BENTHIC FORAMINIFER EVIDENCE FOR OCEAN DEOXYGENATION AND INCREASED ORGANIC CARBON BURIAL DURING THE MIDDLE MIOCENE CLIMATIC OPTIMUM IN THE SALISBURY EMBAYMENT OF THE U.S. ATLANTIC COASTAL PLAIN

The Middle Miocene Climatic Optimum (MCO, 17-14.8 Ma) is typified by atmospheric pCO₂ levels comparable to those projected for the year 2050 (~500 ppm) and a global ocean ~6-8°C warmer than modern, thus the MCO is an analog for anthropogenic climate change. This study uses complementary micropaleontological and geochemical (δ¹³C, U^k’₃₇) records constructed for a sediment core (Haynesville, VA) drilled in the Salisbury Embayment of the Atlantic Coastal Plain to assess the response of coastal ecosystems to ocean warming and fluctuating sea level during the MCO. A major compositional change among benthic foraminifer assemblages coincides with an increase in foraminifer δ¹³C values signaling the onset of the Monterey Event. The faunal shift entailed a temporary change from rich assemblages chiefly composed of taxa (Valvulineria, Cibicides) reflecting oxic bottom waters to depauperate assemblages dominated by buliminids and bolivinids indicating suboxic bottom waters and increased primary productivity. Over much of the Haynesville MCO record, alkenone thermometry (U^k’₃₇) is at its upper limit of temperature detection, suggesting sea-surface temperatures ≥29°C. We therefore attribute the change in local redox conditions to ocean deoxygenation stemming from some combination of temperature-gas solubility effects and a transient rise in sea level that caused an onshore shift of deeper waters with lower dissolved oxygen concentrations over our study site. The collective evidence suggests that rising sea level increased shelf accommodation space for carbon burial during the MCO, which helped curtail the overall increase in atmospheric CO₂ levels fueled by tectonic outgassing at this time.