Dan Hammarlund | Lund University (original) (raw)

Papers by Dan Hammarlund

EGU General Assembly Conference Abstracts, Apr 1, 2015

Global and Planetary Change, Nov 1, 2015

Global and Planetary Change, Nov 1, 2015

High resolution palaeoenvironmental records in Far-Eastern Russia are rare, and the Kamchatka Pen... more High resolution palaeoenvironmental records in Far-Eastern Russia are rare, and the Kamchatka Peninsula is among the least studied areas of the region. This paper describes a record spanning the last ca. 11,000 yr, obtained from a bog in the southern part of Kamchatka. The radiocarbon dated core was analysed for pollen, testate amoebae, charcoal and loss-on-ignition (LOI). The vegetation during the early Holocene was dominated by grasses (Poaceae), birch (Betula) and heath (Ericaceae p. p.). Around 10,300 cal yr BP there was a substantial change in the vegetation cover to shrub alder (Alnus viridis s.I.) stands with sedges and ferns (Polypodiophyta) as well as herbs such as meadow rue (Thalictrum) in the understory. In the surroundings of Utka peatlands started to form. The variations in the vegetation cover were most probably caused by climatic changes. At the beginning of sediment accumulation, before 10,300 cal yr BP, the composition of the vegetation points to cooler summers and/or decreased annual precipitation. Around 10,300 cal yr BP, changes in vegetation occurred due to rising temperatures and/or changed water regimes. Increased abundancies of dry indicating testate amoebae after 9100 cal yr BP point to intermediate to dry soil conditions. Between 8600 and 7700 cal yr BP tree alder (Alnus incana) was widely spread at the site which probably indicates optimal environmental conditions. The tephra layer at 381-384.5 cm (ca. 8500 cal yr BP) produces a strong impact on the testate amoebae assemblages. At 7700 cal yr BP there was a sudden drop of A. incana in the local vegetation. From this time on, A. incana and also A. viridis decrease continuously whereas Betula gradually increases. The upper part of the sequence (after 6300 cal yr BP) shows higher abundancies of meadowsweet (Filipendula) and sweet gale (Myrica) pollen. After 6300 cal yr BP, changes in testate amoebae demonstrate variable soil moisture conditions at the site. Between 3700 and 1800 cal yr BP, wet conditions dominate as dry indicating testate amoebae decrease. After 1800 cal yr BP soil conditions become more variable again but this time with dry dominating testate amoebae. In contrast to surrounding regions, there is no evidence of trees such as spruce or larch growing in the surroundings of the site even though those trees are characteristic of many eastern Siberian sites. This difference might be because of the maritime influence of the Okhotsk Sea. Even dwarf pine (Pinus pumila), which is currently widely dispersed in northern Kamchatka, became part of the local vegetation only during the last 700 yr. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved. (Less)

The Holocene, Sep 1, 2005

A radiocarbon-dated sediment sequence from Lake Berkut in the southern part of the Kola Peninsula... more A radiocarbon-dated sediment sequence from Lake Berkut in the southern part of the Kola Peninsula, northwest Russia, was investigated by means of midge analysis in order to reconstruct the Holocene climatic and environmental history of the region. Past mean July air temperatures at the study site and hypolimnetic oxygen contents of the lake water were inferred from chironomid-based transfer functions. The early Holocene (c. 10 100 Á/8400 cal. yr BP) is characterized by summer temperatures and hypolimnetic oxygen concentrations broadly similar to present-day conditions. The midge records give evidence of a lake-level fall at c. 7000 cal. yr BP, resulting in weakened thermal stratification of the lake water, and improvement of the hypolimnetic oxygen conditions. After c. 4000 cal. yr BP midge assemblages suggest a lake-level rise related to increased effective moisture. A secondary mid-Holocene climatic optimum with inferred mean July air temperatures of c. 12.18C was recorded between 6000 and 4400 cal. yr BP, followed by minimum values (c. 11.38C) between 3500 and 1500 cal. yr BP, accompanied by the onset of natural acidification of the lake. The late-Holocene midge assemblages suggest weakly acidic conditions, low hypolimnetic oxygen contents and a general warming trend during the last c. 1500 years, although paludification and natural acidification may have influenced the temperature reconstruction at this stage. However, the modern chironomid-inferred mean July air temperature (12.58C) is consistent with meteorological data from a nearby station. In general, the midge records reflect relatively warm and moist conditions at Lake Berkut before c. 7000 cal. yr BP, a relatively warm and dry climate at c. 7000 Á/ 4000 cal. yr BP, and a shift to cooler and wetter climatic conditions in the region after c. 4000 cal. yr BP.

Biogeosciences Discussions, 2013

Decadal-scale variations in total organic carbon (TOC) concentration in lake water since AD 1200 ... more Decadal-scale variations in total organic carbon (TOC) concentration in lake water since AD 1200 in two small lakes in southern Sweden were reconstructed based on visible-near infrared spectroscopy (VNIRS) of their recent sediment successions. In order to assess the impacts of local land-use changes and regional variations in sulphur deposition and climate on the inferred changes in TOC concentration, the same sediment records were subjected to multi-proxy palaeolimnological analyses. Changes in lake-water pH were inferred from diatom analysis, whereas pollen-based land-use reconstructions (Landscape Reconstruction Algorithm) together with geochemical records provided information on catchment-scale environmental changes, and comparisons were made with available records of climate and population density. Our long-term reconstructions reveal that TOC concentrations were generally high prior to AD 1900, with second-order variations coupled mainly to changes in agricultural land-use intensity. The last century showed significant changes, and unusually low TOC concentrations were recorded in 1930-1990, followed by a recent increase. Variations in sulphur emissions, with an increase in the early 1900s to a peak around AD 1980 and a subsequent decrease, were most likely the main driver of these dynamics, although processes related to the introduction of modern forestry and recent increases in precipitation and temperature may have contributed. The increase in lake-water TOC concentration from around AD 1980 may therefore reflect a recovery process. Given that the effects of sulphate deposition now subside, other forcing mechanisms related to land management and climate change will possibly become the main drivers of TOC concentration changes in boreal lake waters in the future.

Global and Planetary Change, Nov 1, 2015

We present a synthesis of the results of a multiproxy, multisite, palaeoecological study of Holoc... more We present a synthesis of the results of a multiproxy, multisite, palaeoecological study of Holocene environmental change in Kamchatka, Far East Russia, details of which are presented elsewhere in the volume. We summarise the results of the analyses of pollen, diatom, chironomid, and testate amoebae assemblages, together with stable isotopes of oxygen and carbon, and sediment characteristics from the sediments of five lakes and a peat succession on a latitudinal gradient of the Kamchatka Peninsula, to infer environmental change and establish the major climate forcers and climatic teleconnections. There are synchronous shifts in the assemblage composition of most of the biota and across most sites at 6.5-6.2 ka BP, 5.2 ka BP, 4.0 ka BP, and 3.5 ka BP, suggesting a response to strong regional climate forcing at these times. These dates correspond to the warmest part of the Holocene Thermal Maximum (HTM) (6.5-6.2 ka BP), the beginning of the Neoglacial cooling (5.2 ka BP), the coolest and wettest part of the Neoglacial (4.0 ka BP), and a switch to warmer and drier conditions at 3.5 ka BP. Our results provide evidence for the penetration and domination of different air masses at different periods during the Holocene. Cool and dry periods in winter (e.g., at 6.0 ka BP) were driven by a relatively weak pressure gradient between the Siberian High and the Aleutian Low, whereas cool, wet periods in winter (e.g., the Neoglacial and during the LIA) developed when these two systems increased in strength. Warm, dry, continental periods in summer (e.g., at 2.5 ka BP) were driven by a weakening of the Siberian High. We find that the timing of the HTM in Kamchatka is later than in the Eurasian arctic but similar to northern Europe and the sub-arctic part of eastern Siberia. This progressive onset of the HTM was due to the effects of postglacial ice-sheet decay that modulated the routes of westerly storm tracks in Eurasia. A major ecosystem driver was the Siberian dwarf pine Pinus pumila, which spread northward during the Holocene in response to increasing winter snow, and caused water chemistry changes on arrival in the catchments of our study lakes and a response in diatom and chironomid assemblages. We also detect shortterm responses, especially in diatom assemblages, to water chemistry changes following volcanic ash deposits.

DN DEBATT 15/3. Sedan industrialiseringens borjan har vi anvant omkring fyra femtedelar av den ma... more DN DEBATT 15/3. Sedan industrialiseringens borjan har vi anvant omkring fyra femtedelar av den mangd fossilt kol som far forbrannas for att vi ska klara Parisavtalet. Vi har bara en femtedel kvar och det ar brattom att kraftigt reducera utslappen. Det har Greta Thunberg och de strejkande ungdomarna forstatt. Darfor stodjer vi deras krav, skriver 270 klimatforskare.

Boreas, Jun 7, 2010

Cellulose-inferred lakewater oxygen-isotope records have been obtained from two hydrologically op... more Cellulose-inferred lakewater oxygen-isotope records have been obtained from two hydrologically open basins (Lake Sp˚aime and Lake Svartk¨alstj¨arn), located on a west-east transect across central Sweden, to investigate changes in atmospheric circulation patterns during the Holocene. The Lake Sp˚aime d 18 O record is sensitive to changes in the seasonal distribution of precipitation in the Scandes Mountains of west-central Sweden, and thus generally portrays variations in d 18 O of precipitation (d 18 O P) that are governed predominantly by the influence of air masses originating from the North Atlantic. In contrast, the Lake Svartk¨alstj¨arn d 18 O record appears to reflect the varying influence of air masses delivering moisture from the North Atlantic and the Baltic Sea. A comparison of inferred changes in d 18 O P over the Holocene between the two sites reveals systematic patterns of variability over widely different time scales. These include: (1) a previously recognized long-term regional decline in d 18 O P , possibly in response to the declining vigour of Northern Hemisphere atmospheric circulation related to decreasing summer solar insolation; (2) newly identified inverse centennial-to millennial-scale d 18 O P fluctuations at the two sites that may be linked to changes in modes of atmospheric circulation analogous to those described at interannual to multidecadal time scales by the North Atlantic Oscillation (NAO) index; and (3) a prolonged interval of apparent climatic stability in the mid-Holocene (c. 6300-4200 cal. yr BP) characterized by persistently negative NAO-like circulation.

The Holocene, Sep 1, 2008

We offer an alternative palaeoenvironmental interpretation of oxygen-isotope data obtained on two... more We offer an alternative palaeoenvironmental interpretation of oxygen-isotope data obtained on two early-Holocene stalagmite records from caves in the Scandes Mountains of northern Sweden (Korallgrottan and Labrintgrottan), and the well-known Søylegrotta (Norway) SG93 record with which they are compared, that differs in several respects from that proposed by the authors. Contrary to viewing these as inverted palaeotemperature records, we suggest that they primarily reflect changes in the δ18O of local annual precipitation, modified by secondary temperature-dependent variation in water-calcite oxygen-isotope fractionation, at each of the three sites. This is supported by the striking similarity over the entire Holocene between the SG93 calcite δ18 O record and the lacustrine carbonate δ18O record from Lake Tibetanus (northern Sweden), which implies straightforward transfer of annual precipitation δ18O signals into cave drip waters. Recent studies of drip waters in Korallgrottan also support this model. This further enhances the value of cave deposits as palaeoclimate archives in this region and provides additional evidence of the sensitivity of the precipitation `isotope thermometer' to changes in atmospheric circulation patterns.

Journal of Quaternary Science, Sep 27, 2016

We test the hypothesis that rich occurrences of diatoms observed at transitions between major pea... more We test the hypothesis that rich occurrences of diatoms observed at transitions between major peat units representing different vegetation communities in a peat sequence from subarctic northern Sweden reflect responses to acid deposition from the Samalas AD 1257 and Laki AD 1783/1784 eruptions. We observe sudden changes in the mire ecosystem and thereby in the trophic status and biogeochemical cycling of the peatland. Both the eruptions are known to have been associated with significant acid deposition events and climatic anomalies, as recorded in polar ice cores. To test the hypothesis, new chronological analyses and age modelling were applied to existing biogeochemical and biological records from the peat sequence. This approach yielded modelled age ranges of AD 1239-1284 (1s)/AD 1210-1303 (2s) (median: AD 1260) and AD 1674-1795 (1s)/AD 1665-1875 (2s) (median AD 1743), respectively, for the stratigraphic transitions. Hence, the modelled age ranges bracket the ages of the eruptions in question and the hypothesis could therefore not be rejected. Impacts of acid deposition from the eruptions are assumed to have caused instant acidification, vegetation damage, increased nutrient cycling and blooms of opportunistic epiphytic diatoms. In addition, cooling may have contributed to vegetation changes through permafrost inception, frost heave and thereby altered hydrological conditions.

Quaternary, Apr 1, 2022

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Journal Of Geophysical Research: Biogeosciences, Mar 1, 2022

Increasing iron (Fe) concentrations are found in lakes on a wide geographical scale but exact cau... more Increasing iron (Fe) concentrations are found in lakes on a wide geographical scale but exact causes are still debated. The observed trends might result from increased Fe loading from the terrestrial catchment, but also from changes in how Fe distributes between the water column and the sediments. To get a better understanding of the causes we investigated whether there has been any change in the sediment formation of Fe sulfides (FeS) as an Fe sink in response to declining atmospheric sulfur (S) deposition during recent decades. For our study, we chose Lake Bolmen in southern Sweden, a lake for which we confirmed that Fe concentrations in the water column have strongly increased along with water color during 1966–2018. Our investigations showed that Fe accumulation and speciation varied independently of S accumulation patterns in the Lake Bolmen sediment record. Thus, we were not able to relate the positive trend in Fe concentrations to reduced FeS binding in the sediments. Furthermore, we found that Fe accumulation rates increased along with lake water Fe concentrations, indicating that increased catchment loading rather than a change in the distribution between the sediments and the water column has driven the increase in Fe concentrations. The increased loading may be due to land‐use change in the form of an extensive expansion of coniferous forest during the past century. Altered forest management practices and increased precipitation may have led to enhanced weathering and erosion of organic soil layers under aging coniferous forest.

Chemical Geology, Dec 1, 2021

Abstract The solubility and behavior of iron (Fe) in natural waters is tightly linked to Fe speci... more Abstract The solubility and behavior of iron (Fe) in natural waters is tightly linked to Fe speciation, and Fe speciation likely influences how Fe distributes between the water column and sediments. In this study, the function of a lake as an Fe sink, with focus on the role of Fe speciation, was assessed for Lake Bolmen in southern Sweden. We found that a large fraction of the Fe flowing in to the lake was efficiently lost by sedimentation in the lake basin. Fe in inflowing water was a mix of organically complexed mononuclear Fe, Fe-(oxy)hydroxides and Fe-bearing clays, while surface sediments were composed of Fe-(oxy)hydroxides, Fe-bearing clays, Fe-bearing silicates and Fe sulfides. The absence of organically complexed Fe in the surface sediments indicates that the lake is mainly a sink for minerogenic fractions. Furthermore, while lakes are considered to be sinks of Fe, it has been suggested that this function may be impaired by increasing precipitation and consequently shorter water residence time. In this study there were large within- and between-year variations in precipitation and Fe concentrations. However, rather than smaller Fe losses to the sediments during wet years, within-lake losses tended to be larger due to higher loading of Fe from the catchment. Thus, forecasted increases in precipitation may result in enhanced catchment export and Fe loading to lakes, and subsequently enhanced Fe sequestration in sediments.

Geology, Nov 8, 2016

Here we report a new quantitative mean July temperature reconstruction using non-biting midges (c... more Here we report a new quantitative mean July temperature reconstruction using non-biting midges (chironomids) from the Danish Last Interglacial (LIG) site Hollerup (spanning 127-116 ka). We find that peak mean July temperatures of 17.5 °C, similar to those of the present day (1961-1990 CE), were reached shortly before the onset of the regional Carpinus pollen zone. Through comparison to terrestrial and marine sequences we demonstrate that peak summer warmth took place some three millennia after the onset of LIG warming in Europe, a marked delay in line with records from the North Atlantic. Crucially, the warmest northern European summer temperatures appear to follow maximum Greenland Ice Sheet mass loss, implying that meltwater substantially reduced Atlantic Meridional Overturning Circulation and depressed European temperatures during the early part of the interglacial.

Quaternary Science Reviews, Oct 1, 2013

Oxygen (d 18 O) and carbon (d 13 C) isotope records of calcitic carbonate components (Chara sp. a... more Oxygen (d 18 O) and carbon (d 13 C) 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, southeastern 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 18 O 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 13 C 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 1430yr periodicities, and a gradually stronger impact of Polar air outbreaks across the southern Baltic Sea region with time after ca 6000 cal. BP.

EGU General Assembly Conference Abstracts, Apr 1, 2015

Global and Planetary Change, Nov 1, 2015

Global and Planetary Change, Nov 1, 2015

High resolution palaeoenvironmental records in Far-Eastern Russia are rare, and the Kamchatka Pen... more High resolution palaeoenvironmental records in Far-Eastern Russia are rare, and the Kamchatka Peninsula is among the least studied areas of the region. This paper describes a record spanning the last ca. 11,000 yr, obtained from a bog in the southern part of Kamchatka. The radiocarbon dated core was analysed for pollen, testate amoebae, charcoal and loss-on-ignition (LOI). The vegetation during the early Holocene was dominated by grasses (Poaceae), birch (Betula) and heath (Ericaceae p. p.). Around 10,300 cal yr BP there was a substantial change in the vegetation cover to shrub alder (Alnus viridis s.I.) stands with sedges and ferns (Polypodiophyta) as well as herbs such as meadow rue (Thalictrum) in the understory. In the surroundings of Utka peatlands started to form. The variations in the vegetation cover were most probably caused by climatic changes. At the beginning of sediment accumulation, before 10,300 cal yr BP, the composition of the vegetation points to cooler summers and/or decreased annual precipitation. Around 10,300 cal yr BP, changes in vegetation occurred due to rising temperatures and/or changed water regimes. Increased abundancies of dry indicating testate amoebae after 9100 cal yr BP point to intermediate to dry soil conditions. Between 8600 and 7700 cal yr BP tree alder (Alnus incana) was widely spread at the site which probably indicates optimal environmental conditions. The tephra layer at 381-384.5 cm (ca. 8500 cal yr BP) produces a strong impact on the testate amoebae assemblages. At 7700 cal yr BP there was a sudden drop of A. incana in the local vegetation. From this time on, A. incana and also A. viridis decrease continuously whereas Betula gradually increases. The upper part of the sequence (after 6300 cal yr BP) shows higher abundancies of meadowsweet (Filipendula) and sweet gale (Myrica) pollen. After 6300 cal yr BP, changes in testate amoebae demonstrate variable soil moisture conditions at the site. Between 3700 and 1800 cal yr BP, wet conditions dominate as dry indicating testate amoebae decrease. After 1800 cal yr BP soil conditions become more variable again but this time with dry dominating testate amoebae. In contrast to surrounding regions, there is no evidence of trees such as spruce or larch growing in the surroundings of the site even though those trees are characteristic of many eastern Siberian sites. This difference might be because of the maritime influence of the Okhotsk Sea. Even dwarf pine (Pinus pumila), which is currently widely dispersed in northern Kamchatka, became part of the local vegetation only during the last 700 yr. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved. (Less)

The Holocene, Sep 1, 2005

A radiocarbon-dated sediment sequence from Lake Berkut in the southern part of the Kola Peninsula... more A radiocarbon-dated sediment sequence from Lake Berkut in the southern part of the Kola Peninsula, northwest Russia, was investigated by means of midge analysis in order to reconstruct the Holocene climatic and environmental history of the region. Past mean July air temperatures at the study site and hypolimnetic oxygen contents of the lake water were inferred from chironomid-based transfer functions. The early Holocene (c. 10 100 Á/8400 cal. yr BP) is characterized by summer temperatures and hypolimnetic oxygen concentrations broadly similar to present-day conditions. The midge records give evidence of a lake-level fall at c. 7000 cal. yr BP, resulting in weakened thermal stratification of the lake water, and improvement of the hypolimnetic oxygen conditions. After c. 4000 cal. yr BP midge assemblages suggest a lake-level rise related to increased effective moisture. A secondary mid-Holocene climatic optimum with inferred mean July air temperatures of c. 12.18C was recorded between 6000 and 4400 cal. yr BP, followed by minimum values (c. 11.38C) between 3500 and 1500 cal. yr BP, accompanied by the onset of natural acidification of the lake. The late-Holocene midge assemblages suggest weakly acidic conditions, low hypolimnetic oxygen contents and a general warming trend during the last c. 1500 years, although paludification and natural acidification may have influenced the temperature reconstruction at this stage. However, the modern chironomid-inferred mean July air temperature (12.58C) is consistent with meteorological data from a nearby station. In general, the midge records reflect relatively warm and moist conditions at Lake Berkut before c. 7000 cal. yr BP, a relatively warm and dry climate at c. 7000 Á/ 4000 cal. yr BP, and a shift to cooler and wetter climatic conditions in the region after c. 4000 cal. yr BP.

Biogeosciences Discussions, 2013

Decadal-scale variations in total organic carbon (TOC) concentration in lake water since AD 1200 ... more Decadal-scale variations in total organic carbon (TOC) concentration in lake water since AD 1200 in two small lakes in southern Sweden were reconstructed based on visible-near infrared spectroscopy (VNIRS) of their recent sediment successions. In order to assess the impacts of local land-use changes and regional variations in sulphur deposition and climate on the inferred changes in TOC concentration, the same sediment records were subjected to multi-proxy palaeolimnological analyses. Changes in lake-water pH were inferred from diatom analysis, whereas pollen-based land-use reconstructions (Landscape Reconstruction Algorithm) together with geochemical records provided information on catchment-scale environmental changes, and comparisons were made with available records of climate and population density. Our long-term reconstructions reveal that TOC concentrations were generally high prior to AD 1900, with second-order variations coupled mainly to changes in agricultural land-use intensity. The last century showed significant changes, and unusually low TOC concentrations were recorded in 1930-1990, followed by a recent increase. Variations in sulphur emissions, with an increase in the early 1900s to a peak around AD 1980 and a subsequent decrease, were most likely the main driver of these dynamics, although processes related to the introduction of modern forestry and recent increases in precipitation and temperature may have contributed. The increase in lake-water TOC concentration from around AD 1980 may therefore reflect a recovery process. Given that the effects of sulphate deposition now subside, other forcing mechanisms related to land management and climate change will possibly become the main drivers of TOC concentration changes in boreal lake waters in the future.

Global and Planetary Change, Nov 1, 2015

We present a synthesis of the results of a multiproxy, multisite, palaeoecological study of Holoc... more We present a synthesis of the results of a multiproxy, multisite, palaeoecological study of Holocene environmental change in Kamchatka, Far East Russia, details of which are presented elsewhere in the volume. We summarise the results of the analyses of pollen, diatom, chironomid, and testate amoebae assemblages, together with stable isotopes of oxygen and carbon, and sediment characteristics from the sediments of five lakes and a peat succession on a latitudinal gradient of the Kamchatka Peninsula, to infer environmental change and establish the major climate forcers and climatic teleconnections. There are synchronous shifts in the assemblage composition of most of the biota and across most sites at 6.5-6.2 ka BP, 5.2 ka BP, 4.0 ka BP, and 3.5 ka BP, suggesting a response to strong regional climate forcing at these times. These dates correspond to the warmest part of the Holocene Thermal Maximum (HTM) (6.5-6.2 ka BP), the beginning of the Neoglacial cooling (5.2 ka BP), the coolest and wettest part of the Neoglacial (4.0 ka BP), and a switch to warmer and drier conditions at 3.5 ka BP. Our results provide evidence for the penetration and domination of different air masses at different periods during the Holocene. Cool and dry periods in winter (e.g., at 6.0 ka BP) were driven by a relatively weak pressure gradient between the Siberian High and the Aleutian Low, whereas cool, wet periods in winter (e.g., the Neoglacial and during the LIA) developed when these two systems increased in strength. Warm, dry, continental periods in summer (e.g., at 2.5 ka BP) were driven by a weakening of the Siberian High. We find that the timing of the HTM in Kamchatka is later than in the Eurasian arctic but similar to northern Europe and the sub-arctic part of eastern Siberia. This progressive onset of the HTM was due to the effects of postglacial ice-sheet decay that modulated the routes of westerly storm tracks in Eurasia. A major ecosystem driver was the Siberian dwarf pine Pinus pumila, which spread northward during the Holocene in response to increasing winter snow, and caused water chemistry changes on arrival in the catchments of our study lakes and a response in diatom and chironomid assemblages. We also detect shortterm responses, especially in diatom assemblages, to water chemistry changes following volcanic ash deposits.

DN DEBATT 15/3. Sedan industrialiseringens borjan har vi anvant omkring fyra femtedelar av den ma... more DN DEBATT 15/3. Sedan industrialiseringens borjan har vi anvant omkring fyra femtedelar av den mangd fossilt kol som far forbrannas for att vi ska klara Parisavtalet. Vi har bara en femtedel kvar och det ar brattom att kraftigt reducera utslappen. Det har Greta Thunberg och de strejkande ungdomarna forstatt. Darfor stodjer vi deras krav, skriver 270 klimatforskare.

Boreas, Jun 7, 2010

Cellulose-inferred lakewater oxygen-isotope records have been obtained from two hydrologically op... more Cellulose-inferred lakewater oxygen-isotope records have been obtained from two hydrologically open basins (Lake Sp˚aime and Lake Svartk¨alstj¨arn), located on a west-east transect across central Sweden, to investigate changes in atmospheric circulation patterns during the Holocene. The Lake Sp˚aime d 18 O record is sensitive to changes in the seasonal distribution of precipitation in the Scandes Mountains of west-central Sweden, and thus generally portrays variations in d 18 O of precipitation (d 18 O P) that are governed predominantly by the influence of air masses originating from the North Atlantic. In contrast, the Lake Svartk¨alstj¨arn d 18 O record appears to reflect the varying influence of air masses delivering moisture from the North Atlantic and the Baltic Sea. A comparison of inferred changes in d 18 O P over the Holocene between the two sites reveals systematic patterns of variability over widely different time scales. These include: (1) a previously recognized long-term regional decline in d 18 O P , possibly in response to the declining vigour of Northern Hemisphere atmospheric circulation related to decreasing summer solar insolation; (2) newly identified inverse centennial-to millennial-scale d 18 O P fluctuations at the two sites that may be linked to changes in modes of atmospheric circulation analogous to those described at interannual to multidecadal time scales by the North Atlantic Oscillation (NAO) index; and (3) a prolonged interval of apparent climatic stability in the mid-Holocene (c. 6300-4200 cal. yr BP) characterized by persistently negative NAO-like circulation.

The Holocene, Sep 1, 2008

We offer an alternative palaeoenvironmental interpretation of oxygen-isotope data obtained on two... more We offer an alternative palaeoenvironmental interpretation of oxygen-isotope data obtained on two early-Holocene stalagmite records from caves in the Scandes Mountains of northern Sweden (Korallgrottan and Labrintgrottan), and the well-known Søylegrotta (Norway) SG93 record with which they are compared, that differs in several respects from that proposed by the authors. Contrary to viewing these as inverted palaeotemperature records, we suggest that they primarily reflect changes in the δ18O of local annual precipitation, modified by secondary temperature-dependent variation in water-calcite oxygen-isotope fractionation, at each of the three sites. This is supported by the striking similarity over the entire Holocene between the SG93 calcite δ18 O record and the lacustrine carbonate δ18O record from Lake Tibetanus (northern Sweden), which implies straightforward transfer of annual precipitation δ18O signals into cave drip waters. Recent studies of drip waters in Korallgrottan also support this model. This further enhances the value of cave deposits as palaeoclimate archives in this region and provides additional evidence of the sensitivity of the precipitation `isotope thermometer' to changes in atmospheric circulation patterns.

Journal of Quaternary Science, Sep 27, 2016

We test the hypothesis that rich occurrences of diatoms observed at transitions between major pea... more We test the hypothesis that rich occurrences of diatoms observed at transitions between major peat units representing different vegetation communities in a peat sequence from subarctic northern Sweden reflect responses to acid deposition from the Samalas AD 1257 and Laki AD 1783/1784 eruptions. We observe sudden changes in the mire ecosystem and thereby in the trophic status and biogeochemical cycling of the peatland. Both the eruptions are known to have been associated with significant acid deposition events and climatic anomalies, as recorded in polar ice cores. To test the hypothesis, new chronological analyses and age modelling were applied to existing biogeochemical and biological records from the peat sequence. This approach yielded modelled age ranges of AD 1239-1284 (1s)/AD 1210-1303 (2s) (median: AD 1260) and AD 1674-1795 (1s)/AD 1665-1875 (2s) (median AD 1743), respectively, for the stratigraphic transitions. Hence, the modelled age ranges bracket the ages of the eruptions in question and the hypothesis could therefore not be rejected. Impacts of acid deposition from the eruptions are assumed to have caused instant acidification, vegetation damage, increased nutrient cycling and blooms of opportunistic epiphytic diatoms. In addition, cooling may have contributed to vegetation changes through permafrost inception, frost heave and thereby altered hydrological conditions.

Quaternary, Apr 1, 2022

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Journal Of Geophysical Research: Biogeosciences, Mar 1, 2022

Increasing iron (Fe) concentrations are found in lakes on a wide geographical scale but exact cau... more Increasing iron (Fe) concentrations are found in lakes on a wide geographical scale but exact causes are still debated. The observed trends might result from increased Fe loading from the terrestrial catchment, but also from changes in how Fe distributes between the water column and the sediments. To get a better understanding of the causes we investigated whether there has been any change in the sediment formation of Fe sulfides (FeS) as an Fe sink in response to declining atmospheric sulfur (S) deposition during recent decades. For our study, we chose Lake Bolmen in southern Sweden, a lake for which we confirmed that Fe concentrations in the water column have strongly increased along with water color during 1966–2018. Our investigations showed that Fe accumulation and speciation varied independently of S accumulation patterns in the Lake Bolmen sediment record. Thus, we were not able to relate the positive trend in Fe concentrations to reduced FeS binding in the sediments. Furthermore, we found that Fe accumulation rates increased along with lake water Fe concentrations, indicating that increased catchment loading rather than a change in the distribution between the sediments and the water column has driven the increase in Fe concentrations. The increased loading may be due to land‐use change in the form of an extensive expansion of coniferous forest during the past century. Altered forest management practices and increased precipitation may have led to enhanced weathering and erosion of organic soil layers under aging coniferous forest.

Chemical Geology, Dec 1, 2021

Abstract The solubility and behavior of iron (Fe) in natural waters is tightly linked to Fe speci... more Abstract The solubility and behavior of iron (Fe) in natural waters is tightly linked to Fe speciation, and Fe speciation likely influences how Fe distributes between the water column and sediments. In this study, the function of a lake as an Fe sink, with focus on the role of Fe speciation, was assessed for Lake Bolmen in southern Sweden. We found that a large fraction of the Fe flowing in to the lake was efficiently lost by sedimentation in the lake basin. Fe in inflowing water was a mix of organically complexed mononuclear Fe, Fe-(oxy)hydroxides and Fe-bearing clays, while surface sediments were composed of Fe-(oxy)hydroxides, Fe-bearing clays, Fe-bearing silicates and Fe sulfides. The absence of organically complexed Fe in the surface sediments indicates that the lake is mainly a sink for minerogenic fractions. Furthermore, while lakes are considered to be sinks of Fe, it has been suggested that this function may be impaired by increasing precipitation and consequently shorter water residence time. In this study there were large within- and between-year variations in precipitation and Fe concentrations. However, rather than smaller Fe losses to the sediments during wet years, within-lake losses tended to be larger due to higher loading of Fe from the catchment. Thus, forecasted increases in precipitation may result in enhanced catchment export and Fe loading to lakes, and subsequently enhanced Fe sequestration in sediments.

Geology, Nov 8, 2016

Here we report a new quantitative mean July temperature reconstruction using non-biting midges (c... more Here we report a new quantitative mean July temperature reconstruction using non-biting midges (chironomids) from the Danish Last Interglacial (LIG) site Hollerup (spanning 127-116 ka). We find that peak mean July temperatures of 17.5 °C, similar to those of the present day (1961-1990 CE), were reached shortly before the onset of the regional Carpinus pollen zone. Through comparison to terrestrial and marine sequences we demonstrate that peak summer warmth took place some three millennia after the onset of LIG warming in Europe, a marked delay in line with records from the North Atlantic. Crucially, the warmest northern European summer temperatures appear to follow maximum Greenland Ice Sheet mass loss, implying that meltwater substantially reduced Atlantic Meridional Overturning Circulation and depressed European temperatures during the early part of the interglacial.

Quaternary Science Reviews, Oct 1, 2013

Oxygen (d 18 O) and carbon (d 13 C) isotope records of calcitic carbonate components (Chara sp. a... more Oxygen (d 18 O) and carbon (d 13 C) 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, southeastern 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 18 O 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 13 C 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 1430yr periodicities, and a gradually stronger impact of Polar air outbreaks across the southern Baltic Sea region with time after ca 6000 cal. BP.