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.