Proterozoic Nitrogen cycling in shallow marine basins: A quantitative assessment using shale δ15N values

Abstract

Dissolved nitrogen in the Proterozoic ocean was a key limiting factor for the eukaryotic diversification. In this study, the modes of nitrogen cycling during the Proterozoic Eon have been reconstructed using nitrogen (δ15N) and carbon (δ13C) isotopic data for organic-rich shales from three sedimentary successions of India (Vindhyan (Kajrahat and Bijaigarh Fm), Cuddapah (Tadpatri and Cumbum Fm) and Lesser Himalaya (Lower Tal Fm)). These shales were deposited between late Paleoproterozoic and early Cambrian in proximal to distal shelf settings under anoxic (ferruginous to euxinic) conditions. The total organic carbon (TOC) for these samples varies from 0.6 to 4.1 wt%, with higher TOC/TN ratios (15–61) compared to the Redfield ratio. The phosphorus concentration, its enrichment factor, and TOC/TP together suggest possible P-limitation in the Tadpatri Basin, and sufficient P availability in the Kajrahat, Cumbum, Bijaigarh and Tal Basins. The δ13Corg (−33.1 to −29.2‰) and δ15N (+0.8 to +8.6‰) values exhibit significant variation across basins but minimal variation within individual basins. The N-isotopic signatures (∼ +1–2‰) of the Bijaigarh (∼1210 Ma) and the Lower Tal (∼535 Ma) shales indicate dominance of N2-fixing processes in the basins. By contrast, the enriched (> +3‰) δ15N values observed for Paleoproterozoic (Tadpatri, Kajrahat, and Cumbum) shales suggest the occurrence of aerobic modes of nitrogen cycling in these basins, where processes such as nitrification, denitrification/anammox, and/or ammonium assimilation were active. A steady-state box model involving shale δ15N values suggests that nitrate-assimilators dominated surface productivity in the Paleoproterozoic basins, while nitrogen-fixers dominated the Bijaigarh and Tal Basins. These observations provide new insights into the nitrogen cycling in these Precambrian sections.