Trace element (Mn, Zn, Ni, V) and authigenic uranium (aU) geochemistry reveal sedimentary redox history on the Juan de Fuca Ridge, North Pacific Ocean

Changes in meridional overturning circulation and water mass chemistry can be recorded by oxygen concentrations in the deep ocean. Because the deep Pacific is the largest ocean reservoir, its oxygen concentrations may be related to global climate change. In this study, oxygen conditions in the past are reconstructed by contrasting the sedimentary geochemistry of multiple redox-sensitive trace elements (Mn, Ni, Zn, V corrected for terrigenous and hydrothermal inputs) and authigenic U (aU) from six sediment cores on the Juan de Fuca Ridge from 2.7-2.8 km depth. We find that Mn and Ni are indicators for oxygen-rich conditions, while Zn, V, and aU are indicators for oxygen-poor conditions. Relative Redox Potentials (RRPs) for each core are calculated by converting excess metal fluxes into binary presence/absence designations, weighting each element by the strength and direction of its redox indication, summing the five elements, and then averaging the data in 5kyr bins. Metal depositional histories from all six cores demonstrate low oxygen conditions during interglacial periods, particularly during 100-120 ka (MIS5) but also 200-250 ka (MIS7), and high oxygen conditions during glacial periods (MIS2-4 and MIS6). This redox pattern does not appear to be driven by organic matter flux to the sediment, as reconstructed by three different paleo-productivity proxies (organic carbon, opal, and excess barium). Instead higher oxygen concentrations on the Juan de Fuca Ridge may be a result of better ventilation during glacial periods, possibly due to enhanced North Pacific Intermediate Water formation. Alternatively, sedimentary redox conditions on the Juan de Fuca Ridge may be locally controlled by the deposition of hydrothermal sulfides from nearby vent fields.