Congratulations to Allyson Tessin who successfully defended her dissertation on May 9, 2016.
Advisors: Ingrid Hendy, Nathan Sheldon
Abstract: The Cretaceous period (145−66 million years ago) was characterized by elevated atmospheric pCO2 and an equable, warm climate. High Cretaceous sea levels flooded continental areas producing extensive shallow seaways, including the Western Interior Seaway (WIS) in North America. The resulting sedimentary record of the WIS includes episodic deposition of organic carbon-rich black shales, during a series of Oceanic Anoxic Events (OAEs). Multiple processes are known to control the deposition of marine black shales, including changes in primary productivity, organic matter preservation, and sedimentary dilution. OAEs offer an opportunity to evaluate the relative roles of these forcing factors.
This dissertation centers on understanding the causes of enhanced organic carbon burial within the WIS during a particularly poorly studied OAE, the Coniacian-Santonian OAE 3 (~88−84 Ma). In Chapter 2, I use trace metal records from the USGS Portland core (retrieved near Cañon City, Colorado) to reconstruct the degree of oxygenation before, during and after OAE 3. These data are compared to bulk elemental and isotopic records of organic matter composition to demonstrate that oxygen depletion in pore and bottom waters provided a feedback to carbon burial, by triggering enhanced organic matter preservation within the WIS during OAE 3. In Chapter 3, I examine changes in organic preservation in greater detail using biomarker and compound specific carbon isotope records. My results document distinct changes in OM composition before, during and after the OAE associated with varying degrees of bioturbation and therefore, oxygen exposure time. In Chapter 4, I explore the feedbacks associated with changing iron chemistry in the WIS. Sequential iron extractions, phosphorus, and sulfur results indicate that low reactive Fe availability promoted phosphorus remineralization from sediments, illustrating the importance of internal nutrient recycling during and after OAE 3. In Chapter 5, trends and timing of organic carbon accumulation are examined on a larger spatial scale. Trace metal and foraminiferal records from western Alberta are compared to time equivalent records from the central seaway. The reconstructed seaway-wide patterns indicate that anoxic conditions and the resulting sedimentary elevated organic carbon accumulation closely follow the sea-level transgression and expanding influence of nutrient-rich Tethyan (equatorially-sourced) water.