Assessing past temperature and soil pH estimates from bacterial tetraether membrane lipids: Evidence from the recent lake sediments of Lochnagar, Scotland

Abstract

Past variation in soil pH and air temperature can potentially be reconstructed from the relative abundance in sediments of branched glycerol dialkyl glycerol tetraethers (GDGTs), synthesized by anaerobic bacteria. Specifically, the cyclization of branched tetraethers (CBT) is believed to be a function of pH, whereas temperature can be estimated from a combination of the extent of both the CBT and methylation of branched tetraethers. Here we explore this potential by comparing a recent sedimentary GDGT profile from Lochnagar, Scotland, with reconstructed air temperature (statistically extrapolated from regional instrumental data sets) and diatom‐inferred lake water pH for the past ∼200 years. Branched glycerol dialkyl glycerol tetratether and diatom‐inferred pH generally agree throughout the core, supporting the use of cyclization of branched tetraethers to reconstruct pH. During the period of rapid industrial acidification (∼1860–1970 A.D.), changes in diatom‐inferred pH lag behind those inferred using branched tetraethers by between 10 and 50 years, possibly due to differing nonlinear responses to acid deposition within soil and lake water environments. However, branched‐GDGT‐derived temperatures are both lower than extrapolated mean annual air temperature estimates (by ∼5°C) and exhibit at least double the rate of reconstructed warming (∼2.5°C in 200 years). At Lochnagar, methylation and cyclization of branched tetraethers are closely correlated (r2 = 0.96) suggesting that in this setting the underlying controls over the two indices may not significantly differ. Therefore the validity of branched‐GDGT‐derived temperature is uncertain and further research is required to address the environmental controls over branched glycerol dialkyl glycerol tetratether synthesis and thus their value as palaeoclimate proxies.