Carbon isotope of the Early Cretaceous sediments from the West Pacific and the Sulu orogenic belt: Implying the global atmospheric pCO2 changes during the Early Aptian Oceanic Anoxic Event 1a

Abstract

Multiple lines of evidence revealed that enormous CO2 was input into the ocean-atmosphere system during Oceanic Anoxic Event 1a (OAE 1a), which led to a series of environmental perturbations (e.g. temperature rising, enhanced weathering, promoted primary productivity and marine anoxia). Among them, increased carbon isotope fractionation (Δ13Ccarb-org) between the carbonate (δ13Ccarb) and organic carbon (δ13Corg) in the sediments is one of the most important indicators to the enhanced atmospheric pCO2. However, only a few research reported high-resolution paired carbon isotopes during OAE 1a, which were almost limited around the western Tethys. Here, we report new paired carbon isotopes of the Early Cretaceous sediments from the ODP site 866A in the western Pacific and the Lingshan Island section in the Sulu orogenic belt. The δ13Ccarb curves from both sections correlate well with OAE 1a, in which a positive δ13Ccarb excursion is identified between the C3–C6 segments. The Δ13Ccarb-org displays a ∼7–11‰ positive shift coinciding with the δ13Ccarb excursion, indicating a remarkable increase and a gradual decrease in CO2 in water and atmosphere, which further support significant atmospheric pCO2 changes during OAE 1a on a global scale. The sediments also show a few variations in carbon isotope records during OAE 1a between different locations, which may reflect different influences from local environments, and the deviations of stratigraphic correlation and diagenesis cannot be ruled out either.