Abstract
Abstract. 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. Warm-season temperature indices outperformed MAT in our regressions, with the mean temperature of months above freezing yielding the highest-performing model (adjusted R2 = 0.91, RMSE = 1.97 ∘C, n = 182). The natural logarithm of conductivity had the second-strongest relationship to brGDGT distributions (adjusted R2 = 0.83, RMSE = 0.66, n = 143), notably outperforming pH in our dataset (adjusted R2 = 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.
Highlights
Paleotemperature records from lake sediment archives are highly sought after in studies of terrestrial paleoclimate
The low temperatures are reflected in the branched glycerol dialkyl glycerol tetraethers (brGDGTs) distributions of these sediments (Fig. 4), which contain on average a higher fIIIaFull and lower fIaFull than the other samples in this dataset
We show that two adjustments can be made to significantly improve the correlations between brGDGT fractional abundances (FAs) and temperature in lake sediments: (1) replace mean annual air temperatures (MATs) with a warm-season temperature index and (2) use FAs calculated
Summary
Paleotemperature records from lake sediment archives are highly sought after in studies of terrestrial paleoclimate. Bacterial branched glycerol dialkyl glycerol tetraether (brGDGT) lipids have solidified themselves as an important tool in this pursuit (Fig. A1; Schouten et al, 2013). First isolated from peat (Sinninghe Damsté et al, 2000), these membrane lipids have since been measured in increasingly diverse settings, from marine, soil, lacustrine, and riverine locations (Hopmans et al, 2004; Weijers et al, 2006; Pearson et al, 2011; De Jonge et al, 2014b, respectively) to hot springs, fossil bones, groundwater, deep ocean trenches, and methane seeps Raberg et al.: Revised brGDGT calibrations in lake sediments
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