THE PALEOPROTEROZOIC RISE OF ATMOSPHERIC OXYGEN CAUSED BY ALGAL BLOOMS STIMULATED BY HIGH PHOSPHATE IN THE LOWER ARAVALLI GROUP, INDIA

The accumulation of atmospheric oxygen in the Paleoproterozoic atmosphere (the Great Oxidation Event - GOE) occurred over several hundred millions of years. Multiple sulfur isotopes indicate that this occurred between around 2.5 and 2.2 Ga and that higher levels of seawater sulfate around that time were caused by increased oxidative weathering. Carbon isotope excursions in Paleoproterozoic post-glacial carbonates on most continents indicate a relative increase in burial rates of organic carbon and suggest a significant production of atmospheric oxygen. Because atmospheric oxygen is primarily produced by oxygenic photosynthesis, these observations lead to the suggestion that the cause of atmospheric oxygenation may have been high productivity by cyanobacteria due to the concomitant increased delivery of riverine phosphate. There may thus be a connection between these global biogeochemical changes and the deposition of the oldest major Precambrian phosphorites, which occurred in a few Paleoproterozoic sequences after the GOE. However, the connections between carbon isotope excursions, phosphorites, and atmospheric oxygenation have so far remained unsatisfactorily explained. In this study, we show that carbonates deposited during the oldest phosphogenic event in Paleoproterozoic basins of the Aravalli Supergroup, India, hosted algal blooms that preceded more oxidizing conditions. Restricted basins with stromatolitic phosphorites had large ranges of δ¹³C_org values between -33.3 and -10.1‰ that point to a complex carbon cycle, under eutrophic conditions. Contemporary open marine basins without phosphate deposits had moderate to high δ¹³C_carb excursions with values up to +11.2‰ that suggest high rates of organic carbon burial. The low nitrogen isotope compositions of these rocks point to a dominance of biological nitrogen fixation during algal blooms, which we interpret as a biological response to low nitrate availability caused by high phosphorous availability. We conclude that increased phosphate availability during the Paleoproterozoic supported algal blooms and eutrophic conditions and that this was a major process by which a significant quantity of atmospheric oxygen was produced in the Paleoproterozoic.