Sofia Holmgren - Academia.edu (original) (raw)
Papers by Sofia Holmgren
Earth-Science Reviews, 2013
Stratigraphic boundaries are ideally defined by distinct lithological, geochemical, and palaeobio... more Stratigraphic boundaries are ideally defined by distinct lithological, geochemical, and palaeobiological signatures, to which a chronological framework can be applied. We present a range of observations that illustrate how the Holocene-Anthropocene transition meets these criteria in its expression in sediments from remote arctic and alpine lakes, removed from direct, catchment-scale, anthropogenic influences. In glaciated lake basins, the retreat of glaciers commonly leads to lithological successions from proglacial clastic sedimentation to non-glacial organic deposition. Sediments from the majority of lakes record marked depletions in the nitrogen stable isotopic composition of sediment organic matter, reflecting anthropogenic influences on the global nitrogen cycle. In all cases, siliceous microfossil assemblages (diatoms and chrysophytes) change markedly and directionally, with regional nuances. These stratigraphic fingerprints begin to appear in the sediment record after AD 1850, but accelerate in pulses between AD 1950 and 1970 and again after AD 1980. Our review indicates that recent environmental changes associated with humankind's dominance of key global biogeochemical cycles are sufficiently pervasive to be imprinted on the sediment record of remote lakes. Moreover, these changes are of sufficient magnitude to conclude that the Holocene has effectively ended, and that the concept of Anthropocene more aptly describes current planetary dynamics. The synthesis of these observations pertains directly to ongoing discussions concerning the eventual formalization of a new stratigraphic boundary.
A late Holocene diatom stratigraphy of Bottom Pond (37°05 0 S, 12°17 0 W) was studied and interpr... more A late Holocene diatom stratigraphy of Bottom Pond (37°05 0 S, 12°17 0 W) was studied and interpreted to infer paleolimnological and climate changes on Tristan da Cunha, South Atlantic. The diatom flora of Bottom Pond is dominated by meroplanktonic Aulacoseira alpigena and A. distans and a benthic/epiphytic flora including Achnanthes spp., Naviculadicta spp., Eunotia taxa and Frustulia rhomboides. The flora also includes an important terrestrial and aerophytic component, consisting of several Diadesmis spp., Navicula lapidosa, Nitzschia terrestris and some Pinnularia spp., at times dominating the flora. Peaks of high terrestrial diatom abundances and low abundances and concentrations of Aulacoseira spp., correspond to periods of increased magnetic susceptibility and are interpreted to be the result of increased catchment erosion and/or precipitation leading to enhanced in-wash of species living in the soils around the lake. The diatom record of Bottom Pond most likely registers both local responses to increased erosion in the catchment as well as climatic change of a more regional character. Comparisons with the geochemical record in 2nd Pond on Nightingale Island show a correspondence between the two data sets at 2,000-1,750 cal yrs BP, most likely evidence of increased precipitation in the Tristan da Cunha region. The Tristan da Cunha archipelago is situated at the northern limit of the Southern hemisphere west wind belt and stronger and/ or a northward shift of the westerlies, are probably the main causes of the precipitation changes.
Quaternary Science Reviews, 2012
The full Holocene development of the large (8100 km 2) Vatnajökull ice cap in Iceland with its ma... more The full Holocene development of the large (8100 km 2) Vatnajökull ice cap in Iceland with its many outlet glaciers is poorly known. The idea of an early deglaciation, leading to a glacier-free period in mid-Holocene, followed by the Neoglaciation is still the main concept for the glacial history in the North Atlantic region, including Iceland. We have examined a continuous sediment record from the glacier-fed Lake Lögurinn in eastern Iceland to infer Holocene meltwater variability of Eyjabakkajökull, which is a surge-type outlet glacier of the Vatnajökull ice cap. We focus on the early and mid-Holocene, and our data show that Eyjabakkajökull receded rapidly during the final phase of the last deglaciation, and did not deliver glacial meltwater to Lake Lögurinn by 9000 years BP, suggesting that Eyjabakkajökull was significantly smaller than today at that time. The return of glacial meltwater transport to Lake Lögurinn, and thus a return of Eyjabakkajökull is dated to ca 4400 years BP, suggesting an almost 5000 years long glacier-free period during early and mid-Holocene. During this time period, we infer that the 8.2 ka cold event did not cause a significant expansion of Eyjabakkajökull, however, we note a marked decrease in the aquatic productivity in Lake Lögurinn, which is suggested to be the result of shorter ice-free seasons of Lake Lögurinn. The Holocene Thermal Maximum is inferred by a period of maximum Holocene aquatic productivity, and dated to ca 7900e7000 years BP. Following the reformation of Eyjabakkajökull ca 4400 years BP, we suggest that the glacier reached stable conditions ca 1700 years BP, and remained fairly stable until the later part of the Little Ice Age, when Eyjabakkajökull reached its maximum Holocene extent.
Palaeogeography, Palaeoclimatology, Palaeoecology, 2013
ABSTRACT A Holocene diatom stratigraphy of 2nd Pond, a small, filled pond on Nightingale Island (... more ABSTRACT A Holocene diatom stratigraphy of 2nd Pond, a small, filled pond on Nightingale Island (37° 25 S, 12° 29 W) was analyzed and interpreted to infer paleolimnological changes on Tristan da Cunha, South Atlantic. The diatom assemblage of 2nd Pond has the character of an acidic, oligotrophic wetland and the diatom record suggests that 2nd Pond most likely has been a bog/wetland throughout most of the Holocene. The flora is largely dominated by Pinnularia viridis, a species typically found in peat bogs. The flora also includes benthic and epiphytic Gomphonema sp, Achnanthes saxonica, Frustulia cf. rhomboides, Staurosira venter as well as Pinnularia cf. divergens var. decrescens and Eunotia paludosa var. paludosa. Peaks of aerophytic diatom (i.e. mostly Diadesmis spp. and Luticola spp.) abundances and concentrations correspond to increased magnetic susceptibility and slightly higher C/N ratios and are interpreted to be the result of increased catchment erosion due to precipitation and following increased in-wash of terrestrial diatoms. The diatom record suggests dry conditions in the early Holocene, followed by recurrent periods of increased precipitation in the region at 8600–5600 cal a BP and at 2200–1700 cal a BP. The main causes for these hydrological changes are probably changes in intensity and/or position of the Southern hemisphere west wind belt.
Journal of Quaternary Science, 2013
ABSTRACT Based on their luminescence and infinite radiocarbon ages, organic deposits beneath till... more ABSTRACT Based on their luminescence and infinite radiocarbon ages, organic deposits beneath till at two sites on the Småland peneplain, southern Sweden (Nybygget and Stora Gäddevik), are concluded to have formed before the Middle Weichselian. Applied palaeoecological methods include analyses of pollen, diatoms, charcoal fragments, macroscopic remains of vascular plants and mosses, and insect remains. Pollen‐stratigraphical correlations with previously studied interglacial/interstadial sites in southern Sweden, Denmark and northern Germany suggest that the peat at Nybygget dates from the Brørup interstadial or the final stage of the Eemian interglacial, whereas the lake sediments at Stora Gäddevik probably were emplaced during the middle Eemian. We conclude that the peat was formed in a wetland characterized by both wood swamp and open mire vegetation, and surrounded by semi‐open woodlands dominated by pine, birch and hazel. The middle Eemian sequence at the Stora Gäddevik site provides evidence of a moderately nutrient‐rich to nutrient‐rich lake environment with relatively diverse aquatic vegetation. Regional vegetation, as reconstructed using the REVEALS model, was spruce woodland mixed with pine, alder and birch, but also included more open environments with hazel, oak, grasslands and sedge‐dominated wetlands. Water shield (Brasenia schreberi), now extinct in Europe, was identified in the Eemian lake deposits, from both pollen and macroscopic remains.
A late Holocene diatom stratigraphy of Bottom Pond (37°05 0 S, 12°17 0 W) was studied and interpr... more A late Holocene diatom stratigraphy of Bottom Pond (37°05 0 S, 12°17 0 W) was studied and interpreted to infer paleolimnological and climate changes on Tristan da Cunha, South Atlantic. The diatom flora of Bottom Pond is dominated by meroplanktonic Aulacoseira alpigena and A. distans and a benthic/epiphytic flora including Achnanthes spp., Naviculadicta spp., Eunotia taxa and Frustulia rhomboides. The flora also includes an important terrestrial and aerophytic component, consisting of several Diadesmis spp., Navicula lapidosa, Nitzschia terrestris and some Pinnularia spp., at times dominating the flora. Peaks of high terrestrial diatom abundances and low abundances and concentrations of Aulacoseira spp., correspond to periods of increased magnetic susceptibility and are interpreted to be the result of increased catchment erosion and/or precipitation leading to enhanced in-wash of species living in the soils around the lake. The diatom record of Bottom Pond most likely registers both local responses to increased erosion in the catchment as well as climatic change of a more regional character. Comparisons with the geochemical record in 2nd Pond on Nightingale Island show a correspondence between the two data sets at 2,000-1,750 cal yrs BP, most likely evidence of increased precipitation in the Tristan da Cunha region. The Tristan da Cunha archipelago is situated at the northern limit of the Southern hemisphere west wind belt and stronger and/ or a northward shift of the westerlies, are probably the main causes of the precipitation changes.
… Change in Arctic and …, 2004
Lake sediments from four small lakes on western Spitsbergen (Svalbard Archipelago, Norwegian High... more Lake sediments from four small lakes on western Spitsbergen (Svalbard Archipelago, Norwegian High Arctic) preserve biostratigraphic and isotopic evidence for a complex suite of twentieth century environmental changes. At Lake Skardtjørna and Lake Tjørnskardet on Nordenskiöldkysten, there is a marked diatom floristic change coupled to increased diatom concentrations beginning around 1920. At Lake Istjørna and Lake Istjørnelva, 25 km southwest of Longyearbyen, both diatom total valve and chrysophyte stomatocyst concentrations have increased dramatically since the beginning of the 1900s. The early twentieth century changes are probably related to climate warming after the Little Ice Age. However, the most pronounced changes in diatom assemblages seem to have occurred in the last few decades. At the same time, nitrogen stable isotopes in sediment organic matter in two of the lakes became progressively depleted by *2%, which is consistent with diffuse atmospheric inputs from anthropogenic sources and attendant fertilization. These data suggest that climate change and nitrogen deposition may be acting together in driving these lakes towards new ecological states that are unique in the context of the Holocene.
The Holocene, Sep 1, 2004
A suite of analyses was performed on sediments accumulated during the last 10 700 years in Lake S... more A suite of analyses was performed on sediments accumulated during the last 10 700 years in Lake Spaime, a small, hydrologically open water body in the modem alpine tundra zone of the Scandes Mountains, west-central Sweden. The study aimed to evaluate (1) the nature of climate changes that forced the late-Holocene lowering of altitudinal tree limit in the region, the timing of which is known from prior studies based on radiocarbon dating of subfossil wood, and (2) the impact of these vegetational changes on an aquatic ...
Earth-Science Reviews, 2013
Stratigraphic boundaries are ideally defined by distinct lithological, geochemical, and palaeobio... more Stratigraphic boundaries are ideally defined by distinct lithological, geochemical, and palaeobiological signatures, to which a chronological framework can be applied. We present a range of observations that illustrate how the Holocene-Anthropocene transition meets these criteria in its expression in sediments from remote arctic and alpine lakes, removed from direct, catchment-scale, anthropogenic influences. In glaciated lake basins, the retreat of glaciers commonly leads to lithological successions from proglacial clastic sedimentation to non-glacial organic deposition. Sediments from the majority of lakes record marked depletions in the nitrogen stable isotopic composition of sediment organic matter, reflecting anthropogenic influences on the global nitrogen cycle. In all cases, siliceous microfossil assemblages (diatoms and chrysophytes) change markedly and directionally, with regional nuances. These stratigraphic fingerprints begin to appear in the sediment record after AD 1850, but accelerate in pulses between AD 1950 and 1970 and again after AD 1980. Our review indicates that recent environmental changes associated with humankind's dominance of key global biogeochemical cycles are sufficiently pervasive to be imprinted on the sediment record of remote lakes. Moreover, these changes are of sufficient magnitude to conclude that the Holocene has effectively ended, and that the concept of Anthropocene more aptly describes current planetary dynamics. The synthesis of these observations pertains directly to ongoing discussions concerning the eventual formalization of a new stratigraphic boundary.
A late Holocene diatom stratigraphy of Bottom Pond (37°05 0 S, 12°17 0 W) was studied and interpr... more A late Holocene diatom stratigraphy of Bottom Pond (37°05 0 S, 12°17 0 W) was studied and interpreted to infer paleolimnological and climate changes on Tristan da Cunha, South Atlantic. The diatom flora of Bottom Pond is dominated by meroplanktonic Aulacoseira alpigena and A. distans and a benthic/epiphytic flora including Achnanthes spp., Naviculadicta spp., Eunotia taxa and Frustulia rhomboides. The flora also includes an important terrestrial and aerophytic component, consisting of several Diadesmis spp., Navicula lapidosa, Nitzschia terrestris and some Pinnularia spp., at times dominating the flora. Peaks of high terrestrial diatom abundances and low abundances and concentrations of Aulacoseira spp., correspond to periods of increased magnetic susceptibility and are interpreted to be the result of increased catchment erosion and/or precipitation leading to enhanced in-wash of species living in the soils around the lake. The diatom record of Bottom Pond most likely registers both local responses to increased erosion in the catchment as well as climatic change of a more regional character. Comparisons with the geochemical record in 2nd Pond on Nightingale Island show a correspondence between the two data sets at 2,000-1,750 cal yrs BP, most likely evidence of increased precipitation in the Tristan da Cunha region. The Tristan da Cunha archipelago is situated at the northern limit of the Southern hemisphere west wind belt and stronger and/ or a northward shift of the westerlies, are probably the main causes of the precipitation changes.
Quaternary Science Reviews, 2012
The full Holocene development of the large (8100 km 2) Vatnajökull ice cap in Iceland with its ma... more The full Holocene development of the large (8100 km 2) Vatnajökull ice cap in Iceland with its many outlet glaciers is poorly known. The idea of an early deglaciation, leading to a glacier-free period in mid-Holocene, followed by the Neoglaciation is still the main concept for the glacial history in the North Atlantic region, including Iceland. We have examined a continuous sediment record from the glacier-fed Lake Lögurinn in eastern Iceland to infer Holocene meltwater variability of Eyjabakkajökull, which is a surge-type outlet glacier of the Vatnajökull ice cap. We focus on the early and mid-Holocene, and our data show that Eyjabakkajökull receded rapidly during the final phase of the last deglaciation, and did not deliver glacial meltwater to Lake Lögurinn by 9000 years BP, suggesting that Eyjabakkajökull was significantly smaller than today at that time. The return of glacial meltwater transport to Lake Lögurinn, and thus a return of Eyjabakkajökull is dated to ca 4400 years BP, suggesting an almost 5000 years long glacier-free period during early and mid-Holocene. During this time period, we infer that the 8.2 ka cold event did not cause a significant expansion of Eyjabakkajökull, however, we note a marked decrease in the aquatic productivity in Lake Lögurinn, which is suggested to be the result of shorter ice-free seasons of Lake Lögurinn. The Holocene Thermal Maximum is inferred by a period of maximum Holocene aquatic productivity, and dated to ca 7900e7000 years BP. Following the reformation of Eyjabakkajökull ca 4400 years BP, we suggest that the glacier reached stable conditions ca 1700 years BP, and remained fairly stable until the later part of the Little Ice Age, when Eyjabakkajökull reached its maximum Holocene extent.
Palaeogeography, Palaeoclimatology, Palaeoecology, 2013
ABSTRACT A Holocene diatom stratigraphy of 2nd Pond, a small, filled pond on Nightingale Island (... more ABSTRACT A Holocene diatom stratigraphy of 2nd Pond, a small, filled pond on Nightingale Island (37° 25 S, 12° 29 W) was analyzed and interpreted to infer paleolimnological changes on Tristan da Cunha, South Atlantic. The diatom assemblage of 2nd Pond has the character of an acidic, oligotrophic wetland and the diatom record suggests that 2nd Pond most likely has been a bog/wetland throughout most of the Holocene. The flora is largely dominated by Pinnularia viridis, a species typically found in peat bogs. The flora also includes benthic and epiphytic Gomphonema sp, Achnanthes saxonica, Frustulia cf. rhomboides, Staurosira venter as well as Pinnularia cf. divergens var. decrescens and Eunotia paludosa var. paludosa. Peaks of aerophytic diatom (i.e. mostly Diadesmis spp. and Luticola spp.) abundances and concentrations correspond to increased magnetic susceptibility and slightly higher C/N ratios and are interpreted to be the result of increased catchment erosion due to precipitation and following increased in-wash of terrestrial diatoms. The diatom record suggests dry conditions in the early Holocene, followed by recurrent periods of increased precipitation in the region at 8600–5600 cal a BP and at 2200–1700 cal a BP. The main causes for these hydrological changes are probably changes in intensity and/or position of the Southern hemisphere west wind belt.
Journal of Quaternary Science, 2013
ABSTRACT Based on their luminescence and infinite radiocarbon ages, organic deposits beneath till... more ABSTRACT Based on their luminescence and infinite radiocarbon ages, organic deposits beneath till at two sites on the Småland peneplain, southern Sweden (Nybygget and Stora Gäddevik), are concluded to have formed before the Middle Weichselian. Applied palaeoecological methods include analyses of pollen, diatoms, charcoal fragments, macroscopic remains of vascular plants and mosses, and insect remains. Pollen‐stratigraphical correlations with previously studied interglacial/interstadial sites in southern Sweden, Denmark and northern Germany suggest that the peat at Nybygget dates from the Brørup interstadial or the final stage of the Eemian interglacial, whereas the lake sediments at Stora Gäddevik probably were emplaced during the middle Eemian. We conclude that the peat was formed in a wetland characterized by both wood swamp and open mire vegetation, and surrounded by semi‐open woodlands dominated by pine, birch and hazel. The middle Eemian sequence at the Stora Gäddevik site provides evidence of a moderately nutrient‐rich to nutrient‐rich lake environment with relatively diverse aquatic vegetation. Regional vegetation, as reconstructed using the REVEALS model, was spruce woodland mixed with pine, alder and birch, but also included more open environments with hazel, oak, grasslands and sedge‐dominated wetlands. Water shield (Brasenia schreberi), now extinct in Europe, was identified in the Eemian lake deposits, from both pollen and macroscopic remains.
A late Holocene diatom stratigraphy of Bottom Pond (37°05 0 S, 12°17 0 W) was studied and interpr... more A late Holocene diatom stratigraphy of Bottom Pond (37°05 0 S, 12°17 0 W) was studied and interpreted to infer paleolimnological and climate changes on Tristan da Cunha, South Atlantic. The diatom flora of Bottom Pond is dominated by meroplanktonic Aulacoseira alpigena and A. distans and a benthic/epiphytic flora including Achnanthes spp., Naviculadicta spp., Eunotia taxa and Frustulia rhomboides. The flora also includes an important terrestrial and aerophytic component, consisting of several Diadesmis spp., Navicula lapidosa, Nitzschia terrestris and some Pinnularia spp., at times dominating the flora. Peaks of high terrestrial diatom abundances and low abundances and concentrations of Aulacoseira spp., correspond to periods of increased magnetic susceptibility and are interpreted to be the result of increased catchment erosion and/or precipitation leading to enhanced in-wash of species living in the soils around the lake. The diatom record of Bottom Pond most likely registers both local responses to increased erosion in the catchment as well as climatic change of a more regional character. Comparisons with the geochemical record in 2nd Pond on Nightingale Island show a correspondence between the two data sets at 2,000-1,750 cal yrs BP, most likely evidence of increased precipitation in the Tristan da Cunha region. The Tristan da Cunha archipelago is situated at the northern limit of the Southern hemisphere west wind belt and stronger and/ or a northward shift of the westerlies, are probably the main causes of the precipitation changes.
… Change in Arctic and …, 2004
Lake sediments from four small lakes on western Spitsbergen (Svalbard Archipelago, Norwegian High... more Lake sediments from four small lakes on western Spitsbergen (Svalbard Archipelago, Norwegian High Arctic) preserve biostratigraphic and isotopic evidence for a complex suite of twentieth century environmental changes. At Lake Skardtjørna and Lake Tjørnskardet on Nordenskiöldkysten, there is a marked diatom floristic change coupled to increased diatom concentrations beginning around 1920. At Lake Istjørna and Lake Istjørnelva, 25 km southwest of Longyearbyen, both diatom total valve and chrysophyte stomatocyst concentrations have increased dramatically since the beginning of the 1900s. The early twentieth century changes are probably related to climate warming after the Little Ice Age. However, the most pronounced changes in diatom assemblages seem to have occurred in the last few decades. At the same time, nitrogen stable isotopes in sediment organic matter in two of the lakes became progressively depleted by *2%, which is consistent with diffuse atmospheric inputs from anthropogenic sources and attendant fertilization. These data suggest that climate change and nitrogen deposition may be acting together in driving these lakes towards new ecological states that are unique in the context of the Holocene.
The Holocene, Sep 1, 2004
A suite of analyses was performed on sediments accumulated during the last 10 700 years in Lake S... more A suite of analyses was performed on sediments accumulated during the last 10 700 years in Lake Spaime, a small, hydrologically open water body in the modem alpine tundra zone of the Scandes Mountains, west-central Sweden. The study aimed to evaluate (1) the nature of climate changes that forced the late-Holocene lowering of altitudinal tree limit in the region, the timing of which is known from prior studies based on radiocarbon dating of subfossil wood, and (2) the impact of these vegetational changes on an aquatic ...