ALBERT

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.