Development of a global lacustrine temperature calibration based on 3-hydroxy fatty acid membrane lipids

Lakes play an important role in paleoclimate studies as they archive high-resolution and continuous records. However, the climatic proxies developed for lake settings are limited. Among all, the lipid biomarkers have been widely studied, as they are ubiquitously distributed and efficiently carry environmental information. One prominent example is the branched glycerol dialkyl glycerol tetraethers (brGDGTs), produced by bacteria, use to quantitatively reconstruct the temperature and pH based on lake sediment. However, due to the influence of confounding factors and not fully determined producer species, temperature reconstruction based on brGDGTs could yield uncertainties as large as 4°C. Having complementary and independent temperature proxies appears to be essential.

3-hydroxy fatty acids (3-OH FAs) were recently proposed as temperature and pH proxies in soils and may hold the potential to be also applied to lakes. These compounds are membrane lipids produced by Gram-negative bacteria. Similar to brGDGTs, their distribution was related to environmental variables. To date, 3-OH FAs were mainly investigated in soils [1] with only 3 studies, all in the Chinese region, in lakes. A linear correlation between some of the 3-OH FA isomers (i.e. the Ratio of Anteiso-C₁₃ to Normal-C₁₃  ̶   RAN₁₃)and mean annual air temperature (MAAT) was observed in Chinese lakes  [2]. In contrast, we did not observe such a correlation in 52 lake sediments of the French Alps and 20 lakes of Southern Chile [3]. This suggests that the relationship between MAAT and 3-OH FA distribution in lakes is complex and cannot be systematically reflected by a linear correlation. Nevertheless, this relationship needs to be further investigated using additional samples from all over the world.

This study aims to present the first global analysis of lacustrine 3-OH FAs and their relationship with MAAT. In addition to first studies, we analyzed these lipids in 220 lakes distributed worldwide over a large range of latitude and elevation, with MAATs ranging from -14.2°C to 27.7°C. Principal Component Analysis (PCA) was first applied to the whole dataset (220 lakes) to investigate the changes in 3-OH FA distribution with location. In addition, both linear (including the RAN₁₃ index) and non-linear models (based on machine learning algorithms) are currently used to examine the relationship between 3-OH FA distribution and MAAT. This will bring new insights into the applicability of the 3-OH FAs as lacustrine temperature proxies at the global scale. 3-OH FAs could then be applied to paleotemperature reconstructions from lake sediment cores, complementarily of and independently from existing proxies such as brGDGTs.

References: [1] Véquaud et al. (2021). Biogeosciences 18, 3937-3959. [2]Yang et al. (2021). Org. Geochem. 160, 104277. [3] Ke et al., Org Geochemistry, under revision.