Redox evolution and nutrient cycling during Earth’s middle age

Understanding of the geochemical evolution of the ocean and atmosphere through Earth history has evolved markedly over recent years. Despite recent advances, however, the redox evolution of the oceans and atmosphere in the mid-Proterozoic (1.8–0.8 billion years ago, Ga) remains poorly constrained. Ocean redox structure is commonly considered stratified in the Mesoproterozoic, with oxic surface waters commonly being underlain by euxinic mid-depth waters along some continental margins, with deeper waters dominantly being ferruginous. However, emerging evidence suggests that ocean redox conditions were highly heterogeneous during this time, both spatially and temporally, but little is known about the controlling factor of these redox fluctuations. Previous studies suggest that ocean redox variations could regulate the cycling of phosphorus, an ultimate limiting nutrient for primary productivity, however, our acknowledgement of feedbacks between ocean redox conditions and nutrient cycling in the mid-Proterozoic remains rare. Through a series of geochemical investigations on drill core materials from the ~1.4 Ga Xiamaling Formation, this thesis aims to unveil what controlled redox dynamics and nutrient cycling in the mid-Proterozoic, henceforth shining light on the evolution of Earth’s surface oxygenation at this time. The application of multiple redox proxies, phosphorus phase partitioning, and weathering indicators to two high-resolution sections from the Xiamaling Formation suggests that climatic changes at individual locations exerted a dominant control of regional redox variation and phosphorus cycling on different timescales, leading to ocean redox heterogeneity in the Mesoproterozoic. A biogeochemical model was utilized to examine how weathering inputs at different locations would drive fluctuations in regional ocean biogeochemistry. To quantitatively reconstruct the global ocean redox landscape in the Mesoproterozoic, a new uranium-molybdenum isotope dataset is reported in this thesis. In combination with existing redox geochemistry, these δ238U and δ98Mo data exhibit distinct fractionations under different redox conditions during the deposition of the Xiamaling Formation. A coupled U-Mo isotope mass balance model was then built to quantitatively estimate the spatial extent of anoxia in the Mesoproterozoic ocean. The application of a phosphorus phase partitioning approach across the Xiamaling succession reveals distinct phosphorus behaviour under various redox conditions, with moderately enhanced P recycling under sulfidic and organic carbon-rich ferruginous conditions in the ~1.4 Ga ocean. To investigate the impact of P recycling and changes in the continental weathering P influx, a biogeochemical model that incorporates the C-O-P cycles was further applied, with model results supporting that atmospheric oxygen would have been maintained at a low level in the ferruginous-dominated Mesoproterozoic, unless there was a significantly higher weathering P influx than that of the present-day value. Oxygen levels might experience a drop in the early Neoproterozoic, due to a reduced nutrient availability.