High primary productivity and nitrogen cycling after the Paleoproterozoic phosphogenic event in the Aravalli Supergroup, India

The causes of atmospheric oxygenation in the Paleoproterozoic are multifaceted yet poorly understood. An increase in the level of primary productivity however may have been an important factor in producing large amounts of oxygen in the atmosphere. A likely environmental consequence of atmospheric oxygenation is an increased abundance of seawater nitrate, which would have resulted in a fundamental change from the Archean nitrogen cycle. In this study, we investigate these questions using the carbon and nitrogen isotope compositions of carbonaceous shales from the Paleoproterozoic Aravalli Supergroup, India. The shales occur stratigraphically above one of the oldest phosphogenic events that took place around 2.0 Ga. Shales in the phosphate domain of the Jhamarkotra Formation from the Lower Aravalli Group have distinct characteristics from coeval shales in the non-phosphate domain. Carbonaceous shales of the phosphate domain contain relatively high V and U/Th, typical of oxygen-deficient environments. These shales also have a high organic content of up to 14 wt%, generally homogeneous δ13Corg values around −29‰, C/NWR up to 360 and a large range of δ15N values between −5 and +27‰. Their organic matter has low H/C and O/C and Raman spectra characteristic of lower greenschist to lower amphibolite facies metamorphism, which suggests a moderate alteration of geochemical compositions. Data from these shales also suggest high primary productivity under eutrophic conditions, as well as intense recycling of organic nitrogen through ammonium assimilation and denitrification at the time of sedimentation.In contrast, carbonaceous shales from the non-phosphate domain of the Jhamarkotra Formation contain trace element abundances characteristic of more oxygenated conditions. They contain less than 3 wt% organic matter with δ13Corg values often between −13 and −18‰, whole-rock δ15N values between +5 and +12‰ and C/NWR below 56. Acid-insoluble organic matter from these shales has high H/C and O/C, and Raman spectral characteristics typical of lower greenschist facies metamorphism, which suggests only minor alteration of isotope compositions. Data gleaned from carbonaceous shales of the non-phosphate domain are interpreted to indicate high rates of primary productivity as well as the existence of a modern-like nitrogen cycle in this open-marine environment at the mouth of the Udaipur Epicontinental Sea. The interpretation of high primary productivity in carbonaceous shales from both domains of the Jhamarkotra Formation is consistent with stratigraphically underlying 13C-enriched carbonates and with their associated stromatolitic phosphorites. Therefore, increased fluxes of phosphorous in Paleoproterozoic seawater may have played an important role in stimulating increased levels of primary productivity and the rise of atmospheric oxygen.