High-Resolution Environmental Magnetism Record of Dynamics in Bottom Water Redox Conditions Across the Paleocene-Eocene Thermal Maximum: Zumaia, Spain

Hyperthermal climate events in the geological record, like the Paleocene-Eocene Thermal Maximum (PETM), are widely studied because they provide important analogues to the modern Earth system. The PETM (~56 Ma) is a ~200 ka-long global supergreenhouse event that is characterized by a ~4-5‰ negative carbon isotope excursion (CIE). Mean annual temperatures, sea surface temperatures, and the regional hydrologic conditions changed at all latitudes during the PETM. At Zumaia, Spain, which was subtropical and marine during Paleocene and Eocene time, sedimentological and isotopic observations suggest a dramatic increase in the amount of physical and chemical weathering at the onset of the PETM. We hypothesize that these surface processes will be reflected in the environmental magnetism of offshore deposits at Zumaia, due to, for example, the provenance of the sediments or authigenic and diagenetic conditions within the sediment caused by runoff events during the PETM. Here we present the environmental magnetic and bulk carbonate (δ¹³C_carb) isotope stratigraphy across the onset, core, and recovery of the PETM at Zumaia. This is the first comprehensive environmental magnetic record directly paired with δ¹³C_carb from this section. Our magnetic susceptibility vs. temperature curves, magnetic hysteresis measurements, remanent backfield curves, and FORC diagrams all indicate mixtures of magnetite and hematite in the samples. Stratigraphic plots of the magnetic data show an increase in and high variability of magnetic concentration (M_s), magnetic hardness (B_cr), and hematite concentration (S₃₀₀) at the beginning and throughout the PETM. Magnetite is more abundant in the core of the CIE and in intervals with higher U and Mo enrichment. Hematite is more abundant during the recovery phase of the CIE. Published Rb/Al values, a proxy for fluvial detrital input, decreases during the CIE. We propose a model in which intervals of intense terrestrial run-off, especially at the beginning of the PETM, produced lower oxygen bottom water conditions that favored the preservation of terrestrially-sourced magnetite and hematite; more stable run-off supply and increased bottom water oxygenation during the recovery of the PETM favored hematite authigenesis and preservation of terrestrially-sourced hematite.