Metal interactions with the sulfur cycle in modern and ancient environments

Modelling global biogeochemical cycles requires an understanding of processes at a range of scales. Small-scale redox processes are often modelled as a succession of successively less energy efficient reactions which are predominantly mediated by bacteria. This thesis is a series of discrete studies which examine early diagenetic processes in a peatbog, saltmarsh and an ancient marine sequence. In an ombrotrophic peat bog at Thorne Moors, redox horizons, which influence the behaviour of sulfur and iron, are complex because they are impacted by abiotic and biotic processes, roots, lateral flows and potentially “hotspots” of organic matter (OM) degradation. In these systems the long-term fate of trace metals was found to be predominantly controlled by dust deposition suggesting that they are immobile; this was confirmed by the DET and DGT data. Warham salt marsh results show that due to the rapid sedimentation rates, more reactive iron is buried which then partially oxidises sulfide to elemental sulfur in the deeper sediment and to sulfate in the near-surface. The ancient site (Shales-with-Beef Member) has concretionary horizons that coincide with partial oxidation of sulfide to elemental sulfur and a change in iron chemistry, whilst the organic matter input and anoxic conditions remained the same. The changes in diagenetic conditions arose from slow sedimentation rates which cause the less reactive iron to remain longer in the biogeochemical zone. This study shows that redox zones are much more complex than the series of cascading reactions which successively yield less energy and are predominantly mediated by bacteria

Descent toward the icehouse: Eocene sea surface cooling inferred from GDGT distributions

The TEX86 proxy, based on the distribution of marine isoprenoidal glycerol dialkyl glycerol tetraether lipids (GDGTs), is increasingly used to reconstruct sea surface temperature (SST) during the Eocene epoch (56.0–33.9 Ma). Here we compile published TEX86 records, critically reevaluate them in light of new understandings in TEX86 palaeothermometry, and supplement them with new data in order to evaluate long-term temperature trends in the Eocene. We investigate the effect of archaea other than marine Thaumarchaeota upon TEX86 values using the branched-to-isoprenoid tetraether index (BIT), the abundance of GDGT-0 relative to crenarchaeol (%GDGT-0), and the Methane Index (MI). We also introduce a new ratio, % GDGTRS, which may help identify Red Sea-type GDGT distributions in the geological record. Using the offset between TEX86H and TEX86L(ΔH-L) and the ratio between GDGT-2 and GDGT-3 ([2]/[3]), we evaluate different TEX86 calibrations and present the first integrated SST compilation for the Eocene (55 to 34 Ma). Although the available data are still sparse some geographic trends can now be resolved. In the high latitudes (>55°), there was substantial cooling during the Eocene (~6°C). Our compiled record also indicates tropical cooling of ~2.5°C during the same interval. Using an ensemble of climate model simulations that span the Eocene, our results indicate that only a small percentage (~10%) of the reconstructed temperature change can be ascribed to ocean gateway reorganization or paleogeographic change. Collectively, this indicates that atmospheric carbon dioxide (pCO2) was the likely driver of surface water cooling during the descent toward the icehouse