ANDRILL Targets Coulman High, Ross Sea, Antarctica to Recover History of the West Antarctic Ice Sheet

The ANDRILL Program completed two 1 km+ holes in McMurdo Sound (MIS, SMS), and will now move east of Ross Island to the ice shelf over Coulman High. Here it will utilize new drilling capabilities to operate from a fast moving ice shelf platform (~700 m/year northward) and complete two deep holes. The target for the Coulman High Project is a Cretaceous (?) -Paleogene to lower Miocene section. Drilling at Coulman High will investigate: (1) the evolution of the West Antarctic Ice Sheet in a period of high greenhouse gas levels; (2) the Antarctic environment in warm greenhouse periods; (3) controls on Oligocene and Miocene climate and cryosphere; and (4) tectonic processes within the West Antarctic Rift System. In 2003 and 2004 a marine multichannel seismic grid was completed as close as 500 m from the front of the ice shelf. Since then the ice shelf has migrated north 4 km and now covers some of the survey lines. The plan is to drill through the ice shelf into targets located on those lines. The chosen sites target <600 m of stratified sediments underlain by a major unconformity. The section below the unconformity comprises 350 - 850 m of faulted sediments and basement. Acoustic basement is interpreted to be from 700 to >1500 m below the sea floor. Seismic correlation from DSDP Site 273 within Central Trough and Site 272 in the Eastern Basin to the Coulman High sites implies that the section proposed to be drilled predates 19 Ma. These correlations imply that the section at Coulman High would extend back in time the early Miocene and younger climate record from ANDRILL sites in McMurdo Sound. Drilling will require melting through the ice shelf with a hot water drill. This method was successful during the McMurdo Ice Shelf Project, but the ice shelf above the Coulman High is thicker (~250 vs. 80 m). Operational challenges will result from the movement of the ice shelf northward over the sea floor at ~2 m/day. Deflection of the riser cannot be more than 8% of the water layer thickness of 630 m (below ice base) and constrains drilling depth to a maximum of ~500 m. To reach targets at > 1000 m below sea floor, two options are being considered; to pull out and move the rig back over the site and reenter the hole, or to pull the string at the end of the drilling and wash down to the prior depth at a new offset site and drill and core below that depth. A small-diameter mini-ROV will be used to support the drilling. Surveys required prior to drilling include airborne and ground radar of the ice shelf for nascent cracks and crevasses that threaten either the sites or traverse to the sites, and measurements of currents beneath the ice shelf to model deflection of the riser. An over-ice seismic survey near the sites will improve seismic velocity control to estimate drilling depths. Survey work will take place over the 2010-11 and subsequent seasons with drilling in 2012-14, depending on the commitment of funds by international Antarctic Programs.