Borehole microbial observatory science in basaltic ocean crust: The North Pond area on the western mid-Atlantic Ridge flank at 23° N MICROB II - Cruise MSM37 - March 22 - April 21, 2014 - Las Palmas (Spain) - Cadiz (Spain) (English)

MERIAN cruise MSM37 to North Pond (23°N, 56°W) had two objectives: to conduct operations at the subseafloor observatories (CORKs) and to investigate in detail locations identified during MSM20-5 for suspected hydrothermal discharge sites at the seafloor. These goals were achieved with the main operational tool ROV Jason II from WHOI. In addition, the night program was devoted to detailed acoustic mapping of North Pond and its close vicinity with the ships built-in swath bathymetry system EM122. Hydrologically active ridge flanks may host a large microbial biomass that is most likely supported by oxidation reactions of basalt. To investigate these processes three longterm subseafloor observatories (CORKs) were installed during IODP Expedition 336 (fall 2011) to be able to conduct in situ incubation experiments, to collect fluid samples and to measure in situ formation pressures. A first visit of the sites during MSM20-5 with WHOI ROV Jason II showed that the observatories were fully operational. Ten dives in total were conducted with ROV Jason II during the time at North Pond. Seven of these dives were dedicated to CORK operations at IODP Site 1382, Hole U1382B and IODP Site 1383, Hole U1383B and U1383C, such as pumping crustal fluids with the help of the CORK sampling ports, recovery and installation of osmo samplers and recovery of two previously installed GeoMicrob sleds. In addition, pressure data were downloaded at all CORKs. All operations at all CORKs went very well and without any technical problems. Two about 36 hour long exploration dives at previously identified areas with suspected seafloor discharge did not reveal active discharge although we could identify areas using temperature gradient measurements in the sediment with values of up to 0.9 K/m. This is a clear indication for circulating warm fluids within the permeable upper crust. Diffuse discharge with temperatures just above bottom water temperature may occur but could not be detected due to the subtle temperature signal in the water column. One exploratory dive to the southern termination of a dome-like rift mountain south of North Pond yielded basalt, which may represent the break-away zone of an oceanic core complex. Acoustic mapping revealed new structures due to the increased resolution of the EM122 system compared to the previously used EM120; especially the backscatter data seem to be good enough to identify sediment cover on rough topography. Despite the fact that we lost one day to weather and one to technical problems all objectives were achieved.