Nutrients as the dominant control on the spread of anoxia and euxinia across the Cenomanian‐Turonian oceanic anoxic event (OAE2): Model‐data comparison

Abstract

[1] The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in the oxygen and sulfur cycles of the ocean, potentially resulting from changes in oxygen supply (via oxygen solubility and ocean circulation) and in marine productivity. We assess the relative impact of these mechanisms, comparing model experiments with a new compilation of observations for seafloor dysoxia/anoxia and photic zone euxinia. The model employed is an intermediate-complexity Earth system model which accounts for the main ocean dynamics and biogeochemistry of the Cretaceous climate. The impact of higher temperature and marine productivity is evaluated in the model as a result of higher atmospheric carbon dioxide and oceanic nutrient concentrations. The model shows that temperature is not alone able to reproduce the observed patterns of oceanic redox changes associated with OAE2. Observations are reproduced in the model mainly via enhanced marine productivity due to higher nutrient content (responsible for 85% of the change). Higher phosphate content could have been sustained by increased chemical weathering and phosphorus regeneration from anoxic sediments, which in turn induced an enhanced nitrogen nutrient content of the ocean via nitrogen fixation. The model also shows that the presence of seafloor anoxia, as suggested by black-shale deposition in the proto-North Atlantic Ocean before the event, might be the result of the silled shape and lack of deep-water formation of this basin at the Late Cretaceous. Overall our model-data comparison shows that OAE2 anoxia was quasi-global spreading from 5% of the ocean volume before the event to at least 50% during OAE2.

High precision U–Pb zircon geochronology for Cenomanian Dakota Formation floras in Utah

“…Finally, there is evidence from d 34 S sulfate for an increase in marine sulfate from submarine volcanism, initiated 430 ka prior to OAE2 (Adams et al, 2010). Together, the shifts in these marine proxies are consistent with a major episode of volcanism, interpreted to have modified the chemistry of the oceans and triggered widespread marine anoxia during OAE2 (Jarvis et al, 2011;Monteiro et al, 2012;Du Vivier et al, 2014). There is relatively good agreement for the timing of changes in this set of geochemical proxies (400-600ka prior to OAE2), but extrapolating sediment accumulation rates calculated from within OAE2 to rock intervals outside of OAE2 (where marine conditions were clearly different) is tenuous.…”

Sulfur isotopes track the global extent and dynamics of euxinia during Cretaceous Oceanic Anoxic Event 2

“…17). Recent modeling by Monteiro et al (32) suggests that 50% of the global ocean was anoxic. Nevertheless, the spatial extent of anoxic and more specifically, euxinic deposition, remains poorly constrained, particularly for the Pacific and Indian Oceans.…”

Black shale deposition and early diagenetic dolomite cementation during Oceanic Anoxic Event 1: The mid-Cretaceous Maracaibo Platform, northwestern South America

“…The widespread deposition of black shale intervals during the mid-Cretaceous has been linked to multiple paleoenvironmental perturbations, potentially resulting from the emplacement of large igneous provinces (Arthur and others, 1985;Hay and others, 2006;Tejada and others, 2009;Monteiro andothers, 2012, Naafs andothers, 2016). Such tectono-magmatic pulses augmented atmospheric concentrations of CO 2 , which led to intensified rates of erosion and phosphate delivery to the oceans (Kump and Arthur, 1999).…”