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  • 1.
    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.

  • 2.
    Borgendahl, Johanna
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Westman, Per
    Cyanobacteria as a trigger for increased primary productivity during sapropel formation in the Baltic Sea: a study of the Ancylus/Litorina transition2007In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 38, no 1, p. 1-12Article in journal (Refereed)
    Abstract [en]

    A sediment sequence from the Gotland Basin, representing the Ancylus Lake/Litorina Sea transition, was analysed for pigments, stable isotopes (δ13C and δ15N), diatoms and carbon and nitrogen content. The negative correlation between the cyanobacterial specific pigment zeaxanthin and δ15N supports the hypothesis that cyanobacteria in the early Litorina Sea indeed were nitrogen fixers. Their incorporation of nitrogen could have acted as a trigger for eutrophication during the Litorina Sea stage of the Baltic Sea. As cyanobacteria normally flourish in eutrophicated waters, the increasing concentrations of zeaxanthin during early Litorina Sea also corroborate that high primary production was an important part of the formation of organic rich (sapropel) sediments in the Baltic Sea. The first occurrence pre-dates the formation of laminated sediments, but the peaks of both zeaxanthin and organic carbon are within laminated sequences. This implies that the oxygen conditions of the bottom water also play a major role in the formation of sapropel sediments.

  • 3. Choudhary, Preetam
    et al.
    Routh, Joyanto
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Chakrapani, Govind J.
    Kumar, Bhishm
    Biogeochemical records of paleoenvironmental changes in Nainital Lake, Kumaun Himalayas, India2009In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 42, no 4, p. 571-586Article in journal (Refereed)
    Abstract [en]

    Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns about sediment and water pollution. Lead-210 dated sediment cores from the lake represent similar to 95 years of accumulation and yield a mean sedimentation rate of similar to 4.7 mm year(-1). Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (delta C-13, delta N-15, and delta S-34), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC flux varies from 1.0 to 3.5 g m(-2) year(-1). Sediments are anoxic (Eh -328 to -187 mV) and have low (0.10-0.30 g m(-2) year(-1)) N, but high (0.37-1.0 g m(-2) year(-1)) S flux. Shifts in delta C-13, delta N-15, and delta S-34 suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the lake has remained largely unchanged, and the perceived human-induced impacts are limited.

  • 4.
    Cunningham, Laura
    et al.
    School of Geography & Geosciences, University of St Andrews.
    Bishop, Kevin
    Department of Environmental Assessment, Swedish University of Agricultural Sciences.
    Mettävainio, Ewa
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Rosén, Peter
    Climate Impacts Research Centre, Umeå University.
    Paleoecological evidence of major declines in total organic carbon concentrations since the nineteenth century in four nemoboreal lakes2011In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 45, no 4, p. 507-518Article in journal (Refereed)
    Abstract [en]

    A decade of widespread increases in surface water concentrations of total organic carbon (TOC) in some regions has raised questions about longer term patterns in this important constituent of water chemistry. This study uses near-infrared spectroscopy (NIRS) to infer lake water TOC far beyond the decade or two of observational data generally available. An expanded calibration dataset of 140 lakes across Sweden covering a TOC gradient from 0.7 to 24.7 mg L-1 was used to establish a relationship between the NIRS signal from surface sediments (0-0.5 cm) and the TOC concentration of the water mass. Internal cross-validation of the model resulted in an R (2) of 0.72 with a root mean squared error of calibration (RMSECV) of 2.6 mg L-1. The TOC concentrations reconstructed from surface sediments in four Swedish lakes were typically within the range of concentrations observed in the monitoring data during the period represented by each sediment layer. TOC reconstructions from the full sediment cores of four lakes indicated that TOC concentrations were approximately twice as high a century ago.

  • 5.
    Das, Supriyo Kumar
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Routh, Joyanto
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Roychoudhury, Alakendra N.
    Biomarker evidence of macrophyte and plankton community changes in Zeekoevlei, a shallow lake in South Africa2009In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 41, no 3, p. 507-521Article in journal (Refereed)
    Abstract [en]

    Zeekoevlei is the largest freshwater lake in South Africa and has a century-long history of anthropogenic impact that caused hyper-eutrophic conditions. We used biomarkers (alkanes and pigments), stable isotopes (delta C-13 and delta N-15), rates of primary palaeoproduction and total inorganic carbon (TIC) accumulation rates in the lake sediments to investigate changes in plankton and macrophyte communities in response to anthropogenic activities in this shallow lake. Specific alkanes (I C-15 pound,C-17,C-19, pristane, phytane and n-C-29/n-C-17 ratio) and pigment (chlorophyll a, beta,beta-carotene, echinenone, fucoxanthin and zeaxanthin) concentrations in lake waters indicated the present-day hyper-eutrophic condition and seasonal fluctuations of cyanobacteria, zooplankton and diatom populations. Eutrophic conditions were initiated in the lake with the start of recreational activities and construction of a sewage treatment plant in the early 1920s. The lake transformed from a eutrophic to a hyper-eutrophic waterbody following damming, pondweed eradication and accelerated catchment-derived nutrient input. The change in lake trophic state was recorded by a sharp decline in the terrestrial to aquatic ratio (TAR) of specific n-alkanes, low carbon preference index (CPI) and increased delta C-13 values in the sediment core. In addition, the aquatic macrophyte n-alkane proxy (P-aq) values (similar to 1) indicated a slow takeover by floating macrophytes after the eradication of submerged pondweeds in 1951. Elevated n-alkane (I C-15 pound,C-17,C-19), total alkane and pigment (chlorophyll a, beta,beta-carotene, zeaxanthin and zeaxanthin to beta,beta-carotene ratio) concentrations, low delta N-15 values and low TIC accumulation rates in the upper middle section of the core indicated the beginning of intense cyanobacterial blooms after the dredging in 1983. Although the cyanobacterial population has decreased in recent years, hyper-eutrophic conditions are reflected by low CPI < 0.04 and TAR < 1 values at the top of the sediment core.

  • 6.
    Das, Supriyo Kumar
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Routh, Joyanto
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Roychoudhury, Alakendra N.
    Klump, J. Val
    Elemental (C, N, H and P) and stable isotope (delta N-15 and delta C-13) signatures in sediments from Zeekoevlei, South Africa: a record of human intervention in the lake2008In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 39, no 3, p. 349-360Article in journal (Refereed)
    Abstract [en]

    We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (delta C-13 and delta N-15) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10-12), H/C ratios (>= 1.7) and delta C-13(organic) values (-22 to -19 parts per thousand) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18-22 cm) and top (0-8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured delta(15) N-organic values (similar to 11) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low delta N-15 values (similar to 3 parts per thousand) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (similar to 0) and high TP (100-2,200 mg l(-1)) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12-14 cm) of the core.

  • 7. Engels, Stefan
    et al.
    Self, Angela E.
    Luoto, Tomi P.
    Brooks, Stephen J.
    Helmens, Karin F.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    A comparison of three Eurasian chironomid-climate calibration datasets on a W-E continentality gradient and the implications for quantitative temperature reconstructions2014In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 51, no 4, p. 529-547Article in journal (Refereed)
    Abstract [en]

    Multiple regional chironomid-climate calibration datasets are available to reconstruct quantitatively July air temperatures from fossil chironomid assemblages. We examined the relationship between July air temperature and the 40 most common chironomid taxa in three independent Eurasian calibration (training) sets. The estimated temperature optimum of each chironomid taxon is systematically lower (by similar to 1-2 A degrees C) in a Norwegian calibration set compared to Finnish and Russian calibration sets. This result might partly be explained by the fact that the Norwegian calibration set extends further at the cold end of the temperature gradient. A difference in continentality between the Russian sites and the European sites might also contribute to this pattern. The number of taxa that show a statistically significant unimodal response to temperature is higher in the Norwegian calibration set (34 out of 40 taxa) compared to the modern Finnish (11 of 37 taxa; 3 common taxa absent) and the Russian calibration set (20 of 40 taxa), probably due to the longer temperature gradient incorporated in the Norwegian calibration set. We applied all three calibration sets to fossil chironomid assemblages from the high-latitude study site of Sokli (northeast Finland), a site with a unique series of lacustrine deposits covering (amongst others) the Holocene, part of early MIS 3 (at similar to 53 ka) and MIS 5d-c (at similar to 110-95 ka) and with independent proxy-records for comparison. In the early Holocene and during MIS 5c, the chironomid-based temperature inferences from all three inference models had similar values. Temperature reconstructions based on the Norwegian calibration set are 2-4 A degrees C lower for the late Holocene, early MIS 3 and MIS 5d than the inferred temperatures based on the other calibration sets. Although the lakes included in the Finnish calibration set are located closest to the site of Sokli, evaluation tests and a comparison with independent proxy data suggests that the Norwegian calibration set provides the most suitable analogues for reconstruction purposes for most of the fossil assemblages. Our results imply that when choosing a calibration set for quantitative climate reconstructions on glacial timescales, regional proximity of the fossil site may not be a sufficient basis, and the length of the temperature gradient of the calibration dataset and factors such as the continentality gradient covered by the calibration set must also be considered.

  • 8. 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.

  • 9. Heyng, Alexander M.
    et al.
    Mayr, Christoph
    Luecke, Andreas
    Striewski, Bernd
    Wastegård, Stefan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Wissel, Holger
    Environmental changes in northern New Zealand since the Middle Holocene inferred from stable isotope records (delta N-15, delta C-13) of Lake Pupuke2012In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 48, no 2, p. 351-366Article in journal (Refereed)
    Abstract [en]

    Maar lakes in the Auckland Volcanic Field are important high-resolution archives of Holocene environmental change in the Southern Hemisphere mid-latitudes. Stable carbon and nitrogen isotope analyses were applied on bulk organic matter and the green alga Botryococcus from a sediment core from Lake Pupuke (Auckland, North Island, New Zealand) spanning the period since 7,165 cal. year BP. The origin of organic matter was established using total-organic-carbon-to-nitrogen ratios (TOC/TN) as well as organic carbon (delta C-13(OM)) and nitrogen (delta N-15) isotope composition of potential modern sources. This approach demonstrated that the contribution of allochthonous organic matter to the lake sediment was negligible for most of the record. The sedimentary TOC/TN ratios that are higher than Redfield ratio (i.e. > 7) are attributed to N-limiting conditions throughout the record. Variations of nitrogen and carbon isotopes during the last 7,165 years are interpreted as changes in the dominant processes in the lake. While epilimnetic primary productivity controlled isotope composition before 6,600 cal. year BP, microbial processes, especially denitrification and methane oxidation, caused overall shifts of the delta N-15 and delta C-13 values since the Mid-Holocene. Comparisons with climate reconstructions from the Northern Island suggest that changes in the wind-induced lake overturn and a shift to more pronounced seasonality were the most likely causes for lake-internal changes since 6,600 cal. year BP.

  • 10.
    Holmgren, Karin
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Freudendahl, Johan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Achimo, Mussa
    Ekblom, Anneli
    Mugabe, Joao
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Sitoe, Sandra
    University of Eduardo Mondlane, Mozambique.
    Water-level variations in Lake Nhauhache, Mozambique, during the last 2,300 years2012In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 48, no 2, p. 311-322Article in journal (Refereed)
    Abstract [en]

    Stratigraphic variations in diatom composition and phytolith abundance in a sediment core from a small, hydrologically isolated waterbody, Lake Nhauhache, Mozambique, provide evidence of water-level fluctuations over the past 2,300 years. Ten AMS radiocarbon dates on bulk sediment samples show that the lake came into existence about 2,300 years ago and that it has dried out since then, but only for brief time periods. Changes in the diatom assemblage composition indicate that lake level fluctuated in response to shifting humidity conditions. The changes reflect wetter conditions ca. 300 BC-AD 800, more variable conditions between AD 800 and 1150, a distinct dry phase within the time span AD 1150-1700 and a return thereafter to more humid conditions until present. There is general agreement between the Lake Nhauhache record and other records from the Southern Hemisphere summer rainfall region. This suggests that sediments from small interdunal lakes, which are abundant along the coast of southern Africa, provide reliable, regional paleoenvironmental information about an area from which more such data are needed.

  • 11. Jones, Timothy D.
    et al.
    Lawson, Ian T.
    Reed, Jane M.
    Wilson, Graham P.
    Leng, Melanie J.
    Gierga, Merle
    Bernasconi, Stefano M.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hajdas, Irka
    Bryant, Charlotte L.
    Tzedakis, P. C.
    Diatom-inferred late Pleistocene and Holocene palaeolimnological changes in the Ioannina basin, northwest Greece2013In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 49, no 2, p. 185-204Article in journal (Refereed)
    Abstract [en]

    The character and impact of climate change since the last glacial maximum (LGM) in the eastern Mediterranean region remain poorly understood. Here, two new diatom records from the Ioannina basin in northwest Greece are presented alongside a pre-existing record and used to infer past changes in lake level, a proxy for the balance between precipitation and evaporation. Comparison of the three records indicates that lake-level fluctuations were the dominant driver of diatom assemblage composition change, whereas productivity variations had a secondary role. The reconstruction indicates low lake levels during the LGM. Late glacial lake deepening was underway by 15.0 cal kyr BP, implying that the climate was becoming wetter. During the Younger Dryas stadial, a lake-level decline is recorded, indicating arid climatic conditions. Lake Ioannina deepened rapidly in the early Holocene, but long-term lake-level decline commenced around 7.0 cal kyr BP. The pattern of lake-level change is broadly consistent with an existing lake-level reconstruction at Lake Xinias, central Greece. The timing of the apparent change, however, is different, with delayed early Holocene deepening at Xinias. This offset is attributed to uncertainties in the age models, and the position of Xinias in the rain shadow of the Pindus Mountains.

  • 12.
    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.

  • 13. Martinez Cortizas, Antonio
    et al.
    Lopez-Merino, Lourdes
    Bindler, Richard
    Mighall, Timothy
    Kylander, Malin M
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Atmospheric Pb pollution in N Iberia during the late Iron Age/Roman times reconstructed using the high-resolution record of La Molina mire (Asturias, Spain)2013In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 50, no 1, p. 71-86Article in journal (Refereed)
    Abstract [en]

    Extensive mining took place in Spain during the Iron Age and Roman times, although a detailed chronology is still lacking due to the inherent difficulties in dating mining structures. In this study we sampled and analyzed a core from La Molina mire in the Asturias region, northern Spain. Because more than 100 Roman mines have thus far been found within 20 km of the mire, our aim was to shed light on local mining history, which we can then compare to the wider narrative of early mining pollution in Spain. We focus on the section from similar to 500 BC to AD similar to 600, which has a high temporal resolution of 6-15 year per sample. Geochemical analyses included the determination of major, minor and trace lithogenic elements (Si, Al, Fe, Ti, Ga, Rb, Y, Zr, Th) as markers of mineral content of the peat, and trace metals/metalloids (Mn, Cu, Ni, As, Pb) as well stable Pb isotopes, as potential markers of atmospheric metal pollution. The use of principal components analysis enabled the identification of a dominant geogenic component and a secondary pollution component. The earliest pollution signal of the covered period was recorded by similar to 300 BC, coinciding with the late local Iron Age. Average Pb-206/Pb-207 ratios of samples with ages older than this date was 1.204 +/- A 0.002, while all samples with a younger age had a less radiogenic ratio. Based on the metal pollution component four phases were identified: I, similar to 500 to 300 BC; II, similar to 300 to 20 BC; III, similar to 20 BC-AD 480; IV, AD similar to 480 to 600. The lowest isotopic ratio and highest proportion of pollution Pb (Pb-206/Pb-207 ratio of 1.157 and 89 % of total accumulated Pb) was reached at peak Pb production during Roman times (AD similar to 180 to 340), indicating that this was the period of most intense metal contamination in the area over the studied period. It is remarkable that the La Molina record shows a more extended period (two centuries) of active mining in comparison with other areas in Iberia, and a pattern of repeated shifts in Pb pollution of short duration, which is likely related to the local history of exploitation and exhaustion of mines within the area.

  • 14. Orme, L. C.
    et al.
    Lind, Ewa M.
    Stockholm University, Faculty of Science, Department of Physical Geography. IVL Swedish Environmental Research Institute. Swedish.
    Holm, T. M.
    Kjellman, Sofia E.
    Stockholm University, Faculty of Science, Department of Physical Geography. UiT The Arctic University of Norway, Norway.
    Koinig, K. A.
    Hormes, A.
    Rosqvist, Gunhild C.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Ruppel, M.
    Divine, D. V.
    Husum, K.
    Miettinen, A.
    Isaksson, E.
    Climatic impacts on an Arctic lake since 1300 AD: a multi-proxy lake sediment reconstruction from Prins Karls Forland, Svalbard2023In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 69, no 3, p. 249-266Article in journal (Refereed)
    Abstract [en]

    On the remote Arctic archipelago of Svalbard, there is increasing evidence of environmental impacts from climate change. The analysis of lake sedimentary records can be used to assess how strongly these recent changes have altered lake ecosystems. Sediments deposited during the last millennium from Lake Blokkvatnet, Prins Karls Forland, were analysed using a multiproxy approach, including stable isotope and X-ray fluorescence analysis. The results were interpreted as reflecting variability of (1) soil organic matter inwash, and potentially catchment and lake primary production, and (2) catchment weathering and erosion. Organic content began increasing after 1920 AD to the present, likely in response to warming. Earlier peaks of a similar magnitude occurred on three occasions since 1300 AD, with evidence indicating that these may have coincided with multidecadal-scale periods with higher temperatures, reduced sea ice and negative phases of the North Atlantic Oscillation. Catchment weathering and fluvial erosion began to increase around 1800 AD and peaked during the early twentieth century, potentially due to rising temperatures in autumn and winter causing increased liquid water availability. The records suggest that similar levels of erosion and weathering occurred between approximately 1300 and 1600 AD, spanning the transition from the Medieval Climate Anomaly to the Little Ice Age. 

  • 15.
    Plikk, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Engels, Stefan
    Luoto, Tomi P.
    Nazarova, Larisa
    Salonen, J. Sakari
    Helmens, Karin F.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Chironomid-based temperature reconstruction for the Eemian Interglacial (MIS 5e) at Sokli, northeast Finland2019In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 61, no 3, p. 355-371Article in journal (Refereed)
    Abstract [en]

    The Last Interglacial (Eemian, MIS 5e) can be considered a test-bed for climate dynamics under a warmer-than-present climate. In this study we present a chironomid record from the high latitude Sokli site (N Finland), where a long continuous sediment sequence from the Last Interglacial has been preserved from glacial erosion. The chironomid-analysis shows a diverse fauna, with dominance of warm-water indicators and shifts in assemblage composition that can be attributed to temperature, lake depth, productivity and habitat availability. Quantitative mean July paleotemperature estimates based on the chironomid data indicate overall mean July temperatures up to 1°C warmer than present. Two cooling events can be discerned, the Tunturi event, dated to about 127 ka BP, in the lower part of the sequence, and the Värriö event, dated to about 119 ka BP, associated with the beginning of a cooling trend in the upper part of the record. Warm conditions already at the onset of the interglacial contrast with a recent chironomid-based Last Interglacial temperature reconstruction from Denmark which suggests a late onset of Eemian warming. The relatively small increase in inferred temperatures compared to present day temperatures   differs from other high latitude Eemian sites, and likely reflects the influence of the Atlantic Meridional Overturning Circulation in maintaining already elevated temperatures in Fennoscandia during interglacials.

  • 16.
    Routh, Joyanto
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Choudhary, Preetam
    Meyers, Philip A.
    Kumar, Bhishm
    A sediment record of recent nutrient loading and trophic state change in Lake Norrviken, Sweden2009In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 42, no 3, p. 325-341Article in journal (Refereed)
    Abstract [en]

    Human-induced perturbations in the Lake Norrviken catchment, Sweden, over the last 100+ years have left distinctive geochemical imprints in the sediments. Disposal of sewage, industrial, and agricultural run-off into the lake since the end of the nineteenth century changed the trophic status from eutrophic to hyper-eutrophic. The primary organic matter (OM) source in the lake is in situ algal material. Total organic carbon (TOC) concentrations increased near the mid-section of a short sediment core collected from the deepest part of the lake, reflecting elevated epilimnetic productivity and consequent hypolimnetic anoxia. Accompanying shifts to lighter stable organic C and total N isotopic compositions suggest that cyanobacterial productivity increased during this period. The distribution of pigments in the core indicates a shift in the phytoplankton community to a cyanobacteria-dominated system. Moreover, pigments confirm that N-2-fixing versus non-N-2-fixing phytoplankton varied depending upon the external inputs of N and P. Conditions in the lake improved after sewage input was diverted and the lake is currently mesotrophic.

  • 17.
    Ryner, Maria
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Taylor, David
    A record of vegetation dynamics and lake level changes from Lake Emakat, northern Tanzania, during the last c. 1200 years2007In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 40, no 2, p. 583-601Article in journal (Refereed)
    Abstract [en]

    Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (14C) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS 14C dates, suggesting tree growth within the interval ∼1500–1670 AD. Down-core variations in pollen from terrestrial taxa, particularly the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second millennium AD, could relate to an expansion of human population levels and agricultural activity in the region.

  • 18.
    Shala, Shyhrete
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Helmens, Karin F.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Luoto, Tomi P.
    Väliranta, Minna
    Weckström, Jan
    Salonen, J. Sakari
    Kuhry, Peter
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Evaluating environmental drivers of Holocene changes in water chemistry and aquatic biota composition at Lake Loitsana, NE Finland2014In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 52, no 4, p. 311-329Article in journal (Refereed)
    Abstract [en]

    This study presents a detailed analysis of geochemical and biotic proxies in a lake sediment profile to assess the effects of local and regional environmental drivers on the Holocene development of Lake Loitsana, situated in the northern boreal forest of NE Finland. Multi-proxy studies, in particular those that include a detailed plant macrofossil record, from the part of the northern boreal zone of Fennoscandia which has not been affected by treeline fluctuations, are scarce and few of these records date back to the earliest part of the Holocene. A 9-m sediment sequence of gyttja overlying silts representing the last c. 10,700 cal year, allowed for a high-resolution study with emphasis on the early to mid-Holocene lake history. The lacustrine sediments were studied using lithology, loss-on-ignition and C/N ratios, micro- and macro-fossils of aquatic and wetland taxa, diatoms, chironomids and accelerator mass spectrometry C-14 dating on terrestrial plant macrofossils. Our study shows that the local development at Loitsana was complex and included a distinct glacial lake phase and subsequent drainage, a history of fluvial input affected by nearby wetland expansion, and lake infilling in an eventual esker-fed shallow lake. Enhanced trophic conditions, due to morphometric eutrophication, are recorded as Glacial Lake Sokli drained and open water conditions became restricted to a relatively small Lake Loitsana depression. pH appears to have been stable throughout the Holocene with a well-buffered lake due to the local carbonatite bedrock (Sokli Carbonatite Massif). The fossil assemblage changes are best explained by a complex mixture of drivers, including water-body conditions (i.e. depth, turbidity and turbulence), rate of sediment input, and the general infilling of the lake, highlighting the need to carefully evaluate the possible influence of such local factors as palaeoenvironmental conditions are reconstructed based on aquatic proxies.

  • 19. Yahiaoui, Nassima
    et al.
    Mansour, Bouhameur
    Katrantsiotis, Christos
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Reimer, Paula J.
    Mahboubi, M'hammed
    Early to Middle Holocene hydroclimate changes in the Guern El Louläilet depressions, Algerian Sahara2023In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 69, no 2, p. 161-183Article in journal (Refereed)
    Abstract [en]

    Fossil diatoms and litho-stratigraphic changes in the Guern El Louläilet depressions, NW of the Great Western Erg, Algeria, were analysed to infer paleoenvironmental changes in the northern Algerian Sahara during the Early and Middle Holocene. Analysis was based on calcareous diatomite collected from four outcrops within the depressions. The diatom flora consists of brackish and epiphytic taxa, such as Epithemia argus, with percentages of some freshwater and planktonic species, mainly Cyclotella distinguenda. Results provide evidence for two Holocene lacustrine episodes related to the African Humid Period. The first episode (Early to Middle Holocene) was characterized by abrupt development of shallow-water conditions, with extensive littoral zones and evaporative periods that coincided with high salt concentrations in warm, alkaline water (swampy conditions). A second episode (Middle to Late Holocene?), with brackish water and alkaline conditions, coincided with a decline in lake water level that is attributed to drier conditions. Our findings are consistent with those of other studies from the area and demonstrate similar environmental changes occurred after 9300 cal yr BP at sites within the region. The main drivers of the African Humid Period were the northward shift of the Intertropical Convergence Zone (ITCZ) and expansion of summer monsoonal rains. Our study sites were located in the northern Sahara, where variations in the position of the Intertropical Convergence Zone (ITCZ) did not affect rainfall. Early and Middle Holocene climate fluctuations detected in this study may have been caused by intensification of winter precipitation in the south-central Mediterranean and its penetration southward.

  • 20.
    Öberg, Helena
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Andersen, Thorbjørn J.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    A diatom record of recent environmental change in Lake Duluti, northern Tanzania2012In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 48, no 2, p. 401-416Article in journal (Refereed)
    Abstract [en]

    Lake Duluti is a small, topographically closed crater lake located on the flanks of Mt Meru, northern Tanzania. Analyses of diatoms in three short sediment cores and four modern samples from Lake Duluti were used to infer past environmental changes. 210Pb and 137Cs activity profiles combined with AMS 14C dates provide the chronological framework. Weak agreement between the 210Pb and 14C records, together with dating uncertainty, precludes construction of precise age models. The modern diatom flora, from plankton and three periphytic habitats, is dominated by Aulacoseira ambigua (Grunow) Simonsen, Gomphonema parvulum (Kützing) Grunow and Nitzschia amphibia Grunow. All three cores display similar stratigraphic succession, but the relative ratio of habitats represented by the diatoms varies substantially between cores. Diatoms indicate that the oldest part of the record is characterized by relatively low lake level and swampy vegetation. In the late nineteenth or early twentieth century there was a rapid lake level rise and the swamp turned into an open-water lake. High lake levels have prevailed since that time.

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