After 29 days at sea, the DynAMITE cruise has come to an end, docking at Penno Wharf in Bermuda on Sunday, June 12th. The chief scientist Ruth Curry declared the mission a success and believes the data collected will significantly improve our understanding of deep mixing and circulation in the ocean.
Science Crew of the Dynamite cruise as the R/V Knorr arrives in Bermuda (from left: Kurt Polzin, Bob Petitt, Liz Douglass, Kevin Manganini, George Tupper, Amelia Snow, Fred Thwaites, Carolina Nobre, Dave Wellwood, Terry McKee, Ruth Curry, Leah Trafford)
At ~ 5:37 pm on May 28th, 201, HRP was sent on dive #34 over the rugged seafloor terrain of the Mid Atlantic Ridge. Its dive was programmed for 5495 meters (the readout from the ship's echo sounders), with its altimeter set to turn on at 160 meters above that depth to provide a safety cushion-- it would drop its weights earlier if it detected a shallower bottom. The altimeter never had a chance to perform its job -- HRP hit the bottom at 5330 meters deep, 5 meters before it would have switched on. As it turned out, the bottom was covered with relatively soft, thick and sticky mud. Although HRP did drop its weights when it realized it was not moving, the mud held fast and prevented it from rising to the surface. For the next week, day and night, the ship and science crew dragged a trawl wire over the bottom more than 5000 meters / 3 miles deep, attempting to knock HRP loose from the mud's grasp. By the 7th day, recognizing that we soon had to leave the site and continue on Knorr's schedule, the mood on the ship was somber. A slightly different setup with the trawl wire was employed, but it hung up on the bottom repeatedly. As Chief Mate Dee maneuvered and winch operator, Susan Coleman, raised and lowered the wire attempting to free it, a sudden movement off the starboard quarter caught her eye. Breaching like a whale, HRP broke the surface with strobe flashing! The news traveled quickly through the ship, and everyone turned out on deck to watch as the rubber boat was launched and 3 crew members retrieved the instrument package. A rousing cheer, clapping, hugging and dancing greeted HRP as it was hoisted back on board to the safety of its cradle. Although it sustained damage to external sensors, after replacing these with spares, HRP has returned to work, continuing our mission to explore circulation and mixing in the abyssal depths of the Atlantic.
We have found 2 pathways for the dense flows across 20°N:
1) through a large gap in the ridge between 60-62°W; and
2) over and through a deep channel near 54-57°W.
After transiting the ridge in the western gap, a portion of the dense waters flow eastward along bathymetric contours on the northern flank of the ridge and join the eastern overflow of dense waters. Here they encounter steepening topography of the Mid Atlantic Ridge (MAR) and turbulent mixing begins to intensify.
Our survey includes 6 sites along the MAR between 20° and 31°N to help determine how turbulent mixing changes with latitude and the roughness of seafloor topography.
WHOI Engineers Robert Petitt and Fred Thwaites prepare the High Resolution Profiler (HRP2) for deployment. The engineer's expert knowledge of the instrument has been critical to the many successful deployments on this cruise, during which it has completed missions to depths exceeding 6000 meters. Robert Petitt has been a member of the Applied Ocean Physics and Engineering (AOPE) deparment since 1990. Dr. Fred Thwaites is a WHOI/MIT joint program graduate and has been on the AOPE staff since 1995. Both scientists have been involved in the development of HRP2 since its inception in 2001.
We are presently at sea aboard R/V Knorr using the High Resolution Profiler (HRP), a CTD and SeaBeam to make detailed measurements of deep currents, temperature, salinity, nutrients and bathymetry. We want to determine how the dense cold plume of Antarctic Bottom Waters (AABW) flows around/through/over a topographic ridge at ~20°N that blocks its northward pathway in the western North Atlantic basin. There are virtually no deep measurements in this region prior to our survey. This program is funded by NSF grant# OCE-0926848.
Crew members and science staff deploy HRP for its eleventh cast on May 22, 2011 at 20 45'N , 54 20'W.