Organic carbon burial under greenhouse climate conditions: The regional expression of the Late Cretaceous Oceanic Anoxic Event 3 within the Western Interior Seaway.

[摘要] 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 control marine black shale deposition, including changes in primary productivity, organic matter preservation, and sedimentary dilution. OAEs offer an opportunity to evaluate the relative roles of these forcing factors on marine carbon burial.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 (drilled 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 carbon preservation in greater detail using biomarker and compound specific carbon isotope records. My results document distinct organic matter sources 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 iron 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.

[发布机构] University of Michigan

[效力级别] Oceanic Anoxic Events

[关键词] carbon cycling;Oceanic Anoxic Events;trace metals;North American Western Interior Seaway;Coniacian-Santonian;Geology and Earth Sciences;Science;Geology