Reconstruction of Holocene glacier fluctuations at Kongsbreen based on sediments deposited in lake Sarsvatnet, Ossian Sarsfjellet, Svalbard

The Arctic is warming twice as fast as the global average, and the melting of mountain glaciers and ice caps has accelerated over the last two decades accompanied by reduced sea ice in the Arctic Ocean. Here we combine sedimentological and geochemical approaches to reconstruct changes in glacier extent at the marine terminating glacier Kongsbreen in order to put present-day climate changes into a longer time perspective. Glaciers are highly sensitive climate indicators as they rapidly respond to variations in summer temperature and precipitation, two parameters that are closely linked to atmospheric dynamics. This climate response is recorded by variations in glacier extent and moraine formation and by variations in glacial erosion and hence sedimentation rates in distal glacier-fed lakes. Lake Sarsvatnet is a threshold-lake that only receive glacial derived sediments when the surface of Kongsbreen crosses a local threshold. When the catchment is ice-free, lake sedimentation rate is lower and dominated by material weathered from the immediate proximity and organic-rich sediments. Based on seismic surveying seven coring sites were selected in three different sub-basins in lake Sarsvatnet. Laboratory analyses, including geochemical measurement by XRF scanning and XRD, CT scanning, grain size and measurements of magnetic proxies, were preformed in order to fingerprint the inorganic sediments. Chronological control is based on radiometric dating (¹⁴C, ²¹⁰Pb, and ¹⁰Be). Erratics (n=3, 125-306 m a.s.l.) indicate ice-free conditions since 13.0±1.1 ka (2σ), overlapping with the oldest organic material found in the lake which is 11 860±80 cal. yr BP. Until around 7400 cal. yr BP lake Sarsvatnet is dominated by organic sedimentation. From around 7400 – 6900 cal. yr BP there is evidence for glacial input into the lake indicating the expansion of Kongsbreen and corresponding to the decline in temperature after the HTM. In the following millennia, and entering the Neoglacial period, there is evidence for mulitiple (~20) decadal to centennial-scale periods of glacier expansion, the most recent dated to AD 1650 marking the onset of glacier build-up towards the LIA maximum. This indicate that the Kongsbreen glacier had short lived expansion periods reaching LIA-like extension already during the middle Holocene, as well as multiple times during the Neoglacial.