APPROACHING MARINE ANOXIA FROM DRY LAND: USING PLANTS TO RECONSTRUCT CO2 AND CONSTRAIN THE TRIGGER MECHANISMS FOR OCEAN ANOXIC EVENT 2, 94 MYA

Cretaceous ocean anoxic events (OAEs) represent brief intervals (<1Ma) when the global ocean returned to low oxygen conditions, similar to those of the Proterozoic. Ocean anoxic event 2 (OAE2) at the C-T boundary (94Ma) is a prime example where inhospitable marine conditions lasted for 600-800 ka, causing a second order mass extinction of organisms optimized for oxic conditions. Elevated primary productivity has been proposed as the direct cause of marine anoxia, as sinking organic debris overwhelmed the system’s ability to oxidize carbon, drawing down atmospheric carbon.

A high resolution record of pCO₂ was created to test this hypothesis using the stomatal index proxy on mesofossil plant cuticles. Our data shows that pCO₂ was drawn down at the beginning of OAE2 as expected, declining in phase with both the initial +2‰ and the final +4‰ shift in δ¹³C_org. The stomatal frequency values were measured from two morphospecies in the Lauraceae family, extracted from the nearshore Dakota Fm. of SW Utah, in sections precisely correlated to OAE2 using a new terrestrial δ¹³C_org record. Stomatal densities provide strong support for the stomatal index record, but were lower during ponded intervals, caused by a known biological effect of enhanced water availability, but when filtered produces an equivalent curve. Correlation of the pCO₂ drawdown intervals to the marine record suggests they are coincident with two temperature declines recorded in the N. Atlantic by the TEX₈₆ proxy.

The pCO₂ decreases at the beginning of OAE2 were superimposed on a longer-term CO₂ rise, initiated at least 100 ky before anoxia. Our results are in agreement with recent findings that volcanism was the initial indirect trigger, a hypothesis supported by isotopes systems including lead, osmium, strontium, and sulfur, all shifting prior to the onset of OAE2. This pCO₂ rise is directly coincident with a trend toward more depleted δ¹³C_org values, and is recorded with high fidelity by bulk samples, mixed plant cuticles, and also by the same specimens that produced the pCO₂ estimates. This correspondence of a CO₂ release with a negative isotopic trend is in line with early volcanism initiating a complicated sequence of events that ultimately created anoxic conditions detrimental to oxygen dependent ecosystems.