TY - ABST

T1 - Lake ecosystem response to climate change 8200 years ago. A multi-proxy study at Lake Højby Sø, Denmark

AU - Rasmussen, Peter

AU - Hede, Mikkel Ulfeldt

AU - Noe-Nygaard, Nanna

AU - Clarke, Annemarie L.

AU - Vinebrooke, Rolf D.

N1 - IOP Conf. Series: Earth and Environmental Science 6 (2009) 072053

PY - 2009

Y1 - 2009

N2 - The assessment of present-day climate effects on lake ecosystems in lowland Europe is difficult for tworeasons: i. the absence of long-term monitoring data (usually less than 30 yrs) which makes it difficult todetermine natural variation, and ii. the problem of distinguishing between the effects of climate and theeffects of human activities. These problems also complicate the prediction of possible future climateinfluence on lake ecology. A way of circumventing these problems is the use of lake sediment records whichcontain a wealth of information about past lake history over long time scales and under different climate andland-use regimes. Using a sediment record from Lake Højby Sø, Denmark, the aim of the present study wasto explore ecological response to the most pronounced climate change known from the Holocene: an abruptcooling event around 8200 cal. yr BP, also known as the ‘8.2 kyr cooling event’. The event has beenrecorded as a negative δ18O excursion in the central Greenland ice cores, lasting c. 160 years from8247–8086 cal. yr BP (Thomas et al. 2007, Quat. Sci. Rev. 26, 70–81). In Greenland the maximum coolingwas estimated to be 6 ± 2oC while in southern Fennoscandia and the Baltic countries pollen-basedquantitative temperature reconstructions indicate a maximum annual mean temperature decrease of around1.5 oC (Alley et al. 1997, Geology 25, 483–486; Seppä et al. 2007, Clim. of the past 3, 225–236). Todaythere is a general consensus that the primary cause of the cooling event was the final collapse of theLaurentide ice sheet near Hudson Bay and the associated sudden drainage of the proglacial Lake Agassizinto the North Atlantic Ocean (e.g. Kleiven et al. 2008, Science 319, 60–64). This freshwater outflowreduced the strength of the North Atlantic thermohaline circulation and thereby the heat transported to theNorth Atlantic region, resulting in an atmospheric cooling (e.g. Barber et al. 1999, Nature 400, 344–348).The climatic consequences of this meltwater flood is assumed to be a good geological analogue for futureclimate-change scenarios, as a freshening of the North Atlantic is projected by almost all global-warmingmodels (e.g. IPCC 2007). In Denmark the 8.2 kyr event pre-dates the introduction of agriculture (by morethan two millennia) and any other major human impact on the environment, so allowing the anthropogenicfactor to be disregarded. At Lake Højby Sø the 8.2 kyr event was identified using AMS 14C dating andhigh-resolution multi-proxy studies of the sediments involving geochemistry, magnetic susceptibility,pollen, macrofossil, diatom and algal pigment analysis. The 8.2 kyr climatic anomaly involved a distinctincrease in regional precipitation as inferred from an abrupt increase in catchment soil erosion. This lastedfor at least 250 years and gave rise to a drastic increase in sedimentation rates of inorganic and organicmaterial. The enhanced soil erosion also increased the input of nutrients leading to an increase in lakeproductivity as reflected by high algal pigment accumulation rates in the period c. 8400–7950 cal yr BP.After c. 7950 cal yr BP algal productivity declined somewhat but the lake did not return to its pre-8400 calyr BP conditions remaining a more productive and nutrient rich lake than before the climate-induced change.Thus, at this point the lake seems to have experienced a climate-driven regime shift. The pollen data fromLake Højby Sø reveal a drop in temperature during the 8.2 kyr event as inferred from a reduced pollenproduction from thermophilous deciduous trees. However, a temperature effect was not demonstrable in thelake ecosystem, at least based on the proxies analysed. If present, a temperature forcing on the aquaticecosystem appears to have been fully overridden by climate-induced catchment processes. In conclusion,our study suggests that during the 8.2 kyr cooling event hydrological change was of more importance forlake ecosystem process than the change in air temperature.

AB - The assessment of present-day climate effects on lake ecosystems in lowland Europe is difficult for tworeasons: i. the absence of long-term monitoring data (usually less than 30 yrs) which makes it difficult todetermine natural variation, and ii. the problem of distinguishing between the effects of climate and theeffects of human activities. These problems also complicate the prediction of possible future climateinfluence on lake ecology. A way of circumventing these problems is the use of lake sediment records whichcontain a wealth of information about past lake history over long time scales and under different climate andland-use regimes. Using a sediment record from Lake Højby Sø, Denmark, the aim of the present study wasto explore ecological response to the most pronounced climate change known from the Holocene: an abruptcooling event around 8200 cal. yr BP, also known as the ‘8.2 kyr cooling event’. The event has beenrecorded as a negative δ18O excursion in the central Greenland ice cores, lasting c. 160 years from8247–8086 cal. yr BP (Thomas et al. 2007, Quat. Sci. Rev. 26, 70–81). In Greenland the maximum coolingwas estimated to be 6 ± 2oC while in southern Fennoscandia and the Baltic countries pollen-basedquantitative temperature reconstructions indicate a maximum annual mean temperature decrease of around1.5 oC (Alley et al. 1997, Geology 25, 483–486; Seppä et al. 2007, Clim. of the past 3, 225–236). Todaythere is a general consensus that the primary cause of the cooling event was the final collapse of theLaurentide ice sheet near Hudson Bay and the associated sudden drainage of the proglacial Lake Agassizinto the North Atlantic Ocean (e.g. Kleiven et al. 2008, Science 319, 60–64). This freshwater outflowreduced the strength of the North Atlantic thermohaline circulation and thereby the heat transported to theNorth Atlantic region, resulting in an atmospheric cooling (e.g. Barber et al. 1999, Nature 400, 344–348).The climatic consequences of this meltwater flood is assumed to be a good geological analogue for futureclimate-change scenarios, as a freshening of the North Atlantic is projected by almost all global-warmingmodels (e.g. IPCC 2007). In Denmark the 8.2 kyr event pre-dates the introduction of agriculture (by morethan two millennia) and any other major human impact on the environment, so allowing the anthropogenicfactor to be disregarded. At Lake Højby Sø the 8.2 kyr event was identified using AMS 14C dating andhigh-resolution multi-proxy studies of the sediments involving geochemistry, magnetic susceptibility,pollen, macrofossil, diatom and algal pigment analysis. The 8.2 kyr climatic anomaly involved a distinctincrease in regional precipitation as inferred from an abrupt increase in catchment soil erosion. This lastedfor at least 250 years and gave rise to a drastic increase in sedimentation rates of inorganic and organicmaterial. The enhanced soil erosion also increased the input of nutrients leading to an increase in lakeproductivity as reflected by high algal pigment accumulation rates in the period c. 8400–7950 cal yr BP.After c. 7950 cal yr BP algal productivity declined somewhat but the lake did not return to its pre-8400 calyr BP conditions remaining a more productive and nutrient rich lake than before the climate-induced change.Thus, at this point the lake seems to have experienced a climate-driven regime shift. The pollen data fromLake Højby Sø reveal a drop in temperature during the 8.2 kyr event as inferred from a reduced pollenproduction from thermophilous deciduous trees. However, a temperature effect was not demonstrable in thelake ecosystem, at least based on the proxies analysed. If present, a temperature forcing on the aquaticecosystem appears to have been fully overridden by climate-induced catchment processes. In conclusion,our study suggests that during the 8.2 kyr cooling event hydrological change was of more importance forlake ecosystem process than the change in air temperature.

U2 - 10.1088/1755-1307/6/7/072053

DO - 10.1088/1755-1307/6/7/072053

M3 - Conference abstract for conference

ER -