Book Description

The past 3 million years of Earth history are characterized by dramatic changes, which greatly affected the biogeochemical cycling of elements like carbon, sulfur and phosphorous. Here I investigate the effect of these changes on sulfur fluxes and microbially mediated sulfur cycling in marine sediments. Climate driven sea level fluctuations and dynamic topography have greatly affected the areal extent of the continental shelf during the Quaternary. In turn this affects organic matter burial rates and the relative importance of different organic matter remineralization pathways. Since microbial sulfur cycling is the dominant organic matter re-mineralization pathway in marine sediments, this must have affected marine sulfur cycling. Furthermore, previous studies suggested that Quaternary sea level fluctuations caused a considerable increase in the erosion of shelf sediments, which is closely tied to the re-oxidation of pyrite. Here, I use the marine barite record of sulfur and oxygen isotope ratios (d34S and d18O) of seawater sulfate for the past 4Ma to evaluate the implications of Quaternary sea level changes on the sulfur cycling. Quantitative interpretation of d34S and d18O data suggests that erosion during sea level lowstands was only partly compensated by increased sedimentation during times of rising sea levels and sea level highstands. Furthermore, my findings indicate that shelf systems reached a new equilibrium state about 700 kyr ago, which considerably slowed or terminated erosion of shelf sediments. Modeling results also show that microbial sulfur cycling changes proportionally with shelf area, resulting in a 15% reduction of microbial sulfur cycling over the last 2 Ma. This results in a 1-1.5 / drop in the marine sulfate d18O isotope value. While further work is needed to understand how shelf area changes affect the cycling of carbon, phosphorous and other elements, results presented here highlight the dynamic role of continental shelves in the global biogeochemical cycles.