Unraveling the Biogeochemical Dynamics of Pyrite Formation and Trace Element Incorporation in Marine Sediments

Past studies in paleoenvironmental reconstruction have set out to bridge the gap that limits our understanding of the biogeochemical controls in the past oceans by developing redox proxies based on trace metal content, iron speciation, and pyrite formation. Many of those studies have relied on broad-scale temporal assumptions about ocean redox conditions inferred from idealized chemical processes and classifying past oceans into either oxygenated, ferruginous, or euxinic. The redox threshold values associated with these proxies can vary considerably among depositional systems and, for this reason, geochemical proxies should be scrutinized in multiple modern deposition systems of variable redox characteristics (stable and dynamic). This dissertation applies various geochemical tools to understand the biogeochemical controls on carbon, iron, and sulfur reaction rates during early diagenesis. Specifically, I test, refine, and expand the use of pyrite as a paleoredox proxy by expanding our understanding on the controls on pyrite formation and the incorporation of trace elements in pyrite during early diagenesis by investigating those relationships in modern marine depositional systems. First, I explore the early diagenetic processes occurring in marine sediments with emphasis on the carbon, iron, and sulfur cycle–the three main components in sedimentary pyrite formation. This effort is made by measuring nutrients, organic carbon, and iron and sulfur mineralogical characterization and coupling with reactive-transport diagenetic modelling to understand the diagenetic reactions that lead to iron-sulfide precipitation within the sedimentary profile. Two geographically distinct locations are studied in detail: (1) The Santa Monica Basin (SMB), an exceptionally iron dominated system, and (2) Saanich Inlet, BC, Canada, a fjord with high redox variability and transient euxinic bottom waters. Then, I explore the relationships between the chemical signatures in syngenetic and dia