Initial Observations and Inferences from New Field Work on Pine Island Glacier Ice Shelf

1. 3700 1980’s 2007 Date: 12/21/1989 Initial Observations and Inferences from New Field Work on Pine Island Glacier Ice Shelf Robert Bindschadler, NASA; David Holland, New York University and David Vaughan, British Antarctic Survey • Background • Observations from space and aircraft have demonstrated that the Pine Island Glacier (PIG) is among the most rapidly changing ice covered areas anywhere (Zwally et al., 2006). The spatial pattern of ice thinning and ice flow acceleration suggest an oceanic trigger to these changes (Shepherd et al., 2004). Water from either Circumpolar Deep Water (CDW) or High Salinity Shelf Water (HSSW) (we suggest the former has the greater recent impact) enters the sub-ice shelf cavity, contacts ice at the grounding line and circulates back to the open ocean, perhaps causing further melting as it leaves. • A joint US/UK IPY field project to determine if water beneath the floating ice shelf extension of the PIG is responsible for these changes has begun. More information about this project is available at http://pigiceshelf.nasa.gov 2. Field Reconnaissance (2007-2008) A Twin Otter made the first-ever landing on PIG ice shelf on January 3, 2008. Unfortunately, the surface was too hard to safely attempt subsequent ice shelf landings. Two GPS stations and a fully instrumented AWS were established on nearby tributaries of PIG (see image on left). Power systems designed to provide year-round power and frequent data transmission have initiated surface data collection from this active area. GPS1 is located on floating ice and is moving toward the main ice shelf at 425 m/a. GPS2 is located on a major southern tributary, moving into the main trunk at 356 m/a. The AWS, moving at 780 m/a, measures hourly the air temperature and pressure, wind speed, upward and downward long-wave radiation, surface height and relative humidity and takes two webcam pictures. These data are phoned to NYU and are viewable at http://efdl_5.cims.nyu.edu/ems_pig/. Sample data are at right. GPS site AWS site 3. Offshore Observations The bathymetry of the continental shelf fronting PIG consists of a trough eroded into the seafloor by former advances of PIG (red arrow in figure below). This directs denser waters across the shelf to beneath the PIG ice shelf. 4. Surface Waves British Antarctic Survey airborne profiles of surface elevation and ice thickness reveal significant differences between the two halves of the PIG ice shelf, possibly related to the two-cell character inferred beneath the ice shelf. Profile N (at left) shows a more typical ice shelf longitudinal cross-section: rapid thinning of about 250 meters immediately downstream of the grounding line to a nearly uniform floating slab 500 meters thick with a smooth ice shelf surface. There is an additional 180 meters thinning near the terminus surrounding a thicker section of ice that we do not explain here. Profile S (shown below Profile N) displays a much different longitudinal profile: the initially ungrounded ice flows into a region of lightly grounded ice plain (Corr et al., 2001), before ungrounding again and thinning very rapidly (400 meters ice thickness in 2000 meters distance). This second ungrounding is accompanied by a roughly periodic series of large waves in both the surface and ice bottom. thickness. The waves can be seen in Landsat imagery (upper central figure of poster). Precise coregistration of the waves in the imagery and the airborne profile is difficult due to the changing speed of the ice shelf (Joughin et al., 2003), but a general corroboration exists (see lower plot, this panel). The wavelength (mean=2239 m) is close to the annual motion but variability is high. The aperiodic character of the waves suggests these waves are caused by temporal variations in sub-surface melt on a nearly annual basis. Annually averaged melt rates greater than 100 m/a have been estimated (Payne et al., 2007), sufficient to create such large sub-annual oscillations of the observed magnitude. flotation A bathymetric profile across the ice shelf front displays a shallow shelf to the north and a deeper southern basin (figure at right is from Jacobs et al., 1996, and looks eastward toward the shelf from the ocean). Movement of sub-shelf waters deflects water leftward, setting up possibly two separated cells, one on the shallower bed and the other in the deeper basin (Payne et al., 2004). Landsat imagery from 1972 to 2008 illustrate a sudden shift in 2000 from an ice shelf front either completely covered in sea ice or completely open to a situation where three isolated polynyas persisted at the edges of these circulation cells. flotation ice shelf ice shelf sea ice polynyas Date: 1/4/2001 This observation supports the contention of two distinct circulation cells beneath the ice shelf. It is also likely that the deeper section of the seafloor across the southern third of the ice front, extends upstream beneath the ice shelf. Changing speed of PIG (adapted from Joughin et al., 2003) 5. Future Work Project plans include drilling from the ice shelf surface into the water cavity below, exploration of the cavity with a remote video camera and the placement of up to 4 ocean profilers in the cavity to monitor cavity water conditions. In addition, a surface field program will measure the cavity geometry, use phase sensitive radar to measure basal melt rate and GPS monitoring of surface motion. Logistic constraints have delayed the next field season to no earlier than the 2009-10 austral summer. Future surface work on the ice shelf will be supported by helicopters. 6. References Corr, H.F.J., C.S.M. Doake, A. Jenkins and D.G. Vaughan, 2001. Investigations of an “ice plain” in the mouth of Pine Island Glacier, Antarctica, Journal of Glaciology, Vol. 47, No. 156, pp. 51-57.Jacobs, S.S., H.H. Hellmer and A. Jenkins, 1996. Antarctic ice sheet melting in the Southeast Pacific, Geophysical Research Letters, Vol. 23, No. 9, p. 957-960. Joughin, I, E. Rignot, C.E. Rosanova, B.K. Lucchitta and J. Bohlander, 2003. Timing of recent accelerations of Pine Island Glacier, Antarctica, Geophysical Research Letters, Vol. 30, No. 13, Art. No. 1706, July 11, 2003. Payne, A.J., A. Vieli, A.P. Shepherd, D.J. WIngham and E. Rignot, 2004. Recent dramatic thinning of largest West Antarctic ice stream triggered by oceans, Geophysical Research Letters, Vol. 31, No. 23: Art. No. L23401 DEC 9 2004 Payne, A.j., P.R. Holland, A.P. Shepherd, I.C. Rutt, A. Jenkins and I. Joughin, 2007. Numerical modeling of ocean-ice interactions under Pine Island Bay’s ice shelf, Journal of Geophysical Research, Vol. 112, C10019, doi:10.1029/2006JC003733. Shepherd, A., D.J. Wingham and E. Rignot, 2004. Warm ocean is eroding West Antarctic Ice Sheet, Geophysical Research Letters, Vol. 31, Art. No. L23402 DEC 9 2004. Zwally, H.J, M.B. Giovenetto, J. Li, H.G. Cornejo, M.A. Beckley, A.C. Brenner, J.L. Saba and D. Yi, 2006. Mass Changes of the Greenland and Antarctic Ice Sheets and Shelves and Contributions to Sea Level Rise: 1992 – 2002, Journal of Glaciology, Vol 51 (175), P. 509-527.