AAPG ACE 2018

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From Carbonate Factory Collapse to Recovery: Insights Through Box Modeling of Carbon Isotope Excursions of Oceanic Anoxic Events (OAEs)

Abstract

Oceanic anoxic events (OAE’s) result in profound disturbances of the global carbon budget that preserve measureable shifts (positive and negative) of the carbon isotope record of organic and carbonate oceanic deposits. At least nine OAE’s are recorded for the Phanerozoic. OAE-2 at the Cenomanian-Turonian boundary (95-93 Ma) occurred while deposition of the Comanche Platform along the Cretaceous Texas Gulf Coast transitioned from a thriving photozoan carbonate factory (e.g. Stuart City and Sligo Margins) to the organic-rich shales of the Eagle Ford Formation. Although the trigger mechanism(s) that initiated all OAE’s is debated, volcanism, thermohaline circulation, excess nutrients in the photic zone, and elevated atmospheric CO2 and/or CH4 concentrations are considered to be important drivers. A persistent issue with assigning actual trigger mechanisms to OAE’s is the diachronous timing of carbonate factory collapse, organic-rich shale deposition, carbon isotope excursions, and changes in redox-sensitive trace element distributions. For example, within the Eagle Ford Formation, deposition of organic-rich shale precedes carbon isotope excursions preserved in organics, and the carbon isotope excursion preserved in carbonates is even further delayed. A similar effect is observed at the early Jurassic OAE (Toarcian OAE; ca. 183.1 Ma).

Using box models to account for global carbon and phosphate budgets, and perturbations in atmospheric and deep ocean fluxes (i.e., trigger mechanisms), we investigate responses of the organic and carbonate carbon cycle in a carbonate prone environment through its collapse and ultimate recovery. Although generalized, box model results help to parameterize changes in sources and flux, and build a better geochemical tool to evaluate hydrocarbon resource potential across basins that preserve mud rocks associated with OAE’s. This approach may also bridge geochemical knowledge gaps between ocean, atmosphere and marine deposits.