Deciphering depositional and diagenetic processes in marine systems by application of trace metal and isotope geochemistry

Abstract

While organic-rich shales have been extensively studied as petroleum source rocks, and more recently as unconventional petroleum reservoirs, there is still much that remains unknown about the complex sedimentary and diagenetic processes responsible for black shale formation. Using trace metal concentrations and isotopic inventories, this investigation explores the pathways of trace metal incorporation into organic-rich sediments during deposition and describes the mechanisms which may have been responsible for variations in trace metal contents of three North American black shales deposited during Late Devonian — Early Mississippian time. To interpret ancient ocean redox and productivity, trace metal abundances in marine sediments have been used extensively. However, several oceanographic and diagenetic processes, such as basin restriction or post-depositional oxidation, can modify the primary geochemical signal of the sediment, which in turn may impact paleo-redox and/or -productivity interpretations. Therefore, it is imperative to study the impacts of a dynamic depositional system on trace metal accumulations. On the Namibian Continental Margin (NCM), sediments are deposited in an upwelling zone where large quantities of organic carbon accumulate on the anoxic shelf and are transported to a secondary depocenter on the slope, beneath the oxygen minimum zone. This dynamic environment makes the NCM an ideal location to study an organic-rich depositional system. In this study, the redepositional zone on the NCM was geochemically defined in NCM sediments below the oxygen minimum zone using the concentrations of several redox-sensitive and productivity-related trace metals. It was determined that the U-isotope redox proxy is a particularly useful tool to identify the primary depositional zone of redeposited sediments, despite post-depositional oxidization. The results about the effects of sediment transport and post-depositional oxidation on trace metal contents, the U/TOC ratio, and the U-isotopic composition of organic rich sedimentary deposits presented in this thesis provide new information about how geochemical signals are impacted by post-depositional processes. Understanding the impact of sediment transport and redeposition will assist with the interpretation of geochemical signals in organic-rich shales and may help to identify zones of lateral transport and redeposition related to ancient upwelling margin settings.