The Significance of Covariance between the Carbon Isotopic Composition of Marine Carbonates and Organic Matter: Assumptions, Processes, and Applications

The global carbon cycle refers to a series of processes that transfer carbon between the various reservoirs on Earth, including the atmosphere, the oceans, the sediments, the biosphere, and mantle. Over geologic time scales, the transfer of carbon to and from the atmosphere via volcanism, continental weathering, and the deposition of carbonate sediments works to regulate the concentration of atmospheric CO2, which as a result has a profound influence upon the Earth’s climate. However, on time scales of less than a million years, these same processes can produce imbalances in the amount of carbon in a given reservoir. Such perturbations to the atmosphere, either in the rate of carbon transfer or a change in the source of carbon, are thought to be recorded in the carbon isotopic composition of both marine carbonates and marine organic material produced by photosynthetic organisms. Marine carbonate and organic carbon isotope values are considered some of the best records of changes in the rate of carbon cycling since they are produced in the surface waters of the ocean. Air-sea gas exchange at the interface between the oceans and atmosphere translates perturbations to the atmospheric reservoir to the surface ocean in equilibrium. As a result, deposits of marine carbonates and organic material are thought to directly record changes in the carbon isotope value of the atmosphere over geologic time scales. A common approach used to evaluate changes in the global carbon cycle is to analyze the relationship between carbon isotope values of co-occurring marine carbonates and organic material. The projects presented in this dissertation test the assumptions of this approach. The fundamental assumption is that carbonate and organic carbon isotope values which exhibit covariance, or simultaneous changes in their isotopic composition, are direct records of the carbon isotope value of the CO2 dissolved in the surface waters of the ocean (DIC). In fact, some significant biogeochemical events in Earth history are accompanied by covariant carbon isotope values in co-occurring marine carbonates and organic material, including the Permo-Triassic Boundary, some of the Cretaceous Ocean Anoxic Events, the Paleocene-Eocene Thermal Maximum, and even some of the Precambrian anomalies associated with Snowball Earth conditions. However, the assumption that covariance between carbonate and organic carbon isotope values record changes in the global carbon cycle has not been fully investigated in more recent periods in Earth history where changes in the atmospheric concentration of CO2 and many other environmental parameters like depositional environment, sediment transport pathways, margin type, and post-depositional changes are better constrained. The goals of this dissertation were to 1) assess the influence of these environmental factors on the relationship between carbonate and organic carbon isotope records in a variety of recent depositional environments, including shallow marine platform, slope, and pelagic settings, and 2) to determine whether the assumption that covariance in carbonate and organic carbon isotope values as evidence for a robust record of the carbon isotope values of oceanic DIC is a universally valid assumption. The results of this dissertation suggest that a variety of syn-depositional and post-depositional processes influence the relationship between carbonate and organic carbon isotope records. For example, syn-depositional processes like mixing of isotopically distinct sources produce highly covariant carbonate and organic carbon isotope records in toe of slope and basinal settings adjacent to isolated carbonate platforms (Chapter 3). Post-depositional processes like marine burial diagenesis beneath non-depositional surfaces generate weakly covariant carbonate and organic carbon isotope records from slope sediments adjacent to the Great Barrier Reef (Chapter 4), while repeated episodes of subaerial exposure produce highly covariant carbonate and organic carbon isotope records from the Great Bahama Bank (Chapter 2). Furthermore, analysis of the paired carbonate and organic carbon isotope records from pelagic settings in the Atlantic, Indian, and Pacific Ocean Basins (Chapter 6) show that pelagic depositional environments do not exhibit covariance between carbonate and organic carbon isotope values. In addition to these modern case studies, the analysis of paired carbonate and organic carbon isotope values from four outcrops constituting a transect of shallow-to-deep ramp environments from the Mississippian Madison Limestone in the Western United States demonstrates that large global perturbations can be observed in the carbonate carbon isotope values, but are not captured in the co-occurring organic carbon isotope values (Chapter 5). Finally, the results of these five case studies were synthesized to produce a model of the impact of depositional environment, post-depositional alteration, margin type, and how the influence of these factors on the relationship between carbonate and organic carbon isotope values changed through geologic time (Chapter 6). A significant result of this synthesis is the proposal that icehouse and greenhouse conditions may favor different mechanisms for generating covariance. In concert, these findings have important implications for reconstructing global carbon cycling through geologic time, and may require re-evaluation of some carbon isotope records interpreted to record fundamental biogeochemical changes in the Earth surface system