LACUSTRINE PALEOTHERMOMETRY: SEASONAL VARIABILITY OF BRANCHED GDGTs IN A TEMPERATE LAKE SYSTEM

Quantitative paleoclimate reconstructions are necessary for testing the reliability of models that predict future climates; however, there are few proxies that can quantitatively reconstruct temperatures over much of the terrestrial realm. Recently, a continental paleothermometer has emerged that utilizes bacterial membrane lipids known as branched glycerol dialkyl glycerol tetraethers (GDGTs). Branched GDGTs are found in a variety of terrestrial environments, including peats, soils, and lake sediments, and the relative distribution of the nine different GDGTs responds to changes in the mean annual air temperature and pH of their environment. This demonstrates that branched GDGTs preserved in lake sediments have the potential to be used as a continental temperature proxy, but there is still debate over whether branched GDGTs in lake sediments are derived from the landscape or produced in situ. Furthermore, if in situ production is the main source, it is unclear whether it occurs within the water column or in the sediments themselves.

To elucidate the origins of branched GDGTs in a temperate lake system, we collected sediment trap and water column samples on a biweekly to monthly basis for over two years at South King Pond, a small kettle lake near Montpellier, Vermont. These samples were analyzed for their branched GDGT distributions and abundances and compared to lake surface sediments and soil samples from the catchment. We find that branched GDGT distributions are fairly constant over the seasonal cycle and are very similar between the sediment trap, the water column, and the surface sediment transect. The branched GDGT distributions observed in lake samples are very different from those in the soil samples, indicating that GDGTs in the lakes are primarily produced in situ. Branched GDGT concentrations are much higher in the hypolimnion of the lake, suggesting that branched GDGTs are mainly produced in the anoxic bottom waters over much of the year. However, branched GDGTs fluxes to the sediment trap increase dramatically during the two seasonal mixing periods, especially during fall turnover, when the lake is fully oxygenated. As such, paleotemperature reconstructions utilizing the branched GDGT proxy are likely recording bottom water temperatures and/or lake mixing temperatures in dimictic systems.