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

Abstract

Abstract. Exchange of dissolved substances at the sediment–water interface provides an important link between the short-term and long-term geochemical cycles in the ocean. A second, as yet poorly understood sediment–water exchange is supported by low-temperature circulation of seawater through the oceanic basement underneath the sediments. From the basement, upwards diffusing oxygen and other dissolved species modify the sediment, whereas reaction products diffuse from the sediment down into the basement where they are transported by the basement fluid and released to the ocean. Here, we investigate the impact of this “second” route with respect to transport, release and consumption of oxygen, nitrate, manganese, nickel and cobalt on the basis of sediment cores retrieved from the Clarion Clipperton Zone (CCZ) in the equatorial Pacific Ocean. We show that in this abyssal ocean region characterised by low organic carbon burial and sedimentation rates vast areas exist where the downward- and upward-directed diffusive fluxes of oxygen meet so that the sediments are oxic throughout. This is especially the case where sediments are thin or in the proximity of faults. Oxygen diffusing upward from the basaltic crust into the sediment contributes to the degradation of sedimentary organic matter. Where the sediments are entirely oxic, nitrate produced in the upper sediment by nitrification is lost both by upward diffusion into the bottom water and by downward diffusion into the fluids circulating within the basement. Where the oxygen profiles do not meet, they are separated by a suboxic sediment interval characterised by Mn2+ in the porewater. Where porewater Mn2+ in the suboxic zones remains low, nitrate consumption is low and the sediment continues to deliver nitrate to the ocean bottom waters and basement fluid. We observe that at elevated porewater manganese concentrations, nitrate consumption exceeds production and nitrate diffuses from the basement fluid into the sediment. Within the suboxic zone, not only manganese but also cobalt and nickel are released into the porewater by reduction of Mn oxides, diffusing towards the oxic–suboxic fronts above and below where they precipitate, effectively removing these metals from the suboxic zone and concentrating them at the two oxic–suboxic redox boundaries. We show that not only do diffusive fluxes in the top part of deep-sea sediments modify the geochemical composition over time but also diffusive fluxes of dissolved constituents from the basement into the bottom layers of the sediment. Hence, the palaeoceanographic interpretation of sedimentary layers should carefully consider such deep secondary modifications in order to prevent the misinterpretation of primary signatures.

Impact of Upward Oxygen Diffusion From the Oceanic Crust on the Magnetostratigraphy and Iron Biomineralization of East Pacific Ridge-Flank Sediments

“…This study investigates three sediment cores 22-KL, 42-SL and 81-SL retrieved during RV Sonne cruise SO-240 (Kuhn et al, 2015;Versteegh et al, 2021) to the Clarion-Clipperton-Zone (CCZ) in the eastern low-latitude North Pacific. The coring sites (Figure 1) lie within the German license area "East" for manganese nodule exploration (11.8 °N, 116.5 °W to 13.2 °N, 118.4 °W).…”

“…As the three ex-situ pore-water oxygen concentration profiles (oxygen data for core 22-KL and 81-SL from Kuhn et al, 2017;Versteegh et al, 2021 oxygen data for core 42-SL from Versteegh et al, 2021) in Figure 8 reveal, the sediments at all three coring sites are subject to upward-directed oxygen diffusion from fluids migrating within the underlying oceanic crust as already described by Kuhn et al (2017). The vertical coverage of upward oxygen diffusion is largest by core 81-SL (∼5 m), and lesser (∼2.5 m) in cores 42-SL and 22-KL, a consequence of total sediment thickness and core recovery.…”

“…Consequently, biogenic magnetite would not add much to the sediment's NRM at this point. 2) (Figure 9B) With ongoing sedimentation, the diffusive oxygen flux to the deeper sediment column would be progressively diminished by increasing diffusion lengths and respiratory consumption (e.g., Mogollón et al, 2016;Versteegh et al, 2021). At some point, the oxygen concentration inside the sediment would lower to habitable levels for microaerophilic bacteria (e.g., Petermann and Bleil, 1993;Faivre and Schüler, 2008).…”

“…This leads to the formation of a suboxic zone in the center of the sediment column. Release, outward migration, and re-oxidation of Mn 2+ at the oxygen boundaries further reduce oxygen availability below the Mn precipitation layers (Mogollón et al, 2016;Volz et al, 2020b;Versteegh et al, 2021). These effects result in an initial retreat of the lower and upper oxic/suboxic redox boundaries.…”

“…However, there is also growing interest to understand (bio)geochemical processes and element fluxes in the underlying sediments (e.g., Bischoff and Piper, 1979;Jahnke et al, 1982;Müller et al, 1988;Mewes et al, 2014;Volz et al, 2018). Only recently, ex-situ onboard measurements of sedimentary pore-water oxygen concentrations revealed characteristic bow-shaped oxygen profiles because of diffusive vertical oxygen exchange with low-temperature hydrothermal fluids circulating through the underlying crust (Mewes et al, 2016;Kuhn et al, 2017;Versteegh et al, 2021). Low-temperature hydrothermal circulation in ridge flank settings is a widespread postmagmatic process (e.g., Rudnicki et al, 2001;Wheat et al, 2002;Fisher et al, 2003;Wheat and Fisher, 2008;Winslow and Fisher, 2015;Coogan and Gillis, 2018) and upward oxygen diffusion from the sediment-crust interface may therefore be a common phenomenon in the deeper sections of marine sediments.…”