Revised fractional abundances and warm-season temperatures substantially improve brGDGT calibrations in lake sediments

Distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs) are frequently employed for reconstructing terrestrial paleotemperatures from lake sediment archives. Although brGDGTs are globally ubiquitous, the microbial producers of these membrane lipids remain unknown, precluding a full understanding of the ways in which environmental parameters control their production and distribution. Here, we advance this understanding in three ways. First, we present 43 new high-latitude lake sites characterized by low mean annual air temperatures (MATs) and high seasonality, filling an important gap in the global dataset. Second, we introduce a new approach for analyzing brGDGT data in which compound fractional abundances (FAs) are calculated within structural groups based on methylation number, methylation position, and cyclization number. Finally, we perform linear and nonlinear regressions of the resulting FAs against a suite of environmental parameters in a compiled global lake sediment dataset (n = 182). We find that our approach deconvolves temperature, conductivity, and pH trends in brGDGTs without increasing calibration errors from the standard approach. We also find that it reveals novel patterns in brGDGT distributions and provides a methodology for investigating the biological underpinnings of their structural diversity. Warmseason temperature indices outperformed MAT in our regressions, with the mean temperature of months above freezing yielding the highest-performing model (adjusted R-2 = 0.91, RMSE = 1.97 degrees C, n = 182). The natural logarithm of conductivity had the second-strongest relationship to brGDGT distributions (adjusted R-2 = 0.83, RMSE = 0.66, n = 143), notably outperforming pH in our dataset (adjusted R-2 = 0.73, RMSE = 0.57, n = 154) and providing a potential new proxy for paleohydrology applications. We recommend these calibrations for use in lake sediments globally, including at high latitudes, and detail the advantages and disadvantages of each.

Automated summary

The use of brGDGT distributions in lake sediments for reconstructing terrestrial paleotemperatures is important, but the microbial producers of these membrane lipids remain unknown. Through new high-latitude lake sites and analysis methods, researchers have successfully deciphered the control of temperature, conductivity, and pH on brGDGT, revealed new distribution patterns, and provided a method for exploring the biological underpinnings of their structural diversity.