Description: 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 oxygen profiles do not meet, they are separated by a suboxic sediment interval characterised by Mn2+ in the pore–water. 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 pore–water manganese 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 pore–water manganese concentrations, nitrate consumption exceeds production and the basement becomes a nitrate source. Within the suboxic zone, not only manganese but also cobalt and nickel are released into the pore–water by reduction of oxides, diffuse towards the oxic/suboxic fronts above and below where they precipitate, effectively removing these metals from the suboxic zone and concentrating them at the oxic/suboxic redox boundaries. We show that not only 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, paleoceanographic interpretation of sedimentary layers should carefully consider such deep secondary modifications in order to prevent misinterpretation as primary signatures.

Description: To extend the understanding of dinoflagellate cysts production, excystment and vertical/ lateral transport in the water column, upper water dinoflagellate cyst export production have been compared with cysts associations and concentrations in the subsurface nepheloid layer, bottom nepheloid layer and deeper water column during active upwelling off Cape Blanc (NW Africa) in August 2020. Export production was collected by two drifting trap surveys; one in an active upwelling cell for 4 days and the other in an offshore drifting upwelling filament for 2 days. Subsurface and bottom nepheloid layers as well as the deeper waters were sampled by in-situ pumps along two transects perpendicular to the shelf break. The dataset represents the count data on species level of organic-walled dinoflagellate cysts in drifting trap and in-situ pump collected samples as well as information about the sampling time, sampling volume and collection surface.

Description: We report on the succession of selective degradation of dinoflagellate cyst species that can be considered representative for discrete particulate organic matter (POM) classes of different degradability. The effects of anaerobic and aerobic degradation as well as bioturbation in a natural setting are documented in high resolution by means of palynological and geochemical analyses on Madeira Abyssal Plain A- and F-turbidites. These turbidites are unique as their initial ungraded sediments are affected by a downward penetrating oxydation front. The dataset covers the density, porosity, water content and dinoflagellate cysts content of sediments from the so called A- and F-turbidite.

Description: Long 1, midchain alkane-diols have been analyzed from sediment trap CBeu (off Mauritania) samples collected from 2003-2008 (CBeu1-5). The changes in diol composition and diol indices have been compared with changes in environmental parameters. New and hardly available data are deposited here. These include the diol flux data, the corrected UK'37 record, the relative abundances of upwelling indicating species and the 11 years moving average of wind speed and direction from Nouadhibou Airport. Data that are readily available from the World Ocean Atlas 2018, NOAA and earlier publications on CBeu have not been deposited here. These include solar insolation, sea surface temperatures, as well as water temperature, salinity and phosphate for different water depths.