PP31B-2243
Contrasting Redox Stories from Trace Metal and Iron Proxy Records of the Late Cretaceous Western Interior Seaway

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Allyson C Tessin1, Nathan D Sheldon1, Ingrid L Hendy2 and Anthony Chappaz3, (1)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (2)University of Michigan Ann Arbor, Geological Sciences, Ann Arbor, MI, United States, (3)Central Michigan University, Institute for Great Lakes Research - Dept. of Earth and Atmospheric Sciences - Dept. of Chemistry and Biochemistry, Mount Pleasant, MI, United States
Abstract:
The sedimentary record of the Cretaceous Western Interior Seaway (North America) is characterized by periods of enhanced organic carbon burial known as Oceanic Anoxic Events (OAEs). Compared to more typical OAEs, the last Cretaceous OAE, the Coniacian-Santonian OAE 3 (~86 Ma), is enigmatic and its driving mechanism less understood. While OAE 3 was geographically limited to restricted basins and shallow seaways, it also had a longer duration (~3 myrs) of organic carbon burial than earlier OAEs., Constraining paleoredox conditions during organic carbon burial events, such as OAEs, is important for understanding the relationship between ocean anoxia and carbon burial.

Iron proxies (FeHR/FeT, Fepy/FeHR, and FeT/Al) provide an important tool in the reconstruction of paleoredox conditions through geologic time. Here we present sequential Fe and pyrite Fe measurements from the Niobrara Formation recovered in the USGS #1 Portland core from Cañon City Basin, Colorado. The new results indicate that Fe redox proxies from the Portland core are not always in agreement with other redox proxies. Pyrite Fe measurements indicate a single stepwise change in seaway oxygen limitation, whereas other paleoredox proxies (Mo, Re, U, bioturbation indices) indicate fluctuating redox conditions and a variable degree of euxinia throughout the record. Interpretation of pyrite Fe values may be complicated by the development of a highly reactive Fe limitation. Another commonly used redox proxy, FeT/Al, indicates oxic conditions when other redox proxies suggest anoxia and vice versa. Interpretation FeT/Al may be complicated by a number of factors, including sequestration of Fe in diagenetic carbonate phases, variable supply of external Fe, atypical basin geometry, and the development of shallow marine euxinia. Taken together, these observations clearly support the importance of using a multi proxy approach to accurately reconstruct oceanic redox history.

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