Oceanic organic carbon as a possible first-order control on the carbon cycle during the Bathonian–Callovian

Abstract

Abstract Oceans are the largest, readily exchangeable, superficial carbon reservoir; a current challenge in investigating past and present environments and predict future evolution relates to the role of oceanic carbon in regulating Earths' carbon cycle and climate. At least one paired δ13Ccarb-TOC decoupling event is noted in the Late Bathonian–Early Callovian. Provokingly, we suggest that this decoupling event and other carbon isotopic events in the Bathonian–Callovian resulted from the addition and removal of carbon from the oceanic organic carbon (OOC) reservoir, likely dominated by dissolved and fine particulate oceanic organic carbon. The decoupling event is characterised by a mainly invariant δ13CTOC record and increasingly more positive δ13Ccarb values. The δ13Ccarb-TOC decoupling is tentatively explained by the expansion of the OOC reservoir, which increased the residence time of carbon in the oceans and buffered (slow equilibrium) δ13COOC (approximated by δ13CTOC) to changes in δ13C of oceanic inorganic carbon, comprised mainly of dissolved inorganic carbon (approximated by δ13Ccarb). Reconversion of OOC to CO2 may have resulted in negative δ13C excursions and increased atmospheric pCO2, whereas the change from accumulation of OOC to export of organic carbon to sediments may have resulted in positive δ13C excursions and widespread accumulation of organic matter. It is speculated that other small-scale (~ 1‰) δ13C excursions in the Mesozoic record may have resulted from similar episodes of OOC accumulation and oxidation. Our study calls attention to the potential of the OOC reservoir to act as a critical driver for planetary-scale changes over short geological time intervals.