TEX₈₆ paleothermometry: proxy validation and application in marine sediments

Determination of past sea surface temperature (SST) is of primary importance for the reconstruction of natural climatic changes, modelling of climate and reconstruction of ocean circulation. Recently, a new SST proxy was introduced, the TetraEther indeX of lipids with 86 carbons (TEX₈₆), which is based on temperature-related changes that occur in the composition of creanarchaeotal membrane lipids. An empirical correlation between TEX₈₆ values of sediment core tops from a wide variety of geographical locations and annual mean SST has been used to calibrate the TEX₈₆ proxy. The main objective of the work described in this thesis was to improve, validate and apply the TEX₈₆ proxy. To do this we focused on a) the analytical aspects of the TEX₈₆, b) the transport of GDGTs to the sediment and the influence of diagenesis and c) the application of the proxy in various settings. Regarding the analytical aspects, the detection limit of the GDGTs was established at 0.05 ng GDGTs injected on-column. Furthermore application of SIM improved the analytical reproducibility to 0.3C, and a new synthetic internal GDGT standard was used which improved the accuracy of GDGT abundance measurements increasing the potential of GDGTs in (paleo)ecological studies substantially. The transport and diagenesis of GDGTs were also subject of study as they are essential in understanding and applying the TEX₈₆ as palaeothermometer. This study revealed the presence of GDGTs in both stomachs and, in lower amounts, in the intestines showing uptake of GDGTs by decapods, but TEX₈₆-derived temperatures are not significantly different between stomachs and intestines (1C), suggesting that TEX₈₆ values are not altered during gut transit. To investigate the effect of degradation by long term exposure to oxygen we analyzed GDGTs abundance, TEX₈₆ and Branched versus Isoprenoid Tetraether index (BIT) in several Madeira Abyssal Plain turbidites across an oxidation front. A decrease by several orders of magnitude in the concentrations of GDGTs was found and also substantial shifts in the BIT and TEX₈₆. The shifts are interpreted to be caused by a strong relative increase in soil-derived GDGTs relative to the marine-derived GDGTs. This selective preservation of soil-derived lipids which are chemically nearly identical to that of their marine counterpart strongly suggests that the mineral matrix to which soil organic matter is attached enhanced its preservation. Finally the application of the TEX₈₆ to a range of environments and time resolutions in combination with other SST proxies allowed the validation of the TEX₈₆ proxy as a paleothermometer. While in most studies TEX₈₆ showed a good correlation with SST, work using sediment traps and a sediment core covering the last 150 years in the Santa Barbara Basin revealed that TEX86 values were always substantially lower than sea surface temperatures (SST), indicating a sub-surface origin, likely between 100-150 m. This result highlights the importance of performing calibration studies using sediment traps and core tops before applying the TEX₈₆ temperature proxy in a given study area.