Beschreibung: Die Meteorreise M157 ist die erste Feldexpedition des vom Bundesministerium für Bildung und Forschung (BMBF) geförderten Projekts EVAR (Das Benguela Auftriebssystem im Klimawandel – Effekte von Variabilität im physikalischen Antrieb auf das Budget von Kohlenstoff und Sauerstoff). In diesem bis zum Jahr 2021 laufenden Forschungsprojektuntersucht ein Konsortium von WissenschaftlerInnen des Leibniz-Instituts für Ostseeforschung Warnemünde (IOW), des Bremer Zentrums für Marine Umweltwissenschaften (MARUM) und des Kieler Helmholtz-Zentrums für Ozeanforschung (GEOMAR) zusammen mit ihren KollegInnen von der Universität Namibias und des National Marine Information and Research Centres NatMIRC mögliche Folgen des Klimawandels für das Benguela Auftriebssystem.

Beschreibung: Die Meteorreise M157 ist die erste Feldexpedition des vom Bundesministerium für Bildung und Forschung (BMBF) geförderten Projekts EVAR (Das Benguela Auftriebssystem im Klimawandel – Effekte von Variabilität im physikalischen Antrieb auf das Budget von Kohlenstoff und Sauerstoff). In diesem bis zum Jahr 2021 laufenden Forschungsprojektuntersucht ein Konsortium von WissenschaftlerInnen des Leibniz-Instituts für Ostseeforschung Warnemünde (IOW), des Bremer Zentrums für Marine Umweltwissenschaften (MARUM) und des Kieler Helmholtz-Zentrums für Ozeanforschung (GEOMAR) zusammen mit ihren KollegInnen von der Universität Namibias und des National Marine Information and Research Centres NatMIRC mögliche Folgen des Klimawandels für das Benguela Auftriebssystem.

Beschreibung: Die Meteorreise M157 ist die erste Feldexpedition des vom Bundesministerium für Bildung und Forschung (BMBF) geförderten Projekts EVAR (Das Benguela Auftriebssystem im Klimawandel – Effekte von Variabilität im physikalischen Antrieb auf das Budget von Kohlenstoff und Sauerstoff). In diesem bis zum Jahr 2021 laufenden Forschungsprojektuntersucht ein Konsortium von WissenschaftlerInnen des Leibniz-Instituts für Ostseeforschung Warnemünde (IOW), des Bremer Zentrums für Marine Umweltwissenschaften (MARUM) und des Kieler Helmholtz-Zentrums für Ozeanforschung (GEOMAR) zusammen mit ihren KollegInnen von der Universität Namibias und des National Marine Information and Research Centres NatMIRC mögliche Folgen des Klimawandels für das Benguela Auftriebssystem.

Beschreibung: Die Meteorreise M157 ist die erste Feldexpedition des vom Bundesministerium für Bildung und Forschung (BMBF) geförderten Projekts EVAR (Das Benguela Auftriebssystem im Klimawandel – Effekte von Variabilität im physikalischen Antrieb auf das Budget von Kohlenstoff und Sauerstoff). In diesem bis zum Jahr 2021 laufenden Forschungsprojektuntersucht ein Konsortium von WissenschaftlerInnen des Leibniz-Instituts für Ostseeforschung Warnemünde (IOW), des Bremer Zentrums für Marine Umweltwissenschaften (MARUM) und des Kieler Helmholtz-Zentrums für Ozeanforschung (GEOMAR) zusammen mit ihren KollegInnen von der Universität Namibias und des National Marine Information and Research Centres NatMIRC mögliche Folgen des Klimawandels für das Benguela Auftriebssystem.

Beschreibung: A new approach to predict biogenic particle fluxes to the seafloor is presented, which is based on diffusive oxygen uptake and, in particular, opal fluxes to the seafloor. For this purpose, we used a recently published empirical equation coupling benthic silica to oxygen fluxes, and showing a clear negative correlation between Si and O2 fluxes. Dissolution of biogenic silica mediated by aerobic microbial activity has been inferred at 24 sites along the African and South American continental margins. Based on the assumption that these findings hold essentially for the entire Southern Atlantic Ocean, we applied the silica to oxygen flux ratio to a basin-wide grid of diffusive oxygen uptake extracted from the literature. Assuming that the silica release across the sediment-water interface equals the particulate flux of biogenic opal to the seafloor, we estimated minimum opal rain rates. We combined these calculations with published relationships between aerobic organic carbon mineralization and dissolution rates of calcite above the hydrographical lysocline, thereby assessing the calcite rain rate and particulate organic carbon flux to the seafloor. The addition of the buried fraction completes our budget of biogenic particulate rain fluxes. The combination of such empirical equations provides a powerful and convenient tool which greatly facilitates future investigations of biogenic particle fluxes to the seafloor.

Beschreibung: We investigated gas hydrate in situ inventories as well as the composition and principal transport mechanisms of fluids expelled at the Amsterdam mud volcano (AMV; 2,025 m water depth) in the Eastern Mediterranean Sea. Pressure coring (the only technique preventing hydrates from decomposition during recovery) was used for the quantification of light hydrocarbons in near-surface deposits. The cores (up to 2.5 m in length) were retrieved with an autoclave piston corer, and served for analyses of gas quantities and compositions, and pore-water chemistry. For comparison, gravity cores from sites at the summit and beyond the AMV were analyzed. A prevalence of thermogenic light hydrocarbons was inferred from average C1/C2+ ratios 〈35 and δ13C-CH4 values of −50.6‰. Gas venting from the seafloor indicated methane oversaturation, and volumetric gas–sediment ratios of up to 17.0 in pressure cores taken from the center demonstrated hydrate presence at the time of sampling. Relative enrichments in ethane, propane, and iso-butane in gas released from pressure cores, and from an intact hydrate piece compared to venting gas suggest incipient crystallization of hydrate structure II (sII). Nonetheless, the co-existence of sI hydrate can not be excluded from our dataset. Hydrates fill up to 16.7% of pore volume within the sediment interval between the base of the sulfate zone and the maximum sampling depth at the summit. The concave-down shapes of pore-water concentration profiles recorded in the center indicate the influence of upward-directed advection of low-salinity fluids/fluidized mud. Furthermore, the SO42− and Ba2+ pore-water profiles in the central part of the AMV demonstrate that sulfate reduction driven by the anaerobic oxidation of methane is complete at depths between 30 cm and 70 cm below seafloor. Our results indicate that methane oversaturation, high hydrostatic pressure, and elevated pore-water activity caused by low salinity promote fixing of considerable proportions of light hydrocarbons in shallow hydrates even at the summit of the AMV, and possibly also of other MVs in the region. Depending on their crystallographic structure, however, hydrates will already decompose and release hydrocarbon masses if sediment temperatures exceed ca. 19.3°C and 21.0°C, respectively. Based on observations from other mud volcanoes, the common occurrence of such temperatures induced by heat flux from below into the immediate subsurface appears likely for the AMV.

Beschreibung: Sedimentary molybdenum (Mo) and uranium (U) enrichments have been widely used as a proxy for redox conditions in oxygen-depleted marine paleo-environments. However, in a dynamic upwelling system the seasonal fluctuations from oxic to completely anoxic-sulfidic bottom waters and lateral sediment transport can modify the primary Mo and U signal of the sediment, which in turn may impact paleo-redox interpretations. In this study we present pore water and solid phase data collected at two cross shelf transects during the ‘more oxygenated’ austral winter and ‘anoxic’ austral summer to study the influence of spatially and seasonally contrasting redox conditions on the formation of authigenic Mo and U enrichments in organic carbon (TOC) rich mud belt sediments on the Namibian shelf. A mass balance was established for each element based on diffusive fluxes and element mass accumulation rates to evaluate the respective mechanisms of trace metal delivery, accumulation and recycling. Mo is delivered to the sediment in its dissolved form via diffusion across the sediment–water interface, especially during austral summer when bottom waters are anoxic and surface sediments are highly sulfidic. In the center of the inner shelf mud belt, the benthic Mo fluxes of up to 37 nmol cm−2 yr−1 into sulfidic surface sediments are the highest ever reported for reducing sulfidic systems and agree with the rate of Mo accumulation in the solid phase. Concurrently, high sedimentation rates and low terrigenous input limit solid phase Mo accumulation on the Namibian shelf. In ancient marine sediments, this mode of Mo cycling can be identified by low Mo/TOC ratios of ∼2 similar to those found in sediments deposited below the perennial oxygen minimum zone on the Peruvian shelf and to those found in deposits of the Cretaceous Oceanic Anoxic Event 2. Diffusive U fluxes into the sediment are generally too low to account for the sedimentary enrichment leading to the conclusion that U is delivered mainly in particulate form. In areas with anoxic bottom water, shallow dissolved U maxima directly below the sediment water interface and rather low sedimentary U content indicate that particulate U is recycled and largely released back into the bottom water. At sites where bottom water oxygen concentrations vary from anoxic to completely oxic on seasonal timescales, the depth at which Mo and U are removed from pore waters moves vertically within the sediment column thus defining a layer between the sediment surface and ∼20 cm depth, in which Mo and U accumulate in the solid phase. Our results emphasize the importance of short-term redox fluctuations in the bottom waters and underlying sediments, as well as lateral sediment transport for the authigenic enrichment of redox-sensitive trace metals in reducing shelf sediments. The relative enrichment patterns identified might be useful for the reconstruction of open marine anoxia in the geological past.