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. Exchange of dissolved substances at the sediment–waterinterface provides an important link between the short-term and long-termgeochemical cycles in the ocean. A second, as yet poorly understoodsediment–water exchange is supported by low-temperature circulation ofseawater through the oceanic basement underneath the sediments. From thebasement, upwards diffusing oxygen and other dissolved species modify thesediment, whereas reaction products diffuse from the sediment down into thebasement where they are transported by the basement fluid and released tothe ocean. Here, we investigate the impact of this “second” route withrespect to transport, release and consumption of oxygen, nitrate, manganese,nickel and cobalt on the basis of sediment cores retrieved from the ClarionClipperton Zone (CCZ) in the equatorial Pacific Ocean. We show that in thisabyssal ocean region characterised by low organic carbon burial andsedimentation rates vast areas exist where the downward- and upward-directeddiffusive fluxes of oxygen meet so that the sediments are oxic throughout.This is especially the case where sediments are thin or in the proximity offaults. Oxygen diffusing upward from the basaltic crust into the sedimentcontributes to the degradation of sedimentary organic matter. Where thesediments are entirely oxic, nitrate produced in the upper sediment bynitrification is lost both by upward diffusion into the bottom water and bydownward diffusion into the fluids circulating within the basement. Wherethe oxygen profiles do not meet, they are separated by a suboxic sedimentinterval characterised by Mn2+ in the porewater. Where porewaterMn2+ in the suboxic zones remains low, nitrate consumption is low andthe sediment continues to deliver nitrate to the ocean bottom waters andbasement fluid. We observe that at elevated porewater manganeseconcentrations, nitrate consumption exceeds production and nitrate diffusesfrom the basement fluid into the sediment. Within the suboxic zone, not onlymanganese but also cobalt and nickel are released into the porewater byreduction of Mn oxides, diffusing towards the oxic–suboxic fronts above andbelow where they precipitate, effectively removing these metals from thesuboxic zone and concentrating them at the two oxic–suboxic redoxboundaries. We show that not only do diffusive fluxes in the top part ofdeep-sea sediments modify the geochemical composition over time but alsodiffusive fluxes of dissolved constituents from the basement into the bottomlayers of the sediment. Hence, the palaeoceanographic interpretation ofsedimentary layers should carefully consider such deep secondarymodifications in order to prevent the misinterpretation of primary signatures.

biogeosciences

Geochemical consequences of oxygen diffusion from the oceanic crust into overlying sediments and its significance for biogeochemical cycles based on sediments of the northeast Pacific