Ca Isotope Evidence for Changes in the Carbonate Geochemistry of Seawater Across OAE 2

Abstract:

We used a new high-precision MC-TIMS method to generate Ca isotope records (δ^44/40Ca; 2SD = ± 0.04‰) for three sections spanning OAE 2: central Colorado, USA (Portland #1 core), southeastern France (Pont d’Issole), and Eastbourne, England (Eastbourne Chalk). The Eastbourne Chalk δ^44/40Ca values are higher than previously reported but define a similar trend, where δ^44/40Ca values increase prior to OAE 2 and then decrease at the onset. The Portland core and the Pont d’Issole section display comparable δ^44/40Ca values, which increase by ~0.10 – 0.15‰ immediately at the onset of OAE 2 and then decrease to near-initial values across the event. According to a numerical model of the marine Ca cycle, reasonable changes to the riverine and hydrothermal input fluxes cannot explain the rate and magnitude of the positive isotope excursion. A decrease in the magnitude of the carbonate fractionation factor provides the best explanation for the data. Because a decrease in the fractionation factor corresponds to an increase in the Ca/CO₃ ratio of seawater, we tentatively attribute the positive Ca isotope excursion to transient ocean acidification, i.e., a reduction in the concentration of CO₃ during CO₂ uptake. Other studies implicate eruption of the Caribbean Large Igneous Province as the most likely source of CO₂.