Plattform Platform	Reise-Nr. Cruise-No.	Zeitraum Period	Projekt Project	Arbeitsgebiet Working area
Maria S. Merian	MSM24	27.12.2012 - 21.01.2013		South Atlantic Ocean

Fahrtleiter Chief Scientist	Institut Institute	Auslaufhafen Port of departure	Einlaufhafen Port of return	Stationskarte Station map	Schiffsroute Trackchart
Geissler Wolfram	Alfred-Wegener-Institut fur Polar- und Meeresforschung	Walvis Bay	Cape Town

Ziel der Reise / Objectives of Cruise:

According to classical plume theory, the Tristan da Cunha hotspot is thought to have played a major role in the rifting of the South Atlantic margins and the creation of the aseismic Walvis Ridge by impinging at the base of the continental lithosphere shortly before or during the breakup of the South Atlantic margins. However, Tristan da Cunha is enigmatic as it cannot be clearly identified as a hot-spot but may also be classified as a more shallow type of anomaly that may actually have been caused by the opening of the South Atlantic. The equivocal character of Tristan is largely due to a lack of geophysical data in this region. It is of central importance to characterize the region around Tristan da Cunha with geophysical data in a more coherent way to understand the tectonic processes of the opening of the South Atlantic and the formation of the Walvis Ridge, i.e. to understand whether Tristan da Cunha is the cause or the consequence of the rifting. We therefore staged a multi-disciplinary geophysical study of the region by acquiring passive marine electromagnetic and seismic data, bathymetric data as well as gravity data from which we will derive an electrical resistivity, seismic velocity and density model down to a depth of several hundred kilometers. These models will be interpreted in the context of geochemical data and tectonic models developed within the SPP1375 South Atlantic Margin Processes and Links with onshore Evolution (SAMPLE). On the cruise MSM 24 we acquired bathymetric data within the Tristan region and recovered 26 out of 26 ocean-bottom magnetotelluric stations (OBMT) and 22 out off 24 broadband ocean-bottom seismometers (BB-OBS) as well as two seismic and one MT land stations on the uninhabited Nightingale Island, which were deployed one year ago during cruise MSM 20-2. The cruise also offered the opportunity for a colleague from the University Heidelberg to conduct geological sampling on Tristan da Cunha. We left Walvis Bay in the afternoon of December 27th, 2012. Bathymetry and ATLAS Parasound data acquisition was initiated as soon as we left the exclusive economic zone of Namibia on December 28th, 2012. In the morning of January 2nd, 2013 we arrived to the first recovery station, where one BB-OBS and on OBMT had to be recovered. To allow a topographic correction of the MT data, we carried out additional bathymetric surveys around the station sites. Furthermore, we went along parallel tracks to the last year ones to get more complete bathymetry coverage of the region. Recovery of stations was interrupted by short visits to Tristan da Cunha and Nightingale Island between January 7th and 9th, 2013. Together with three local guides we recovered two seismometers and one magnetotelluric station from Nightingale Island. In the meantime bathymetry around the island was mapped using the shallow-water EM1002 multi-beam echo sounder. We left Tristan da Cunha in the evening of January 9th, 2013 and continued with the recovery of the ocean-bottom instruments. Unfortunately, the next ocean-bottom seismometer (TDC25) could not be released from the seafloor even if it answered all release commands. After several failed attempts to release the instrument the schedule forced us to head for the next recovery site. Until the morning of January 13rd, 2013 we recovered the remaining BB-OBS and OBMT, with the exception of the last BB-OBS (TDC17). Bathymetric and Parasound measurements were conducted until January 19th, 2013 arriving close to the South African exclusive economic zone. Additionally, we also operated the ADCP 38 kHz system to measure currents in the ocean. We arrived to Cape Town already in the morning of January 20th, 2013, one day earlier than previously planned.

Messungen / Measurements