Organic carbon, sea level and carbonate weathering controls on marine carbon isotope perturbations across the Hirnantian glaciation

The Hirnantian isotopic carbon excursion (HICE) records a dramatic perturbation to the global carbon cycle across the Late Ordovician Hirnantian glaciation. The HICE is characterized by variable duration and amplitude in global records, but its ultimate driver, and controls on the observed variability, are poorly understood. Here, we present a comprehensive compilation of geochemical data and paired carbon and calcium isotope records for two continuous Hirnantian sections at Wanhe and Shuanghe on the Yangtze Shelf (South China). Our results reveal a lateral gradient of up to 4 ‰ in organic carbon isotope (δ¹³C_org) values across the shelf, decreasing from nearshore to offshore. Carbonate carbon isotope (δ¹³C_carb) data from the Wanhe and Shuanghe sections also show pronounced variability, which cannot be fully explained by primary mineralogical changes or early marine diagenesis. We therefore interpret the spatial δ¹³C_org patterns as reflecting, at least in part, shelf-scale heterogeneity in seawater dissolved inorganic carbon isotope (δ¹³C_DIC) values. Our compilation further suggests that substantial organic carbon burial likely triggered the HICE, while sea-level change subsequently drove the asynchronous, spatially heterogeneous changes in seawater δ¹³C_DIC values. Additionally, enhanced carbonate weathering, linked to falling syn-glacial sea-level, amplified the spatial heterogeneity in regional marine δ¹³C_DIC values. Moderate δ¹³C_org changes (<+2 ‰) are observed in central shelf areas across the Hirnantian glaciation, suggesting that the global carbon cycle perturbations during this ice age were of more modest amplitude than often suggested.