Ca isotope stratigraphy across the Cenomanian-Turonian OAE 2: Links between volcanism, seawater geochemistry, and the carbonate fractionation factor

The Ca isotope composition of marine carbonate rocks offers potential to reconstruct drivers of environmental change in the geologic past. This study reports new, high-precision Ca isotope records (δCa44/40; 2σSD=±0.04‰) for three sections spanning a major perturbation to the Cretaceous ocean-climate system known as Ocean Anoxic Event 2 (OAE 2): central Colorado, USA (Portland #1 core), southeastern France (Pont d'Issole), and Hokkaido, Japan (Oyubari, Yezo Group). In addition, we generated new data for selected samples from Eastbourne, England (English Chalk), where a previous Ca isotope study was completed using different methodology (Blättler et al., 2011). Strata of the Yezo Group contain little carbonate (~1 wt.% on average) and accordingly did not yield a clear δCa44/40 signal. The Portland core and the Pont d'Issole section display comparable δCa44/40 values, which increase by ~0.10-0.15‰ at the onset of OAE 2 and then decrease to near-initial values across the event. The Eastbourne δCa44/40 values are higher than previously reported. They are also higher than the δCa44/40 values for the Portland core and the Pont d'Issole section but define a similar pattern. According to a numerical model of the marine Ca cycle, elevated hydrothermal inputs have little impact on seawater δCa44/40 values. Elevated riverine (chemical weathering) inputs produce a transient negative isotope excursion, which significantly differs from the positive isotope excursions observed in the Portland, Pont d'Issole, and Eastbourne records. A decrease in the magnitude of the carbonate fractionation factor provides the best explanation for a positive shift in δCa44/40 values, especially given the rapid nature of the excursion. 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 CO32- during CO₂ uptake. Recent studies utilizing a variety of isotope proxies, e.g., Nd, Os, and Pb, implicate eruption of the Caribbean Large Igneous Province as a likely source of increased CO₂. Moreover, integration of C, Ca, and Os isotope data reveals new information about the timing of events during the onset of OAE 2.