DECADAL SUMMER LAKE WATER TEMPERATURE VARIABILITY IN SOUTHERN GREENLAND OF THE LAST 2 MILLENNIA: DEVELOPMENT AND APPLICATION OF A SITE-SPECIFIC BRGDGT CALIBRATION

The application of branched glycerol dialkyl glycerol tetraethers (brGDGTs) for reconstructing past temperatures remains rare in Arctic settings, largely because a complete understanding of the environmental controls on brGDGT distributions is lacking in the high latitudes. Here, we investigate brGDGTs in a dimictic lake located in southern Greenland by examining a sediment core, surface sediments, catchment soils and 3-years of settling particle samples. We aim to improve the application of the brGDGT paleothermometer in Arctic region and establish a high-resolution temperature record for southern Greenland. In July 2016, a sediment core was collected from Lake 578 (W45°36’, N61°04’) in sparsely populated southern Greenland, and the age model is based on ²¹⁰Pb and ¹³⁷Cs activity and ¹⁴C dating of terrestrial macrofossils. Settling particles were collected from multiple water depths over three years, and brGDGT distributions are compared to measured lake water temperatures. Our results indicate that sedimentary brGDGTs are mainly produced in the water column, and the MBT’_5ME (Methylation of Branched Tetraethers) of in situ brGDGTs is significantly correlated with measured integrated summer water temperature. Thus, we provide the first water temperature-brGDGTs calibration based on MBT’_5ME index. The RMSE of 0.58 °C is improved relative to previous global lake core-top calibrations. We apply this calibration to the sediment core to produce a decadal-resolution summer water temperature record over the past 1,700 years. Our reconstruction reveals a gradual cooling trend in the late Holocene, in agreement with the decreasing trend of summer insolation at 61°N. Interestingly, our result demonstrates a moderate temperature during the Medieval Climate Anomaly (950 to 1250 CE), instead of a distinct, long-term anomalous climate. Overall, this successful application of brGDGTs to a lacustrine sediment core sheds light on the use of this relatively new paleothermometer to quantitatively reconstruct temperature from Arctic lakes.