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 information about dinoflagellate cysts (dinocysts) and planktonic foraminifera production, succession, excystment and transport in the upper water column, we investigated their fluxes during a 7-day survey in the active upwelling off Cape Blanc (NW Africa) in November 2018 with drifting traps at 100 m, 200 m, and 400 m water depth. The 7-day survey covered a successive change from active upwelling toward stratified conditions. Highest production of organic dinocysts and planktonic foraminifera were observed under active upwelling conditions and decreased drastically towards the end of the survey. Calcareous dinocysts appeared later during upwelling relaxation. Collected cytoplasm bearing (full) dinocysts and foraminifera were produced in the water column above the traps (〈100 m depth). Part of the organic-walled empty dinocysts were resuspended implying that sediments below the survey site are likely to contain both local and allochthonous cyst assemblages. We are the first to show that excystment in the upper water column is species-specific. Brigantedinium excysted in the upper water column before reaching deeper depths whereas no upper water column excystment was observed for the other dinoflagellate species. Dinoflagellate and planktonic foraminifera associations showed a clear succession. Echinidinium zonneveldiae, Brigantedinium spp., other peridinioids, Echinidinium spp., cysts of Pentapharsodinium dalei and other photosynthetic organic-walled dinocysts” as well as Neogloboquadrina incompta and Globigerinella calida were collected during active upwelling. Lingulodinium machaerophorum was produced during upwelling relaxation and Gymnodiniaceae cysts (G. microreticulatum, G. catenatum) as well as the foraminifera Globigerina bulloides and Orbulina universa were collected during stratified conditions. Apart from enhancing the biological knowledge of these species, our observations will allow more detailed reconstructions of upwelling history in the Cape Blanc region based on sedimentary archives using fossilised dinoflagellate and planktonic foraminifera assemblages.

Description: Porewater and solid phase data for manganese, nickel, and cobalt as well as porewater data on oxygen and nitrate are presented for 15 sediment cores from the Clarion-Clipperton Zone, NE Pacific. The cores have been obtained during research cruise SONNE240 (Kuhn, et al., 2015; doi: 10.2312/cr_so240). The maximum age of the sediments recovered by the cores is the age of the ocean crust which is 20 Ma. The cores were taken to investigate the regional pattern of seawater circulation within the basaltic seafloor underneath its sediment cover. For this purpose, for each of the cores the composition of porewaters and the solid phase has been investigated. Core names, coordinates, and lengths are provided in the sediment_cores_overview.csv file. The oxygen measurements were performed onboard (May-June 2015) using using amperometric Clark-type oxygen sensors. All other analyses were performed after the cruise. The porewater analyses for cobalt, nickel have been performed using an inductively coupled plasma optical emission spectrometer (ICP-OES). For the porewater nitrate and Mn2+ analyses a QuAAtro Continuous Segmented Flow Analyser was used. The solid phase data were obtained after digestion of the sediment by ICP-OES, ICP-Mass Spectrometry (ICP-MS) and X-ray Fluorescence Spectroscopy (XRF). Cross calibration, was achieved by quantifying the same elements with ICP-MS, ICP-OES and XRF for a few cores. The datafiles with core data all start with the core name followed by an underscore.

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.

Description: Auxiliary data include one file with alkenone-derived UK'37 data and sea surface temperatures (SST). On these data Figs. 7 and 8 of the manuscript are based. The SST are derived from UK'37 by using the transfer function: SST = 29.876 UK'37 - 1.334 of Conte et al. (2006). The data are against the ages (in A.D.) of samples derived from cores GT91-1 (39[deg]59'23"N, 17[deg]45'25"E), GT89-3 and GT90-3 (both 39[deg]45'43"N, 17[deg]53'55"E ). Also included are composite records for UK'37 and SST. For creating the composite records, GT-89-3 was taken as reference core. In the overlapping period the GT89-3 data seem in general lower than the GT91-1 data. To accommodate for this in the composite record, the average difference (0.0343 UK'37 units; equivalent to 1.023 [deg]C) was subtracted from the GT91-1 record. Hereafter, for each depth in the overlapping interval the respective values (UK'37 or SST) of GT89-3 and GT91-1 were averaged. We have also averaged with 16 additional alkenone measurements, from 1793 to 1851, performed in the GT90-3 core.

Description: High abundances of mangrove pollen have been associated with transgressive cycles on tropical margins, but the detailed relations between systems tracts and the taphonomy of the pollen are unclear. We report here the occurrence and high abundance of Rhizophora pollen, in association with taraxerol, a Rhizophora-sourced biomarker, from a high-resolution Congo fan core covering the last deglaciation. An age model based on 14C dates enables the temporal changes in taraxerol content and the percentage frequencies and flux (pollen grains (pg)/cm**2/(10**3 yr)) of mangrove pollen to be compared quantitatively with the lateral rate of transgression across the flooding surface (derived from glacio-hydro-isostatic model output and the bathymetry of the margin). Rhizophora pollen concentrations and taraxerol content of the sediment are very strongly positively correlated with the lateral rate of transgression and indicate, independently of any sequence stratigraphic context, that mangrove pollen spikes are associated with the transgressive systems tract rather than the highstand systems tract or maximum flooding surface. Lower-resolution longer-term records from this margin indicate an association between taraxerol concentrations and transgressive rather than regressive phases. The flux of these materials to the Congo fan is interpreted as a function of the erosion of flooded mangrove swamp on the shelf and, less importantly, changing extent of mangrove habitat, during sea-level rise. Congo River palaeoflood events also result in reworking of mangrove pollen and supply to the fan, but this mechanism is subdominant. Rhizophora pollen has been underestimated in many palynological studies undertaken on cores from the African margin because of inappropriate sieve mesh size used during laboratory preparation.

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.