Map shows the Western Divide of West Antarctica, which separates the Amundsen Sea and the Ross Sea sectors of the ice sheet. The upcoming WAIS Divide core is located 24 km off the divide on the Ross Sea side. The main goal of this research was to identify the exact location for the new, upcoming WAIS Divide core in central West Antarctica. We completed ground-based geophysical surveys of the region during the 2002-03 and 2003-04 field seasons. Results and recommendations were presented to the community at the Ice Core Working Group meeting in March 2005. View the report here. Drilling started at the site during the 2007-08 season: metadata and drilling progress are here. Site selection was completed in 2006; the project was supported by the US National Science Foundation (OPP-0087345). We also thank Raytheon Polar Services, Air National Guard and Kenn Borek Air for logistical support.

Participants:
Principal Investigators: Howard Conway and Ed Waddington (U. Washington) and David Morse (U. Texas).
Other contributors: Ginny Catania, Maurice Conway, Dan Dixon, Kenny Matsuoka, Paul Mayewski, Tom Neumann, Felix Ng, Erin Pettit, Donovan Power, Steve Price, Eric Steig, Ken Taylor and Erin Wharton.

As a part of the site selection work we established a preliminary depth-age relationship for the WAIS core by tracking tracking continuous radar-detected layers from dated cores at Byrd and ITASE 00-1. The deepest continuous layer that we could trace from Byrd corresponds to a "strong acid-deposition event", dated at 17,400 yrs BP. We then used a time-dependent thermo-mechanical ice-flow model to interpolate the depth-age relationship between radar-detected tie points, and to extrapolate it from the 17,400 yr tie point to the bed.

Figure shows preliminary depth-age relationship for the WAIS Divide core. Ice-equivalent thickness is 3465m, and surface accumulation is 21.5 cm/yr (ice equivalent). Depth-age tie points (denoted by circles) come from radar layers traced from dated cores at Byrd and ITASE 00-1. Prior to 17,500 yrs BP (dashed lines), the time scale is estimated by prescribing histories of ice thickness and accumulation. Model input requires knowledge of the geothermal flux and histories of ice thickness, surface temperature, and accumulation, all of which are not well known. However, model results suggest that basal melting at the core site is likely if the geothermal flux exceeds 70 mW-2 (Conway et al., 2005).

In other work we used the upper, well-dated section and the ice-flow model to establish histories of accumulation and ice sheet dynamics for the past 8,000 yrs. Results show that accumulation was approximately 30% higher than today from 5,000 to 3,000 yrs ago. In addition, results indicate that the ice divide has been moving around, and/or it has been sliding at the bed for at least 8,000 years (Neumann et al., 2008). To further investigate perigrinations of the ice divide we analyzed our measurements of surface velocity field, calculated from repeat GPS measurements of poles set in the ice, and ice thickness using ice-penetrating radar. The rate of ice thickness change, calculated from the difference between the ice flowing out of the region and the accumulation coming into the region, indicate strong thinning on the Amundsen Sea side of the divide, little or no thickness change at the WAIS core site, and thickening farther SSW on the Ross Sea side (Conway and Rasmussen, Submitted). This asymmetric pattern of thickness change is causing the divide to migrate toward the Ross Sea at a rate of 10 m/yr.

Figure shows thinning and migration of the divide that would occur if the present-day pattern of ice-thickness change persists for 50, and 100 years: ongoing monitoring is needed to determine whether this is a transitory response to changing dynamics of the adjacent catchments, or whether it signals a new stage of ongoing draw down of the interior of the West Antarctic Ice Sheet.

2. Ice-flow history of Thwaites Glacier, West Antarctica

Image shows present-day surface velocities, derived from InSAR, for Thwaites Glacier (TG) and Pine Island Glacier (PIG) - from (Joughin et al., 2009). This new project is a collaboration between Howard Conway from U. Washington and colleagues Ginny Catania and Don Blankenship from U.Texas. We are also working closely with CReSIS scientists who are collecting new data from the region. Our collaborative research is supported by the US National Science Foundation (ANT-0739372). In this study we seek to understand the recent response of Thwaites Glacier to changes at its grounding line within the context of possible long-term variability in mass balance. Our plan is to study the controls on fast flow in this area and search for evidence that will evaluate the effects of recent observed changes in elevation and grounding-line retreat on the position of the margin within the context of longer time scales. Our plan is to leverage data that have already been collected from the region (airborne data from UTIG and ground-based data from ITASE), and data from upcoming surveys (PSU/CReSIS) to address the following hypotheses:

• Margin migration: Observed changes in grounding line position and ice sheet elevation of Thwaites Glacier have forced outward migration of its lateral margins over the past few decades indicating that the basal boundary conditions that permit fast flow are not fixed in space and time;



• Past variability: The divide between Thwaites and Pine Island Glaciers contains stratigraphy indicating that the adjacent glaciers have been active but fixed spatially for several millenia;



• Controls on fast flow: A layer of deformable, wet till underlies the fast-moving trunk regions of Thwaites Glacier but not beneath the slow-moving ridge between Thwaites and Pine Island Glacier. It is this wet, deformable till that permits fast flow in the trunk region.

To test these hypotheses we will analyze ice-penetrating radar data that have already been collected from the region, and we will also work closely with CReSIS scientists who will be acquiring new data during the 2009/10 field season. We will use the data to define transitions in ice thickness, internal layer geometry and basal properties. The geometry of internal layers provides insight into the flow history of ice sheets, and changes in bed return amplitude can be diagnostic of transitions from melted to frozen conditions.