The international Antarctic Geological Drilling (ANDRILL) Program, which recently recovered long (> 1000 m) rock and sediment cores from two sites in McMurdo Sound, aims to recover new stratigraphic sections from sites beneath the Ross Ice Shelf east of Ross Island on the Coulman structural High (~77.46 S; 171.23 – 171.68 E). ANDRILL will utilize new drilling capabilities to operate from a fast moving ice shelf platform (~700 m/year northward) and complete two deep holes. The drilling target for the Coulman High Project is a Paleogene to lower Miocene section (Figure 1). Recovery of these strata will allow the ANDRILL team to investigate: (1) the behavior of ice sheets in West Antarctica during periods of moderate to high greenhouse gas levels (Figures 1, 2); (2) the Antarctic environment in warm greenhouse periods (Figures 1, 2); and (3) tectonic processes within the West Antarctic Rift System (Figures 2, 3, 4, 5a,b, 6).
In 2003 and 2004 a marine multichannel seismic grid was completed across Coulman High as close as 500 m from the front of the Ross Ice Shelf (Figure 6). The ice shelf has advanced 4 km north and now sits over several marine seismic lines providing a platform from which to drill into seafloor sites located on those lines. Selected drill sites target ~600 m of laterally continuous sediments underlain by a major regional unconformity and 350 – 850 m of faulted sediments and basement beneath the unconformity (Figure 3). Acoustic basement is interpreted to be from 700 to >1500 m below the sea floor (Figure 3). Seismic correlation from Deep Sea Drilling Project Sites 272 and 273 to the Coulman High sites implies that the section proposed to be drilled predates 19 Ma (Figures 4, 5a,b, 6).
Photo credit: Bruce Luyendyk
Several new operational challenges exist at the Coulman High sites and work is underway to modify existing technology and develop new approaches to accommodate these challenges. Access to the sea-floor requires melting through 250 meters of ice shelf using the ANDRILL hot water drill, which has previously been used to maintain an open hole through 80 meters of ice (Figures 7a,b). The amount of lateral deflection that ANDRILL’s sea-riser can accommodate is limited by water column thickness and amount of ice shelf movement. These parameters constrain drilling depth to a maximum of ~500 m at the Coulman High sites if existing technology were utilized. To reach targets at > 1000 m below sea floor, two drilling options are being considered: pull out at ~500 m, move the rig back over the drill hole, re-occupy the hole via a re-entry cone, and commence coring; or pull out, allow the string to swing back to vertical, wash drill to ~500 m at the new offset site and commence coring below that depth.
An extensive field program in Antarctica will be underway during the austral summer of 2010-2011 to prepare for drilling. Key requirements are locating a safe route to the site from McMurdo Station, assessing the ocean currents beneath the ice shelf and the tides at the site, determining sea floor composition and conditions, and improving the seismic velocity control at the drill sites. Prior activities included airborne radar profiling to measure ice shelf thickness and to search for nascent cracking and crevassing. Field work during the 2010-11 field season will include hot water drilling to provide access holes through the ~250 meter-thick ice shelf. Access holes will be used for deployment/recovery of current meters and other ocean sensors to measure tides and ocean currents, to assess conditions in the sub-ice shelf cavity using a remotely operated vehicle (e.g., SCINI ROV), and to deploy a near bottom hydrophone for a seismic experiment. The survey team will also measure lateral and vertical movements of the ice shelf in the vicinity of CH with GPS and evaluate site meteorology. The seismic experiment will acquire reflection and refraction data to improve the velocity-depth model used to interpret drilling targets from existing marine seismic reflection data. The oceanographic data will be used to model the behavior of the drilling riser in response to the observed tides and currents. All of these data collection efforts and modeling results will be used to finalize CH drill site selection and determine the most robust drilling strategy for the CH Project. More information about the 2010-2011 field season is available in this presentation.
The data collected this season will be utilized to develop a full drilling proposal through the ANDRILL SMO. A new international partnership for drilling in 2012-14 is currently being developed. Interested scientists and their national Antarctic programs are encouraged to get involved by filling out the Coulman High Solicitation of Interest form.
Imagery
The applet below shows high resolution ENVISAT imagery and 2010-2011 field work sites for the Coulman High project. Click and drag your mouse to move and scroll to zoom in and out.
Figures
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Figure 1: 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 (Fig. 1). Proxy data for the past 60 million years : A. Eustatic sea level. Horizontal line 1 is the amount of global sea-level rise if both the Greenland Ice Sheet and West Antarctic Ice Sheet were melted and line 2 is amount of global sea-level rise if all present day ice on Earth was melted; B. Atmospheric carbon dioxide levels. The horizontal dashed red line is CO2 level projected for 2100 AD for the IPCC A2 scenario; C. Global atmospheric temperature curve (bold red) based on deep-sea oxygen isotope records. A compilation of isotope data is in black. Red bands are periods of global warmth. Panel D: Gross scale ice sheet history from deep-sea oxygen isotope records and geologic data from the circum-Antarctic. Panel E: bar chart indicating the geologic time periods previously recovered by ANDRILL (black) and targeted by future drilling projects. The pale yellow box (dashed yellow border) is the period when CO2 levels and atmospheric temperatures were last at the levels projected for 2100 AD by the IPCC A2 scenario.
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Figure 2: Maps of Antarctic topography with restorations for removal of the load of modern ice only (a, BEDMAP adj.), and additionally for erosion, sedimentation, thermal contraction, and horizontal plate motion in geologically active regions (b, from Wilson & Luyendyk (2009); WL09). Models correcting only for ice load have been used in all simulations of ice growth (c) to date, but significant changes result from additionally correcting for other geologic processes (d). Star in a and b shows location of CH site. e) Wilson and Luyendyk tectonic model output for present day and f) BEDMAP 3D model of present topography, g) tectonic model for 34 ma (from Wilson and Luyendyk (2009), h) Topographic 3D model of 34 ma.
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Figure 3: In 2003 and 2004 a marine multichannel seismic grid was completed as close as 500 m from the front of the Ross Ice Shelf (Fig. 5,6). 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. Total proposed maximum drilling depth with optimum conditions for each location is approximately 1200 mbsf.
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Figure 4: Seismic correlation line from CH to DSDP 272, 270 showing (i) stratigraphic position of interpreted prominent reflectors between DSDP sites and CH; (ii) interpreted outcrop of reflector RSU5 (orange) on sea floor north of proposed CH drill site CH-1; (iii) continuation of reflector RSU5A (light blue) over proposed CH-1 site (iv) correlation of both RSU5 and RSU5A occurring below base of DSDP Site 273 (not shown, track in Fig. 5a) and 272, indicating that both reflectors are significantly older than interpreted ages from DSDP Sites 273, 272. The upper section to be cored at CH-1 is therefore early Miocene and Oligocene in age. In the Central Trough a refection below CHMU was identified as RSU6 in ANTOSTRAT (1995) but this cannot be tied to the east across basement highs to DSDP 270.
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Figure 5a: Regional detail of Coulman High study area on MODIS image (250 m resolution) from November 2010 showing locations of: i) CH-1, CH-2 drill sites (yellow); ii) northern oceanographic mooring and field season GPS station (light blue); iii) southern oceanographic mooring and winter-over GPS station (dark blue); iv) experimental test hole for deployment of ROV-SCINI, gravity cores, and oceanographic experiments and field season GPS station (green); v) field season GPS station (brown); vi) NBP0301 marine seismic line array; vii) correlation path to CH from DSDP sites 270, 272, 273 (heavy red line); viii) USGS marine seismic lines 403/404; ix) locations of previous drill sites MIS, SMS, CRP, CIROS (green/blue stars); x) and the polar traverse route (heavy black line), Coulman High traverse route (thin brown line), Ross Island and McMurdo Station (labeled). Black box around primary Coulman High study area indicates areal extent of enlarged map shown on Fig 5B.
Figure 5b: Enlarged study area map on November, 2010 ENVISAT image (60 m resolution) showing locations of: i) CH-1, CH-2 drill sites (yellow); ii) Vertical Seismic Profile (VSP) survey layout (blue line = shot holes, brown line = geophones); iii) sea floor hydrophone for VSP (black); iv) gravity survey stations (brown/black dots); v) field season oceanographic mooring and GPS station (light blue); vi) field season oceanographic mooring and winter-over GPS station (dark blue); vii) oceanographic experiment station, ROV-SCINI deployment hole, and field season GPS (green); viii) field season GPS (brown); ix) CH traverse route (thin brown line); x) NBP0301 marine seismic array line names indicated by adjacent alpha-numeric tags (1A0, 12A, 2B, 11A1, etc.). Ice shelf advance of roughly 5 km over southernmost marine seismic line location from 2003 is apparent.
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Figure 6: Regional overview of RS area showing structural basins and highs (basins in green). Inset box is location of proposed drill sites (CH-1. CH-2) and is shown in detail on Figs 5a and 5b. Regional seismic correlations lines (BGR, IFP, US) from the Eastern Basin (EB), Central Trough (CT) and VLB to CH are shown. Inset map shows location of region on Antarctic continent.
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Figure 7a and 7b: Drilling will require melting through the ice shelf with a hot water drill. Operational challenges will result from the movement of the ice shelf northward over the sea floor at ~2 m/day. This is faster than experienced at ANDRILL site MIS near McMurdo Station. 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 (Fig. 7b; d = days).
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