), quartz, high sulfide and sulfosalt content ( > 30% vol.), As-Fe-W-Pb-Bi-Au-Ag veins; and, 3) Potato Hills Style-2 (PHS-2), high sulfide and sulfosalt content ( > 30% vol.), Fe-Pb-Sb-Zn-Cu-Au-Ag veins. In the ES and PHS-1 veins, Au is present both as native gold (Au, Ag) and as invisible gold in arsenopyrite, whilst in the PHS-2 veins, Au is present as invisible gold in pyrite. Native gold micrograins (individual grains, 1-100's mu m in size) are observed associated with Pb minerals (in anhedral-toglobular cosalite (Pb2Bi2S5) in ES veins, or galena (PbS) in the PHS-1 veins]. Native gold is also observed as micrograins along arsenopyrite margins and in quartz fractures. We suggest a variation on the hydrothermal Bi melt collector model to explain the Au-Pb +/- Bi association. The Au, Ag, Pb, and Bi are interpreted to have been locally remobilized from arsenopyrite, which shows textures and trace element distribution patterns consistent with fluid-and-deformation assisted recrystallization. We suggest Pb and Bi were mobilized either as immiscible nanodroplets that coalesced to form larger Pb +/- Bi liquid accumulations or into the hydrothermal fluid and subsequently exsolved to form immiscible Pb +/- Bi liquids. We propose that remobilized Au and Ag were collected by these Pb +/-Bi immiscible liquids. Subsequent retrograde alteration (sulfidation) is interpreted to have converted the Au-Ag-Pb +/- Bi alloys to native gold and cosalite (ES), and native gold and galena mineral assemblages (PHS-1). The similarity of the Au/Ag ratios in native gold and arsenopyrite supports a local source for the native gold micrograins. Temperatures required to attain liquid Pb-0.Bi-5(0).(5) ( > 145.2 degrees C) and Pb (> 327.5 degrees C), are consistent with arsenopyrite geothermometry (ES 345-405 degrees C; PHS-1 approximate to 380 degrees C). These suggested new variations (Pb-Bi and Pb) on the hydrothermal melt (Bi) collector model are important, given the common association of native gold with Pb +/- Bi sulfosalts in many gold deposits, and the relatively low temperatures required to maintain these liquids and collect Au from the hydrothermal fluid.

  • 153. Cave, Ben J.
    et al.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Craw, Dave
    Large, Ross R.
    Thompson, Jay M.
    Johnson, Sean C.
    A metamorphic mineral source for tungsten in the turbidite-hosted orogenic gold deposits of the Otago Schist, New Zealand2017In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 52, no 4, p. 515-537Article in journal (Refereed)
    Abstract [en]

    The orogenic gold deposits of the Otago Schist, New Zealand, are enriched in a variety of trace elements including Au, As, Ag, Hg, W and Sb. We combine laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) traverses and images to show that detrital rutile is the most important host mineral for W in the subgreenschist facies rocks. Furthermore, the prograde metamorphic recrystallisation of detrital rutile to titanite releases significant amounts of W (potentially 0.41 g/tonne of rock). Scheelite development closely follows the progression of this W-liberating reaction. Scheelite micrograins form early within the fabric of the rock evolving to locally and regionally sourced scheelite-bearing veins. Scheelite from syn-metamorphic veins at Fiddlers Flat and Lake HAwea shows distinct differences in composition compared with scheelite from late-metamorphic veins at the Macraes Mine, the latter of which is enriched in REEs, Y and Sr. We suggest that the scheelite at Macraes became enriched due to the liberation of these elements during alteration of the Ca-silicate minerals epidote and titanite by the ore-forming fluid. These results are supportive of recent models for orogenic gold mineralisation in the Otago Schist, whereby prograde metamorphic recrystallisation of diagenetic or detrital metal-rich mineral phases (pyrite to pyrrhotite: Au, As, Ag, Hg and Sb; rutile to titanite: W) releases significant amounts of metals into the concurrently developing metamorphic fluids that can be subsequently focussed into regional structures and form significant tungsten-bearing orogenic gold deposits.

  • 154.
    Chabangborn, Akkaneewut
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Asian monsoon over mainland Southeast Asia in the past 25 000 years2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The objective of this research is to interpret high-resolution palaeo-proxy data sets to understand the Asian summer monsoon variability in the past. This was done by synthesizing published palaeo-records from the Asian monsoon region, model simulation comparisons, and analysing new lake sedimentary records from northeast Thailand.

    Palaeo-records and climate modeling indicate a strengthened summer monsoon over Mainland Southeast Asia during the Last Glacial Maximum (LGM), compared to dry conditions in other parts of the Asian monsoon region. This can be explained by the LGM sea level low stand, which exposed Sundaland and created a large land-sea thermal contrast. Sea level rise ~19 600 years before present (BP), reorganized the atmospheric circulation in the Pacific Ocean and weakened the summer monsoon between 20 000 and 19 000 years BP.

    Both the Mainland Southeast Asia and the East Asian monsoon hydroclimatic records point to an earlier Holocene onset of strengthened summer monsoon, compared to the Indian Ocean monsoon. The asynchronous evolution of the summer monsoon and a time lag of 1500 years between the East Asian and the Indian Ocean monsoon can be explained by the palaeogeography of Mainland Southeast Asia, which acted as a land bridge for the movement of the Intertropical Convergence Zone.

    The palaeo-proxy records from Lake Kumphawapi compare well to the other data sets and suggest a strengthened summer monsoon between 10 000 and 7000 years BP and a weakening of the summer monsoon thereafter. The data from Lake Pa Kho provides a picture of summer monsoon variability over 2000 years. A strengthened summer monsoon prevailed between BC 170-AD 370, AD 800-960 and since AD 1450, and was weaker about AD 370-800 and AD 1300-1450. The movement of the mean position of the Intertropical Convergence Zone explains shifts in summer monsoon intensity, but weakening of the summer monsoon between 960 and 1450 AD could be affected by changes in the Walker circulation.

  • 155.
    Chabangborn, Akkaneewut
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The Asian monsoon - 50-7 ka BP2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The Asian monsoon is one of the largest climatic systems on Earth. It covers an area from the Arabian Sea to the South China Sea and from northern Australia to northern China with the world’s highest population density. Moreover, the Asian monsoon transports heat energy and humidity to higher latitudes. In order to better understand the behaviour of the Asian monsoon and its environmental impact, its variability between 50 and 7 ka BP is analysed using paleo-data compilation, data-model comparisons, and lake sediment analysis.

    The main results presented here are from the compilation of the Asian monsoon variability during the last glacial maximum (LGM) (23 - 19 ka BP) which is presumed to be under persistence cool and dry climatic conditions. The pattern of reconstructed and simulated precipitation agrees well in most of the region. However, the data-model discrepancies show in some areas, which may come from low resolution of the model or the local topographic effect. The reconstructed SSTs are well correlation with simulated SSTs, except in the Arabian Sea. The LGM Asian monsoon changes around 20 – 19 ka BP. The simulated ITCZ varies between 5°N and 15°N in the west and the east of the Asian monsoon region. However, the reconstructed ITCZ is ~5°N in the Arabian Sea, shifts northward in the Bay of Bengal, reaches ~30°N over central of China and migrates southward in the South China Sea. The ITCZ is likely shift northward after 20 ka BP. The climatic change might have been triggered by several factors, e.g., an increased land-sea thermal contrast and a variation of Pacific water inflow.

  • 156.
    Chabangborn, Akkaneewut
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Brandefelt, Jenny
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Asian monsoon climate during the Last Glacial Maximum: palaeo-data–model comparisons2013In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 43, no 1, p. 220-242Article in journal (Refereed)
    Abstract [en]

    The Last Glacial Maximum (LGM) (23-19ka BP) in the Asian monsoon region is generally described as cool and dry, due to a strong winter monsoon. More recently, however, palaeo-data and climate model simulations have argued for a more variable LGM Asian monsoon climate with distinct regional differences. We compiled, evaluated, and partly re-assessed proxy records for the Asian monsoon region in terms of wet/dry climatic conditions based on precipitation and effective moisture, and of sea surface temperatures. The comparison of the palaeo-data set to LGM simulations by the Climate Community System Model version 3 (CCSM3) shows fairly good agreement: a dry LGM climate in the western and northern part due to a strengthened winter monsoon and/or strengthened westerly winds and wetter conditions in equatorial areas, due to a stronger summer monsoon. Data-model discrepancies are seen in some areas and are ascribed to the fairly coarse resolution of CCSM3 and/or to uncertainties in the reconstructions. Differences are also observed between the reconstructed and simulated northern boundaries of the Intertropical Convergence Zone (ITCZ). The reconstructions estimate a more southern position over southern India and the Bay of Bengal, whereas CCSM3 simulates a more northern position. In Indochina, the opposite is the case. The palaeo-data indicate that climatic conditions changed around 20-19ka BP, with some regions receiving higher precipitation and some experiencing drier conditions, which would imply a distinct shift in summer monsoon intensity. This shift was probably triggered by the late LGM sea-level rise, which led to changes in atmosphere-ocean interactions in the Indian Ocean. The overall good correspondence between reconstructions and CCSM3 suggests that CCSM3 simulates LGM climate conditions over subtropical and tropical areas fairly well. The few high-resolution qualitative and quantitative palaeo-records available for the large Asian monsoon region make reconstructions however still uncertain

  • 157.
    Chabangborn, Akkaneewut
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Climate over mainland Southeast Asia 10.5–5 ka2014In: Journal of Quaternary Science, ISSN 0267-8179, E-ISSN 1099-1417, Vol. 29, no 5, p. 445-454Article in journal (Refereed)
    Abstract [en]

    We assembled and evaluated Holocene palaeo-vegetation records regarding semi-quantitative precipitation and temperature for mainland Southeast Asia and compared these with precipitation reconstructions for the Indian Ocean (IOM) and East Asian (EAM) monsoon sub-systems. Our results indicate that temperatures and precipitation in mainland Southeast Asia generally exceeded 18 °C and 1100 mm a−1 during the Holocene. Mainland Southeast Asia experienced cool/wet climatic conditions between 10.5 and 10 ka BP, a warmer/drier climate between 10 and 9 ka BP, cooler/wetter conditions between 9 and 7 ka BP, and moderately warmer/drier conditions since 7 ka BP. The reconstructed summer monsoon intensity compares well with the reconstructed hydroclimate for the EAM region, but diverges from that of the IOM region between 10.5 and 9 ka BP and 7–6.5 ka BP. This discrepancy is explained by differences in land–sea configuration, and regional sea-level history. A strengthening/weakening of the Asian summer monsoon between 9 and 7 ka BP and after 6.5 ka BP, respectively, is observed across the whole Asian monsoon region. Our new data sets support an asynchronous onset of the Asian summer monsoon optimum.

  • 158. Chabangborn, Akkaneewut
    et al.
    Yamoah, Kweku K. A.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Phantuwongraj, Sumet
    Choowong, Montri
    Climate in Sundaland and Asian monsoon variability during the last deglaciation2018In: Quaternary International, ISSN 1040-6182, E-ISSN 1873-4553, Vol. 479, p. 141-147Article in journal (Refereed)
    Abstract [en]

    The lack of semi-quantitative paleo data from mainland Southeast Asia, especially from the emergent Sundaland, creates a marked difficulty in following the past Asian monsoon variability. The published pollen records from the adjacent sites to the Sundaland were selected in this study and evaluated for the relationship between taxa and plant functional types (PFTs), and subsequently between PFTs and biome, to eventually reconstruct the semi-quantitative temporal temperature and precipitation profile. In order to comprehend the Asian monsoon modification between 18.5 and 11 ka BP, the derived pollen based temperature and precipitation records were further analyzed together with other selected speleothem records from the Asian monsoon region, with reference to the Greenland and Antarctic ice cores. The warmer temperature in the Southern Hemisphere caused a southward shift of the mean position of the Intertropical Convergent Zone (ITCZ) and weakened the summer monsoon in the Asian monsoon region between 18.5 and 15 ka BP. The Northern Hemisphere temperature played an important role in the Asian monsoon modification between 15 and 13.5 ka BP, where the warmer Northern Hemisphere conditions strengthened the summer monsoon intensity in the Asian monsoon region and moved the mean position of the ITCZ to the north. However, the opposing precipitation pattern between the East and the West Indian Ocean suggested the potential influence of the Walker circulation on the Sundaland climate from 13.5 to 11 ka BP.

  • 159. Chang, Kuang-Yu
    et al.
    Riley, William J.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Grant, Robert F.
    Rich, Virginia I.
    Saleska, Scott R.
    Large carbon cycle sensitivities to climate across a permafrost thaw gradient in subarctic Sweden2019In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 13, no 2, p. 647-663Article in journal (Refereed)
    Abstract [en]

    Permafrost peatlands store large amounts of carbon potentially vulnerable to decomposition. However, the fate of that carbon in a changing climate remains uncertain in models due to complex interactions among hydrological, biogeochemical, microbial, and plant processes. In this study, we estimated effects of climate forcing biases present in global climate reanalysis products on carbon cycle predictions at a thawing permafrost peatland in subarctic Sweden. The analysis was conducted with a comprehensive biogeochemical model (ecosys) across a permafrost thaw gradient encompassing intact permafrost palsa with an ice core and a shallow active layer, partly thawed bog with a deeper active layer and a variable water table, and fen with a water table close to the surface, each with distinct vegetation and microbiota. Using in situ observations to correct local cold and wet biases found in the Global Soil Wetness Project Phase 3 (GSWP3) climate reanalysis forcing, we demonstrate good model performance by comparing predicted and observed carbon dioxide (CO2) and methane (CH4) exchanges, thaw depth, and water table depth. The simulations driven by the bias-corrected climate suggest that the three peatland types currently accumulate carbon from the atmosphere, although the bog and fen sites can have annual positive radiative forcing impacts due to their higher CH4 emissions. Our simulations indicate that projected precipitation increases could accelerate CH4 emissions from the palsa area, even without further degradation of palsa permafrost. The GSWP3 cold and wet biases for this site significantly alter simulation results and lead to erroneous active layer depth (ALD) and carbon budget estimates. Biases in simulated CO2 and CH4 exchanges from biased climate forcing are as large as those among the thaw stages themselves at a landscape scale across the examined permafrost thaw gradient. Future studies should thus not only focus on changes in carbon budget associated with morphological changes in thawing permafrost, but also recognize the effects of climate forcing uncertainty on carbon cycling.

  • 160. Chatterjee, Sayantan
    et al.
    Bhatnagar, Gaurav
    Dugan, Brandon
    Dickens, Gerald R.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Rice University, USA.
    Chapman, Walter G.
    Hirasaki, George J.
    The impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments2014In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 119, no 9, p. 6705-6732Article in journal (Refereed)
    Abstract [en]

    Gas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two-dimensional (2-D) numerical model that accounts for mass transfer over geological timescales. The model extends a previously documented one-dimensional (1-D) model such that lateral variations in permeability (k) become important. Various simulations quantitatively demonstrate how focused fluid flow through high-permeability zones affects local hydrate accumulation and saturation. Simulations that approximate a vertical fracture network isolated in a lower permeability shale (k(fracture) >>k(shale)) show that focused fluid flow through the gas hydrate stability zone (GHSZ) produces higher saturations of gas hydrate (25-70%) and free gas (30-60%) within the fracture network compared to surrounding shale. Simulations with a dipping, high-permeability sand layer also result in elevated saturations of gas hydrate (60%) and free gas (40%) within the sand because of focused fluid flow through the GHSZ. Increased fluid flux, a deep methane source, or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high-permeability zones. Permeability anisotropy, with a vertical to horizontal permeability ratio on the order of 10(-2), enhances transport of methane-charged fluid to high-permeability conduits. As a result, gas hydrate concentrations are enhanced within these high-permeability zones. The dip angle of these high-permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects. Hydrate and free gas saturations can be characterized by a local Peclet number (localized, vertical, focused, and advective flux relative to diffusion) relative to the methane solubility gradient, somewhat analogous to such characterization in 1-D systems. Even in lithologically complex systems, local hydrate and free gas saturations might be characterized by basic parameters (local flux and diffusivity).

  • 161.
    Chatziemmanouil, Jannis P.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The Upper Cretaceous of the Vomb Trough, southern Sweden1982Doctoral thesis, monograph (Other academic)
  • 162. Chauhan, T.
    et al.
    Rasmussen, T. L.
    Noormets, R.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hogan, K. A.
    Glacial history and paleoceanography of the southern Yermak Plateausince 132 ka BP2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 92, p. 155-169Article in journal (Refereed)
    Abstract [en]

    The southern Yermak Plateau (YP) is situated at the entrance to the Arctic Ocean in the narrow MarginalIce Zone (MIZ) between the Polar and Arctic Fronts, north-west of Svalbard. A gravity core JM10-02GChas been analysed in order to reconstruct paleoceanographic conditions and the movement of the seaice margin as well as the glacier ice conditions of the SvalbardeBarents Sea Ice Sheet (SBIS) during theLast InterglacialeGlacial cycle. The distribution of planktic and benthic foraminifera, planktic and benthicoxygen and carbon isotopes and variations in ice-rafted debris (IRD) has been investigated. The sedimentcore covers the time interval from the Marine Isotope Stage (MIS) 6/5e transition (Termination II, c.132 ka BP) to the early Holocene. During Termination II (TII), the SBIS retreated and the sea ice marginwas in distal position whereas during MIS 5 to MIS 4 the sea ice margin was close to the core site. Severalcore intervals interpreted as representing MIS 5e, MIS 5c, MIS 5a, MIS 3 and MIS 1 were barren ofcalcareous microfossils whereas the intervals representing MIS 4 and MIS 2 were characterised by highproductivity (HP) of planktic and benthic foraminifera. These “glacial” HP zones were associated with theopen water conditions resulting from the advection of Atlantic Water (AW) and retreat of the sea icemargin. The barren zones during MIS 5, MIS 3 and MIS 1 resulted from the proximity of the sea icemargin whereas during MIS 2 the likely cause was an advance of the SBIS.

  • 163.
    Chawchai, Sakonvan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Paleoenvironmental and paleoclimatic changes in northeast Thailand during the Holocene2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The long-term climatic and environmental history of Southeast Asia is still fragmentary. This thesis therefore aims at studying lake sediment/peat sequences using a multi-proxy approach to reconstruct the environmental history and the impact of past changes in monsoon variability and intensity on lake ecosystems in Thailand. The study focuses on two lakes located in northeast Thailand: the larger Lake Kumphawapi and the smaller Lake Pa Kho.

    The comparison of multiple sediment sequences and their proxies from Kumphawapi suggests a strengthening of the summer monsoon between c. 10,000 and 7000 cal yr BP. Parts of the lake had been transformed into a wetland/peatland by c. 7000 cal yr BP, while the deeper part of the basin still contained areas of shallow water until c. 6600 cal yr BP. This gradual lowering of the lake level can point to a weakening of the summer monsoon. Paleoenvironmental information for the time interval between 6200 and 1800 cal yr BP is limited due to a several thousand-year long hiatus. This new investigation demonstrates that arguments using the phytolith and pollen record of Lake Kumphawapi to support claims of early rice agriculture in the region or an early start of the Bronze Age are not valid, because these were based upon the assumption of continuous deposition. The lithostratigraphy and multi-proxy reconstructions for Pa Kho support a strengthened summer monsoon between 2120-1580 cal yr BP, 1150-980 cal yr BP, and after 500 cal yr BP; and a weakening of the summer monsoon between 1580-1150 cal yr BP and between 650-500 cal yr BP. The increase in run-off and higher nutrient supply after AD 1700 can be linked to agricultural intensification in the region. Conclusively, the Holocene records from northeast Thailand add important paleoclimatic information for Southeast Asia and allow discussing past monsoon variability and movements of the Intertropical Convergence Zone in greater detail.

  • 164.
    Chawchai, Sakonvan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The Holocene climate history of Lake Kumphawapi, northeast Thailand2012Licentiate thesis, comprehensive summary (Other academic)
  • 165.
    Chawchai, Sakonvan
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Chulalongkorn University, Thailand.
    Chabangborn, Akkaneewut
    Stockholm University, Faculty of Science, Department of Geological Sciences. Chulalongkorn University, Thailand.
    Fritz, Sherilyn
    Valiranta, Minna
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Blaauw, Maarten
    Reimer, Paula J.
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Lowemark, Ludvig
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hydroclimatic shifts in northeast Thailand during the last two millennia - the record of Lake Pa Kho2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 111, p. 62-71Article, review/survey (Refereed)
    Abstract [en]

    The Southeast Asian mainland is located in the central path of the Asian summer monsoon, a region where paleoclimatic data are still sparse. Here we present a multi-proxy (TOC, C/N, delta C-13, biogenic silica, and XRF elemental data) study of a 1.5 m sediment/peat sequence from Lake Pa Kho, northeast Thailand, which is supported by 20 AMS C-14 ages. Hydroclimatic reconstructions for Pa Kho suggest a strengthened summer monsoon between BC 170-AD 370, AD 800-960, and after AD 1450; and a weakening of the summer monsoon between AD 370-800, and AD 1300-1450. Increased run-off and a higher nutrient supply after AD 1700 can be linked to agricultural intensification and land-use changes in the region. This study fills an important gap in data coverage with respect to summer monsoon variability over Southeast Asia during the past 2000 years and enables the mean position of the Intertropical Convergence Zone (ITCZ) to be inferred based on comparisons with other regional studies. Intervals of strengthened/weaker summer monsoon rainfall suggest that the mean position of the ITCZ was located as far north as 35 degrees N between BC 170-AD 370 and AD 800-960, whereas it likely did not reach above 17 degrees N during the drought intervals of AD 370-800 and AD 1300-1450. The spatial pattern of rainfall variation seems to have changed after AD 1450, when the inferred moisture history for Pa Kho indicates a more southerly location of the mean position of the summer ITCZ.

  • 166.
    Chawchai, Sakonvan
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chabangborn, Akkaneewut Nut
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kylander, Malin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Loewemark, L.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Blaauw, M.
    Klubseang, W.
    Reimer, P. J.
    Fritz, S. C.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lake Kumphawapi - an archive of Holocene palaeoenvironmental and palaeoclimatic changes in northeast Thailand2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 68, p. 59-75Article in journal (Refereed)
    Abstract [en]

    The long-term climatic and environmental history of Southeast Asia, and of Thailand in particular, is still fragmentary. Here we present a new C-14-dated, multi-proxy sediment record (TOC, C/N, CNS isotopes, Si, Zr, K, Ti, Rb, Ca elemental data, biogenic silica) for Lake Kumphawapi, the second largest natural lake in northeast Thailand. The data set provides a reconstruction of changes in lake status, groundwater fluctuations, and catchment run-off during the Holocene. A comparison of multiple sediment sequences and their proxies suggests that the summer monsoon was stronger between c. 9800 and 7000 cal yr BP. Lake status and water level changes around 7000 cal yr BP signify a shift to lower effective moisture. By c. 6500 cal yr BP parts of the lake had been transformed into a peatland, while areas of shallow water still occupied the deeper part of the basin until c. 5400-5200 cal yr BP. The driest interval in Kumphawapi's history occurred between c. 5200 and 3200 cal yr BP, when peat extended over large parts of the basin. After 3200 cal yr BP, the deepest part of the lake again turned into a wetland, which existed until c. 1600 cal yr BP. The observed lake-level rise after 1600 cal yr BP could have been caused by higher moisture availability, although increased human influence in the catchment cannot be ruled out. The present study highlights the use of multiple sediment sequences and proxies to study large lakes, such as Lake Kumphawapi in order to correctly assess the time transgressive response to past changes in hydroclimate conditions. Our new data set from northeast Thailand adds important palaeoclimatic information for a region in Southeast Asia and allows discussing Holocene monsoon variability and ITCZ movement in greater detail.

  • 167. Chawchai, Sakonvan
    et al.
    Kylander, Malin E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chabangborn, Akkaneewut
    Löwemark, Ludvig
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Testing commonly used XRF core scanning based proxies for organic rich lake sediments and peat2016In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 45, no 1, p. 180-189Article in journal (Refereed)
    Abstract [en]

    X-ray fluorescence (XRF) core scanning has become widely available for geological studies during the last decade. The data obtained from XRF core scanning, however, may be strongly influenced by the amount of organic matter, water content, density and porosity of the sediment matrix. In this study we discuss the usefulness of XRF core scanning to distinguish different kinds of organic-rich sediments and peat based on examples from tropical Lakes Kumphawapi and Nong Leng Sai in Thailand. We examined how sedimentary factors influence XRF core scanning analyses by comparing elemental and scattering ratios to lithological changes and quantitative LOI, TOC, biogenic silica (BSi) and grain-size values. Our comparison suggests that the (inc/coh) scattering ratio is of limited use as an indicator for variations in LOI and TOC in peaty gyttja or peat. In Lake Kumphawapi's sediments, Si/Ti ratios reflect clastic input associated with grain-size variations rather than BSi contents. The Ti-normalized ratios of Si, Zr, Sr, K and Rb are linked to mineral input and associated grain-size variations. We conclude that XRF core scanning of organic-rich tropical lake sediments and peat is useful to infer palaeoenvironmental conditions. However, XRF core scanning data does not stand-alone and needs to be underpinned by additional proxies.

  • 168.
    Chawchai, Sakonvan
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Chulalongkorn University, Thailand.
    Yamoah, Kweku Afrifa
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kurkela, Janita
    Väliranta, Minna
    Chabangborn, Akkaneewut
    Stockholm University, Faculty of Science, Department of Geological Sciences. Chulalongkorn University, Thailand.
    Blaauw, Marten
    Fritz, Sherilyn C.
    Reimer, Paula J.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lake Kumphawapi revisited – a synthesis of Holocene environmental and climatic changes for NE Thailand2016In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 26, no 4, p. 614-626Article in journal (Refereed)
    Abstract [en]

    Kumphawapi, which is Thailand’s largest natural freshwater lake, contains a >10,000-year-long climatic and environmental archive. New data sets (stratigraphy, chronology, hydrogen isotopes, plant macrofossil and charcoal records) for two sedimentary sequences are here combined with earlier multi-proxy studies to provide a comprehensive reconstruction of past climatic and environmental changes for Northeast Thailand. Gradually higher moisture availability due to a strengthening of the summer monsoon led to the formation of a large shallow lake in the Kumphawapi basin between >10,700 and c. 7000 cal. BP. The marked increase in moisture availability and lower evaporation between c. 7000 and 6400 cal. BP favoured the growth and expansion of vegetation in and around the shallow lake. The increase in biomass led to gradual overgrowing and infilling, to an apparent lake level lowering and to the development of a wetland. Multiple hiatuses are apparent in all investigated sequences between c. 6500 and 1400 cal. BP and are explained by periodic desiccation events of the wetland and erosion due to the subsequent lake level rise. The rise in lake level, which started c. 2000 cal. BP and reached shallower parts c. 1400 cal. BP, is attributed to an increase in effective moisture availability. The timing of hydroclimatic conditions during the past 2000 years cannot be resolved because of chronological limitations.

  • 169. Chen, H.-F.
    et al.
    Song, S.-R.
    Lee, T.-Q.
    Löwemark, Ludvig
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chi, Z.
    Yong, W.
    A multiproxy lake record from Inner Mongolia displays a late Holocene teleconnection between central Asian and North Atlantic climate2010In: Quaternary International, ISSN 1040-6182, E-ISSN 1873-4553, Vol. 227, no 2, p. 170-182Article in journal (Refereed)
    Abstract [en]

    In order to study how the Holocene Central Asian climate is coupled to the global climate system, a 4.24 m long lake core from western Inner Mongolia in China was studied using a multiproxy approach. Sedimentology and geochemical parameters such as gypsum and dolomite content, presence of lakeshore sand changing to aeolian sand, and changes in paleomagnetic properties bear witness to a trend toward a generally drier climate over the late Holocene. Aridification is linked to the southward retreat of the northern boundary of the Asian summer monsoon, leaving central Asia under the influence of the westerly belt. The weakening of the Asian summer monsoon in turn was caused by an orbitally driven decrease in summer insolation. The weakening summer insolation also likely increased the intensity of the Siberian High pressure system, further promoting aridification of central Asia. On a shorter time scale, the multiproxy record shows the climate to have been relatively dry during the Medieval Warm Period (AD 800–1100) with the ensuing humid environment at the end of this period gradually turning to become extremely dry (AD 1100–1550) at the Little Ice Age Maximum. Switches in the North Atlantic Oscillation caused these changes through a teleconnection in the form of westerlies. These westerlies provided most of central Asia’s moisture after the retreat of the Asian summer monsoon. The central Asian climate therefore corresponds closely with late Holocene European climate changes.

  • 170.
    Chi Fru, Ernest
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
    Arvestal, Emma
    Callac, Nolwenn
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    El Albani, Abderrazak
    Kilias, Stephanos
    Argyraki, Ariadne
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arsenic stress after the Proterozoic glaciations2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 17789Article in journal (Refereed)
    Abstract [en]

    Protection against arsenic damage in organisms positioned deep in the tree of life points to early evolutionary sensitization. Here, marine sedimentary records reveal a Proterozoic arsenic concentration patterned to glacial-interglacial ages. The low glacial and high interglacial sedimentary arsenic concentrations, suggest deteriorating habitable marine conditions may have coincided with atmospheric oxygen decline after similar to 2.1 billion years ago. A similar intensification of near continental margin sedimentary arsenic levels after the Cryogenian glaciations is also associated with amplified continental weathering. However, interpreted atmospheric oxygen increase at this time, suggests that the marine biosphere had widely adapted to the reorganization of global marine elemental cycles by glaciations. Such a glacially induced biogeochemical bridge would have produced physiologically robust communities that enabled increased oxygenation of the ocean-atmosphere system and the radiation of the complex Ediacaran-Cambrian life.

  • 171.
    Chi Fru, Ernest
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Cardiff University, UK.
    Callac, Nolwenn
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Posth, Nicole R.
    Argyraki, Ariadne
    Ling, Yu-Chen
    Ivarsson, Magnus
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kilias, Stephanos P.
    Arsenic and high affinity phosphate uptake gene distribution in shallow submarine hydrothermal sediments2018In: Biogeochemistry, ISSN 0168-2563, E-ISSN 1573-515X, Vol. 141, no 1, p. 41-62Article in journal (Refereed)
    Abstract [en]

    The toxicity of arsenic (As) towards life on Earth is apparent in the dense distribution of genes associated with As detoxification across the tree of life. The ability to defend against As is particularly vital for survival in As-rich shallow submarine hydrothermal ecosystems along the Hellenic Volcanic Arc (HVA), where life is exposed to hydrothermal fluids containing up to 3000 times more As than present in seawater. We propose that the removal of dissolved As and phosphorus (P) by sulfide and Fe(III)(oxyhydr)oxide minerals during sediment-seawater interaction, produces nutrient-deficient porewaters containing<2.0ppb P. The porewater arsenite-As(III) to arsenate-As(V) ratios, combined with sulfide concentration in the sediment and/or porewater, suggest a hydrothermally-induced seafloor redox gradient. This gradient overlaps with changing high affinity phosphate uptake gene abundance. High affinity phosphate uptake and As cycling genes are depleted in the sulfide-rich settings, relative to the more oxidizing habitats where mainly Fe(III)(oxyhydr)oxides are precipitated. In addition, a habitat-wide low As-respiring and As-oxidizing gene content relative to As resistance gene richness, suggests that As detoxification is prioritized over metabolic As cycling in the sediments. Collectively, the data point to redox control on Fe and S mineralization as a decisive factor in the regulation of high affinity phosphate uptake and As cycling gene content in shallow submarine hydrothermal ecosystems along the HVA.

  • 172.
    Chi Fru, Ernest
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hemmingsson, Christoffer
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Callac, Nolwenn
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pérez, Nathalie
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Panova, Elena G.
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    El Albani, Abderrazzak
    Atmospheric weathering of Scandinavian alum shales and the fractionation of C, N and S isotopes2016In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 74, p. 94-108Article in journal (Refereed)
    Abstract [en]

    Subaerial exposure and oxidation of organic carbon (C-org)-rich rocks is believed to be a key mechanism for the recycling of buried C and S back to Earth's surface. Importantly, processes coupled to microbial C-org oxidation are expected to shift new biomass delta C-13(org) composition towards more negative values relative to source. However, there is scarcity of information directly relating rock chemistry to oxidative weathering and shifting delta C-13(org) at the rock-atmosphere interface. This is particularly pertinent to the sulfidic, C-org-rich alum shale units of the Baltoscandian Basin believed to constitute a strong source of metal contaminants to the natural environment, following subaerial exposure and weathering. Consistent with independent support, we show that atmospheric oxidation of the sulfidic, C-org-rich alum shale sequence of the Cambrian-Devonian Baltoscandian Basin induces intense acid rock drainage at the expense of progressive oxidation of Fe sulfides. Sulfide oxidation takes priority over microbial organic matter decomposition, enabling quantitative massive erosion of C-org without producing a delta C-13 shift between acid rock drainage precipitates and shale. Moreover, C-13 enrichment in inorganic carbon of precipitates does not support microbial C-org oxidation as the predominant mechanism of rock weathering upon exposure. Instead, a Delta S-34 = delta S-34(shale) - delta S-34(precipitates) approximate to 0, accompanied by elevated S levels and the ubiquitous deposition of acid rock drainage sulfate minerals in deposited efflorescent precipitates relative to shales, provide strong evidence for quantitative mass oxidation of shale sulfide minerals as the source of acidity for chemical weathering. Slight delta N-15 depletion in the new surface precipitates relative to shale, coincides with dramatic loss of N from shales. Collectively, the results point to pyrite oxidation as a major driver of alum black shale weathering at the rock-atmosphere interface, indicating that quantitative mass release of C-org, N, S, and key metals to the environment is a response to intense sulfide oxidation. Consequently, large-scale acidic weathering of the sulfide-rich alum shale units is suggested to influence the fate and redistribution of the isotopes of C, N, and S from shale to the immediate environment.

  • 173.
    Chi Fru, Ernest
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
    Hemmingsson, Christoffer
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
    Holm, Mikaela
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chiu, Beverly
    Stockholm University, Faculty of Science, Department of Geological Sciences. Rutgers University, USA.
    Iniguez, Enrique
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arsenic-induced phosphate limitation under experimental Early Proterozoic oceanic conditions2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 434, p. 52-63Article in journal (Refereed)
    Abstract [en]

    Comparison of phosphorus concentrations associated with modern hydrothermal Fe(III)(oxyhydr)oxides and ancient Fe(III) oxide-rich iron formations, is used to estimate bioavailable Precambrian marine phosphorus (P) concentrations. This led to the proposition of a low dissolved P budget of similar to 10-25% of present-day levels, before similar to 1.9 billion years ago. Estimates incorporating ancient marine Si levels >= 0.67 mM instead suggested global dissolved P levels greater than today. Here we unite current experimental models that have considered NaCl solutions containing elevated dissolved Fe(II), Si, Ca2+ and Mg2+ ions in the incorporation of P in Precambrian marine Fe(III)(oxyhydr)oxides, in addition to arsenic as a hydrothermal proxy. We show that the coprecipitation of dissolved P and Fe(III)(oxyhydr)oxides from arsenic-rich marine waters produces an average P distribution coefficient of similar to 0.072 (+/- 0.01) mu M-1. This is comparable to the similar to 0.07 mu M-1 predicted for Fe(III)(oxyhydr)oxides in modern arsenic-rich, submarine hydrothermal settings, from which the lower Early Proterozoic dissolved marine P concentrations were predicted. As/P molar ratios below modern seawater ratios removed the negative feedback effect high Si impose on P scavenging by Fe(III)(oxyhydr)oxides. The binding of As(III) to Fe(III)(oxyhydr)oxides exhibits a lower competitive influence on P fixation. As(V) that likely became prominent in the surficially oxidized Early Proterozoic oceans induced dissolved P limitation because of preferential P sequestration at the expense of dissolved As(V) enrichment. The control of As on P scavenging by the precipitating Fe(III)(oxyhydr)oxides is strong regardless of common seawater cations (Mg2+ and Ca2+). The data suggest that the application of Si and Fe(III)(oxyhydr)oxides as an ancient seawater P proxy should consider chemical variability between depositional basins, taking into account the rather strong role hydrothermal arsenic has on the distribution of P in Fe(Ill)(oxyhydr)oxides. We propose that the generalized lower dissolved P budgets estimated from Early Proterozoic iron formations are consistent with oceans thought to be at least 3-4 times more hydrothermally active than at present.

  • 174.
    Chi Fru, Ernest
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History, Sweden.
    Ivarsson, M.
    Kilias, S. P.
    Frings, P. J.
    Hemmingsson, Christoffer
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bengtson, S.
    Chatzitheodoridis, E.
    Biogenicity of an Early Quaternary iron formation, Milos Island, Greece2015In: Geobiology, ISSN 1472-4677, E-ISSN 1472-4669, Vol. 13, no 3, p. 225-244Article in journal (Refereed)
    Abstract [en]

    A 2.0-million-year-old shallow-submarine sedimentary deposit on Milos Island, Greece, harbours an unmetamorphosed fossiliferous iron formation (IF) comparable to Precambrian banded iron formations (BIFs). This Milos IF holds the potential to provide clues to the origin of Precambrian BIFs, relative to biotic and abiotic processes. Here, we combine field stratigraphic observations, stable isotopes of C, S and Si, rock petrography and microfossil evidence from a 5-m-thick outcrop to track potential biogeochemical processes that may have contributed to the formation of the BIF-type rocks and the abrupt transition to an overlying conglomerate-hosted IF (CIF). Bulk C-13 isotopic compositions lower than -25 parts per thousand provide evidence for biological contribution by the Calvin and reductive acetyl-CoA carbon fixation cycles to the origin of both the BIF-type and CIF strata. Low S levels of 0.04 wt.% combined with S-34 estimates of up to 18 parts per thousand point to a non-sulphidic depository. Positive Si-30 records of up to +0.53 parts per thousand in the finely laminated BIF-type rocks indicate chemical deposition on the seafloor during weak periods of arc magmatism. Negative Si-30 data are consistent with geological observations suggesting a sudden change to intense arc volcanism potentially terminated the deposition of the BIF-type layer. The typical Precambrian rhythmic rocks of alternating Fe- and Si-rich bands are associated with abundant and spatially distinct microbial fossil assemblages. Together with previously proposed anoxygenic photoferrotrophic iron cycling and low sedimentary N and C potentially connected to diagenetic denitrification, the Milos IF is a biogenic submarine volcano-sedimentary IF showing depositional conditions analogous to Archaean Algoma-type BIFs.

  • 175. Chi Fru, Ernest
    et al.
    Ivarsson, Magnus
    Kilias, Stephanos P.
    Bengtson, Stefan
    Belivanova, Veneta
    Marone, Federica
    Fortin, Danielle
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stampanoni, Marco
    Fossilized iron bacteria reveal a pathway to the biological origin of banded iron formation2013In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 4, p. 2050-Article in journal (Refereed)
    Abstract [en]

    Debates on the formation of banded iron formations in ancient ferruginous oceans are dominated by a dichotomy between abiotic and biotic iron cycling. This is fuelled by difficulties in unravelling the exact processes involved in their formation. Here we provide fossil environmental evidence for anoxygenic photoferrotrophic deposition of analogue banded iron rocks in shallow marine waters associated with an Early Quaternary hydrothermal vent field on Milos Island, Greece. Trace metal, major and rare earth elemental compositions suggest that the deposited rocks closely resemble banded iron formations of Precambrian origin. Well-preserved microbial fossils in combination with chemical data imply that band formation was linked to periodic massive encrustation of anoxygenic phototrophic biofilms by iron oxyhydroxide alternating with abiotic silica precipitation. The data implicate cyclic anoxygenic photoferrotrophy and their fossilization mechanisms in the construction of microskeletal fabrics that result in the formation of characteristic banded iron formation bands of varying silica and iron oxide ratios.

  • 176.
    Chi Fru, Ernest
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Cardiff University, UK.
    Kilias, Stephanos
    Ivarsson, Magnus
    Rattray, Jayne E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gkika, Katerina
    McDonald, Iain
    He, Qian
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sedimentary mechanisms of a modern banded iron formation on Milos Island, Greece2018In: Solid Earth, ISSN 1869-9510, E-ISSN 1869-9529, Vol. 9, no 3, p. 573-598Article in journal (Refereed)
    Abstract [en]

    An early Quaternary shallow submarine hydrothermal iron formation (IF) in the Cape Vani sedimentary basin (CVSB) on Milos Island, Greece, displays banded rhythmicity similar to Precambrian banded iron formation (BIF). Field-wide stratigraphic and biogeochemical reconstructions show two temporal and spatially isolated iron deposits in the CVSB with distinct sedimentological character. Petrographic screening suggests the presence of a photoferrotrophic-like microfossil-rich IF (MFIF), accumulated on a basement consisting of andesites in a similar to 150 m wide basin in the SW margin of the basin. A banded nonfossiliferous IF (NFIF) sits on top of the Mn-rich sandstones at the transition to the renowned Mn-rich formation, capping the NFIF unit. Geochemical data relate the origin of the NFIF to periodic submarine volcanism and water column oxidation of released Fe(II) in conditions predominated by anoxia, similar to the MFIF. Raman spectroscopy pairs hematite-rich grains in the NFIF with relics of a carbonaceous material carrying an average delta C-13 org signature of similar to-25%0. A similar delta C-13 org signature in the MFIF could not be directly coupled to hematite by mineralogy. The NFIF, which postdates large-scale Mn deposition in the CVSB, is composed primarily of amorphous Si (opal-SiO2 center dot nH(2)O) while crystalline quartz (SiO2) predominates the MFIF. An intricate interaction between tectonic processes, changing redox, biological activity, and abiotic Si precipitation are proposed to have collectively formed the unmetamorphosed BIF-type deposits in a shallow submarine volcanic center. Despite the differences in Precambrian ocean-atmosphere chemistry and the present geologic time, these formation mechanisms coincide with those believed to have formed Algoma-type BIFs proximal to active seafloor volcanic centers.

  • 177.
    Chi Fru, Ernest
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rodriguez, Nathalie P.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Partin, Camille A.
    Lalonde, Stefan V.
    Andersson, Per
    Weiss, Dominik J.
    El Albani, Abderrazak
    Rodushkin, Ilia
    Konhauser, Kurt O.
    Cu isotopes in marine black shales record the Great Oxidation Event2016In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 113, no 18, p. 4941-4946Article in journal (Refereed)
    Abstract [en]

    The oxygenation of the atmosphere similar to 2.45-2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth's redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (delta Cu-65(ERM-AE633)) in organic carbon-rich shales spanning the period 2.66-2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in Cu-65, along with the preferential removal of Cu-65 by iron oxides, left seawater and marine biomass depleted in Cu-65 but enriched in Cu-63. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy delta Cu-65 values coincides with a shift to negative sedimentary delta Fe-56 values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary delta Cu-65 values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past.

  • 178. Chiu, Pin-Yao
    et al.
    Chao, Weng-Si
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Li, Hong-Chun
    Löwemark, Ludvig
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    New constraints on Arctic Ocean Mn stratigraphy from radiocarbon dating on planktonic foraminifera2017In: Quaternary International, ISSN 1040-6182, E-ISSN 1873-4553, Vol. 447, p. 13-26Article in journal (Refereed)
    Abstract [en]

    Variations in the abundance of manganese (Mn) in Arctic Ocean sediments are used as a tool to identify glacial and interglacial periods. This study aims to provide new insight into the applicability of Mn as a stratigraphic tool in the topmost sediment and to investigate the occurrence of Mn peaks in sediments within the range of radiocarbon dating. In combination with variations in ice-rafted debris (IRD), radiocarbon dating is used to better constrain the stratigraphic occurrence of Mn peaks, and the synchroneity between multiple records, especially during the late glacial and the Holocene. We find that a hiatus spanning MIS 2 is widely observed in most of our cores, resulting in a merging of Mn peaks of Holocene age and the later part of MIS 3. The Holocene Mn peak is usually high amplitude but short, while the MIS 3 Mn peak has a lower amplitude and is protracted. Where preserved, MIS 2 sediments form a 2-3 cm thick layer characterized by a light color, low Mn content, sparse IRD and low foraminiferal abundance. IRD variations provide a powerful tool to identify the boundary of the Holocene and late MIS 3 in cores with a MIS 2 hiatus. Because the IRD content displays a general increment from the start of MIS 3, and both the Holocene and MIS 2 show small IRD variations, the end of MIS 3 can be pinpointed to the point of decrease in IRD. The hiatus of MIS 2 is widely observed in our cores, suggesting extensive persistent sea ice coverage during the peak of the last glacial cycle, with sharply reduced sedimentation throughout the Arctic Ocean. Identifying similar events during previous glacial periods may be an important step towards constructing longer and more accurate chronologies for Arctic Ocean sediments.

  • 179.
    Chong, Ting-Fung
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mineralogy and petrology of a drill core section from Borg, SW Norrköping: Fracture fillings and tentative mineral reactions2019Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    During drilling at Borg, ca. 5 km southwest of central Norrköping, an unidentified fine-grained grey rock was encountered in contact with dark red granitic gneiss at 35m depth. The fine-grained grey rock occurs above the red granitic gneiss. Our understanding prior to testing is that the rock may have been formed by fluid alteration of a mafic rock, as neither metamorphic nor sedimentary textures were observed. Understanding this rock and the formation processes related to it, is of interest for the general understanding of the region and of geotechnical importance since fracture fillings may affect the stability of the rock during and after construction projects. This study uses the methods petrography, XRD and XRF to define the fine-grained mafic rock, black vein in the fine-grained mafic rock, the contact zone and the granitic gneiss. Results show that the fine-grained grey rock has a composition of muscovite (36.1%), quartz (24.6%), albite (20.8%), sericite (10.8) and montmorillonite (0.5%). The contact is mainly made of quartz veins and calcite veins. The protolith of the fine-grained mafic rock is suggested to be iron-rich. Further testing on additional unaltered mafic rock samples from the area would provide a more accurate picture of the protolith evolution.

  • 180. Choudhary, Preetam
    et al.
    Routh, Joyanto
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chakrapani, Govind J.
    Organic geochemical record of increased productivity in Lake Naukuchiyatal, Kumaun Himalayas, India2010In: ENVIRONMENTAL EARTH SCIENCES, ISSN ISSN 1866-6299, Vol. 60, no 4, p. 837-843Article in journal (Refereed)
    Abstract [en]

    Organic geochemical proxies have been studied in a 45-cm-long core retrieved from Lake Naukuchiyatal in Kumaun Himalayas, India. Increase in TOC, N, hydrocarbons and pigments concentration from bottom to surface sediments of the core indicates increase in the lake productivity. Stable isotopes (δ13C and δ 15 N), biomarkers (TAR, CPI and n-ΣC15,17,19) and C/N atomic (between 9 and 12) suggest dominance of algal derived organic matter in these sediments. Decrease in organic δ13C values (between −27 and −31‰) in surface sediments indicate influence of sewage and land runoff in shifting organic δ13C values, whereas low (between −0.23 and 2.2‰) δ15N values together with high pigment concentrations (zeaxanthin and echinenone) represent dominance of cyanobacteria in the lake.

  • 181. Christensen, Torben R.
    et al.
    Jackowicz-Korczynski, Marcin
    Aurela, Mika
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Heliasz, Michal
    Mastepanov, Mikhail
    Friborg, Thomas
    Monitoring the multi year carbon balance of a subarctic palsa mire with micrometeorological techniques2012In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 41, p. 207-217Article in journal (Refereed)
    Abstract [en]

    This article reports a dataset on 8 years of monitoring carbon fluxes in a subarctic palsa mire based on micrometeorological eddy covariance measurements. The mire is a complex with wet minerotrophic areas and elevated dry palsa as well as intermediate sub-ecosystems. The measurements document primarily the emission originating from the wet parts of the mire dominated by a rather homogenous cover of Eriophorum angustifolium. The CO2/CH4 flux measurements performed during the years 2001-2008 showed that the areas represented in the measurements were a relatively stable sink of carbon with an average annual rate of uptake amounting to on average -46 g C m(-2) y(-1) including an equally stable loss through CH4 emissions (18-22 g CH4-C m(-2) y(-1)). This consistent carbon sink combined with substantial CH4 emissions is most likely what is to be expected as the permafrost under palsa mires degrades in response to climate warming.

  • 182. Ciummelli, Marina
    et al.
    Raffi, Isabella
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Biostratigraphy and evolution of Miocene Discoaster spp. from IODP Site U1338 in the equatorial Pacific Ocean2017In: Journal of Micropalaeontology, ISSN 0262-821X, E-ISSN 2041-4978, Vol. 36, p. 137-152Article in journal (Refereed)
    Abstract [en]

    Assemblages of upper lower through upper Miocene Discoaster spp. have been quantified from Integrated Ocean Drilling Program (IODP) Site U1338 in the eastern equatorial Pacific Ocean. These assemblages can be grouped into five broad morphological categories: six-rayed with bifurcated ray tips, six-rayed with large central areas, six-rayed with pointed ray tips, five-rayed with bifurcated ray tips and five-rayed with pointed ray tips. Discoaster deflandrei dominates the assemblages prior to 15.8 Ma. The decline in abundance of D. deflandrei close to the early-middle Miocene boundary occurs together with the evolution of the D. variabilis group, including D. signus and D. exilis. Six-rayed discoasters having large central areas become a prominent member of the assemblages for a 400 ka interval in the late middle Miocene. Five-and six-rayed forms having pointed tips become prominent in the early late Miocene and show a strong antiphasing relationship with the D. variabilis group. Discoaster bellus completely dominates the Discoaster assemblages for a 400 ka interval in the middle late Miocene. Abundances of all discoasters, or discoasters at the species level, show only (surprisingly) weak correlations to carbonate contents or oxygen and carbon isotopes of bulk sediment when calculated over the entire sample interval.

  • 183. Claremar, Björn
    et al.
    Wällstedt, Teresia
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rutgersson, Anna
    Omstedt, Anders
    Deposition of acidifying and neutralising compounds over the Baltic Sea drainage basin between 1960 and 20062013In: Boreal environment research, ISSN 1239-6095, E-ISSN 1797-2469, Vol. 18, no 6, p. 425-445Article in journal (Refereed)
    Abstract [en]

    This study produced a gridded database of acidifying and eutrophying deposition in the Baltic Sea and its drainage basin for the period 1960-2006. Data from various data sets were combined to generate monthly atmospheric (wet) deposition of cations (Ca2+, Mg2+, Na+, K+ and NH4+) and anions (SO42-, NO3- and Cl-). Output of a chemical transport model and interpolated measurements were used, and when these were not available, trends and seasonal cycles were constructed from historical emissions and deposition data. These methods lose some spatial patterns, but the mean trends reflect the influence of east-European emissions more than earlier studies with more westerly-centred observations. The calculated depositions of sulphur, nitrogen and calcium (correlated with sulphur emission) increased from 1960 to 1990 and then decreased until 2006. The trend is most evident for sulphur with a 100% increase followed by a 73% decrease.

  • 184. Clark, Chris D.
    et al.
    Ely, Jeremy C.
    Greenwood, Sarah L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hughes, Anna L. C.
    Meehan, Robert
    Barr, Iestyn D.
    Bateman, Mark D.
    Bradwell, Tom
    Doole, Jenny
    Evans, David J. A.
    Jordan, Colm J.
    Monteys, Xavier
    Pellicer, Xavier M.
    Sheehy, Michael
    BRITICE Glacial Map, version 2: a map and GIS database of glacial landforms of the last British-Irish Ice Sheet2018In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 47, no 1, p. 11-27Article in journal (Refereed)
    Abstract [en]

    During the last glaciation, most of the British Isles and the surrounding continental shelf were covered by the British-Irish Ice Sheet (BIIS). An earlier compilation from the existing literature (BRITICE version 1) assembled the relevant glacial geomorphological evidence into a freely available GIS geodatabase and map (Clark etal. 2004: Boreas 33, 359). New high-resolution digital elevation models, of the land and seabed, have become available casting the glacial landform record of the British Isles in a new light and highlighting the shortcomings of the V.1 BRITICE compilation. Here we present a wholesale revision of the evidence, onshore and offshore, to produce BRITICE version 2, which now also includes Ireland. All published geomorphological evidence pertinent to the behaviour of the ice sheet is included, up to the census date of December 2015. The revised GIS database contains over 170000 geospatially referenced and attributed elements - an eightfold increase in information from the previous version. The compiled data include: drumlins, ribbed moraine, crag-and-tails, mega-scale glacial lineations, glacially streamlined bedrock (grooves, roches moutonnees, whalebacks), glacial erratics, eskers, meltwater channels (subglacial, lateral, proglacial and tunnel valleys), moraines, trimlines, cirques, trough-mouth fans and evidence defining ice-dammed lakes. The increased volume of features necessitates different map/database products with varying levels of data generalization, namely: (i) an unfiltered GIS database containing all mapping; (ii) a filtered GIS database, resolving data conflicts and with edits to improve geo-locational accuracy (available as GIS data and PDF maps); and (iii) a cartographically generalized map to provide an overview of the distribution and types of features at the ice-sheet scale that can be printed at A0 paper size at a 1:1250000 scale. All GIS data, the maps (as PDFs) and abibliography of all published sources are availablefor download from: https://www.sheffield.ac.uk/geography/staff/clark_chris/britice

  • 185. Clark, Chris D.
    et al.
    Hughes, Anna L. C.
    Greenwood, Sarah L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jordan, Colm
    Sejrup, Hans Petter
    Pattern and timing of retreat of the last British Irish ice sheet2012In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 44, p. 112-146Article in journal (Refereed)
    Abstract [en]

    During the last glacial the ice sheet that subsumed most of Britain, Ireland and the North Sea attained its maximum extent by 27 ka BP and with an ice volume sufficient to raise global sea level by ca 2.5 m when it melted. We reconstruct the demise of this British-Irish Ice Sheet (BIIS) and present palaeo-glaciological maps of retreat stages between 27 and 15 ka BR The whole land area was investigated using remote sensing data and we present maps of moraines, meltwater channels, eskers, and drumlins and a methodology of how to interpret and bring them together. For the continental shelf, numerous large moraines were discovered recording an extensive pattern of retreat stretching from SW Ireland to the Shetland Isles. From an integration of this new mapping of glacial geomorphology (>26,000 landforms) with previously published evidence, compiled in the BRITICE database, we derive a pattern of retreat for the whole BIIS. We review and compile relevant dates (881 examples) that constrain the timing of retreat. All data are held within a Geographic Information System (GIS), and are deciphered to produce a best-estimate of the combined pattern and timing of retreat. Pattern information reveals an ice sheet mainly comprised of a shelf-parallel configuration from SW Ireland to NE Scotland but it spread far enough to the south to incorporate outlying ice domes over Wales, the Lake District and Kerry. Final disintegration was into a number of separate ice caps, rather than reduction as a single mass, and paradoxically, retreat was not always back to high ground. By 23 ka BP ice withdrew along its northern boundaries at the same time as the southern margins were expanding, including transient ice streaming down the Irish Sea and advances of lobes in the Cheshire Basin, Vale of York and east coast of England. Ice divides migrated south. By 19 ka the ice sheet was in crisis with widespread marine-based ice losses, particularly in the northern North Sea and the Irish Sea. Considerable dynamic-thinning occurred during this phase. Final collapse of all marine sectors occurred by 17 ka BP and with most margins beginning to back-step onshore. Disintegration of the North Sea 'ice bridge' between Britain and Norway remains loosely constrained in time but the possibility of catastrophic collapse of this sector is highlighted. The North Channel and Irish Sea ice streams had finally cleaved the ice sheet into separate Irish and Scottish ice sheets by 16 ka BP. Rates of ice loss were found to vary widely over space and time (e.g., 65-260 km(3) per year). The role of ice streams and calving losses of marine-based sectors are examined. Retreat rates of up to ca 150 ma(-1) were found for some ice stream margins. That large parts (2/3) of the BIIS were marine-based, drained by ice streams, and possibly with fringing ice shelves in places, makes it a useful analogue for the West Antarctic Ice Sheet (WAIS). This is especially so because the BIIS deglaciated in response to rising temperatures and a rising sea level (driven by melting of other ice masses) which are the current forcings that might cause collapse of the WAIS. Our reconstruction, when viewed from the opposite perspective, documents when fresh land became exposed for exploitation by plants, animals and Man, and records for how long such land has been available for soil and geochemical development and ecological succession.

  • 186.
    Colleoni, Florence
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Joseph Fourier University, France.
    Krinner, G.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The role of an Arctic ice shelf in the climate of the last glacial maximum of MIS 6 (140 ka)2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 25-26, p. 3590-3597Article in journal (Refereed)
    Abstract [en]

    During the last decade, Arctic icebreaker and nuclear submarine expeditions have revealed large-scale Pleistocene glacial erosion on the Lomonosov Ridge, Chukchi Borderland and along the Northern Alaskan margin indicating that the glacial Arctic Ocean hosted large Antarctic-style ice shelves. Dating of sediment cores indicates that the most extensive and deepest ice grounding occurred during Marine Isotope Stage (MIS) 6. The precise extents of Pleistocene ice shelves in the Arctic Ocean are unknown but seem comparable to present existing Antarctic ice shelves. How would an Antarctic-style ice shelf in the MIS 6 Arctic Ocean influence the Northern Hemisphere climate? Could it have impacted on the surface mass balance (SMB) of the MIS 6 Eurasian ice sheet and contributed to its large southward extent? We use an Atmospheric General Circulation Model (AGCM) to investigate the climatic impacts of both a limited MIS 6 ice shelf covering portions of the Canada Basin and a fully ice shelf covered Arctic Ocean. The AGCM results show that both ice shelves cause a temperature cooling of about 3 °C over the Arctic Ocean mainly due to the combined effect of ice elevation and isolation from the underlying ocean heat fluxes stopping the snow cover from melting during summer. The calculated SMB of the ice shelves are positive. The ice front horizontal velocity of the Canada Basin ice shelf is estimated to ≈ 1 km yr−1 which is comparable to the recent measurements of the Ross ice shelf, Antarctica. The existence of a large continuous ice shelf covering the entire Arctic Ocean would imply a mean annual velocity of icebergs of ≈12 km yr−1 through the Fram Strait. Our modeling results show that both ice shelf configurations could be viable under the MIS 6 climatic conditions. However, the cooling caused by these ice shelves only affects the Arctic margins of the continental ice sheets and is not strong enough to significantly influence the surface mass balance of the entire MIS 6 Eurasian ice sheet.

  • 187.
    Colleoni, Florence
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Joseph Fourier University, France.
    Liakka, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Krinner, Gerhard
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Masina, Simona
    Peyaud, Vincent
    The sensitivity of the Late Saalian (140 ka) and LGM (21 ka) Eurasian ice sheets to sea surface conditions2011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 37, no 3-4, p. 531-553Article in journal (Refereed)
    Abstract [en]

    This work focuses on the Late Saalian (140 ka) Eurasian ice sheets’ surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). An Atmospheric General Circulation Model (AGCM), forced with two preexisting Last Glacial Maximum (LGM, 21 ka) SST reconstructions, is used to compute climate at 140 and 21 ka (reference glaciation). Contrary to the LGM, the ablation almost stopped at 140 ka due to the climatic cooling effect from the large ice sheet topography. Late Saalian SST are simulated using an AGCM coupled with a mixed layer ocean. Compared to the LGM, these 140 ka SST show an inter-hemispheric asymmetry caused by the larger ice-albedo feedback, cooling climate. The resulting Late Saalian ice sheet SMB is smaller due to the extensive simulated sea ice reducing the precipitation. In conclusion, SST are important for the stability and growth of the Late Saalian Eurasian ice sheet.

  • 188. Cory, Neil
    et al.
    Buffam, Ishi
    Laudon, Hjalmar
    SUBBjorkvald, Louise
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Koehler, Stephan
    Bishop, Kevin
    Particulate aluminium in boreal streams: Towards a better understanding of its sources and influence on dissolved aluminium speciation2009In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 24, no 9, p. 1677-1685Article in journal (Refereed)
    Abstract [en]

    The adverse impacts of the inorganic labile monomeric Al (Al-i) fraction on aquatic organisms have meant that Al (Al-tot) determination and even speciation has become a routine part of environmental monitoring and assessment. However, if samples are not filtered prior to analysis then particulate Al (Al-tot(p)) could influence the determination of Al-tot, and therefore the determination of the more toxicologically important (Al-i), both when it is measured analytically or modelled from Al-tot. This paper shows that the Al/DOC ratio in unfiltered samples can identify the Al-tot(p) fraction, and thus improve the speciation of Al-i. These findings are based on data from a study in a 67 km(2) catchment in northern Sweden during the snowmelt-driven spring flood of two consecutive years. Filtered and unfiltered samples were studied to determine the spatial and temporal patterns in Al-tot(p). The concentrations of Al-tot(p) were greatest in larger downstream sites where significant silt deposits are located. The sites with no silt in their drainage area showed a mean difference between filtered (Al-tot(f)) and unfiltered (Al-tot(uf)) samples of 6%, while sites with silt deposits had a mean difference of 65%. The difference between filtered and unfiltered samples was greatest at peak flow. Spikes in Al-tot(p) did not behave consistently during fractionation with a cation exchange column, resulting in increases in either measured Al-i(f) or non-labile monomeric Al (Al-o(f)). Al-tot(p) spikes were associated with sharp increases in the Al:DOC ratio. The baseflow Al:DOC ratio could be used to model filtered Al-tot from DOC with a Spearman rho of 0.75. 

  • 189.
    Coxall, Helen K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Huck, Claire E.
    Huber, Matthew
    Lear, Caroline H.
    Legarda-Lisarri, Alba
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sliwinska, Kasia K.
    van de Flierdt, Tina
    de Boer, Agatha M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Zachos, James C.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Export of nutrient rich Northern Component Water preceded early Oligocene Antarctic glaciation2018In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 11, no 3, p. 190-196Article in journal (Refereed)
    Abstract [en]

    The onset of the North Atlantic Deep Water formation is thought to have coincided with Antarctic ice-sheet growth about 34 million years ago (Ma). However, this timing is debated, in part due to questions over the geochemical signature of the ancient Northern Component Water (NCW) formed in the deep North Atlantic. Here we present detailed geochemical records from North Atlantic sediment cores located close to sites of deep-water formation. We find that prior to 36 Ma, the northwestern Atlantic was stratified, with nutrient-rich, low-salinity bottom waters. This restricted basin transitioned into a conduit for NCW that began flowing southwards approximately one million years before the initial Antarctic glaciation. The probable trigger was tectonic adjustments in subarctic seas that enabled an increased exchange across the Greenland-Scotland Ridge. The increasing surface salinity and density strengthened the production of NCW. The late Eocene deep-water mass differed in its carbon isotopic signature from modern values as a result of the leakage of fossil carbon from the Arctic Ocean. Export of this nutrient-laden water provided a transient pulse of CO2 to the Earth system, which perhaps caused short-term warming, whereas the long-term effect of enhanced NCW formation was a greater northward heat transport that cooled Antarctica.

  • 190.
    Coxall, Helen K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wilson, Paul A.
    Early Oligocene glaciation and productivity in the eastern equatorial Pacific: Insights into global carbon cycling2011In: Paleoceanography, ISSN 0883-8305, E-ISSN 1944-9186, Vol. 26, p. PA2221-Article in journal (Refereed)
    Abstract [en]

    The onset of sustained Antarctic glaciation across the Eocene-Oligocene transition (EOT) marks a pivotal change in Earth's climate, but our understanding of this event, particularly the role of the carbon cycle, is limited. To help address this gap we present the following paleoceanographic proxy records from Ocean Drilling Program Site 1218 in the eastern equatorial Pacific (EEP): (1) stable isotope (delta(18)O and delta(13)C) records generated in epifaunal benthic foraminifera (Cibicidoides spp.) to improve (double the resolution) the previously published records; (2) delta(18)O and delta(13)C records measured on Oridorsalis umbonatus, a shallow infaunal species; and (3) a record of benthic foraminifera accumulation rate (BFAR). Our new isotope data sets confirm the existence at Site 1218 of a two-step delta(18)O increase. They also lend support to the hypothesized existence of a late Eocene transient delta(18)O increase and early Oligocene Oi-1a and Oi-1b glacial maxima. Our record of BFAR indicates a transient (similar to 500 kyr) twofold to threefold peak relative to baseline Oligocene values associated with the onset of Antarctic glaciation that we attribute to enhanced biological export production in the EEP. This takes the same general form as the history of opal accumulation in the Southern Ocean, suggesting strong high-to-low-latitude oceanic coupling. These findings appear to lend support to the idea that the EOT delta(13)C excursion is traceable to increased organic carbon (C(org)) burial. Paradoxically, early Oligocene sediments in the EEP are extremely C(org)-poor, and proxy records of atmospheric pCO(2) indicate a transient increase associated with the EOT.

  • 191. Craw, Dave
    et al.
    Mortensen, Jim
    Mackenzie, Doug
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Contrasting geochemistry of orogenic gold deposits in Yukon, Canada and Otago, New Zealand2015In: Geochemistry: Exploration, Environment, Analysis, ISSN 1467-7873, E-ISSN 2041-4943, Vol. 15, no 2-3, p. 150-166Article in journal (Refereed)
    Abstract [en]

    The Yukon-Tanana Terrane (YTT) of western Yukon Territory in NW Canada and Otago Schist belt (OSB) of South Island, New Zealand share similar geological evolutionary histories as convergent orogenic belts. Both belts host orogenic gold deposits of mainly Jurassic to Early Cretaceous age. Jurassic mineralization in the YTT occurred during convergent orogenesis and stacking of previously-metamorphosed (Palaeozoic) greenschist-amphibolite facies metasediments, metavolcanic rocks, and metagranitoids. Early Cretaceous OSB mineralization occurred in the latter stages of terrane accretion of un-metamorphosed turbidites with minor basaltic rocks. Metamorphism of the OSB turbidites mobilised background levels of Au (0.6-1.3 ppb), As (2-20 ppm), Sb (0.1-1 ppm), and W (< 10 ppm), primarily under greenschist to lower amphibolite facies conditions when diagenetic pyrite (Au c.0.5-2 ppm; As c.500-10000 ppm) transformed to pyrrhotite on a regional scale. In contrast, the previously-metamorphosed YTT rocks had generally low background As contents (1-2 ppm) apart from some As-rich quartzites (up to 100 ppm As). Consequently, there was less As available for orogenic mobilisation, and YTT Au deposits generally have lower concentrations of this pathfinder element compared to the OSB. YTT host rocks, especially metagranitoids, have anomalous levels of Mo (10-300 ppm), and many orogenic deposits contain elevated Mo, locally including molybdenite. OSB turbidites have elevated Mo (2-200 ppm), along with elevated Au and As, in diagenetic pyrite, but this Mo became largely dispersed through the metamorphic pile as metamorphic grade increased and pyrite transformed to pyrrhotite. OSB orogenic deposits have only marginally elevated Mo (c.1 ppm), no molybdenite, and accessory scheelite in these deposits is distinctly Mo-poor. Only minor mobilisation of base metals occurred in these orogenic belts, and orogenic Au deposits contain sparse base metal sulphides. Orogenic deposits in the YTT and OSB differ in that Au (and other associated elements) in many of the orogenic deposits in the YTT was remobilised from relatively local sources (e.g. pre-existing Cu-Mo-Au porphyry or volcanogenic sulphide mineralization) whereas Au in the OSB was mobilised from larger volumes of homogeneous rock at depth.

  • 192.
    Cronin, T.
    et al.
    US Geological Survey.
    Dwyer, G.S.
    Briggs Jr., W.M.
    Farmer, J.
    Bauch, Henning
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Spielhagen, Robert
    Stepanova, A.
    Arctic Ocean Temperature History since 60 ka based on ostracode Mg/Ca ratios2011In: APEX Fith International Conference and Workshop: Quaternary Glacial and Climate Extremes, The University Centre in Svalbard (UNIS) , 2011, p. 22-22Conference paper (Other academic)
  • 193. Cronin, T. M.
    et al.
    Dwyer, G. S.
    Farmer, J.
    Bauch, H. A.
    Spielhagen, R. F.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Briggs, W. M., Jr.
    Stepanova, A.
    Deep Arctic Ocean warming during the last glacial cycle2012In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 5, no 9, p. 631-634Article in journal (Refereed)
    Abstract [en]

    In the Arctic Ocean, the cold and relatively fresh water beneath the sea ice is separated from the underlying warmer and saltier Atlantic Layer by a halocline. Ongoing sea ice loss and warming in the Arctic Ocean(1-7) have demonstrated the instability of the halocline, with implications for further sea ice loss. The stability of the halocline through past climate variations(8-10) is unclear. Here we estimate intermediate water temperatures over the past 50,000 years from the Mg/Ca and Sr/Ca values of ostracods from 31 Arctic sediment cores. From about 50 to 11 kyr ago, the central Arctic Basin from 1,000 to 2,500 m was occupied by a water mass we call Glacial Arctic Intermediate Water. This water mass was 1-2 degrees C warmer than modern Arctic Intermediate Water, with temperatures peaking during or just before millennial-scale Heinrich cold events and the Younger Dryas cold interval. We use numerical modelling to show that the intermediate depth warming could result from the expected decrease in the flux of fresh water to the Arctic Ocean during glacial conditions, which would cause the halocline to deepen and push the warm Atlantic Layer into intermediate depths. Although not modelled, the reduced formation of cold, deep waters due to the exposure of the Arctic continental shelf could also contribute to the intermediate depth warming.

  • 194. Cronin, T. M.
    et al.
    Gemery, L.
    Briggs, W. M.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Polyak, L.
    Brouwers, E. M.
    Quaternary Sea-ice history in the Arctic Ocean based on a new Ostracode sea-ice proxy2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 25-26, p. 3415-3429Article in journal (Refereed)
    Abstract [en]

    Paleo-sea-ice history in the Arctic Ocean was reconstructed using the sea-ice dwelling ostracode Acetabulastoma arcticum from late Quaternary sediments from the Mendeleyev, Lomonosov, and Gakkel Ridges, the Morris Jesup Rise and the Yermak Plateau. Results suggest intermittently high levels of perennial sea ice in the central Arctic Ocean during Marine Isotope Stage (MIS) 3 (25–45 ka), minimal sea ice during the last deglacial (16–11 ka) and early Holocene thermal maximum (11–5 ka) and increasing sea ice during the mid-to-late Holocene (5–0 ka). Sediment core records from the Iceland and Rockall Plateaus show that perennial sea ice existed in these regions only during glacial intervals MIS 2, 4, and 6. These results show that sea ice exhibits complex temporal and spatial variability during different climatic regimes and that the development of modern perennial sea ice may be a relatively recent phenomenon.

  • 195.
    Cronin, Thomas
    et al.
    USGS.
    Gemery, L.
    Jakobsson, M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Polyak, Leonid
    Brouwers, E.M.
    Briggs, W.M.
    An ostracode sea-ice proxy for reconstructing Quaternary sea-ice history in the Arctic Ocean2010In: Abstract volume: Fourth Conference on Arctic Palaeoclimate and its Extremes, University of Iceland , 2010, p. 27-27Conference paper (Other academic)
  • 196. Cronin, Thomas M.
    et al.
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pearce, Christof
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gemery, Laura
    Toomey, Michael
    Semiletov, Igor
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Deglacial sea level history of the East Siberian Sea and Chukchi Sea margins2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 9, p. 1097-1110Article in journal (Refereed)
    Abstract [en]

    Deglacial (12.8-10.7 ka) sea level history on the East Siberian continental shelf and upper continental slope was reconstructed using new geophysical records and sediment cores taken during Leg 2 of the 2014 SWERUS-C3 expedition. The focus of this study is two cores from Herald Canyon, piston core SWERUS-L2-4-PC1 (4-PC1) and multicore SWERUS-L2-4-MC1 (4-MC1), and a gravity core from an East Siberian Sea transect, SWERUS-L2-20-GC1 (20GC1). Cores 4-PC1 and 20-GC were taken at 120 and 115m of modern water depth, respectively, only a few meters above the global last glacial maximum (LGM; similar to 24 kiloannum or ka) minimum sea level of similar to 125-130 meters below sea level (m b.s.l.). Using calibrated radiocarbon ages mainly on molluscs for chronology and the ecology of benthic foraminifera and ostracode species to estimate paleodepths, the data reveal a dominance of river-proximal species during the early part of the Younger Dryas event (YD, Greenland Stadial GS-1) followed by a rise in river-intermediate species in the late Younger Dryas or the early Holocene (Preboreal) period. A rapid relative sea level rise beginning at roughly 11.4 to 10.8 ka (similar to 400 cm of core depth) is indicated by a sharp faunal change and unconformity or condensed zone of sedimentation. Regional sea level at this time was about 108m b.s.l. at the 4-PC1 site and 102m b.s.l. at 20-GC1. Regional sea level near the end of the YD was up to 42-47m lower than predicted by geophysical models corrected for glacio-isostatic adjustment. This discrepancy could be explained by delayed isostatic adjustment caused by a greater volume and/or geo-graphical extent of glacial-age land ice and/or ice shelves in the western Arctic Ocean and adjacent Siberian land areas.

  • 197. Cuzzone, Joshua K.
    et al.
    Clark, Peter U.
    Carlson, Anders E.
    Ullman, David J.
    Rinterknecht, Vincent R.
    Milne, Glenn A.
    Lunkka, Juha-Pekka
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Marcott, Shaun A.
    Caffee, Marc
    Final deglaciation of the Scandinavian Ice Sheet and implications for the Holocene global sea-level budget2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 448, p. 34-41Article in journal (Refereed)
    Abstract [en]

    The last deglaciation of the Scandinavian Ice Sheet (SIS) from similar to 21,000 to 13,000 yr ago is well constrained by several hundred Be-10 and C-14 ages. The subsequent retreat history, however, is established primarily from minimum-limiting C-14 ages and incomplete Baltic-Sea varve records, leaving a substantial fraction of final SIS retreat history poorly constrained. Here we develop a high-resolution chronology for the final deglaciation of the SIS based on 79 Be-10 cosmogenic exposure dates sampled along three transects spanning southern to northern Sweden and Finland. Combining this new chronology with existing Be-10 ages on deglaciation since the Last Glacial Maximum shows that rates of SIS margin retreat were strongly influenced by deglacial millennial-scale climate variability and its effect on surface mass balance, with regional modulation of retreat associated with dynamical controls. Ice-volume estimates constrained by our new chronology suggest that the SIS contributed 8 m sea-level equivalent to global sea-level rise between similar to 14.5 ka and 10 ka. Final deglaciation was largely complete by similar to 10.5 ka, with highest rates of sea-level rise occurring during the Bolling-Allerod, a 50% decrease during the Younger Dryas, and a rapid increase during the early Holocene. Combining our SIS volume estimates with estimated contributions from other remaining Northern Hemisphere ice sheets suggests that the Antarctic Ice Sheet (AIS) contributed 14.4 +/- 5.9 m to global sea-level rise since 13 ka. This new constraint supports those studies that indicate that an ice volume of 15 m or more of equivalent sea-level rise was lost from the AIS during the last deglaciation.

  • 198. Dahl, Tais W.
    et al.
    Hammarlund, Emma
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Do large predatory fish track ocean oxygenation?2011In: Communicative & Integrative Biology, ISSN 1942-0889, E-ISSN 1942-0889, Vol. 4, no 1, p. 92-94Article in journal (Refereed)
    Abstract [en]

    The Devonian appearance of 1-10 meter long armored fish (placoderms) coincides with geochemical evidence recording a transition into fully oxygenated oceans.1 A comparison of extant fish shows that the large individuals are less tolerant to hypoxia than their smaller cousins. This leads us to hypothesize that Early Paleozoic O2 saturation levels were too low to support >1 meter size marine, predatory fish. According to a simple model, both oxygen uptake and oxygen demand scale positively with size, but the demand exceeds supply for the largest fish with an active, predatory life style. Therefore, the largest individuals may lead us to a lower limit on oceanic O2 concentrations. Our presented model suggests 2-10 meter long predators require >30-50% PAL while smaller fish would survive at <25% PAL. This is consistent with the hypothesis that low atmospheric oxygen pressure acted as an evolutionary barrier for fish to grow much above ~1 meter before the Devonian oxygenation.

  • 199. Dahl, Tais W.
    et al.
    Hammarlund, Emma U.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Anbar, Ariel D.
    Bond, David P. G.
    Gill, Benjamin C.
    Gordon, Gwyneth W.
    Knoll, Andrew H.
    Canfield, Donald E.
    Reply to Butterfield: The Devonian radiation of large predatory fish coincided with elevated athospheric oxygen levels2011In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 108, no 9, p. E29-Article in journal (Refereed)
    Abstract [en]

    We welcome this opportunity to clarify the conclusions and implications of our recent publication in PNAS. Butterfield (1) raises four issues regarding the oxygenation of the Paleozoic Earth's surface and its correlation to animal evolution. Our geochemical and paleontological data supported ocean oxygenation in the Silurian-Early Devonian (2), a critical transition in Earth history that influenced biogeochemical cycles and biological systems.

    First, Butterfield suggests that evidence of charcoal in late Silurian rocks is incompatible with our claim that the earlier Paleozoic atmosphere had oxygen levels below 50% PAL (present-day atmospheric level). This counterargument rests on the assumption that the “fire window” of 62–166% PAL oxygen is well defined, but this is not the case (3).

  • 200. Dahl, Tais W.
    et al.
    Hammarlund, Emma U.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Anbar, Ariel D.
    Bond, David P. G.
    Gill, Benjamin C.
    Gordon, Gwyneth W.
    Knoll, Andrew H.
    Nielsen, Arne T.
    Schovsbo, Niels H.
    Canfield, Donald E.
    Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish2010In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 107, no 42, p. 17911-17915Article in journal (Refereed)
    Abstract [en]

    The evolution of Earth's biota is intimately linked to the oxygenation of the oceans and atmosphere. We use the isotopic composition and concentration of molybdenum (Mo) in sedimentary rocks to explore this relationship. Our results indicate two episodes of global ocean oxygenation. The first coincides with the emergence of the Ediacaran fauna, including large, motile bilaterian animals, ca. 550-560 million year ago (Ma), reinforcing previous geochemical indications that Earth surface oxygenation facilitated this radiation. The second, perhaps larger, oxygenation took place around 400 Ma, well after the initial rise of animals and, therefore, suggesting that early metazoans evolved in a relatively low oxygen environment. This later oxygenation correlates with the diversification of vascular plants, which likely contributed to increased oxygenation through the enhanced burial of organic carbon in sediments. It also correlates with a pronounced radiation of large predatory fish, animals with high oxygen demand. We thereby couple the redox history of the atmosphere and oceans to major events in animal evolution.

1234567 151 - 200 of 1127
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf