Decreased marine organic carbon burial during the Hirnantian glaciation

Abstract

A significant glaciation occurred during the Late Ordovician Hirnantian Age, coinciding with the substantial positive Hirnantian Isotope Carbon Excursion (HICE, up to +7 ‰). Modeling studies suggest that both events were driven by increased organic carbon burial; however, this hypothesis has not been substantiated with global sedimentary organic carbon data, hampering our understanding of the underlying causes of the Hirnantian glaciation and the HICE. In this study, we compiled data from 25 globally distributed sections along continental shelves, each stratigraphically constrained by graptolite biozonation, to elucidate the evolution of global organic carbon burial and the marine carbon cycle during the Late Ordovician to Early Silurian (Dicellograptus complexus to Akidograptus ascensus biozones, 449.13 to 443.07 Ma). Our results indicate a rapid decline in organic carbon burial from the late Katian to the early Hirnantian, coinciding with the onset of glaciation and the rising limb of the HICE, which is in contrast to the increased organic carbon burial indicated by modeling. This discrepancy between the geological record and modeling results suggests that variations in organic carbon burial may not be the primary driver of the Hirnantian glaciation and the HICE, and necessitates a reassessment of the global carbon cycling during the Late Ordovician. The diminished organic carbon burial during the glaciation is likely associated with redox-dependent remineralization of organic matter and increased organic matter degradation timescales, indicating that the organic carbon cycle functioned as a negative feedback mechanism to global cooling during the Late Ordovician.