Multidecadal Delay Between Deglaciation and Formation of a Proglacial Lake Sediment Record

Proglacial lake sediments are widely recognised as accurate and high-resolution archives of climate and glacier variability. Sediments deposited in proglacial lakes are frequently varved, which offers the possibility to generate precisely-dated records, and their basal ages are often used to constrain deglaciation histories. It is often assumed that lake sedimentation starts immediately after deglaciation. With this in mind, we studied the onset of lake sedimentation in a recently deglaciated lake (Calluqueo Lake, Chilean Patagonia) to investigate the possible delay between proglacial lake formation and establishment of a continuous sediment record. Calluqueo Lake is a 3.5 km long lake composed of a large 220 m deep proximal basin, separated from a smaller 50 m deep distal basin by a 40 m deep sill. The lake is bordered by steep lateral moraines that contain large boulders. Aerial images and historical data show that Calluqueo Glacier entirely covered the lake basin until 1941. Since then, it rapidly receded until it became land-based in 1985. Side Scan sonar images and grab sampling shows that the sediment cover is limited to the small distal basin, which was entirely deglaciated by 1978. By comparison, no sediment was found in the deepest proximal basin although it has been ice free for at least three decades. Varve counting of sediments deposited in the distal basin shows that the stratigraphic record starts in 1996 ± 4 CE, i.e., that the first 20 – 50 years of the glacier’s retreat are not represented in the sediments of Calluqueo Lake. We hypothesize that the fine-grained sediments that are discharged into the lake immediately after its formation first start accumulating between the large boulders that compose the ablation moraine on the lake floor. The continuous stratigraphic record only starts forming after the coarse moraine deposits are buried under fine-grained particles. Our results have important implications for the use of proglacial lake sediments in paleoclimate and paleoenvironmental research. They suggest that proglacial lake sediment records lack the first 20 – 50 years of sedimentation. Although this delay may be negligible for reconstructions of deglaciation histories based on basal radiocarbon ages, it becomes significant for the use of lake sediment records from recently deglaciated environments.