Gunhild Rosqvist

Holocene climatic change in Swedish Lapland inferred from an oxygen-isotope record of lacustrine biogenic silica

Holocene climatic variability was studied in a 9500-year lake-sediment sequence from the Abisko region in Swedish Lapland, using the oxygen-isotope ratio in diatom biogenic silica (d18Osi). Oxygen-and hydrogen-isotope ratios of waters from the Abisko area suggest that in this region the evaporative flux is small and the isotopic composition of most lakes reflects that of the local precipitation. The hydrological setting of the region and sensitivity analysis of isotopic response to changing climatic parameters such as humidity, inflow and evaporation show that the downcore diatom d18Osi record is primarily controlled by changes in the summer isotopic composition of the lake water. The overall 3.5‰ depletion in d18Osi since the early Holocene is interpreted as an increase in the influence of the Arctic polar continental air mass that carries depleted precipitation. We estimate that this change is associated with a 2.5–4°C cooling that has occurred since the early Holocene. In general, the diatom d18Osi record resembles the average annual air temperature reconstructed for the Greenland ice core GISP2, especially during the past 4000 years, with a pronounced cooling starting at 2000 years BP.

Late glacial to middle Holocene climatic record of lacustrine biogenic silica oxygen isotopes from a Southern Ocean island

Late glacial to middle Holocene climatic record of lacustrine biogenic silica oxygen isotopes from a Southern Ocean island

Millennial-scale climate changes on South Georgia, Southern Ocean

Abstract The location of South Georgia (54°S, 36°W) makes it a suitable site for the study of the climatic connections between temperate and polar environments in the Southern Hemisphere. Because the mass balance of the small cirque glaciers on South Georgia primarily responds to changes in summer temperature they can provide records of changes in the South Atlantic Ocean and atmospheric circulation. We use grey scale density, weight-loss-on-ignition, and grain size analyses to show that the proportion of glacially eroded sediments to organic sediments in Block Lake was highly variable during the last 7400 cal yr B.P. We expect that the glacial signal is clearly detectable above noise originating from nonglacial processes and assume that an increase in glacigenic sediment deposition in Block Lake has followed Holocene glacier advances. We interpret proglacial lake sediment sequences in terms of summer climate warming and cooling events. Prominent millennial-scale features include cooling events between 7200 and 7000, 5200 and 4400, and 2400 and 1600 cal yr B.P. and after 1000 cal yr B.P. Comparison with other terrestrial and marine records reveals that the South Georgian record captures all the important changes in Southern Hemisphere Holocene climate. Our results reveal a tentative coupling between climate changes in the South Atlantic and North Atlantic because the documented temperature changes on South Georgia are anti-phased to those in the North Atlantic.

GLACIER FLUCTUATIONS, SUSPENDED SEDIMENT FLUX AND GLACIO-LACUSTRINE SEDIMENTS

ABSTRACT. In order to perform inverse modelling of climate variability based on palaeoclimate proxy records, the complexity of intermediate steps in the chain of processes from the climate forcing to the responding proxy has to be considered. In reconstructing climate‐forced glacier fluctuations from proglacial lacustrine sediments it is important to understand how climate affects glacier dynamics. A glacier system is complex with many factors influencing sediment production, transport and deposition. Fluvial and mass movement processes in the proglacial environment may affect lake sedimentation substantially. We argue that it is easy to over‐interpret glaciolacustrine sediment variability by ignoring these complications. The sediment records may contain individual layers resulting from single precipitation or melt events, as well as persistent changes in climate‐forced glacier dynamics. We conclude that it is necessary to consider all possible influencing factors in order to derive reliable palaeoclimate data from lacustrine sediment sequences.

Quaternary glaciations in Africa

Abstract Glaciers were active on the African mountains during several periods of the Quaternary. Evidence exists of three old glaciations (before 100 ka BP) on Kilimanjaro, and two on Mount Kenya and Ruwenzori. The Last Glaciation, which was synchronous with the last major glacial phase in temperate latitudes, affected Kilimanjaro, Mount Kenya, Ruwenzori, Nyandarua Range, Mount Elgon, the Ethiopian Highlands and several of the high mountains within and around the Sahara Desert. The ice disappeared from most of these areas during the Early Holocene, except from Kilimanjaro, Mount Kenya and Ruwenzori, where glaciers exist at the present day.

North Atlantic region atmospheric circulation dynamics inferred from a late-Holocene lacustrine carbonate isotope record, northern Swedish Lapland

The first high-resolution record of climate variation based on the oxygen and carbon isotope composition of authigenic carbonate for northern Scandinavia is presented. Modern lake-water isotope data indicate that controls on its oxygen and hydrogen (δ18Ow and δDw) composition are unlikely to be evaporation or temperature, and its variations must therefore reflect changes in, or at the source of, precipitation. Substantial and persistent changes of the isotopic composition of the precipitation are required to change the mean annual isotope composition of lake surface water. For this reason we argue that the recorded changes were significant and that the recurrence of such changes would greatly affect future regional climate conditions in the North Atlantic region. Oxygen isotope (δ18O) minima occurring at ∼ 200, 500, 1300, 1600 and at 2900 cal. yr BP all coincide with major peaks in North Atlantic ice rafted debris deposition. We suggest that the depletion events in δ18O cycles recorded in several lakes in northern Swedish Lapland are caused by the same climatic shifts as those noted in the North Atlantic marine records. This is because changes of atmospheric circulation pattern and the lower ocean and atmospheric temperatures associated with the IRD events help to explain why 18O depletion of precipitation occurred during these events. Our findings indicate that the recorded changes in North Atlantic ice drift and surface hydrography are coupled to changes in atmospheric circulation.

Influence of geomorphological setting, fluvial-, glaciofluvial- and mass-movement processes on sedimentation in alpine lakes

Lacustrine sediments are often used for paleoclimate reconstructions as continuous archives of several physical and biological proxies. The relation between autochthonous and allochthonous sedimentation in alpine lakes is a complex system that may cause difficulties when interpreting biological and physical parameters. Results from previous studies of alpine lakes in northern Sweden have demonstrated that non-glacial processes produce minerogenic lake deposits with similar physical characteristics (density, LOI, magnetic susceptibility, grain-size) as those that have been associated with glacier fluctuations in proglacial lakes. In this study of two consecutive proglacial alpine lakes we show that fluvial redeposition of alluvial fan deposits significantly affects the Holocene lake sedimentation. Depending on the geomorphological setting, such fluvial redeposition signals may actually overprint a glaciofluvial signal. We also show that minerogenic laminations of fluvial origin are impossible to separate from the type of laminations usually used to infer glacier activity using the most common lithological sediment parameters. This emphasizes the complexity of sediment transport system in proglacial (paraglacial) settings where redeposition of older glacial sediment is of major importance. Our results highlight the need for thorough understanding of the geomorphological setting before inferences are made about climate variations from sedimentation in alpine lakes. Both lakes in this study contain sediment sequences with both episodic (turbidites) and continuously deposited sediments. Unfortunately we have too few radiocarbon dates to exactly date the turbidites but it is clear that turbidite layers in any case should be excluded from age model constructions since episodic sedimentation significantly influences the sediment age—depth relationship. In our age-model turbidites cause a potential dating error of several hundred, up to a thousand, years.

Late-Holocene temperature and precipitation changes in Vindelfjällen, mid-western Swedish Lapland, inferred from chironomid and geochemical data

In this article, we present results from a palaeolimnological study from Lake Vuoksjávrátje in the mountain tundra region in the Vindelfjällen Mountains, northwest Sweden. We suggest that the influence of precipitation may be one of the factors causing discrepancies between chironomid-based late-Holocene July temperature (JulyT) reconstructions from Fennoscandia. We combine quantitative temperature reconstruction using chironomids for the last 5100 years with qualitative analysis of chironomid composition and geochemical analyses, such as x-ray fluorescence (XRF), total organic carbon (TOC) and C/N analysis. The studied sequence is dated by 210Pb, 137Cs and 11 14C datings from terrestrial macrofossils. The aim of the study was to use chironomids to reconstruct late-Holocene summer temperature variation on a multi-centennial to centennial timescale and to use geochemical data to identify periods during which the changes in chironomid composition might have been forced by environmental variables other than temperature, such as within lake processes or precipitation. Based on ordination techniques, and a comparison between chironomid-inferred JulyTs and changes in minerogenic sedimentation with regional temperature and wetness records, it is concluded that the JulyT signal was modulated by precipitation. The proxies indicate that both JulyT and annual precipitation have influenced the chironomid communities in Lake Vuoksjávrátje, and that catchment-related processes caused by enhanced precipitation have overridden the summer temperature signal between 3000 and 2200 cal. yr BP, and between 1050 and 100 cal. yr BP.

The sensitivity of a small icecap to climatic fluctuations

The glacial chronology, obtained from proglacial lacustrine sediments, shows that Riukojietna, a small ice cap, disappeared or was small and inactive in the early-mid Holocene. A reactivation of the ice cap occurred around 2000 B.P. Riukojietna has retreated rapidly after a distinct maximum in extent in the beginning of the twentieth century. Measurements yielded negative net balances between 1985 and 1988. Differences in net balance seem to be caused primarily by fluctuating summer balances. Since Riukojietna has a relatively small vertical extent and is relatively low lying compared with cirque glaciers, it is much more sensitive to changes in the climate. Riukojietna is far from being in balance with the existing climate and will, if present trends continue, finally disappear.

Seasonal and interannual variability of elemental carbon in the snowpack of Storglaciaren, northern Sweden

Abstract We studied the variability of elemental carbon (EC) over 3 years (2009–11) in the winter snowpack of Storglaciären, Sweden. The goal of this study was to relate the seasonal variation in EC to specific snow accumulation events in order to improve understanding of how different atmospheric circulation patterns control the deposition of EC. Specifically, we related meteorological parameters (e.g. wind direction, precipitation) to snow physical properties, EC content, stable-isotope δ18O ratios and anion concentrations in the snowpack. The distribution of EC in the snowpack varied between years. Low EC contents corresponded to a predominance of weather systems originating in the northwest, i.e. North Atlantic. Analysis of single layers within the snowpacks showed that snow layers enriched in heavy isotopes coincided predominantly with low EC contents but high chloride and sulfate concentration. Based on this isotopic and geochemical evidence, snow deposited during these events had a strong oceanic, i.e. North Atlantic, imprint. In contrast, snow layers with high EC content coincided with snow layers depleted in heavy isotopes but high anion concentrations, indicating a more continental source of air masses and origin of EC from industrial emissions.