BRANCHED GDGT DISTRIBUTIONS IN SEBAGO LAKE, ME: DISENTANGLING SOIL AND LACUSTRINE INPUTS TO THE SEDIMENTARY RECORD

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are compounds that are useful for examining past environments. Two proxies based on brGDGTs, the methylation of branched tetraethers (MBT) and the cyclization of brGDGTs (CBT), are used to reconstruct continental air temperature and soil pH, respectively, when measured in soils. Recent research on brGDGTs indicates that these compounds are also produced in-situ in the water columns of lakes and rivers. Due to brGDGTs often being abundant in lake sediments, MBT is widely applied as a paleothemometer. Presently, numerous outstanding questions exist with regard to MBT and CBT, particularly when applied to lacustrine settings. First, the dominant producer of brGDGTs remains unknown, although studies suggest they are likely members of the Acidobacteria. Second, numerous lacustrine calibrations between MBT ratios of lake surface sediments and temperature exist yet most are based on a relatively small numbers of sites and all are currently calibrated to mean annual air temperature instead of lake water temperatures, as the latter are not available in many instances. Mean annual air temperature is a poor indicator of water column temperature for certain types of lakes, especially high-latitude lakes. Third, it remains largely unknown where in the water column most brGDGTs are produced, and if they are present throughout the year or only during specific seasons. Furthermore, as brGDGTs are produced in both soils and lakes, it remains unknown how much of an influence terrestrial inputs may have on a lacustrine brGDGT record. To address these outstanding questions, we examine a suite of catchment soil and surface sediment samples from Sebago Lake, ME. This relatively large lake has a surface area of 117 km² and is >100m deep. Sebago Lake serves as the water supply for the city of Portland, ME and has been monitored since the late 1970s (including in-situ temperature measurements). We collected surface sediment samples along two transects across the lake, one of which extends from the Crooked River at the north end of the lake, south across the central deep basin. Here we examine differences in brGDGT distributions within Sebago Lake and discuss implications for MBT and CBT paleoclimate reconstructions.