Revised brGDGT fractional abundances and warm-season temperatures strengthen relationships between brGDGTs and environmental variables

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are an important tool for reconstructing terrestrial paleotemperatures from lake sediments. In addition to temperature, however, the distribution of these bacterial membrane lipids is influenced by other environmental variables such as pH, conductivity, and dissolved oxygen. Furthermore, though most brGDGT calibrations are performed against mean annual air temperature (MAT), there is considerable evidence that their distributions are more closely tied to warm-season conditions. Here, we present a new method for analyzing brGDGT data that deconvolves the influences of temperature, conductivity, and pH. Additionally, we measure brGDGT distributions in surface sediments from 43 high-latitude lakes with low MAT and high seasonality. In combination with a globally compiled lake sediment dataset, these samples show a clear warm-season bias in brGDGT-derived temperatures. They also show lake water conductivity to be the second-most important variable in controlling brGDGT distributions. We use the compiled dataset and new fractional abundances to generate brGDGT calibrations for warm-season air temperatures and lake water conductivity and pH for use in lake sediments globally.