Interpreting the global distribution of Glycerol Dialkyl Glycerol Tetraethers

Abstract

Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are a diverse group of tetraether lipids produced by Archaea and Bacteria which have been detected across all mesophilic environments on Earth. The distribution of these compounds can provide insight into past environmental parameters. However, the global distribution of GDGTs is not well constrained. The aim of this investigation is to constrain this distribution and understand the factors that drive the global variation of these compounds.

A global dataset of GDGT abundance across soils, peats, lakes, and marine sediment was created using High Performance Liquid Chromatography-Mass Spectrometry, before statistical analysis was undertaken to identify key patterns in the GDGT distribution. Principle component analysis was used to explore the key parameters that influence GDGT distributions. This showed that marine and terrestrial settings give the largest separation along PC1 (50% loading), plotting a linear correlation with the branched index of tetraethers (BIT) (r² = 0.81). Comparison of PC2 (25% loading) to temperatures calculated using the appropriate temperature proxy and calibration for each environment, showed linear correlation in most environments, highlighting that temperature is another main driver of GDGT diversity. I then looked at the factors that influences specific groups of GDGTs.

Butanediol dialkyl glycerol tetraethers (BDGTs) were detected in some marine sediments, peats, and lakes in relatively low concentrations (<2% relative abundance), suggesting a global distribution. BDGTs are thought to be produced by 7^th Order Methanogens, but the fact that BDGTs show no correlation with the methane index (MI), suggests that the 7^th order methanogens are independent of overall methanogenic activity. Furthermore BDGTs show potential to correlate with temperature but further work is required to establish this.

Next isoprenoid glycerol dialkyl diethers (isoGDDs) were investigated, for which there is an ongoing discussion on whether these compounds are degradation products or actively biosynthesised by Archaea. IsoGDDs were detected in almost all environments. Based on the strong linear correlation between the ring indexes of isoGDGTs and isoGDDs, these compounds are likely degradation products of isoGDGTs. However some evidence also suggests active biosynthesis occurs in low levels.

Finally the isoGDGT distribution at four marine sediment cores was investigated to determine whether they were suitable for TEX86-based sea surface temperature reconstructions. The results showed that only one site was suitable for TEX₈₆ reconstructions. One site contained elevated branched GDGTs, indicating a significant contribution of terrestrial derived GDGTs that makes it unsuitable. In two further sites, the GDGT distribution was found to be disrupted by dry soil input, undetected by the branched Index of tetraethers. Together these results highlight the need for caution when applying TEX₈₆.

Date of Award	20 Jun 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	B D A Naafs (Supervisor) & Jonathan Clayden (Supervisor)