DEGLACIATION CHRONOLOGY IN THE CENTRAL UUMMANNAQ FJORD SYSTEM (WESTERN GREENLAND) DURING THE HOLOCENE

The goal of this study is to determine the timing of deglaciation of the Greenland Ice Sheet margin in the central Uummannaq Fjord system using lake sediment cores and ¹⁰Be dating of erratic boulders and bedrock. Two sediment cores from proglacial-threshold lakes and a third core from a non-glacial lake were characterized with Itrax-derived elemental analysis, Geotek-Core-Scanner spectroscopy, magnetic susceptibility, and bulk density, along with moisture and organic matter (loss on ignition) content. We obtained eight radiocarbon ages from the sediment cores, and ages from basal organic-rich sediments from the two connected proglacial-threshold lakes are 10.1±0.1 ka cal yr BP and 9.6±0.7 ka cal yr BP. The non-glacial lake has a radiocarbon age of 10.6±0.1 ka cal yr BP near the basal contact with deglacial sediments. In addition, two erratic boulders and a sample of ice-sculpted bedrock have overlapping ¹⁰Be ages, yielding an average of 10.8±0.7 ka. The close correspondence of the basal radiocarbon ages from lake sediments and the ¹⁰Be ages of the surrounding catchments gives us confidence that deglaciation occurred between 11 and 10 ka. Our chronology is different than existing chronologies of marine terminating glaciers in the northern and southern portions of the Uummannaq the Fjord system. Lane et al. (2014, Quaternary Science Reviews, v. 92, p. 324-344.) found that deglaciation of Rink Isbræ occurred ~5.3 ka, and Roberts et al. (2013, Journal of Geophysical Research: Earth Surface, v. 118, no. 2, p. 519-541.) found that deglaciation occurred at ~8.7 ka near Store Gletscher. The three studies show a wide variation of the timing of deglaciation within the inner parts of the UISS, and point to either error in chronologies or large differences in the timing of deglaciation of nearby glacier systems. These results highlight the need to better constrain the variability of ice-sheet-wide deglaciation chronology.