Description: Continental shelves are predominately (~70%) covered with permeable, sandy sediments. While identified as critical sites for intense oxygen, carbon, and nutrient turnover, constituent exchange across permeable sediments remains poorly quantified. The central North Sea largely consists of permeable sediments and has been identified as increasingly at risk for developing hypoxia. Therefore, we investigate the benthic O2 exchange across the permeable North Sea sediments using a combination of in situ microprofiles, a benthic chamber, and aquatic eddy correlation. Tidal bottom currents drive the variable sediment O2 penetration depth (from ~3 to 8 mm) and the concurrent turbulence-driven 25-fold variation in the benthic sediment O2 uptake. The O2 flux and variability were reproduced using a simple 1-D model linking the benthic turbulence to the sediment pore water exchange. The high O2 flux variability results from deeper sediment O2 penetration depths and increased O2 storage during high velocities, which is then utilized during low-flow periods. The study reveals that the benthic hydrodynamics, sediment permeability, and pore water redox oscillations are all intimately linked and crucial parameters determining the oxygen availability. These parameters must all be considered when evaluating mineralization pathways of organic matter and nutrients in permeable sediments.

Description: The upwelling area off North-West Africa is characterized by high export production, high nitrate and low oxygen concentration in bottom waters. The underlying sediment consists of sands that cover most of the continental shelf. Due to their permeability sands allow for fast advective pore water transport and can exhibit high rates of nitrogen (N) loss via denitrification as reported for anthropogenically eutrophied regions. However, N loss from sands underlying naturally eutrophied waters is not well studied, and in particular, N loss from the North-West African shelf is poorly constrained. During two research cruises in April/May 2010/2011, sediment was sampled along the North-West African shelf and volumetric denitrification rates were measured in sediment layers down to 8 cm depth using slurry incubations with 15N-labeled nitrate. Areal N loss was calculated by integrating volumetric rates down to the nitrate penetration depth derived from pore water profiles. Areal N loss was neither correlated with water depth nor with bottom water concentrations of nitrate and oxygen but was strongly dependent on sediment grain size and permeability. The derived empirical relation between benthic N loss and grains size suggests that pore water advection is an important regulating parameter for benthic denitrification in sands and further allowed extrapolating rates to an area of 53,000 km2 using detailed sediment maps. Denitrification from this region amounts to 995 kt yr-1 (average 3.6 mmol m-2 d-1) which is 4 times higher than previous estimates based on diffusive pore water transport. Sandy sediments cover 50-60% of the continental shelf and thus may contribute significantly to the global benthic N loss.

Description: Summary During this cruise a detailed multi-disciplinary research program was conducted at the Peruvian oxygen minimum zone (OMZ) within the framework of the Kiel SFB 754. Investigations were primarily conducted along a depth transect at 12° S. Major aim was to advance understanding of how OMZ´s are maintained and to determine feedbacks of benthic nutrient release on the currently expanding Peruvian OMZ with a major focus on i. variability of benthic nutrient release in response to hydrodynamic forcing and regional differences in bottom water levels of oxygen (O2), nitrate (NO3-), nitrite (NO2-), and sedimentary carbon content, ii. diapycnal and advective fluxes of excess dinitrogen (N2), ammonium (NH4+), phosphorous (P), iron (Fe), silicate (Si), and radium isotopes between the benthic boundary layer, and the stratified interior ocean as well as their entrainment into the surface mixed layer and iii. processes involved in the respective benthic N, Fe, and P cycles. To achieve this goal, physical and biogeochemical measurements were conducted in the water column as well as at the sea floor. For investigations in the water column a total of 84 CTD casts, 41 micro-structure CTD, 20 in situ pump and 12 GoFlo deployments were performed. Sediment samples were obtained during 50 multiple corer casts, 12 gravity corers and 10 benthic chamber lander deployments. Furthermore a profiler lander was used to determine in situ microprofiles of O2, NO3- and nitrous oxide (N2O) in situ. Microprofiles were obtained using glass-microsensors that were pushed into the sediment in 300 μm increments. In order to obtain time series data on the oxygen distribution and the current regime oceanographic moorings were distributed along the 12°S transect in addition to four benthic satellite-landers each equipped with upward looking ADCPs. Lastly, a glider swarm was established at 12°S. These instruments were deployed for the duration of cruise M92 as well as for the subsequent M93 cruise. Deviating from the cruise proposal, more time was spent for station works at the depth transect at 12° S. Major aim of this cruise was to obtain a coherent data set of all involved groups, which however took slightly more time than originally planned, yet bears a high scientific potential. Additionally, it was discovered that at 12° S in shallow waters sulphide was released from the seabed into the bottom water. Furthermore, in water depths from about 120 to 200 m nitrite in addition to nitrate was available in high concentrations which affects the benthic nitrogen cycle to a hitherto unknown extent. Hence these stations were more intensely investigated than originally planned. Weather conditions were fine and all deployments of the scientific gear went very well. It is expected that after analyses and synthesis of the different data sets from the different disciplines the scientific questions above can be addressed to broad extent.

Description: Summary A detailed multi-disciplinary research program was conducted at the Mauritanian oxygen minimum zone (OMZ). Investigations were primarily performed along a depth transect at 18°20’ N. In this area upwelling of cold, nutrient-rich deep water is strongly seasonal, predominating from April until December. Major aim was to advance understanding of how OMZs are maintained and to determine feedbacks of benthic nutrient release on the currently expanding Mauritanian OMZ under such conditions. Major focus was on (i) variability of benthic nutrient release in response to hydrodynamic forcing and regional differences in geochemistry, (ii) diapycnal and advective fluxes of nutrients, trace metals, and radio-tracer between the sediments and the stratified interior ocean as well as their entrainment into the surface mixed layer and (iii) processes involved in the respective benthic and pelagic N, Fe, and P cycles. The working program in the water column comprised a total of 73 CTD casts, 38 microstructure CTD- and 17 in situ pump deployments. Moorings and Glider were deployed at 18°20’ N and 19°50’ N. Furthermore, in the northern working area ADCP-transects and casts of Underway CTDs were conducted to follow upwelling-induced frontal systems. In situ benthic fluxes of nutrients and oxygen were conducted using the Biogeochemical Observatories BIGO I and BIGO II comprising a total of 9 deployments. Further sediment samples for biogeochemical, investigations were obtained during the deployment of 22 casts of a video guided Multiple Corer (MUC). All deployments were successful and the envisaged data and samples were collected.

Description: The magnitude of nutrient and trace metal release from oxygen minimum zone (OMZ) sediments as well as their fate in the water column is of utmost importance for the pelagic nutrient budget and consequently for the ongoing expansion of OMZs. The major aim of this research cruise that was conducted within the framework of the Kiel collaborative research center SFB 754 (Climate – Biogeochemistry Interactions in the Tropical Ocean), was to study the effects of variable environmental conditions on benthic element turnover and exchange with the bottom water under natural conditions of austral fall/winter but also during experiments. This allows the quantitative simulation of benthic-pelagic nutrient- and trace metal budgets over longer time periods. The experimental investigations aimed to specifically resolve the contribution of sulfur bacteria and denitrifying foraminifera controlling benthic N, P and S fluxes. The investigation of mixing processes in the bottom boundary layer (BBL) and quantification of diapycnal and advective fluxes across the BBL and the stratified water column will be used to resolve the fate of these substances. The multidisciplinary study mainly focused on a depth transect at 12°S and is scientifically closely linked to the METEOR cruises M135, M136 and M138. In order to achieve the scientific goals an intense physical, biogeochemical and biological working program was conducted in the water column and at the seafloor accompanied by shipboard experiments. Studies in the water column comprised 92 CTD, 65 microstructure CTD, 18 TM-CTD and 14 in situ pump deployments. Sediment samples were obtained during 47 mulitple-corer and 12 Lander deployments. Additionally, lander deployments were performed to obtain time series of physical parameters and the current regime in water depths of 76 and 128m. The deployment of these instruments covered the time period of cruises M136 and M137. We slightly deviated from the cruise proposal and spent a minor amount of the station time along a zonal transect at 12.3°S in order study the biogeochemistry during eddy formation. Eddy formation cannot be predicted and hence planned in a cruise proposal, however their study bears a high scientific potential and is a central part of the SFB745 research activities. Due to the good weather conditions all deployments were successful, hence all the data and sample material aimed for has been achieved. It is to expect that as planned all scientific questions can be addressed. Especially, the joined synthesis including the data of the other recent SFB cruises M135, M136 und M138 and their comparison with the earlier SFB-cruises M77, M92 harbor a high scientific potential.

Description: An extensive data set of biogenic silica (BSi) fluxes is presented for the Peruvian oxygen minimum zone (OMZ) at 11ºS and 12ºS. Each transect extends from the shelf to the upper slope (∼1000 m) and dissects the permanently anoxic waters between ∼200 – 500m water depth. BSi burial (2100 mmol m‐2 yr‐1) and recycling fluxes (3300 mmol m‐2 yr‐1) were highest on the shelf with mean preservation efficiencies (34±15%) that exceed the global mean of 10 – 20%. BSi preservation was highest on the inner shelf (up to 56%), decreasing to 7% and 12% under anoxic waters and below the OMZ, respectively. The data suggest that the main control on BSi preservation is the rate at which reactive BSi is transported away from undersaturated surface sediments by burial and bioturbation to the underlying saturated sediment layers where BSi dissolution is thermodynamically and/or kinetically inhibited. BSi burial across the entire Peruvian margin between 3ºS to 15ºS and down to 1000m water depth is estimated to be 0.1 – 0.2 Tmol yr‐1; equivalent to 2 – 7% of total burial on continental margins. Existing global data permit a simple relationship between BSi rain rate to the seafloor and the accumulation of unaltered BSi, giving the possibility to reconstruct rain rates and primary production from the sediment archive in addition to benthic Si turnover in global models.

Description: Bacterial sulfate reduction (SR) is often determined by radiotracer techniques using 35S‐labeled sulfate. In environments featuring simultaneous sulfide oxidation, SR can be underestimated due to re‐oxidation of 35S‐sulfide. Recycling of 35S‐tracer is expected to be high in sediment with low concentrations of pore‐water sulfide and high abundance of giant filamentous sulfur‐oxidizing bacteria (GFSOB). Here, we applied a sulfide‐spiking method, originally developed for water samples, to sediments along a shelf‐slope transect (72, 128, 243, 752 m water depth) traversing the Peruvian oxygen minimum zone. Sediment spiked with unlabeled sulfide prior to 35S‐sulfate injection to prevent radiotracer recycling was compared to unspiked sediment. At stations characterized by low natural sulfide and abundant GFSOB (128 and 243 m), the method revealed 1–3 times higher SR rates in spiked sediment. Spiking had no effect on SR in sediment with high natural sulfide despite presence of GFSOB (72 m). Bioturbated sediment devoid of GFSOB (752 m) showed elevated SR in spiked samples, likely from artificial introduction of sulfidic conditions. Sulfide oxidation rates at the 128 and 243 m station, derived from the difference in SR between spiked and unspiked sediment, approximated rates of dissimilatory nitrate reduction to ammonium by GFSOB. Gross SR contributed considerably to benthic dissolved inorganic carbon fluxes at the three shallowest station, confirming that SR is an important process for benthic carbon respirations within the oxygen minimum zone. We recommend to further explore the spiking method to capture SR in sediment featuring low sulfide concentrations and high sulfur cycling by GFSOB.