Reconstructing summer water temperature from a southern Greenland lake over the Common Era: a site-specific calibration for brGDGTs

Abstract:

The Arctic has warmed about twice as fast compared to the global mean since the start of the industrial revolution. Arctic climate shows strong sensitivity to the global climate system; therefore, quantitative temperature reconstructions from the Arctic are essential to put the current global warming into a long-term context. Here, we investigate branched glycerol dialkyl glycerol tetraethers (brGDGTs) in a dimictic lake located in sparsely populated southern Greenland, including sediment core, surface sediments, catchment soils and 3-year settling particle samples, aiming to improve the application of this paleothermometer in Arctic region, and establish high resolution temperature records from this area. In July 2016, the sediment core was collected from Lake 578 (W45°36’, N61°04’) in southern Greenland, and the age model is based on ²¹⁰Pb and ¹³⁷Cs activity and ¹⁴C dating of terrestrial macrofossils. Sediment traps with mounted thermistors were deployed at 5 m, 10 m and 14 m water depths collected settling particles, and we collected settling particulate matter in 2017, 2018 and 2019 summer. Our results indicate that sedimentary brGDGTs are mainly produced in the water column, and the MBT’_5ME (Methylation of Branched Tetraethers) of these brGDGTs is significantly correlated with measured summer water temperature. Thus, we provide the first water temperature-brGDGT 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 sedimentary brGDGTs and acquire a decadal resolution summer water temperature record over the past 1700 years. Our reconstruction reveals a gradual cooling trend in late Holocene, which is in accordance with the decreasing trend of summer insolation at 61°N. Interestingly, our result demonstrates a moderate temperature during the Medieval Climate Anomaly, instead of a distinct, long-term anomalous climate. Overall, our new Arctic MBT’_5ME calibration and resulting temperature record validate the utility of this relatively new paleothermometer from Arctic lakes.