Spatiotemporal variations of sedimentary carbon and nitrogen isotopic compositions in the Yangtze Shelf Sea across the Ordovician-Silurian boundary

Abstract

The Hirnantian Glaciation resulted in a perturbation in global carbon cycling, which is recognized as a positive excursion in the δ13C values of both carbonates and organic matter. To investigate the factors controlling the carbon and nitrogen cycles from the late Ordovician to the early Silurian, the C and N isotopic and geochemical compositions of marine sediments deposited on the Yangtze Platform were measured. The majority of rocks in the Wukemuchang section exhibit high δ15Nbulk values, reflecting aerobic nitrogen cycling in the inner shelf. The global spatial variation in δ15Nbulk values likely reflects the occurrence of nitrate gradients in seawater, implying that nitrate was limited in offshore environments that were characterized by oxic conditions. More eukaryotes may be restricted to the inner shelf environments, where large amounts of bioavailable nitrate were accessible. The limited availability of nitrate may have affected the fossil distributions and the complexity of animal ecosystems, thereby influencing the late Ordovician extinction and early Silurian biotic recovery. The δ13Corg profiles from the Yangtze Platform are consistent with earlier reports of Hirnantian shelf gradients in sedimentary δ13CDIC values. We suggest that the global variation in δ13Corg values during the late Ordovician was related to the amount of nitrate because productivity is limited when bioavailable N is scarce. In addition to intense carbonate weathering, the high level of photosynthesis in the inner shelf caused by abundant nitrate and nutrients may have enhanced the dissolved inorganic carbon gradient on shelves globally, driving sedimentary δ13Corg values even higher than those of sediments deposited in the outer shelf where carbon exchange with the surface ocean was less restricted