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

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.

Near-universal trends in brGDGT lipid distributions in nature

“…These discrepancies have led to independent modern calibrations for different sample types [e.g., soils and peats (36), lacustrine sediments (37) or SPM (38), bones (39), speleothems (40), and marine sediments (23)] and regions [e.g., East Africa (41) and China (42)] and have necessitated much effort to disentangle the allochthonous versus autochthonous sources of brGDGTs to sedimentary archives (2,23,(43)(44)(45)(46)(47)(48). Further complicating the matter, brGDGT distributions are affected by a wide array of other environmental parameters, including oxygen levels (26,27,29,(49)(50)(51)(52)(53)(54), salinity/electrical conductivity (55,56), seasonality (27,36,46,57), nutrient availability (58,59), and soil chemistry (28), which can mask or override relationships with temperature or pH. Last, an observed bias in brGDGT-derived temperatures toward warmer seasons has proven difficult to quantify, with various studies finding summer air temperature (60), the mean air temperature of months above freezing (MAF) (36,37,55), growing degree days above freezing (61), or other temperature indices to provide the strongest correlations in modern training sets.…”

“…Further complicating the matter, brGDGT distributions are affected by a wide array of other environmental parameters, including oxygen levels (26,27,29,(49)(50)(51)(52)(53)(54), salinity/electrical conductivity (55,56), seasonality (27,36,46,57), nutrient availability (58,59), and soil chemistry (28), which can mask or override relationships with temperature or pH. Last, an observed bias in brGDGT-derived temperatures toward warmer seasons has proven difficult to quantify, with various studies finding summer air temperature (60), the mean air temperature of months above freezing (MAF) (36,37,55), growing degree days above freezing (61), or other temperature indices to provide the strongest correlations in modern training sets. These discrepancies might be expected, as these air temperature indices are themselves only proxies for the in situ temperatures that are relevant for brGDGT-producing microbes.…”

“…Relationships between temperature and brGDGT FAs for each compound (A to L) in the Meth set. The Meth set is formed by grouping brGDGTs by cyclization number and isomer designation (schematics at top); FAs are calculated within each group(55). Plots of IIa′, IIb′, and IIc′ exactly mirror those of IIIa′, IIIb′, and IIIc′, respectively, and are therefore not shown.…”

Calibration and Application of Branched GDGTs to Tibetan Lake Sediments: The Influence of Temperature on the Fall of the Guge Kingdom in Western Tibet, China

“…In (a) and (b), bottom left scatterplots show bias of different calibrations derived temperatures across different latitudes (compiled and revised from Martínez‐Sosa et al., 2021), blue dot, MBT′ 6Me ‐MAAT calibration from this study; yellow dot, MBT′ 6Me ‐MAF calibration from this study; gray dot, calibration from Raberg et al. (2021); and red dot, calibration from Martínez‐Sosa et al. (2021).…”

Supplemental Material: Differences between soil and air temperatures: Implications for geological reconstructions of past climate

“…Similarly, several report that brGDGTs in lakes record temperatures above freezing (e.g., Dang et al, 2018;Martínez-Sosa et al, 2021;Raberg et al, 2021;. brGDGTs that accumulate in sediment in lakes, however, bring the additional complication that some are produced by bacteria in the lake column and some are washed in from surrounding soils (e.g., Colcord et al, 2015;Günther et al, 2014;Li et al, 2016Li et al, , 2017Loomis et al, 2011;Russell et al, 2018;Tierney and Russell, 2009;van Bree et al, 2020;Wang et al, 2012;Weber et al, 2015).…”