Pliocene–Pleistocene Redox Evolution of the Peruvian Margin: A Multi‐Proxy Approach

Abstract Due to upwelling of nutrient‐rich waters, the Peruvian Margin is one of the most productive regions of the global ocean, hosting one of the most pronounced and shallowest oxygen minimum zones. This combination of high productivity and low oxygen makes the Peruvian margin a critical site for reconstructing past environmental changes. In this study, we investigated the evolution of redox conditions, productivity, and sedimentary processes at ODP Site 680 since the Pliocene using multiple geochemical proxies, including iron speciation, redox‐sensitive element contents, and δ 98 Mo xs , δ 15 N bulk , and δ 13 C org . Our findings reveal that over the past 15 marine isotope stages, the mechanism of molybdenum accumulation in the sediments varied. During interglacial periods, elevated Mo, high δ 15 N bulk , and positive correlations with reactive Fe suggest that particulate delivery was the main mode of Mo accumulation, potentially driven by Fe (oxyhydr)oxide precipitation in the water column coupled to denitrification. Glacial periods lack these correlations, and Mo accumulation appears to have been primarily controlled by diffusion from bottom waters. We observe a coupling between proxies for surface productivity and the redox state of both bottom waters and the overlying water column during interglacial intervals, indicating that during these periods, productivity controlled redox conditions. During the warm and humid Pliocene, comparably low U and Mo concentrations were accompanied by elevated ratios of reactive iron to total iron. We propose that elevated Fe HR /Fe T values were produced by enhanced delivery of reactive iron from a highly weathered terrigenous sediment source. However, an additional redox control on Fe speciation remains possible.