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Miller, Gifford H  INSTAAR.
Geirsdottir, Aslaug  University of Iceland.

Subtle shifts in either atmospheric or oceanic circulation produce strong changes in the terrestrial environment of Iceland. Despite the large amplitude of climate change expected for Iceland during the Holocene, and numerous large ice caps that would respond to these changes, there are no complete records of terrestrial environmental change for the Holocene of Iceland, and the status of Icelandic glaciers in the early Holocene remains debated. It is not known whether Iceland’s large ice caps disappeared in the early Holocene, and if they did, when they re-grew. To answer these questions, four continuous cores were recovered in Summer 2003 from a deep, high-sedimentation-rate lake basin (Hvítárvatn), selected because the sediments were expected to provide high-resolution, quantitative evidence of environmental change over the past 10 ka.

Hvítárvatn is a glacier dominated lake located on the eastern margin of Langjökull Ice Cap in central-western Iceland. The Hvítárvatn cores have three major periods: a glacially dominated lake system (LIA), an early neoglacial period that is still influenced by Langjökull Ice Cap, and the thermal optimum with no apparent glacial signal in the sediments and a high organic content. Preliminary studies of these sediments revealed distinctive varve and diatom assemblages that reflect these broad changes. In progress is a detailed varve and diatom study to test if the early Holocene summer insolation maximum was sufficient to completely melt Iceland’s large ice caps and to capture a sub-decadal record of late Holocene climate variability. Icelandic lakes are particularly well suited to answer these questions because: 1) Glacial erosion and soft bedrock result in high lacustrine sedimentation rates, 2) Diagnostic tephras of known age aid the geochronology. Iceland’s sensitivity to changes in North Atlantic circulation is expected to produce clear signals in key environmental proxies (diatoms, varve thickness) preserved in lacustrine sequences, and 4) Ice-cap profiles are relatively flat; consequently, small changes in the equilibrium line altitude (ELA) result in large changes in accumulation area. Hence, large changes in ice-sheet margins during the Holocene are likely, and these will impact sedimentation in glacier-dominated lakes and the diatom assemblages at those times.

Smear slides revealed the presence of large concentrations of diatoms. The presence of such a plentiful number of diatoms throughout the cores was a surprise, as the silt-laden nature of the glacial lake was a deterrent to the presence of almost any other form of life (TOC<0.3%). Further study revealed the uppermost 3 to 4 m of all Hvítárvatn cores display sediment characteristics of a glacially dominated lake system. These sediments have large packages of ice-rafted detritus (IRD) and have thick annually laminated varves with white clay caps representing the winter layer when ice cover allowed the fine clay particles to settle out. The diatom assemblages here are dominated by planktonic, silica-demanding taxa that suggest a high dissolved silica and turbid water environment that would be consistent with high fluxes of glacial flour. Deeper in the cores are what are likely early Neoglacial sediment, deposited when Langjökull Ice Cap was active, but outlet glaciers were not in contact with the lake. The varves are predominantly gray in color with much thinner clay caps, and there is very little IRD. The diatom assemblage resembles the uppermost sediments, but a much closer look at the diatoms is necessary in order to qualify the differences. The bottom 1 m of sediment shows a change in the sedimentation of the cores. The sediments are lightly stratified with no evidence of varves or clay caps. There is a much higher organic component to this part of the core, with diatoms comprising a large portion of the sediments. The diatom assemblage here is a diverse suite of taxa dominated by benthic diatoms suggesting clear water with long growing seasons likely found in an environment with warmer summers than present and with no glacial erosion occurring. We believe these lowermost sediments were deposited during the Holocene Thermal Maximum when Iceland’s ice caps completely disappeared. However, we need to prove this and believe that diatoms and an in depth varve study hold the key. To test the sensitivity of the glacial lake to climate parameters, the uppermost 50 years of varves will be compared to instrumental records from a nearby weather station. This work is currently in progress.

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