Development of an in situ branched GDGT calibration in Lake 578, southern Greenland

Abstract

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are abundant in lake sediments and can potentially be used as proxies for evaluating past environmental conditions. The degree of methylation of brGDGTs has a strong relationship with the ambient temperature, and numerous brGDGT temperature reconstructions have been generated ranging from the tropics to high latitudes and over a variety of timescales. However, this proxy still requires calibration and validation in Arctic settings, which are among the most climatically sensitive regions on Earth. Here, we investigate Lake 578, a dimictic lake located in southern Greenland, and examine brGDGTs in a sediment core, surface sediments, catchment soils, and settling particulate matter (SPM). Our results suggest that sedimentary brGDGTs are mainly produced in the water column, and the SPM MBT′5ME (Methylation of Branched Tetraethers) values are significantly correlated with depth-integrated summer water temperature. Here, we provide the first lake water temperature-brGDGT calibration based on the MBT′5ME index, with an improved root mean square error (RMSE) of 0.52 °C. Like previous studies, we find that watershed soils differ in brGDGT distributions from those of the SPM or lake sediments, and we conclude that most brGDGT production in Lake 578 is occurring in situ. In the sediment core, we observe a significant cooling trend in the uppermost 7 cm, but this result is unrealistic because it conflicts with the instrumental temperature record from this region. This “core-top cooling” is associated with increasing relative abundances of brGDGT-IIa and IIIa, and with decreasing brGDGT-Ia, and we speculate that it may be driven by diagenetic effects or changing microbial communities in the sediment subsurface. The anomalous “core-top cooling” phenomenon, which is noted in numerous other published lacustrine brGDGT records, highlights the need for further studies of brGDGTs in modern and recent lake sediments. Constraining the source organisms of brGDGTs and better understanding the relationship between brGDGTs and environmental factors are essential, prior to their use in paleoclimate reconstructions.