Changes in magnetic mineral assemblages as a proxy for redox conditions around the Cenomanian-Turonian boundary

Changes in the environment occurring around the Cenomanian-Turonian boundary are associated with Oceanic Anoxic Event 2 (OAE2), a hyperthermal event that occurred approximately 94 Mya. Cretaceous volcanism caused CO2 levels to surge, impacting ocean chemistry, weathering rates, and the health of the marine biosphere. The Holland Park core from Virginia contains a marine record of the Atlantic Coastal Plain (ACP) over the Cenomanian and Turonian. Preliminary work suggests that the ACP did not reach full anoxia during OAE2. Therefore, the ACP may have had the potential to preserve magnetofossils, the magnetic remains of iron biomineralizing organisms that are sensitive to oxygen levels in aquatic environments. Magnetofossils and other magnetic minerals can inform us about ocean chemistry, the iron cycle, weathering, and sediment input. We measured first-order reversal curve (FORC) datasets to analyze the magnetic minerals in the Holland Park sediments. FORCs are a family of magnetization curves that are used to characterize the coercivity, domain state, and spatial distribution of assemblages of magnetic minerals. We used the statistical method of FORC-PCA (principal component analysis) to describe changes in magnetic minerals across FORC datasets from 46 samples spanning the Cenomanian-Turonian boundary. Our analysis shows that ultrafine magnetite is the dominant magnetic component during the Cenomanian/OAE2 interval, which suggests a bottom water environment with more reducing conditions. Single domain magnetite, likely magnetofossils, characterizes the beginning of the Turonian, whereas fine-grained hematite is dominant over the rest of the Turonian interval we studied. Taken together this indicates a transition to more oxic bottom waters, at the beginning of which microaerobic bottom waters prevailed creating ideal conditions for magnetofossils. In summary, magnetic minerals in the Holland Park core record changes in redox conditions on the North American continental margin. Our results have implications toward understanding how continental shelf environments behaved during OAE2 as well as the role of magnetofossils in hyperthermal events.