Deep-time oceanic chemistry


   Oceanic chemistry is one of the most important properties impacting ecosystems. In particular, seawater sulfate exerts an important impact on global carbon cycling and climate, and quantification of seawater sulfate concentration is an important issue to explore the co-evolution of organisms with environments. The MGP measured the sulfur isotope composition in carbonate-associated sulfate (34SCAS) across the Permian-Triassic boundary at 252 million years ago and found the large amplitude of 34SCAS variation; this implies the occurrence of a small sulfate pool in contemporaneous seawater. On the basis of the biogeochemical model, he found the seawater sulfate concentration at 252 Ma is only 15% of that in modern oceans. This is in striking contrast with the previous knowledge stating that the seawater sulfate concentration shows maximum values at 252Ma in Earth history. On the basis of the measurement of nitrogen isotope composition in organics, the MGP found the organic nitrogen isotope composition ranges from -1~1‰ with a mean value comparable with that of atmospheric N2 of modern days, indicating the enhanced nitrogen-fixation during that period. This explains the low concentration of bioavailable nitrogen at anoxic conditions in association with biotic crisis. At Bulla section in Italy, cyanobacterial biomarkers show a correlation with the organic nitrogen isotope composition, showing the main nitrogen-fixers are cyanobacteria. These microbes provide nitrogen source for the other organisms surviving at the deteriorated environments.


     After Great Oxidation Event I in early Proterozoic, eukaryotes remain a protracted evolution and the isotopic composition of carbonate carbon was surprisingly constant known as the boring billion. The MGP found that carbon isotopic composition of organics is low and constant in subtidal and deeper environments but relatively enriched and more variable in shallower intertidal and supratidal environments, and attributed the facies-dependent variation to the presence of a shallow chemocline. The MGP further found that early Mesoproterozoic seawater sulfate concentration is of <0.1 to 0.35 mM on the basis of multiple-sulfur isotope compositions. Both the organic carbon isotope and sulfur isotope compositions document a distinct early Mesoproterozoic perturbation in ocean chemistry that may have been related to a decline in atmospheric pO2 after Great Oxidation Event I, an interval that coincided with the assembly of the Nuna supercontinent.