Combining local lithofacies and global geochemical signals to test the acidification hypothesis for the onset of Oceanic Anoxic Event 2 in the U.S. Western Interior Basin

Abstract:

Strata preserving Oceanic Anoxic Event 2 (OAE2), which span the Cenomanian-Turonian (C/T; Late Cretaceous), exhibit evidence of widespread anoxia, a major perturbation to the global carbon cycle, and increased biotic turnover rates. It has been hypothesized that a major volcanic (LIP) eruption, increased CO₂ levels, and significant climate warming triggered the event. Recently, OAE2 has also been cited as a potential example of ocean acidification in Earth history and therefore has potential to offer predictive insights on impacts of increasing modern pCO₂ levels. As part of an effort to test this hypothesis, the 131-m Smoky Hollow #1 (SH-1) core was drilled near Big Water, Utah during the summer of 2014. The core recovered an expanded stratigraphic record of OAE2 from the mud-rich western margin of the Western Interior Seaway. A high-resolution stable carbon isotope record from bulk organic carbon (δ¹³C_org) indicates near-continuous preservation of OAE2 with a sustained +2.5‰ excursion that is over 5 times the thickness of the same excursion at the C/T GSSP in Pueblo, Colorado. Notably, this record is characterized by a 1-m thick carbonate-barren interval at the δ¹³C excursion’s onset. This may indicate an episode of ocean acidification driving suppressed carbonate sedimentation or carbonate dissolution. An alternative interpretation is that variations in carbonate concentrations are unrelated to changes in ocean chemistry and are instead driven by changes in local sedimentation patterns (e.g. transgressive-regressive parasequences).

To test these hypotheses, a regional lithostratigraphic correlation to the nearshore Cottonwood Canyon section is constructed to assess whether prograding sandy parasequences may have altered carbonate sedimentation rates at the SH-1 locality. Initial osmium and δ¹³C chemostratigraphies are also developed to constrain the timing of perturbations in global geochemical cycles at the initiation of OAE2, including the onset of large igneous province volcanism and enhanced global organic matter burial, respectively. By combining a refined knowledge of local depositional environments with isotope geochemistry it is better possible to evaluate the impact of ocean acidification on the stratigraphic record through this critical Earth-life transition.