Ocean Deoxygenation Conference | Kiel 2018

Major perturbations of the global carbon cycle are globally imprinted as prominent carbon isotope excursions (CIEs) in Mesozoic sedimentary successions. These CIEs have been linked to the widespread enhancement of primary productivity in ocean basins, which strongly impacted the oxygenation of subsurface water masses. We investigate changes in redox-conditions in a 265 m continuous succession of hemipelagic deposits spanning the Cenomanian to early Turonian, which was recently drilled in the Tarfaya Basin (SW Morocco). X-ray fluorescence core scanner derived logarithmic ratios are integrated with stable isotope data on bulk and organic material and discrete geochemical analysis to establish a robust chronology and to document the detailed evolution of sub-surface oxygenation across the Oceanic Anoxic Event 2 (OAE2), one of the largest CIEs, and across its precursor, the mid-Cenomanian Event (MCE). Enhanced upwelling, inducing deteriorating redox-conditions in the Tarfaya Basin, started in the early Cenomanian and was in part associated with eustatic sea level changes occurring on orbital and tectonic timescales. We speculate that oxygen depletion controlled by increased productivity during the Cenomanian CIEs allowed leakage of the limiting nutrient phosphorus from the sediments, as recorded by sudden increases in C$_{org}$/P$_{react}$ and N$_{total}$/P$_{react}$, which further enhanced primary productivity during the later phases of OAE2. Increased carbon burial controlled by this feedback loop caused atmospheric CO$_2$-drawdown resulting in prominent cooling episodes (i.e., the Plenus Cold Event during OAE2) that influenced the latitudinal distribution pattern of marine organisms. The major decrease in subsurface water oxygenation also caused prominent global extinctions of thermocline dwelling foraminifera during OAE2, which serve as useful biostratigraphic correlation tools. Evaluating the scale of environmental changes during Mesozoic CIEs is relevant to better understand the processes associated with the burial of excess anthropogenic CO$_2$, demonstrating that extreme climate change can impact marine biota on a global scale.