T145 Sedimentary, Paleobiologic, and Geochemical Studies of Deep Time Ocean-Climate Perturbations II: Honoring the Scientific Contributions of Michael A. Arthur

Monday, 26 September 2016
1:30 PM - 5:30 PM
Room 402

THE STEPWISE EVOLUTION OF MARINE DE-OXYGENATION DURING A CRETACEOUS OAE2

Paper number: T145-7
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

Abstract

The Mesozoic Era is punctuated by numerous episodes of widespread organic-carbon deposition and coeval carbon-isotope excursions. Many of these events are regarded as oceanic anoxic events (OAEs) due to supposition that this organic-carbon deposition occurred under anoxic conditions and their apparent basin if not global scale. Understanding the causes and consequences of OAEs is paramount to better understand the links between climate, biology and oceanography. These events had a major geochemical and ecological impact on their environment and biosphere, and represent possible analogs to recent and future expansion of coastal hypoxia. The mechanisms surrounding the largest and most well-studied of these events – OAE2 (~94 Ma) – continue to be debated.

Trace metals and thallium isotopes from several sites suggest that ocean de-oxygenation occurred prior the start of the accompanying carbon isotope excursion, which signals the initiation of the enhanced burial of organic carbon on a large scale. Along these lines shallow water facies from multiple locations show depleted oxygen contents in surface waters prior to and during the event based on I/Ca ratios in carbonate rocks. These lines of evidence point to the suggestion that the expansion of reducing conditions occurred prior to the traditionally defined onset of the OAE.

Further, although it appears that the extent of euxinia (anoxic and sulfide containing water columns) increased at the onset of OAE2, there are now multiple lines of evidence, δ34SCAS and molybdenum isotopes, that suggest only limited spatial distribution; estimates from these proxies range from 2 to 10% seafloor euxinia – such conditions were far more widespread compared to the modern ocean. Modeling studies suggest these redox conditions were reducing enough to drive depletion in the marine inventories of bioessential trace metals on a global scale. Biomarker evidence suggests the depletion of marine trace metal inventories had a large effect on ecological communities and was possible a driver for biotic turnover during the event. In the end, it is possible that the sequential expansion of more reducing conditions is an important feedback terminating the OAE, where trace metals become limiting nutrients, lessening productivity, and returning the system back to previous ocean redox states.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-286244

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