Borehole microbial observatory science in basaltic ocean crust: The North Pond area on the western Mid-Atlantic Ridge flank at 23°N - Cruise No. MSM20/5 - April 11-May 10, 2012 - Freeport (Bahamas)-St. John's (Canada) (English)

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The extent and activity of microbial life in the upper ocean crust is unknown, but hydrologically active, young ridge flanks may host a large microbial biomass that is possibly supported by oxidative alteration reactions of basalt. MARIA S. MERIAN cruise 20/5 had the primary objective of conducting operations on subseafloor observatories (CORKs) installed to examine hydrological-geochemical-microbiological interactions in a sedimented area (North Pond) on the western flank of the mid-Atlantic Ridge at 22°45'N, 46°05'W in 4450 m water depth. The remotely operated vehicle (ROV) Jason of the Woods Hole Oceanographic Institution was the main operational tool, used to carry out installations of a shallow observatory and numerous operations on two deep observatories, installed during Expedition 336 of the Integrated Ocean Drilling Program in the fall of 2011. In nine Jason lowerings, the following objectives were achieved: At observatories in Holes U1382A and U1383C, seafloor Osmo samplers were retrieved and new ones were installed, GeoMICROBE sleds were deployed and attached to fluid sampling lines, basement fluid samples were recovered from all observatory zones in the subseafloor (down to 331.5 m subbasement in Hole U1383C) and sediment samples were pushcored. In Hole U1383B, a scaled-down observatory was fully installed by ROV operations to seal off the seafloor near close-by Hole U1383C and set up for future sampling and interborehole experimentation. Pressure data were downloaded from all holes. A partially installed observatory in Hole 395A was inspected and prepared for future operations. Seafloor mapping using the ROV's Multibeam system, rock sampling from the steep slopes surrounding the sedimented area around the drill holes, further sediment sampling and heat flow surveys complemented the ROV-based work conducted during the cruise. Moreover, a 5500 km**2 area around our North Pond study site was mapped using the Ship's EM 120 Multibeam echosounding system. Despite lost time due to the delayed delivery of a container with critical observatory gear and rough weather around mid-cruise, all major cruise objectives were achieved. The observatories are fully operational and the North Pond experiment is underway.