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  • 1.
    Ahmed, Engy
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Science for Life Laboratory (SciLifeLab).
    Parducci, Laura
    Unneberg, Per
    Ågren, Rasmus
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rattray, Jayne E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Han, Lu
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences. Columbia University, USA.
    Pedersen, Mikkel W.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Afrifa Yamoah, Kweku
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Slotte, Tanja
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. Stockholm University, Science for Life Laboratory (SciLifeLab).
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Archaeal community changes in Lateglacial lake sediments: Evidence from ancient DNA2018In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 181, p. 19-29Article in journal (Refereed)
    Abstract [en]

    The Lateglacial/early Holocene sediments from the ancient lake at Hasseldala Port, southern Sweden provide an important archive for the environmental and climatic shifts at the end of the last ice age and the transition into the present Interglacial. The existing multi-proxy data set highlights the complex interplay of physical and ecological changes in response to climatic shifts and lake status changes. Yet, it remains unclear how microorganisms, such as Archaea, which do not leave microscopic features in the sedimentary record, were affected by these climatic shifts. Here we present the metagenomic data set of Hasseldala Port with a special focus on the abundance and biodiversity of Archaea. This allows reconstructing for the first time the temporal succession of major Archaea groups between 13.9 and 10.8 ka BP by using ancient environmental DNA metagenomics and fossil archaeal cell membrane lipids. We then evaluate to which extent these findings reflect physical changes of the lake system, due to changes in lake-water summer temperature and seasonal lake-ice cover. We show that variations in archaeal composition and diversity were related to a variety of factors (e.g., changes in lake water temperature, duration of lake ice cover, rapid sediment infilling), which influenced bottom water conditions and the sediment-water interface. Methanogenic Archaea dominated during the Allerod and Younger Dryas pollen zones, when the ancient lake was likely stratified and anoxic for large parts of the year. The increase in archaeal diversity at the Younger Dryas/Holocene transition is explained by sediment infilling and formation of a mire/peatbog.

  • 2.
    Ampel, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Bigler, Christian
    Department of Ecology and Environmental Science, Umeå University.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Lotter, André F.
    Institute of Environmental Biology, Laboratory of Palaeobotany and Palynology, Utrecht University.
    Veres, Daniel
    “Emil Racovita” Speleological Institute, Clinicilor 5, 400006 Cluj-Napoca, Romania.
    Modest summer temperature variability during DO cycles in western Europe2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 11-12, p. 1322-1327Article in journal (Refereed)
    Abstract [en]

    Abrupt climatic shifts between cold stadials and warm interstadials, termed Dansgaard-Oeschger (DO) cycles, occurred frequently during the Last Glacial. Their imprint is registered in paleorecords worldwide, but little is known about the actual temperature change both annually and seasonally in different regions. A recent hypothesis based on modelling studies, suggests that DO cycles were characterised by distinct changes in seasonality in the Northern Hemisphere. The largest temperature change between stadial and interstadial phases would have occurred during the winter and spring seasons, whereas the summer seasons would have experienced a rather muted temperature shift. Here we present a temporally high-resolved reconstruction of summer temperatures for eastern France during a sequence of DO cycles between 36 and 18 thousand years before present. The reconstruction is based on fossil diatom assemblages from the paleolake Les Echets and indicates summer temperature changes of ca 0.5–2 °C between stadials and interstadials. This study is the first to reconstruct temperatures with a sufficient time resolution to investigate DO climate variability in continental Europe. It is therefore also the first proxy record that can test and support the hypothesis that temperature changes during DO cycles were modest during the summer season.

  • 3.
    Ampel, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kylander, Malin E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Steinthorsdottir, Margret
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Abrupt climate change and early lake development - the Lateglacial diatom flora at Hasseldala Port, southeastern Sweden2015In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 44, no 1, p. 94-102Article in journal (Refereed)
    Abstract [en]

    The fossil diatom record from the Hasseldala Port palaeolake, southeastern Sweden, offers an excellent opportunity to investigate how past climatic shifts influenced catchment conditions and early lake development. The record, dating to between 13900 and 11200 cal. a BP, covers a climatically dynamic period, starting with deglaciation followed by oscillations between warmer and colder climate states. The stratigraphical changes in the fossil diatom assemblages show a trend of less open-water taxa and a successively more complex periphytic community as the lake shallows and the aquatic habitat structure develops. A diatom-based reconstruction of lake water pH indicates a natural acidification trend early in the record from 13900 to 12500 cal. a BP. From 12500 cal. a BP, coincident with the start of climate cooling, to 11300 cal. a BP this trend is disrupted and lake waters become more alkaline. A cooler and drier climate most likely resulted in reduced soil organic matter build-up as well as more frozen ground that impeded hydrological flow and decreased the input of dissolved organic matter and organic acids into the lake system. This study demonstrates the importance of the hydrological system as a link between terrestrial and aquatic ecosystems during early lake ontogeny.

  • 4.
    Ampel, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Veres, Daniel
    Paleolimnological response to millennial and centennial scale climate variability during MIS 3 and 2 as suggested by the diatom record in Les Echets, France2008In: Quaternary Science Reviews, ISSN 0277-3791, Vol. 27, no 15-16, p. 1493-1504Article in journal (Refereed)
    Abstract [en]

    A 27 m long sediment sequence retrieved from the central part of the Les Echets basin in France has been analysed in sub-centennial resolution for biogenic silica and fossil diatom remains. The sequence corresponds to the later part of Marine Isotope Stage (MIS) 3 and to most of MIS 2. Distinct changes in diatom productivity, diversity and taxonomic composition between 36.2 and 31.7 kyr BP appear to relate to Dansgaard–Oeschger (DO) climate variability. Intervals characterized by low diversity, productivity and small-sized benthic diatom taxa are most likely a response to colder conditions in relation to DO stadials. In contrast, higher diversity, productivity and a high abundance of planktonic taxa indicate a response to warmer temperatures during DO interstadials. The time interval between 30.3 and 15.7 kyr BP is characterized by continuous low diatom productivity and a benthic dominated community with intermediate species richness, suggesting a transition to more stable conditions. Three time intervals with extremely low concentrations of diatom valves (46.1–36.2, 31.7–30.3 and 26.3–23.6 kyr BP) overlap with ages reported for Heinrich (H) events 4, 3, and 2. We speculate that the lake at Les Echets suffered from severe ecological stress as a response to H events. This is the first detailed study exemplifying the response of a lake, based on diatoms, to climate variability during late part of MIS 3 and most of MIS 2 in Europe.

  • 5.
    Ampel, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Veres, Daniel
    Leng, Melanie
    Kaislahti Tillman, Päivi
    Diatom assemblage dynamics during abrupt climate change: The response oflacustrine diatoms to Dansgaard-Oeschger cycles during the last glacialperiod2010In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 44, no 2, p. 397-404Article in journal (Refereed)
    Abstract [en]

    The sedimentary record from the paleolake at Les Echets in eastern France allowed a reconstruction of the lacustrine response to several abrupt climate shifts during the last glacial period referred to as Dansgaard-Oeschger (DO) cycles. The high-resolution diatom stratigraphy has revealed distinct species turnover events and large fluctuations in stable oxygen isotope values in diatom frustules, as a response to DO climate variability. More or less identical species compositions became re-established during each DO stadial and interstadial phases, respectively. However, the relative abundance of the most dominant species within these assemblages varies and might indicate differences in climatic conditions. Interstadial phases are characterized by identical species successions. Transitions from stadial to interstadial conditions show a distinct Fragilaria-Cyclotella succession, which resembles the diatom regime shifts that have been recognized in some lakes in the Northern Hemisphere since the mid-nineteenth century.

  • 6. Banks, W. E.
    et al.
    d'Errico, F.
    Dibble, H.
    Krishtalka, L.
    West, D.
    Olszewski, D.
    Townsend Peterson, A.
    Gillam, J. C.
    Montet-White, A.
    Crucifix, M.
    Marean, C. W.
    Sánchez-Goñi, M. F.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Vanhaeren, M.
    Eco-Cultural Niche Modeling: new tools for reconstructing the geography and ecology of past human populations2006In: PaleoAnthropology, p. 68-83Article in journal (Other (popular science, discussion, etc.))
  • 7. Blaauw, M.
    et al.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Christen, J.
    Ampel, Linda
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Veres, Daniel
    Hughen, K. A.
    Preusser, F.
    Svensson, A.
    Were last glacial climate events simultaneous between Greenland and France?2010In: Journal of Quaternary Science, ISSN 0267-8179, E-ISSN 1099-1417, Vol. 25, no 3, p. 387-394Article in journal (Refereed)
    Abstract [en]

    Several large abrupt climate fluctuations during the last glacial have been recorded in Greenland ice cores and archives from other regions. Often these Dansgaard–Oeschger events are assumed to have been synchronous over wide areas, and then used as tie-points to link chronologies between the proxy archives. However, it has not yet been tested independently whether or not these events were indeed synchronous over large areas. Here, we compare Dansgaard–Oeschger-type events in a well-dated record from southeastern France with those in Greenland ice cores. Instead of assuming simultaneous climate events between both archives, we keep their age models independent. Even these well-dated archives possess large chronological uncertainties that prevent us from inferring synchronous climate events at decadal to multi-centennial time scales. If possible, comparisons between proxy archives should be based on independent, non-tuned time-scales.

  • 8. Brandefelt, J.
    et al.
    Kjellstrom, E.
    Naslund, J. -O
    Strandberg, G.
    Voelker, A. H. L.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    A coupled climate model simulation of Marine Isotope Stage 3 stadial climate2011In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 7, no 2, p. 649-670Article in journal (Refereed)
    Abstract [en]

    We present a coupled global climate model (CGCM) simulation, integrated for 1500 yr to quasi-equilibrium, of a stadial (cold period) within Marine Isotope Stage 3 (MIS 3). The simulated Greenland stadial 12 (GS12; similar to 44 ka BP) annual global mean surface temperature (T(s)) is 5.5 degrees C lower than in the simulated recent past (RP) climate and 1.3 degrees C higher than in the simulated Last Glacial Maximum (LGM; 21 ka BP) climate. The simulated GS12 is evaluated against proxy data and previous modelling studies of MIS3 stadial climate. We show that the simulated MIS 3 climate, and hence conclusions drawn regarding the dynamics of this climate, is highly model-dependent. The main findings are: (i) Proxy sea surface temperatures (SSTs) are higher than simulated SSTs in the central North Atlantic, in contrast to earlier simulations of MIS 3 stadial climate in which proxy SSTs were found to be lower than simulated SST. (ii) The Atlantic Meridional Overturning Circulation (AMOC) slows down by 50% in the GS12 climate as compared to the RP climate. This slowdown is attained without freshwater forcing in the North Atlantic region, a method used in other studies to force an AMOC shutdown. (iii) El-Nino-Southern Oscillation (ENSO) teleconnections in mean sea level pressure (MSLP) are significantly modified by GS12 and LGM forcing and boundary conditions. (iv) Both the mean state and variability of the simulated GS12 is dependent on the equilibration. The annual global mean T(s) only changes by 0.10 degrees C from model years 500-599 to the last century of the simulation, indicating that the climate system may be close to equilibrium already after 500 yr of integration. However, significant regional differences between the last century of the simulation and model years 500-599 exist. Further, the difference between simulated and proxy SST is reduced from model years 500-599 to the last century of the simulation. The results of the ENSO variability analysis is also shown to depend on the equilibration.

  • 9.
    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

  • 10.
    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.

  • 11.
    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.

  • 12.
    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.

  • 13. 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.

  • 14.
    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.

  • 15. Clark, Peter, U.
    et al.
    Dyke, A. S.
    Shakun, J. D.
    Carlson, A. E.
    Clark, A. E.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Mitrovica, J. X.
    Hostetler, S. W.
    McCabe, A. M.
    The Last Glacial Maximum2009In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 325, no 5941, p. 710-714Article in journal (Refereed)
    Abstract [en]

    We used 5704 14C, 10Be, and 3He ages that span the interval from 10,000 to 50,000 years ago (10 to 50 ka) to constrain the timing of the Last Glacial Maximum (LGM) in terms of global ice-sheet and mountain-glacier extent. Growth of the ice sheets to their maximum positions occurred between 33.0 and 26.5 ka in response to climate forcing from decreases in northern summer insolation, tropical Pacific sea surface temperatures, and atmospheric CO2. Nearly all ice sheets were at their LGM positions from 26.5 ka to 19 to 20 ka, corresponding to minima in these forcings. The onset of Northern Hemisphere deglaciation 19 to 20 ka was induced by an increase in northern summer insolation, providing the source for an abrupt rise in sea level. The onset of deglaciation of the West Antarctic Ice Sheet occurred between 14 and 15 ka, consistent with evidence that this was the primary source for an abrupt rise in sea level ~14.5 ka.

  • 16. 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.

  • 17. Davies, Siwan
    et al.
    Elmquist, Marie
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Bergman, Jonas
    Wohlfarth, Barbara
    Hammarlund, Dan
    Developing age models for recent lacustrine sequences spanning the last 200 years: two case studies from west central Sweden2007In: The Holocene, Vol. 17, no 3, p. 1-13Article in journal (Refereed)
  • 18. Davies, S.M.
    et al.
    Elmquist, M.
    Bergman, J.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Wohlfarth, Barbara
    Department of Geology and Geochemistry. Department of Geology and Geochemistry.
    Hammarlund, D.
    Developing age models for recent lacustrine sequences spanning the last 200 years: two case studies from west central Sweden2007In: The Holocene, Vol. 17, no 3, p. 319-330Article in journal (Refereed)
  • 19. Dutton, Andrea
    et al.
    Webster, Jody M.
    Zwartz, Dan
    Lambeck, Kurt
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tropical tales of polar ice: evidence of Last Interglacial polar ice sheet retreat recorded by fossil reefs of the granitic Seychelles islands2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 107, p. 182-196Article in journal (Refereed)
    Abstract [en]

    In the search for a record of eustatic sea level change on glacial-interglacial timescales, the Seychelles ranks as one of the best places on the planet to study. Owing to its location with respect to the former margins of Northern Hemisphere ice sheets that wax and wane on orbital cycles, the local-or relative-sea level history is predicted to lie within a few meters of the globally averaged eustatic signal during the Last Interglacial period. We have surveyed and dated Last Interglacial fossil corals to ascertain peak sea level and hence infer maximum retreat of polar ice sheets during this time interval. We observe a pattern of gradually rising sea level in the Seychelles between similar to 129 and 125 thousand years ago (ka), with peak eustatic sea level attained after 125 ka at 7.6 +/- 1.7 m higher than present. After accounting for thermal expansion and loss of mountain glaciers, this sea-level budget would require similar to 5-8 m of polar ice sheet contribution, relative to today's volume, of which only similar to 2 m came from the Greenland ice sheet. This result clearly identifies the Antarctic ice sheet as a significant source of melt water, most likely derived from one of the unstable, marine-based sectors in the West and/or East Antarctic ice sheet. Furthermore, the establishment of a +5.9 +/- 1.7 m eustatic sea level position by 128.6 +/- 0.8 ka would require that partial AIS collapse was coincident with the onset of the sea level highstand.

  • 20. Fernandez, Marilen
    et al.
    Björck, Svante
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Maidana, Nora I.
    Unkel, Ingmar
    Van der Putten, Nathalie
    Diatom assemblage changes in lacustrine sediments from Isla de los Estados, southernmost South America, in response to shifts in the southwesterly wind belt during the last deglaciation2013In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 50, no 4, p. 433-446Article in journal (Refereed)
    Abstract [en]

    Isla de los Estados (54A degrees 45'S, 63A degrees 10'aEuro64A degrees 46'W) lies east of the main island of Tierra del Fuego and is the southeastern-most point in Argentina. Because of its geographic position near the latitudes of the Southern Hemisphere Westerlies and the strong influence of the Antarctic Circumpolar Current (ACC), the area is suitable for paleoecological and paleoclimate research. The island is not far north of the Subantarctic Front, which limits the northern boundary of the ACC. Paleoenvironmental study in this geographic location can shed light on past changes in atmospheric and marine circulation patterns. Diatom analysis of the lower part of a sediment sequence from Laguna Cascada (54A degrees 45' 51.3''S, 64A degrees 20' 20.07''W) enabled inference of changing lake conditions between 16 and 11.1 cal ka BP. Between 16 and 14.4 cal ka BP fragilarioid diatom species, often a pioneer group, dominated the record. Their presence shows seasonally open-water conditions from the onset of sedimentation. In zone II (14.4-12.8 cal ka BP), the dominance of planktonic/tychoplanktonic Aulacoseira spp. might represent longer ice-free periods and windier conditions, which would have kept this heavy species suspended in the water column. This period corresponds to the Antarctic Cold Reversal, when the Southern Hemisphere Westerlies were possibly centered on the latitudes of Tierra del Fuego, resulting in windy and wet conditions. Zone III (12.8-11.1 cal ka BP) is dominated by benthic diatom taxa that are mainly associated with peat and wetland vegetation. This suggests that climate conditions had become milder and less windy, favoring aquatic productivity and terrestrial vegetation development. This change in environmental conditions may have been a consequence of the southward movement of the Southern Hemisphere Westerlies at the start of the Antarctic Holocene thermal optimum.

  • 21. Feurdean, A.
    et al.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Björkman, L.
    Tantau, I.
    Bennike, O.
    Willis, Katherine J.
    Farcas, Sorina
    Robertsson, Ann-Marie
    Department of Physical Geography and Quaternary Geology.
    The influence of refugial population on Lateglacial and early Holocene vegetational changes in Romania2007In: Review of Paleobotany and Palynology, Vol. 145, p. 305-320Article in journal (Refereed)
  • 22. Feurdean, Angelica
    et al.
    Klotz, Stefan
    Brewer, S.
    Mosbrugger, V.
    Tamas, T.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Lateglacial climate development in NW Romania – Comparative results from three quantitative pollen-based methods.2008In: Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 265, p. 121-133Article in journal (Refereed)
  • 23. Feurdean, Angelica
    et al.
    Klotz, Stefan
    Moosbrugger, V.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Pollen-based quantitative reconstruction of Holocene climate variability in NW Romania.2008In: Palaeogeography, Palaeoclimatology, Paleeoecology, Vol. 260, p. 494-504Article in journal (Refereed)
  • 24.
    Gouirand, Isabelle
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Linderholm, Hans W.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Department of Meteorology.
    Wohlfarth, Barbara
    Department of Geology and Geochemistry.
    On the spatio-temporal characteristics of Fennoscandian tree-ring based summer temperatures reconstructions2008In: Theoretical and Applied Climatology, Vol. 91, p. 1-25Article in journal (Refereed)
    Abstract [en]

    We analyse the spatial representation of five previously published multi-century to millennial length dendroclimatological reconstructions of Fennoscandian summer temperatures. The reconstructions, ranging from local to regional scale, were based on either tree-ring width (TRW) or maximum latewood density (MXD) data or on a combination of the two. TRW chronologies are shown to provide reasonably good spatial information mainly for July temperatures, but a combination of TRW and MXD yields a better spatial representation for the whole summer season (June–August). A multiple-site reconstruction does not necessarily provide better spatial representation than a single site reconstruction, depending on the criterion for selecting data and also on the strength of the climate signal in the tree-ring data. In a new approach to analyse the potential for further developing Fennoscandian temperature reconstructions, we selected from a network of TRW and MXD chronologies those having the strongest temperature information a priori, to obtain a strong common climate signal suitable for a regional-scale reconstruction. Seven separate, but not independent, reconstructions based on progressively decreasing numbers of chronologies were created. We show that it is possible to improve the spatial representation of available reconstructions back to around AD 1700, giving high correlations (>0.7) with observed summer temperatures for nearly the whole of Fennoscandia, and even higher correlations (>0.85) over much of central-northern Fennoscandia. Further sampling of older trees (e.g. dry-dead and subfossil wood) would be needed to achieve the same high correlations prior to AD 1700. Our analysis suggests that it should be possible to select a few key sites for improving the reconstructions before AD 1700. Since tree-ring data from northern Fennoscandia are used in all available hemispheric-scale temperature reconstructions for the last millennium, there is also a potential for slightly improving the quality of the hemispheric-scale reconstructions, by including an improved reconstruction for Fennoscandia. However, adding new chronologies from previously unsampled regions would potentially improve hemispheric-scale temperature reconstructions more substantially.

  • 25.
    Helmens, Karin
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Bos, J.A.A.
    Engels, S.
    van Meerbeeck, C.J.
    Bohncke, S.J.P.
    Renssen, H.
    Heiri, O.
    Brooks, S.J.
    Seppä, H.
    Birks, H.J.B.
    Wohlfarth, Barbara
    Department of Geology and Geochemistry.
    Present-day temperatures in northern Scandinavia during the Last Glaciation2007In: Geology, Vol. 35, no 11, p. 987-990Article in journal (Refereed)
  • 26. Houmark-Nielsen, Michael
    et al.
    Björck, Svante
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Cosmogenic 10Be ages on Pommeranian Moraine, Poland’: Comments2006In: Boreas, Vol. 35, p. 1-5Article in journal (Other (popular science, discussion, etc.))
  • 27. Kjellström, Erik
    et al.
    Brandefelt, Jenny
    Näslund, Jens-Ove
    Smith, Ben
    Strandberg, Gustav
    Swedish Meteorological and Hydrological Institute, Sweden.
    Voelker, Antje H. L.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Simulated climate conditions in Europe during the Marine Isotope Stage 3 stadial2010In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 39, no 2, p. 436-456Article in journal (Refereed)
    Abstract [en]

    State-of-the-art climate models were used to simulate climate conditions in Europe during Greenland Stadial (GS) 12 at 44 ka BP. The models employed for these simulations were: (i) a fully coupled atmosphere-ocean global climate model (AOGCM), and (ii) a regional atmospheric climate model (RCM) to dynamically downscale results from the global model for a more detailed investigation of European climate conditions. The vegetation was simulated off-line by a dynamic vegetation model forced by the climate from the RCM. The resulting vegetation was then compared with the a priori vegetation used in the first simulation. In a subsequent step, the RCM was rerun to yield a new climate more consistent with the simulated vegetation. Forcing conditions included orbital forcing, land-sea distribution, ice-sheet configuration, and atmospheric greenhouse gas concentrations representative for 44 ka BP. The results show a cold climate on the global scale, with global annual mean surface temperatures 5 degrees C colder than the modern climate. This is still significantly warmer than temperatures derived from the same model system for the Last Glacial Maximum (LGM). Regional, northern European climate is much colder than today, but still significantly warmer than during the LGM. Comparisons between the simulated climate and proxy-based sea-surface temperature reconstructions show that the results are in broad agreement, albeit with a possible cold bias in parts of the North Atlantic in summer. Given a prescribed restricted Marine Isotope Stage 3 ice-sheet configuration, with large ice-free regions in Sweden and Finland, the AOGCM and RCM model simulations produce a cold and dry climate in line with the restricted ice-sheet configuration during GS 12. The simulated temperature climate, with prescribed ice-free conditions in south-central Fennoscandia, is favourable for the development of permafrost, but does not allow local ice-sheet formation as all snow melts during summer.

  • 28. Kjellström, Erik
    et al.
    Brandefelt, Jenny
    Näslund, Jens-Ove
    Smith, Ben
    Strandberg, Gustav
    Swedish Meteorological and Hydrological Institute, Sweden.
    Voelker, Antje H. L.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Simulated climate conditions in Fennoscandia during a MIS 3 stadial2010In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 39, no 2, p. 436-456Article in journal (Refereed)
    Abstract [en]

    State-of-the-art climate models were used to simulate climate conditions in Europe during Greenland Stadial (GS) 12 at 44 ka BP. The models employed for these simulations were: (i) a fully coupled atmosphere–ocean global climate model (AOGCM), and (ii) a regional atmospheric climate model (RCM) to dynamically downscale results from the global model for a more detailed investigation of European climate conditions. The vegetation was simulated off-line by a dynamic vegetation model forced by the climate from the RCM. The resulting vegetation was then compared with the a priori vegetation used in the first simulation. In a subsequent step, the RCM was rerun to yield a new climate more consistent with the simulated vegetation. Forcing conditions included orbital forcing, land–sea distribution, ice-sheet configuration, and atmospheric greenhouse gas concentrations representative for 44 ka BP. The results show a cold climate on the global scale, with global annual mean surface temperatures 5 °C colder than the modern climate. This is still significantly warmer than temperatures derived from the same model system for the Last Glacial Maximum (LGM). Regional, northern European climate is much colder than today, but still significantly warmer than during the LGM. Comparisons between the simulated climate and proxy-based sea-surface temperature reconstructions show that the results are in broad agreement, albeit with a possible cold bias in parts of the North Atlantic in summer. Given a prescribed restricted Marine Isotope Stage 3 ice-sheet configuration, with large ice-free regions in Sweden and Finland, the AOGCM and RCM model simulations produce a cold and dry climate in line with the restricted ice-sheet configuration during GS 12. The simulated temperature climate, with prescribed ice-free conditions in south-central Fennoscandia, is favourable for the development of permafrost, but does not allow local ice-sheet formation as all snow melts during summer.

  • 29.
    Kylander, Malin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ampel, Linda
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Veres, Daniel
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    High‐Resolution XRF Core Scanning Analysis of Les Echets (France) Sedimentary Sequence: New Insights from Chemical Proxies2011In: Journal of Quaternary Science, ISSN 1099-1417, Vol. 26, no 1, p. 109-117Article in journal (Refereed)
    Abstract [en]

    The Les Echets sediment sequence has recently been the subject of a high-resolution, multi-proxy study which revealed shifts in lake productivity linked to Greenland stadials and interstadials over the last 40 kyr (Wohlfarth et al., 2008. Rapid ecosystem response to abrupt climate changes during the last glacial period in western Europe, 40-16 ka. Geology 36:407-410). Here we present new elemental data for this sequence as acquired using an XRF core scanning system which provides in situ high-resolution, continuous, multi-element analyses. It was found that the strength of associations between the studied elements (Ti, Rb, K, Zr, Si, Ca, Sr, Mn and Fe) varied over time with changes in lake status which are ultimately driven by changes in climate. Increases in fine grained, detrital input (as indicated by Ti, Rb, K and Zr/Rb) overlap with independently established periods of lower lake productivity and are interpreted to represent more arid conditions. Several of these arid periods are coincident with low diatom concentrations and the timing of Heinrich events H4, H3 and H2. The duration of the environmental impacts linked to the H events varied based on the proxy used with the elemental data (Ti and Zr/Rb) estimating shorter events than the diatom data. Periods of lower detrital input and coarser grain sizes agreed in time with periods of higher lake productivity. The elemental data provides new insight into hydrological changes and related sediment processes within the catchment, and highlights the need for multi-element and multi-proxy approaches when reconstructing climate change using lacustrine sediment sequences. 

  • 30.
    Kylander, Malin
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Coulter, Sarah
    Klimaschewski, Andrea
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    A New Micro-Analytical Technique for the Identification of Tephra Layers in Sediments2008Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Tephrochronology uses material emitted during volcanic eruptions as isochronous markers for linking paleoclimatic records. Cryptotephra (i.e., those tephra not visible to the naked eye) layers can be as little as one to two shards thick while the shards themselves can be smaller than 10 m. The methods used to locate cryptotephra are often time consuming and laborious (e.g., sieving and point counting, density separation techniques) and in some cases may not be applicable (e.g., magnetic susceptibility of tephra may not differ from the sediment). XRF is a well-established analytical tool that has recently taken a new direction with the introduction of core scanning systems. The ITRAX XRF core scanning system is non-destructive, can acquire continuous data for a broad range of elements in situ in both marine and lacustrine sediments as well as take radiographic images. We test the possibility of screening sediment cores to flag the potential location of cryptotephra layers thereby eliminating lengthy laboratory work and saving on often precious sample. The method was tested by seeding a sediment core with tephra shards in layers of increasing thickness and then scanning the core with the ITRAX. The method was then further tested on sequences from two different sites: Hässeldala Port in Southern Sweden where up to 5 different cryptotephra have been detected and Lake Pechora from Russia which has several visible tephra layers. We show that using the ITRAX it is possible to locate most tephra layers without using any sample and within a minimal amount of time (a 1 m core can be analyzed reasonably in 12 hours and it requires essentially no sample preparation).

  • 31.
    Kylander, Malin M
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Klaminder, Jonatan
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Loewemark, Ludvig
    Geochemical responses to paleoclimatic changes in southern Sweden since the late glacial: the Hasseldala Port lake sediment record2013In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 50, no 1, p. 57-70Article in journal (Refereed)
    Abstract [en]

    There is a relatively good understanding of the paleoenvironmental changes that have occurred in southern Sweden since the Late Glacial. A main exception, however, is the sedimentary response of lacustrine systems during this period of rapid climate shifts. To address this, high-resolution X-ray fluorescence core scanning, Total Organic Carbon (TOC), C/N and delta C-13 analyses were made on a core from Hasseldala Port, a paleolake in the region. Site-specific geochemical analyses documented variations in silicate inputs (Zr/Ti, Si/Ti, K/Ti and K/Rb), productivity (TOC, Ca/Ti and Sr/Ti), as well as redox conditions in the sediment (delta C-13, Mn/Ti and Fe/Ti), which were then linked to the regional climatic framework. During the Bolling/Older Dryas sediment accumulation was at its highest, particularly prior to colonization by terrestrial vegetation, and hydrological transport dominated. No clear signal of the Older Dryas was detected in the elemental chemistry. The Allerod was a period of relatively constant sediment accumulation, with the exception of during the Gerzensee oscillation when rates increased. There is evidence for increased within-lake and -catchment productivity and a change in silicate source during parts of the Allerod. As opposed to other records from the region, constant sediment accumulation rates were found during the Younger Dryas. Other proxies also suggest that this was a rather static period at Hasseldala Port. A gradual change in productivity and hydrological activity was observed from 12,000 cal year BP. The Preboreal section is rather short but the geochemical response was similar to that seen during other periods with milder climate conditions. The geochemical record archived in the sediments at Hasseldala Port was found to be the integrated result of physical erosion, landscape and soil development, vegetation changes, basin hydrology and moisture and temperature variations and it fills an important information gap in our understanding of the geochemical response of lake sediments to past climate change.

  • 32.
    Kylander, Malin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Potucek, Martina
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bindler, Richard
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Using Peatlands as Archives of Dust Deposition: A Preliminary Record from Southern Sweden2010In: European Geosciences Union: General Assembly 2010, 2010, p. 2677-Conference paper (Refereed)
    Abstract [en]

    Our understanding of the complex role that mineral dust and its feedbacks play in the Earth's climate system is limited by its highly variable nature in space and time and a lack of data reflecting this variability. Paleorecords of dust provide a means by which we can examine the response of dust over time under very different climatic conditions. These records also supply the data (dust deposition rates, grain size, mineralogy and provenance) necessary to validate models of the dust cycle. Ombrotrophic peat is hydrologically isolated and therefore, like ice, records atmospheric deposition alone. Peatlands have a wide distribution globally, particularly in formerly glaciated regions, and are established environmental archives which provide continuous, high-resolution, datable records of atmospheric deposition and climate change. The use of peat for reconstructing dust deposition has been demonstrated, but as yet, not systematically examined. Their use as an archive of dust deposition must be further tested before they can be used to improve on the relatively poor spatial coverage of current terrestrial dust records (e.g., loess, lake sediments, dune building records). We present here a first focussed effort to reconstruct past changes in dust deposition through loss on ignition (LOI), bulk density, humification and inorganic geochemistry data using a peat sequence from Store Mosse in southern Sweden. This deposit has been studied previously and reveals a record that extends back some 5000 years. LOI data, in combination with elemental chemistry of the samples, aids in identifying those depths where the observed signal is atmospheric in source only. Humification analyses and bulk density are used as a proxy for surface wetness and allows for the linking of broader climatic changes (precipitation, evaporation, temperature) with fluctuations in dust deposition rates and changes in grain character (grain size, surface roughness). The inorganic geochemistry provides a means to quantify dust deposition as well as make a first attempt at source tracing of the deposited materials; this is important in terms of reconstructing changes in paleo wind regimes. Past variations in aeolian activity in Scandinavia are relatively unknown. Dune building records from Denmark and elsewhere around Europe show several periods of inland sand invasion and dune building during the Holocene. These are linked to cooler and stormier climates caused by shifts in the North Atlantic Oscillation and/or movement of the polar front. We compare the dust deposition record from Store Mosse with available dune building records and records of storminess from the region in order to make a preliminary assessment of the use of peatlands as paleo records of dust deposition.

  • 33. Lascu, Ioan
    et al.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Onac, Bogdan P.
    Björck, Svante
    Kromer, Bernd
    A Late Glacial paleolake record from an up-dammed river valley in northern Transylvania, Romania2015In: Quaternary International, ISSN 1040-6182, E-ISSN 1873-4553, Vol. 388, p. 87-96Article in journal (Refereed)
    Abstract [en]

    Lake sediments from the Magherus, Valley in the lowlands of northern Transylvania provide new evidence of paleoenvironmental development in Romania during the Late Glacial. The studied sediments were deposited as a result of the damming of a small river valley following a series of mass wasting events that occurred during the deglaciation period. A continuous sedimentary sequence belonging to the former lake is preserved in the banks of the stream, and contains evidence of paleoenvironmental changes associated with the Late Glacial Interstadial (Bolling-Allerod or Greenland Interstadial 1, GI-1) and the Late Glacial Stadial (Younger Dryas or Greenland Stadial 1, GS-1), implying that the hallmark climatic episodes of northwestern Europe are also expressed in Eastern Europe. We employ a multiproxy approach based on the analysis of sediment composition and texture, mineral magnetism, organic macrofossils, and radiocarbon dating. The reconstructed paleoenvironmental evolution at the site captures the warm and humid conditions associated with GI-1, as well as the subsequent cooling concomitant with the onset of GS-1. These climatic events are paralleled by vegetation shifts in the region, as deduced from comparisons with pollen sequences from Magherus, and neighboring locations in Transylvania and the Carpathians. The Late Glacial Interstadial was warmer and wetter, as evidenced by increased organic matter content in the lake, decreased erosion in the catchment, development of palustrine plant communities in the proximity of the lake, and expansion of spruce during the Allerod (GI-1c-a). The Late Glacial Stadial was colder and dryer, as indicated by low sedimentary organic matter content, an increase in erosion markers, and the decline of spruce and its replacement by birch. The landscape was more open, as herbs and grasses also expanded during this time. These fluctuations imply that Late Glacial climatic events are well expressed not only in upland areas of Romania, but also in lowland regions such as the Transylvanian Basin, where climatic effects are expected to be more muted.

  • 34.
    Muschitiello, Francesco
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andersson, August
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The C20 highly branched isoprenoid biomarker – a new diatom-sourced proxy for summer trophic conditions2015In: Organic Geochemistry, ISSN 0146-6380, E-ISSN 1873-5290, Vol. 81, p. 27-33Article in journal (Refereed)
    Abstract [en]

    The exact biological source of the C20 highly branched isoprenoid (HBI) present in sediments from aquatic systems is unclear. We therefore examined the relationship between the distribution of fossil diatoms and the concentration of the C20 HBI in a Late Glacial sedimentary record from the Hässeldala Port paleolake in southern Sweden. Using Bayesian multiple linear regression analysis, we show that its concentration is linked primarily to the production of the diatom taxon Gomphonema acuminatum, which accounts for the largest proportion of the temporal variability in the biomarker. By analogy with modern observations, we argue that an increasing amount of G. acuminatum biomass in our sedimentary record reflects increasing oligotrophy in the paleolake during the summer growing season, especially at times defined by subdued hydrologic flow. Our conclusions are corroborated by the δ13C composition of the C20 HBI biomarker, which points to a negative photosynthetic fractionation between atmospheric CO2 and the pool of dissolved inorganic carbon during diatom bloom, a distinct phenomenon at times of inhibited hydrological flow. Accordingly, we suggest that the C20 HBI biomarker can be effectively used to reconstruct the trophic state of the paleolake at Hässeldala Port, while its stable isotope composition can provide physicochemical information about the lake conditions during the dry summer season.

    Moreover, we note that the major hydrological shifts recorded in the G. acuminatum-C20 HBI stratigraphy do not coincide with the pollen zone boundaries. We thus infer that aquatic and terrestrial environmental responses to climate change are substantially decoupled through the hydrological system, which highlights the necessity for multi-proxy investigations to decipher past climate events.

  • 35.
    Muschitiello, Francesco
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lea, James M.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Greenwood, Sarah L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nick, Faezeh M.
    Brunnberg, Lars
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Macleod, Alison
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Timing of the first drainage of the Baltic Ice Lake synchronous with the onset of Greenland Stadial 12016In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 45, no 2, p. 322-334Article in journal (Refereed)
    Abstract [en]

    Glacial varves can give significant insights into recession and melting rates of decaying ice sheets. Moreover, varve chronologies can provide an independent means of comparison to other annually resolved climatic archives, which ultimately help to assess the timing and response of an ice sheet to changes across rapid climate transitions. Here we report a composite 1257-year-long varve chronology from southeastern Sweden spanning the regional late AllerOd-late Younger Dryas pollen zone. The chronology was correlated to the Greenland Ice-Core Chronology 2005 using the time-synchronous Vedde Ash volcanic marker, which can be found in both successions. For the first time, this enables secure placement of the Lateglacial Swedish varve chronology in absolute time. Geochemical analysis from new varve successions indicate a marked change in sedimentation regime accompanied by an interruption of ice-rafted debris deposition synchronous with the onset of Greenland Stadial 1 (GS-1; 12846years before AD 1950). With the support of a simple ice-flow/calving model, we suggest that slowdown of sediment transfer can be explained by ice-sheet margin stabilization/advance in response to a significant drop of the Baltic Ice Lake level. A reassessment of chronological evidence from central-western and southern Sweden further supports the hypothesis of synchronicity between the first (penultimate) catastrophic drainage of the Baltic Ice Lake and the start of GS-1 in Greenland ice-cores. Our results may therefore provide the first chronologically robust evidence linking continental meltwater forcing to rapid atmosphere-ocean circulation changes in the North Atlantic.

  • 36.
    Muschitiello, Francesco
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Columbia University, USA; Uni Research Climate, Norway.
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology . University of Quebec in Montreal, Canada.
    Lea, James M.
    Mair, Douglas W. F.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Enhanced ice sheet melting driven by volcanic eruptions during the last deglaciation2017In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 8, article id 1020Article in journal (Refereed)
    Abstract [en]

    Volcanic eruptions can impact the mass balance of ice sheets through changes in climate and the radiative properties of the ice. Yet, empirical evidence highlighting the sensitivity of ancient ice sheets to volcanism is scarce. Here we present an exceptionally well-dated annual glacial varve chronology recording the melting history of the Fennoscandian Ice Sheet at the end of the last deglaciation (similar to 13,200-12,000 years ago). Our data indicate that abrupt ice melting events coincide with volcanogenic aerosol emissions recorded in Greenland ice cores. We suggest that enhanced ice sheet runoff is primarily associated with albedo effects due to deposition of ash sourced from high-latitude volcanic eruptions. Climate and snow-pack mass-balance simulations show evidence for enhanced ice sheet runoff under volcanically forced conditions despite atmospheric cooling. The sensitivity of past ice sheets to volcanic ashfall highlights the need for an accurate coupling between atmosphere and ice sheet components in climate models.

  • 37.
    Muschitiello, Francesco
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Watson, Jenny E.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Salih, Abubakr A. M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brooks, Stephen J.
    Whitehouse, Nicola J.
    Karlatou-Charalampopoulou, Artemis
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fennoscandian freshwater control on Greenland hydroclimate shifts at the onset of the Younger Dryas2015In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, article id 8939Article in journal (Refereed)
    Abstract [en]

    Sources and timing of freshwater forcing relative to hydroclimate shifts recorded in Greenland ice cores at the onset of Younger Dryas, similar to 12,800 years ago, remain speculative. Here we show that progressive Fennoscandian Ice Sheet (FIS) melting 13,100-12,880 years ago generates a hydroclimate dipole with drier-colder conditions in Northern Europe and wetter-warmer conditions in Greenland. FIS melting culminates 12,880 years ago synchronously with the start of Greenland Stadial 1 and a large-scale hydroclimate transition lasting similar to 180 years. Transient climate model simulations forced with FIS freshwater reproduce the initial hydroclimate dipole through sea-ice feedbacks in the Nordic Seas. The transition is attributed to the export of excess sea ice to the subpolar North Atlantic and a subsequent southward shift of the westerly winds. We suggest that North Atlantic hydroclimate sensitivity to FIS freshwater can explain the pace and sign of shifts recorded in Greenland at the climate transition into the Younger Dryas.

  • 38.
    Muschitiello, Francesco
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Schwark, L.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sturm, Christophe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hammarlund, D.
    New evidence of Holocene atmospheric circulation dynamics based on lake sediments from southern Sweden: a link to the Siberian High2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 77, p. 113-124Article in journal (Refereed)
    Abstract [en]

    Oxygen (delta O-18) and carbon (delta C-13) isotope records of calcitic carbonate components (Chara sp. algal encrustations and Bithynia tentaculata gastropod opercula) from a lake-sediment succession on the Baltic Sea island of Gotland, south-eastern Sweden, have been obtained to investigate regional climate dynamics during the Holocene. The hydrological sensitivity of the small lake, particularly in terms of spring snowmelt contribution to the local water budget, provides a means of tracing past changes in the influence of snow-bearing easterly winds across the Baltic Sea Proper, which signifies the wintertime strength of the Siberian High. Repeated episodic depletions in O-18 at the centennial scale correlate with events of increased potassium concentration in the GISP2 ice-core record from Greenland, which indicates a coupling to large-scale fluctuations in atmospheric circulation patterns. A corresponding correlation with simultaneous depletions in C-13 suggests repeated responses of the local lake hydrology to snow-rich winters through decreasing water residence time, perhaps augmented by methanogenesis due to prolonged ice-cover seasons under the influence of an expanding Siberian High. Frequency analysis of the isotopic records reveals well-defined fluctuations at quasi-500-520-, 670-, 830- and 1430-yr periodicities, and a gradually stronger impact of Polar air outbreaks across the southern Baltic Sea region with time after ca 6000 cal. BP.

  • 39.
    Muschitiello, Francesco
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Time-transgressive environmental shifts across Northern Europe at the onset of the Younger Dryas2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 109, p. 49-56Article in journal (Refereed)
    Abstract [en]

    Until lately, it has commonly been assumed that the last major reorganization of the North Atlantic ocean-atmosphere system, the Younger Dryas climatic reversal, spread synchronously on continental to hemispheric scales. This assumption arose because reliable chronologies, which would allow capturing the complexity surrounding local responses to abrupt climate change, were lacking. To better understand the temporal structure at the inception of the Younger Dryas across the North Atlantic, we revised, updated and compared the chronological framework of four Northern European sediment sequences (Lake Krakenes, Lake Madtjarn, Lake Gammelmose, Sluggan Bog) by applying classical Bayesian modelling. We found distinct and spatially consistent age differences between the inferred ages of the Allerod interstadial - Younger Dryas stadial pollen zone boundaries among the four sites. Our results suggest an earlier vegetation response at sites along latitude 56-54 degrees N as compared to sites located at 60 -58 degrees N. We explain this time lag by a gradual regional cooling that started as early as c. 12,900 -13,100 cal. BP. This phenomenon was probably linked to cooling around the Nordic Seas as a result of enhanced iceberg calving from the Fennoscandian Ice Sheet during the final stage of the Aneroid inter-stadial. By contrast, vegetation shifts at sites located further north occurred significantly later and in concert with the establishment of full stadial climate conditions (c. 12,600-12,750 cal. BP). Our study emphasizes the need to develop solid regional C-14 chronologies and to employ the same age modelling approach to determine the temporal and spatial response to a climatic shift.

  • 40. Näslund, Jens-Ove
    et al.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Alexanderson, Helena
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Helmens, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hättestrand, Martina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Jansson, Peter
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kleman, Johan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Lundqvist, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Brandefelt, Jenny
    Houmark-Nielsen, Michael
    Kjellström, Erik
    Strandberg, Gustav
    Knudsen, Karen-Luise
    Krog Larsen, Nikolai
    Ukkonen, Pirkko
    Mangerud, Jan
    Fennoscandian paleo-environment and ice sheet dynamics during Marine Isotope Stage (MIS) 3: Report of a workshop held September 20–21, 2007 in Stockholm, Sweden2008Report (Other academic)
  • 41.
    Schenk, Frederik
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. KTH Royal Institute of Technology, Sweden.
    Väliranta, Minna
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Cambridge, UK; Columbia University, USA.
    Tarasov, Lev
    Heikkilä, Maija
    Björck, Svante
    Stockholm University, Faculty of Science, Department of Geological Sciences. Lund University, Sweden.
    Brandefelt, Jenny
    Johansson, Arne V.
    Näslund, Jens-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Swedish Nuclear Fuel and Waste Management Company (SKB), Sweden.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Warm summers during the Younger Dryas cold reversal2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 1634Article in journal (Refereed)
    Abstract [en]

    The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with global climatic impacts. The sudden cooling is typically linked to an abrupt slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in response to meltwater discharges from ice sheets. However, inconsistencies regarding the YD-response of European summer temperatures have cast doubt whether the concept provides a sufficient explanation. Here we present results from a high-resolution global climate simulation together with a new July temperature compilation based on plant indicator species and show that European summers remain warm during the YD. Our climate simulation provides robust physical evidence that atmospheric blocking of cold westerly winds over Fennoscandia is a key mechanism counteracting the cooling impact of an AMOC-slowdown during summer. Despite the persistence of short warm summers, the YD is dominated by a shift to a continental climate with extreme winter to spring cooling and short growing seasons.

  • 42.
    Steinthorsdottir, Margret
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    de Boer, Agatha M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Oliver, Kevin I. C.
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Blaauw, Maarten
    Reimer, Paula J.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Synchronous records of pCO(2) and Delta C-14 suggest rapid, ocean-derived pCO(2) fluctuations at the onset of Younger Dryas2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 99, p. 84-96Article in journal (Refereed)
    Abstract [en]

    Just before the onset of the Younger Dryas (YD) cold event, several stomatal proxy-based pCO(2) records have shown a sharp increase in atmospheric CO2 concentration (pCO(2)) of between ca 50 and 100 ppm, followed by a rapid decrease of similar or even larger magnitude. Here we compare one of these records, a high-resolution pCO(2) record from southern Sweden, with the IntCal13 record of radiocarbon (Delta C-14). The two records show broadly synchronous fluctuations at the YD onset. Specifically, the IntCal13 record documents decreasing Delta C-14 just before the YD onset when pCO(2) peaks, consistent with a source of old CO2 from the deep ocean. We propose that this fluctuation occurred due to a major ocean flushing event. The cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor. We confirm that the earth system can produce such large multi-decadal timescale fluctuations in pCO(2) through simulating an artificial ocean flushing event with the GENIE Earth System Model. We suggest that sharp transitions of pCO(2) may have remained undetected so far in ice cores due to inter-firn gas exchange and time-averaging. The stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change.

  • 43.
    Steinthorsdottir, Margret
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    de Boer, Agatha M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Oliver, Kevin I. C.
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Blaauw, Maarten
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Response to: Comment on Synchronous records of pCO(2) and Delta C-14 suggest rapid, ocean-derived pCO(2) fluctuations at the onset of Younger Dryas (Steinthorsdottir et al., 2014, Quaternary Science Reviews 99, 84-96)2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 107, p. 270-273Article in journal (Refereed)
  • 44.
    Steinthorsdottir, Margret
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kylander, Malin E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Blaauw, Maarten
    Reimer, Paula J.
    Stomatal proxy record of CO2 concentrations from the last termination suggests an important role for CO2 at climate change transitions2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 68, p. 43-58Article in journal (Refereed)
    Abstract [en]

    A new stomatal proxy-based record of CO2 concentrations ([CO2]), based on Betula nana (dwarf birch) leaves from the Hasseldala Port sedimentary sequence in south-eastern Sweden, is presented. The record is of high chronological resolution and spans most of Greenland Interstadial 1 (GI-1a to 1c, Allerod pollen zone), Greenland Stadial 1 (GS-1, Younger Dryas pollen zone) and the very beginning of the Holocene (Preboreal pollen zone). The record clearly demonstrates that i) [CO2] were significantly higher than usually reported for the Last Termination and ii) the overall pattern of CO2 evolution through the studied time period is fairly dynamic, with significant abrupt fluctuations in [CO2] when the climate moved from interstadial to stadial state and vice versa. A new loss-on-ignition chemical record (used here as a proxy for temperature) lends independent support to the Hasseldala Port [CO2] record. The large-amplitude fluctuations around the climate change transitions may indicate unstable climates and that tipping-point situations were involved in Last Termination climate evolution. The scenario presented here is in contrast to [CO2] records reconstructed from air bubbles trapped in ice, which indicate lower concentrations and a gradual, linear increase of [CO2] through time. The prevalent explanation for the main climate forcer during the Last Termination being ocean circulation patterns needs to re-examined, and a larger role for atmospheric [CO2] considered.

  • 45. Unkel, I.
    et al.
    Björck, Svante
    Lund University.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Deglacial environmental changes on Isla de los Estados (54.4 degrees S), southeastern Tierra del Fuego2008In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 27, no 15-16, p. 1541-1554Article in journal (Refereed)
    Abstract [en]

    The island of Isla de los Estados is situated at 54.5°S, 64°W, east of Argentinian Tierra del Fuego, and is located in a sensitive geographic position in relation to the zonal circulation between Antarctica and South America. Its terrestrial records of the last deglaciation, recording atmospheric conditions but within an oceanic setting, can help to clarify changes of regional circulation patterns, both atmospheric and marine. Here, we present geochemical analyses from 16–10 ka cal BP of a peat core from Lago Galvarne Bog at the northern coast of the island, and a lake sediment core from Laguna Cascada 3 km further south. The data comprise TC, TN, loss on ignition analyses and continuous XRF scanning on both cores as well as age–depth modeling based on AMS-14C dating. Deglaciation and onset of peat formation in the coastal areas began before 16 ka cal BP followed by a rapid glacial retreat and the start of lacustrine sedimentation further inland. Data suggest initially windy conditions with permafrost succeeded by gradually warmer and wetter conditions until ca 14.5 ka cal BP. The warming trend slows down until ca 13.5 ka cal BP, followed by arid conditions culminating around 12.8 ka cal BP. Our data suggest fairly warm conditions and the establishment of denser peat and forest vegetation ca 10.6 ka cal BP, contemporaneous with the onset of the Antarctic thermal optimum. This indicates large-scale shifts in the placement of zonal flow and the Westerlies at the beginning of the Holocene.

  • 46. Van Meerbeeck, C. J.
    et al.
    Renssen, H.
    Roche, D. M.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bohncke, S. J. P.
    Bos, J. A. A.
    Engels, S.
    Helmens, Karin F.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Sanchez-Goni, M. F.
    Svensson, A.
    Vandenberghe, J.
    The nature of MIS 3 stadial-interstadial transitions in Europe: New insights from model-data comparisons2011In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 30, no 25-26, p. 3618-3637Article in journal (Refereed)
    Abstract [en]

    15 abrupt warming transitions perturbed glacial climate in Greenland during Marine Isotope Stage 3 (MIS 3, 60-27 ka BP). One hypothesis states that the 8-16 degrees C warming between Greenland Stadials (GS) and Interstadials (GI) was caused by enhanced heat transport to the North Atlantic region after a resumption of the Atlantic Meridional Overturning Circulation (AMOC) from a weak or shutdown stadial mode. This hypothesis also predicts warming over Europe, a prediction poorly constrained by data due to the paucity of well-dated quantitative temperature records. We therefore use a new evidence from biotic proxies and a climate model simulation to study the characteristics of a GS-GI transition in continental Europe and the link to enhanced AMOC strength. We compare reconstructed climatic and vegetation changes between a stadial and subsequent interstadial - correlated to GS15 and GI14 (similar to 55 ka BP) - with a simulated AMOC resumption using a three-dimensional earth system model setup with early-MIS 3 boundary conditions. Over western Europe (12 degrees W-15 degrees E), we simulate twice the annual precipitation, a 17 degrees C warmer coldest month, a 8 degrees C warmer warmest month, 1300 degrees C-day more growing degree days with baseline 5 degrees C (GDD5) and potential vegetation allowing tree cover after the transition. However, the combined effect of frequent killing frosts, <20 mm summer precipitation and too few GDD5 after the transition suggest a northern tree limit lying at similar to 50 degrees N during GI14. With these 3 climatic limiting factors we provide a possible explanation for the absence of forests north of 48 degrees N during MIS 3 interstadials with mild summers. Finally, apart from a large model bias in warmest month surface air temperatures, our simulation is in reasonable agreement with reconstructed climatic and vegetation changes in Europe, thus further supporting the hypothesis.

  • 47.
    Veres, Daniel
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Davies, Siwan M
    Wohlfarth, Barbara
    Department of Geology and Geochemistry. Department of Geology and Geochemistry.
    Preusser, Frank
    Wastegård, Stefan
    Ampel, Linda
    Hormes, Anne
    Possnert, Göran
    Raynal, Jean-Paul
    Vernet, Gérard
    Age, origin and significance of a new middle MIS 3 tephra horizon identified within a long-core sequence from Les Echets, France2008In: Boreas, Vol. 37, no 434-443Article in journal (Refereed)
    Abstract [en]

    A new tephra has been identified within a long core (EC 3) sequence recovered from Les Echets, near Lyon, France. This visible tephra was discovered as part of a high resolution multiproxy re-investigation of the Les Echets sequence. Independent chronological information suggests that the tephra is c. 2 000-45 000 years old, and geochemical analysis indicates that it is of basanitic composition. The latter suggests a possible origin in the Eifel; however, as yet, no other volcanic events or deposits can be correlated to the Les Echets tephra. New sedimentological and chronological data are presented indicating that the tephra falls within an interval that most likely correlates with Dansgaard-Oeschger events 12-9. Thus, this tephra could potentially be an important middle MIS 3 marker horizon in central Europe if it can be traced in other palaeorecords.

  • 48. Veres, Daniel
    et al.
    Lallier-Vergès, Elisabeth
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Lacourse, Terri
    Keravis, Didier
    Björck, Svante
    Preusser, Frank
    Andrieu-Ponel, Valerie
    Ampel, Linda
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Climate-driven changes in lake conditions during late MIS 3 and MIS 2: a high-resolution geochemical record from les Echets, France2009In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 38, no 2, p. 230-243Article in journal (Refereed)
    Abstract [en]

    A new set of geochemical data obtained from a long sediment sequence from the Les Echets basin provides a new record of palaeoenvironmental change for western Europe. The sequence covers the late part of Marine Isotope Stage (MIS) 3 and MIS 2 ( c. 46.1–15.0 cal. kyr BP) and extends over 30 m of sediment, allowing for high sampling resolution. Distinct cyclic fluctuations in aquatic productivity proxies suggest a tempo of environmental change at Les Echets that resembles the one established for Dansgaard-Oeschger cycles and Heinrich events. Increases in organic carbon are paralleled by high hydrogenous contents and enriched δ<sup>13</sup>C<sub>org</sub> and C<sub>org</sub>/N<sub>tot</sub> ratios, implying a mixture of aquatic and land-derived constituents. The stable isotope values are directly linked to primary aquatic productivity and the input of terrestrial organic matter, enriched during warm, productive, periods, and drastically depleted during cold periods, particularly Heinrich events. Phases of relatively high productivity correlate with Dansgaard-Oeschger interstadials 8–2, while Heinrich events 3 and 2 are also clearly delineated by distinct lithological horizons with very low organic contents, high dry densities and enhanced organic matter oxidation. A large hiatus occurring about the time of Heinrich event 4 suggests that the cold and dry climate reconstructed for this event in the North Atlantic region also greatly affected Les Echets and its catchment.

  • 49.
    Veres, Daniel
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Andrieu-Ponel, Valérie
    Björck, Svante
    De beaulieu, Jacques-Louis
    Digerfeldt, G.
    Ponel, P.
    Ampel, Linda
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Davies, S.
    Gandouin, E.
    Belmecheri, S.
    The lithostratigraphy of the Les Echets basin, France: tentative correlation between cores2007In: Boreas, Vol. 36, p. 326-340Article in journal (Other (popular science, discussion, etc.))
  • 50.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ice-free conditions in Sweden during Marine Oxygen Isotope Stage 3?2010In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 39, no 2, p. 377-398Article, review/survey (Refereed)
    Abstract [en]

    Published and unpublished 14C dates for Sweden older than the Last Glacial Maximum ice advance were evaluated. Acceptable 14C dates indicate that age ranges for interstadial organic material in northern and central Sweden are between c. 60 and c. 35 cal. kyr BP and for similar deposits in southern Sweden are between c. 40 and c. 25 cal. kyr BP, which is in good agreement with recently derived Optical Stimulated Luminescence ages. 14C dates on mammoth remains show a larger scatter, possibly as a result of incomplete laboratory pretreatment. A possible scenario based on calibrated 14C dates from interstadial deposits is that central and northern Sweden was ice-free during the early and middle part of Marine Oxygen Isotope Stage 3 and that southern Sweden remained ice-free until c. 25 cal. kyr BP. A first ice advance into northern and central Sweden might have occurred as late as around 35 cal. kyr BP, more or less coeval with the Last Glacial Maximum ice advance onto the Norwegian shelf. To test the conclusions drawn here, new multi-proxy and high-resolution investigations of several key sites in north, central and south Sweden are required.

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