Dissertation/Thesis Abstract

Testing The Driving Mechanisms For Ocean Anoxic Event 2 (94Ma) Using pCarbon Dioxide Estimates And Carbon Isotopes Derived
by Barclay, Richard Shern, Ph.D., Northwestern University, 2011, 336; 3456521
Abstract (Summary)

The main goal of this dissertation was to construct a record of atmospheric CO2 concentration (pCO2) using the stomatal proxy method on fossil plant cuticle in the Dakota Formation of Utah. A complimentary δ13Corg record was developed from the same samples to constrain the mechanisms that produced marine anoxia during Ocean Anoxic Event 2 (OAE2). This ~600ka event in the late Cenomanian (94 Ma) represented one of most significant shifts among interdependent biogeochemical cycles known from the geologic record, including a +4 / excursion in δ 13Corg used to define the event globally. Using the established marine to terrestrial correlation, it was possible to assess the response of marine and terrestrial systems over a one million year period that bracketed the onset of OAE2.

Stomatal index values from two cuticle morphospecies were used to create a continuous sequence, converted to pCO2 values using two modern analogue species of Lauraceae that matched the fossil material. This analysis demonstrated that decreases in pCO2 closely tracked the global δ13Corg record from fossil plant material. The data in this dissertation supports the hypothesis that elevated primary productivity led to enhanced burial of marine organic carbon and resulted in two significant decreases in pCO 2 at the onset of OAE2. This suggested that the atmospheric, terrestrial, and marine carbon reservoirs were tightly coupled during the Cenomanian.

The pCO2 decreases within OAE2 were overlain on a longer term pCO2 rise that began as much as 500 ka before the event. Background pCO2 levels of ∼370 +100/-70 ppm increased to a peak of ∼500 +400/-180 ppm in the early stages of OAE2. The steady increase in pCO2 began 22 m below the onset of OAE2 and was coincident with a large negative δ13C org excursion. This was followed by a consistent -1.56 / shift in δ 13CorgC that tracked the gradual 90 ppm increase pCO2 leading into OAE2. A mass balance approach that used the δ13Corg and pCO 2 shifts as constraints on the isotopic value of the atmosphere provided support for the volcanic initiation hypothesis prior to OAE2, but suggested that the CO2 originated as thermogenic methane or oxidized terrestrial organic carbon.

Indexing (document details)
Advisor: Sageman, Bradley B.
Commitee: Jacobson, Andrew D., McElwain, Jennifer C., McInerney, Francesca A.
School: Northwestern University
Department: Earth and Planetary Sciences
School Location: United States -- Illinois
Source: DAI-B 72/08, Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Paleontology, Biogeochemistry, Paleoclimate Science
Keywords: Carbon dioxide, Carbon isotopes, Dakota formation, Fossil plant, Ocean anoxic event 2, Paleobotany
Publication Number: 3456521
ISBN: 978-1-124-66083-7
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