The paleolimnologist's guide to compound-specific stable isotope analysis - An introduction to principles and applications of CSIA for quaternary lake sediments

The stable isotope composition of key chemical elements for life on Earth (e.g., carbon, hydrogen, nitrogen, oxygen, sulfur) tracks changes in fluxes and turnover of these elements in the biogeosphere. Over the past 15-20 years, the potential to measure these isotopic compositions for individual, source-specific organic molecules (biomarkers) and to link them to a range of environmental conditions and processes has been unlocked and amplified by increasingly sensitive, affordable and wide-spread analytical technology. Paleoenvironmental research has seen enormous step-changes in our understanding of past ecosystem dynamics. Vital to these paradigm shifts is the need for well-constrained modern and recent analogues. Through increased understanding of these environments and their biological pathways we can successfully unravel past climatic changes and associated ecosystem adaption. With this review, we aim to introduce scientists working in the field of Quaternary paleolimnology to the tools that compound-specific isotope analysis (CSIA) provides for the gain of information on biogeochemical conditions in ancient environments. We provide information on fundamental principles and applications of novel and established CSIA applications based on the carbon, hydrogen, nitrogen, oxygen and sulfur isotopic composition of biomarkers. While biosynthesis, sources and associated isotope fractionation patterns of compounds such as n-alkanes are relatively well-constrained, new applications emerge from the increasing use of functionalized alkyl lipids, steroids, hopanoids, isoprenoids, GDGTs, pigments or cellulose. Biosynthesis and fractionation are not always fully understood

Production and preservation of archaeal glycerol dibiphytanyl glycerol tetraethers as intact polar lipids in marine sediments: Implications for their use in microbial ecology and TEX86 paleothermometry

For climate modelling in order to predict climate change scenarios, it is important to consider past climate conditions and their impacts. The study of past climates, paleoclimatology, makes frequent use of molecular fossils, molecules that are preserved over geological time scales. Ratios of different compounds can be used, for example, as paleothermometers in order to determine temperatures of air and sea millions of years ago. One of these paleothermometers is the so-called TEX86 which is based on compounds present in the membranes of microorganisms called Thaumarchaeota, that thrive in the ocean. Their membrane lipids are glycerol diether glycerol tetraethers with hydrophilic headgroups (IPL-GDGTs), which are degraded after cell death to core lipid (CL-) GDGTs by release of the head groups, and settle to the sea floor packaged in organic matter particles. After deposition, the distribution of the different GDGTs on which the TEX86 is based can potentially be altered through several process, thereby altering the temperature signal. Sedimentary Thaumarchaeota could produce GDGTs within the sediment, and degradation processes, especially exposure to oxygen, could degrade some GDGTs faster than others. The aim of this thesis was to investigate the effect of post-depositional processes in marine sediments have on the GDGT distribution and thus the TEX86. This was achieved by analysis of IPL- and CL-GDGT distributions, and by conducting stable isotope probing experiments. Results showed that GDGTs from pelagic Thaumarchaeota are deposited and preserved in the sediment as both IPL- and CL-GDGTs. This suggests that IPL-GDGTs are not representative for live Thaumarchaeota and can, dependent on their headgroups, be preserved over geological time scales. Furthermore, it was discovered that GDGT-distributions differ per type of head group, and preferential degradation of more labile head group-containing IPL-GDGTs can thus result in small distributional changes of the IPL-GDGTs. However, these changes are not large enough to change the TEX86 of CL-GDGTs, which is used in paleotemperature determinations. A lack of incorporation of 13C from different 13C-labelled substrates – phytodetritus, amino acids, glucose, pyruvate and bicarbonate – confirmed the slow turnover of IPL-GDGTs and indicated that, if sedimentary Thaumarchaeota are indeed active in sediments, they are only present in relatively low numbers and growing and metabolizing only slowly (up to 100s of years of doubling times). In situ produced GDGTs were shown to degrade completely rather than just lose their head groups and pass over into the CL-pool. Sedimentary processes thus hardly influence the TEX86. IPL-GDGT distributions can change with proceeding degradation, but CL-GDGT distributions are not affected by this. Activity and biomass of sedimentary Archaea have thus previously been overestimated, as the majority of IPL-GDGTs present in sediments are fossil

Impact of sedimentary degradation and deep water column production on GDGT abundance and distribution in surface sediments in the Arabian Sea: Implicationsfor the TEX₈₆ paleothermometer

The TEX86 is a widely used paleotemperature proxy based on isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs) produced by Thaumarchaeota. Archaeal membranes are composed of GDGTs with polar head groups (IPL-GDGTs), most of which are expected to be degraded completely or transformed into more recalcitrant core lipid (CL)-GDGTs upon cell lysis. Here, we examined the differences in concentration and distribution of core lipid (CL)- and intact polar lipid (IPL)-GDGTs in surface sediments at different deposition depths, and different oxygen bottom water concentrations (<3–83 µmol L-1). Surface sediments were sampled from 900 to 3000 m depth on a seamount (Murray Ridge), whose summit protrudes into the oxygen minimum zone of the Arabian Sea. Concentrations of organic carbon, IPL- and CL-GDGTs decreased linearly with increasing maximum residence time in the oxic zone of the sediment (tOZ), suggesting increasing sedimentary degradation of organic matter and GDGTs. IPL-GDGT-0 was the only exception and increased with tOZ, indicating that this GDGT was probably produced in situ in the surface sediment. Concentrations of crenarchaeol with glycosidic headgroups decreased with increasing tOZ, while crenarchaeol with a hexose, phosphohexose head (HPH) group, in contrast, showed an increase with increasing tOZ, indicating that the concentration of HPH crenarchaeol was primarily determined by in situ production in surficial sediments. TEX86 values of both IPL-derived GDGTs and CL-GDGTs decreased by ~0.08 units with increasing water depth, in spite of the sea surface temperatures being identical for the restricted area studied. In situ production in sediments could be excluded as the main cause, due to the slow production rates of GDGTs in sediments, and previous observations of the same trends in TEX86 in sediment trap material. Instead, the incorporation of GDGTs produced in the oxygen minimum zone (with high TEX86 values) and their preferential degradation during the sinking through the water column, or differential degradation of IPL-GDGTs per head group could be the causes for the observed change in TEX86 values. The effect of differential degradation might cause differences between oxic and anoxically deposited sediments, and, together with a potential deep water contribution on TEX86 values, could translate into changes in reconstructed temperature of <3 °C, which might have to be accounted for in TEX86 calibration and paleotemperature studies of deep water sedimentary record