186-7 Epicontinental C[h]aOs: Using Calcium, Osmium, and Stratigraphic Analysis of Western Interior Seaway and Saxony Basin Cores to Evaluate Spatial and Temporal Variability in Late Cretaceous Volcanism and Ocean Acidification

Abstract:

The global chemostratigraphic record of Late Cretaceous Ocean Anoxic Event 2 (OAE2) features a pronounced shift to unradiogenic ¹⁸⁷Os/¹⁸⁸Os (Os_i) values followed closely by a well-defined positive carbon isotope (δ¹³C_org) excursion at its onset. These patterns are interpreted to reflect an episode of globally-enhanced organic carbon burial and marine anoxia caused by increased submarine volcanism.

Despite the relative global homogeneity of the OAE2 δ¹³C_org excursion, geochemical expressions of the event vary within and between epicontinental basins. Proposed explanations for differences in the shape and degree of the OAE2 unradiogenic shift in Os_i across these sites include differences in basin connectivity and circulation, water depth, and proximity to shoreline weathering inputs.

Another proxy of interest in epicontinental settings is δ^44/40Ca. Ocean acidification likely occurred as ana consequence of increased volcanic inputs to the Earth system during OAE2, and documented changes in carbonate δ^44/40Ca interpertedinterpreted to reflect decreases in precipitation rates have been identified. Moreover, published δ^44/40Ca records for several OAE2 sites display positive excursions consistent with acidification, but global variations in the signal and the role of diagenetic alteration in certain facies have not been extensively constrained.

We present Os_i and δ^44/40Ca records for epicontinental sites at the southern opening of the Western Interior Seaway (the USGS GC-3 core) and the Lower Saxony Basin (the Wunstorf core) that feature volcanically-driven Os_i signals as well as shifts in δ^44/40Ca. We demonstrate that diagenetically-induced δ^44/40Ca shifts in OAE2 sediments can be constrained and isolated from acidification signals using facies analysis and δ¹³C_carb measurements. We also argue that similarities between the shape of published Os_i records for IODP site 1260B (Demerara rise) and new Os_i data for the GC-3 core constitute further evidence for at least partial connectivity with the open ocean during this dynamic period in Earth’s history.

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