Oxygen Supply Tracer Release Experiment (OSTRE) - Cruise No. M97 - May 25 - June 28, 2013 - Mindelo (Cape Verde) - Fortaleza (Brazil) (English)

The cruise M97 to the Tropical Eastern North Atlantic was a contribution to the DFG Sonderforschungsbereich (SFB) 754, Climate - Biogeochemistry Interactions in the Tropical Ocean. The cruise was the first cruise to map the deliberately released tracer that was injected to the area in December of 2012 as the "Oxygen Supply Tracer Release Experiment (OSTRE)". The results from these measurements will constrain the diapycnal and lateral mixing rates in the region with the aim of quantifying and better understanding the supply of oxygen to the oxygen minimum zone (OMZ) of the Tropical Atlantic. Toward this goal we measured the tracer content of the water column at 171 CTD stations, focused on the area close to the density at which the tracer was released (sigma Θ=27.03). The sampling was focused along a grid pattern with the borders 8 to 12°N and 23 to 19°W where we found tracer at about 2/3 of the stations. This allowed for a detailed mapping of the tracer within the area, both in the vertical as in the horizontal. The sampling at the CTD staions also included oxygen measurements that provided observations on temporal variability and inventory of the oxygen concentration within the OMZ. A zooplankton sampling program aimed to elucidate the role of zooplankton in biogeochemical cycles, in particular their role for the transport of organic and inorganic matter from the surface layer to OMZ depths via diurnal vertical migrations. Zooplankton was sampled with different nets and also measured by a Underwater Vision Profiler at all stations. Additionally experiments aiming at quantifying the nitrogen fixation were carried out during the cruise. During the first day of the cruise a deep CTD profile was conducted at the Cape Verde Ocean Observatory (CVOO), where also a mooring was deployed .Continuous surface water measurements of pCO2 was conducted throughout the entire cruise.