“MODERN COOLING” SIGNAL FROM LAKE SEDIMENTS AND POSSIBLE CAUSES
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are group of membrane lipids that thought to be produced by bacteria. These compounds are abundant in lake sediments and changes in their degree of methylation and cyclization can be used to reconstruct past temperature and pH indicators. brGDGTs have been used to generate past lake temperature records via the MBT’5ME proxy from numerous locations spanning from the tropics to the poles and over a variety of timescales (years to hundreds of millions of years). Interestingly, a growing body of evidence indicates that at some sites brGDGT reconstructed temperatures from surface to near-surface sediments yields cooling trend, and this phenomenon disagrees with the meteorological data. Here, we investigate a dimictic lake located in sparsely populated southern Greenland, as well as a corresponding sediment core, watershed soil and sediment trap samples. We note a significant “modern cooling” in the uppermost 7 cm but this observation conflicts with instrumental air temperature measurements. This “modern cooling” trend in the MBT’5ME proxy is mathematically associated with increasing brGDGT-IIa, IIIa and decreasing brGDGT-Ia of their relative abundance, demonstrating the importance of better understanding the source of specific brGDGTs. Principal component analysis of brGDGTs abundances indicates the prominent brGDGTs pattern changes in the near-surface samples, which is related to the group brGDGT-II, III. By comparing the brGDGTs from soils, sediment traps and sediment core, we find that the relative abundance of brGDGT-IIIa and IIIa’ in the sediment core are much higher than in soil samples, indicating that the in situ brGDGTs have a significant contribution to the total amount of brGDGTs in the lake sediments. A plausible reason is that a brGDGT producer living in deep waters, at the sediment-water interface, and/or in shallow sediments will add “cold biased” brGDGTs signal into the surface sediments. This study highlights the importance of understanding brGDGTs from the shallow sediments prior to applying the MBT’5ME proxy for down-core paleoclimate reconstruction, especially because some of the calibrations are based on the relationship between brGDGT relationships insurface sediments and ambient air temperature.