C-isotope stratigraphy and paleoenvironmental changes across OAE2 (mid-Cretaceous) from shallow-water platform carbonates of southern Mexico

Abstract

The stratigraphic and geochemical record of the mid-Cretaceous (Cenomanian–Turonian) Oceanic Anoxic Event 2 (OAE2) has been studied in numerous Tethyan and proto-Atlantic hemi-pelagic/pelagic successions, but little data comes from nearshore carbonate successions from the proto-Pacific region. Here we present the results of a combined stratigraphic and δ 13C study of C–T platform carbonates from southern Mexico, which were deposited within the proto-Pacific. Two scales of sedimentary cyclicity are recognized. High-frequency peritidal and subtidal cycles (0.4–8 m) display little evidence of cycle-capping subaerial exposure and are not correlative between sections; these relationships suggest that the amplitudes of high-frequency sea-level changes were minimal during the peak mid-Cretaceous greenhouse. Longer-term transgressive-regressive sequences (18–40 + m) are correlated between sections, and using δ 13C trends, can be correlated with sequences developed in northern Europe and India. The Mexican successions were sampled at a high resolution (~ 10 ky) for stable isotopes (inorganic, organic carbon and oxygen), total organic carbon, insoluble residues, and trace metals. The δ 13C carb curve matches global trends (including 6 distinct isotopic stages) permitting identification of OAE2 despite the lack of characteristic anoxic facies. Using the δ 13C carb trends, we tie the previously identified ammonite, planktonic foram, and nannofossil biostratigraphy from England and the Western Interior seaway of Colorado into the Mexican sections. The initiation of OAE2, defined by an abrupt positive 3–4‰ δ 13C shift, coincides with a long-term sea-level rise, though the sedimentary expression of the deepening is no greater than that observed for any of the other sea-level events across the studied interval. OAE2 termination (transition from gradually decreasing to background δ 13C values) is not associated with a particular sea-level trend. Stratigraphic changes in insoluble residues (proxy for continental sediment discharge) across OAE2 are not correlative between sections and do not show consistent systematic relationships with δ 13C or sea-level variations, therefore do not support the hypothesis that OAE2 was associated with increased continental-derived nutrient influx. Two peaks in trace metal concentrations coincide with the abrupt increase in δ 13C ratios (onset of OAE2) and during the transition from elevated-to-decreasing δ 13C values (near the C–T stage boundary). These trends are similar to those recorded in coeval deposits of the Western Interior seaway, and are consistent with the hypothesis that OAE2 development was related to the release of reduced metals during the short-lived (< 1 My) Caribbean oceanic plateau basalt eruption. In this scenario, oxidation of the metals depleted the existing low dissolved-O 2 concentrations and thermally-buoyant plumes of seawater enriched in biolimiting elements mixed with surface waters, stimulated primary productivity, and further reduced O 2 concentrations leading to widespread anoxia and a large positive δ 13C shift.