SoMARTherm: The Mid-Atlantic Ridge 13-33°S - Cruise No. MSM25, January 24 - March 5, 2013 - Cape Town (South Africa) - Mindelo (Cape Verde)

Before the present cruise, no hydrothermal vent sites had been explored in the south Atlantic south of 13°S. As a result of the work in SPP1144 (From Mantle to Ocean: Energy-, Materialand Lifecycles at Spreading Axes), Devey et al. (2010) proposed a model for the relation between volcanism and hydrothermalism. The aim of the cruise was to use a systematic approach to explore the Mid-Atlantic Ridge between 13° and 33°S for hydrothermal activity and plume dispersal by using combined AUV deployments and ship-based CTD casts. The cruise had the following scientific goals: - Testing the link between volcanism and hydrothermalism: The model proposed by Devey et al. (2010) suggests that hydrothermal activity will be almost constant on marked within-axis volcanic highs (as at Turtle Pits) but relatively seldom at deeper parts of the axis. As the time-scale of activity on the deeper parts of the ridges is likely to be on the order of 1000's of years we needed to use geography as a proxy for time - surveying multiple segments for hydrothermal activity to find one in the eruptive phase. - Testing the link between ridge morphology and strength of oceanic diapycnal mixing: The sampling of a large variety of ridge morphology should allow a better understand of the relation between mixing strength, flow properties, and bathymetric roughness. The strength of mixing above the ridge crest and in the axial valley determines to a large extent the vertical exchange of chemical species from the hydrothermal fluids and the ocean interior. - Large scale mapping of plume dispersal: Combined along-axis CTD (Conductivity, Temperature, Depth), plume chemistry (helium, methane, metals, rare elements), and current measurement can allow characterization of the large scale, particularly along axis, displacement of plume material and identification of possible pathways for larvae dispersal. - Closing the gap between 10°S and the Antarctic ridges of ridge crest explored for hydrothermal activity, a region of high importance for the definition of biogeographical boundaries. - Refining hydrothermal plume hunting techniques: The long-range AUV (Autonomous Underwater Vehicles) deployments coupled with ship-based CTD casts offered the possibility to survey whole segments in relatively short time-periods. Although a thorough analysis of the cruise results (particularly for physical oceanography) will require extensive post-cruise work, it is already clear that: 1. The marked within-axis highs show the expected chronic hydrothermal activity, confirming and reinforcing the proposed model. In addition we found the most southerly-known axial oceanic core complex and found it also to be associated with hightemperature venting. 2. Using co-registered AUV and CTD data over these highs we were able to map, for the first time, the 3D extent of hydrothermal plumes (see cover illustration). The combination of the vertical resolution of the CTD Tow-Yo with the horizontal resolution of a near-bottom (50m altitude) AUV raster gave an almost synoptic view of the venting over the entire summit. Together with the high-resolution side-scan data recorded by the AUV (which allowed us to identify and locate individual chimneys associated with the hydrothermal plumes and determine their geological setting) this provides the ideal basis for preparing to return to these biogeographically important sites to sample the ecosystems they sustain and determine the oceanographic features relevant for local larval dispersal. 3. On ridges without within-axis highs we also found occasional signs of hydrothermal activity, proving the efficiency of the combined AUV+CTD exploration method. An additional bonus from the AUV deployments was that we were able to simultaneously collect a single swath of high-frequency side-scan sonar data along the axial valley and hence derive information about the style and relative age of volcanism along-axis.