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The distribution of brGDGT lipids produced by soil bacteria has been used to reconstruct temperatures in marine and terrestrial settings as far back as the Cretaceous period. However, modern calibrations of this proxy have primarily relied on air rather than in situ soil temperatures, which can differ by more than 10 ◦C. Furthermore, the influence of other parameters such as temperature seasonality and soil chemistry on brGDGT lipids is not fully understood. We measured brGDGT distributions, in situ soil temperatures, pH, soil water content, and electrical conductivity on soils from the Eastern Canadian Arctic and Iceland. We compiled our results with those of published soil brGDGT studies that also provide in situ soil temperatures and ancilliary measurements and generated global temperature and pH calibrations from the resulting dataset. Soil temperatures outperformed air temperatures in these calibrations, with mean summer soil temperature providing the highest-performing fit among the 10 tested soil temperature parameters. When applied to a loess/paleosol sequence from the Chinese Loess Plateau, these new calibrations produced paleotemperature and paleo-pH histories consistent with the results of previous studies, encouraging the application of our new calibrations on a broader scale. We also detected 7-methyl and IIIa’’ brGDGT isomers in our Eastern Canadian Arctic and Iceland soils, which have been shown in lakes to relate to salinity and anoxia, respectively. While neither correlated with bulk soil properties such as conductivity, soil water content, or pH, these brGDGT isomers did correlate with seasonality and winter soil temperature. We hypothesize that these compounds are generated in winter by bacteria in habitable niches of more saline, sometimes anoxic liquid water in the otherwise frozen soil matrix. Finally, we report the presence of overly branched GDGTs with m/z = 1064 and suggest that these heptamethylated tetraethers should be investigated as a potential tool for improving brGDGT calibrations. Overall, our results expand our understanding of the seasonality of brGDGT production, especially at high latitudes, and provide in situ soil temperature and pH calibrations for global use. 

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 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. 

Abstract Paleotemperature histories derived from lake sediment archives provide valuable context for modern and future climate changes. Branched glycerol dialkyl glycerol tetraether (brGDGT) lipids are a valuable tool in such pursuits due to their empirical correlation with temperature and near ubiquity in nature. However, the relative contributions of terrestrial and lacustrine sources of brGDGTs to lake sediments is site‐dependent and difficult to constrain. Here, we explored the potential for intact brGDGTs—the complete lipids with polar head groups (HGs) still attached—to provide insight into the sources of brGDGTs on the landscape and their contributions to the sedimentary record in a set of Arctic lakes. We measured core and intact brGDGTs in soils, surface and downcore sediments, water filtrates, and sediment traps across five lake catchments in the Eastern Canadian Arctic, with an emphasis on Lake Qaupat (QPT), Baffin Island. Soils were dominated by brGDGTs with a monoglycosyl (1G) HG, while lacustrine samples contained more phosphohexose (PH) brGDGTs, providing evidence for in situ brGDGT production in both settings. Core‐ and PH‐brGDGT‐IIIa were more abundant in sediments than in the soils or water column, implying an additional post‐depositional source of brGDGTs. A hierarchical clustering analysis indicated that core brGDGTs in Lake QPT sediments were largely lacustrine in origin, while 1G‐brGDGTs were primarily soil‐derived. Additionally, we found evidence for preservation of intact brGDGTs—especially 1G‐brGDGTs—downcore on thousand‐year timespans, though in situ production deeper in the sediment column cannot be ruled out. Finally, we explored the possibility of reconstructing 1G‐brGDGT‐derived soil temperatures and core‐brGDGT‐derived lake temperatures in tandem from sedimentary archives.