U.S. Geological Survey and The National Academies; USGS OF-2007-1047, Extended Abstract 153 New aeromagnetic results from the Thwaites Glacier catchment, West Antarctica J. W. Holt,1 D. D. Blankenship,1 F. Ferraccioli,2 D. G. Vaughan,2 D. A. Young,1 S. D. Kempf,1 and T. M. Diehl1 1Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J.J. Pickle Research Campus, Building 196, 10100 Burnet Road, Austin, TX 78758-4445, USA ([email protected]) 2British Antarctic Survey, High Cross, Madingley Road, CB3 0ET, Cambridge-UK Summary The Amundsen Sea Embayment (ASE) of the West Antarctic Ice Sheet (WAIS) is a recent focus of attention due to observed changes indicating a negative mass balance associated with glacial thinning and grounding line retreat. These changes are likely driven by oceanic and/or atmospheric processes, but the future response of the ice sheet to changes at the margins will be dictated in large part to the ice sheet's underlying geological character including bed slope, roughness, heat flux, and both sediment and water distribution. A large-scale multi-instrumented airborne survey of this region conducted in 2004-05 provides important new information on these conditions. We will present new aeromagnetic data resulting from this survey to reveal constraints on the subglacial geology in this important region and potential controls on ice dynamics. Citation: Holt, J.W., D.D. Blankenship, F. Ferraccioli, D.G. Vaughan, D.A. Young, S.D. Kempf, and T.M. Diehl (2007), New aeromagnetic results from the Thwaites Glacier catchment, West Antarctica: in Antarctica: A Keystone in a Changing World – Online Proceedings of the 10th ISAES, edited by A.K. Cooper and C.R. Raymond et al., USGS Open-File Report 2007-1047, Extended Abstract 153, 3 p. Introduction Recent observations of change within the ice sheet of the Amundsen Sea Embayment (ASE) of the West Antarctic Ice Sheet (Figure 1) have focused attention within the glaciological community on this remote and relatively little- studied region. Over the past decades, the grounding lines of both Pine Island Glacier (Rignot, 1998) and Thwaites Glacier (Rignot, 2001) have retreated, and ice in both catchments has thinned (Wingham et al., 1998; Shepherd et al., 2002). Pine Island Glacier has accelerated (Rignot et al., 2002) while Thwaites Glacier has likely widened (Rignot et al., 2002). Ice flux estimates show that ASE glaciers are significantly out of balance, with Thwaites having the highest flux and contributing a mass imbalance of -36 ± 7 km3/yr (Rignot et al., 2004). These observations imply a recent dynamical change, possibly due to the warming of adjacent ocean water (Payne et al., 2004; Shepherd et al., 2004). The potential instability of the ASE with continued grounding line retreat (Hughes, 1981; Stuiver et al., 1981) and its ice equivalent to ~ 1.5 meters of global sea level rise make it particularly important for predicting future sea level rise that may result from climate change. Ice sheet models constrained by accurate and well-sampled sub-ice topography are needed to understand the possible response of the ice sheet to such changes (e.g., Vieli and Payne, 2005). However, data from early radar sounding studies (Drewry, 1983) were sparse (Fig. 1c) and only sufficient to reveal the general morphology of the region. In order to begin to remedy this significant lack of observational data, a collaborative US/UK aerogeophysical campaign surveyed the ASE during the austral summer of 2004/2005 using two Twin Otter aircraft operating from two bases. Measurements included surface elevations, ice thickness, subglacial bed elevations, gravity anomalies and magnetic anomalies. This presentation focuses on new aeromagnetic results for the Thwaites, Smith, Kohler, Pope, and Haynes Glacier catchments obtained primarily by an aircraft configured and operated by the University of Texas Institute for Geophysics (UTIG). Geological Setting Prior to the recent airborne work in the ASE, early ground traverses and sparse airborne radar sounding profiles combined with limited outcrop studies formed the foundation for our understanding of the subglacial geologic framework for the glaciers of the ASE (Dalziel and Elliot, 1982). The interior drainage basins of both Thwaites and Pine Island Glacier are dominated by the Byrd Subglacial Basin, which is thought to be the eastern extension of the West Antarctic Rift System (Behrendt et al., 1992). It is thought that this rift system was formed by Cretaceous stretching with possible reactivation in the Cenozoic (Dalziel and Lawver, 2001). The divide between the Ross and Amundsen Sea Embayments lies over what was thought to be a sinuous ridge bisecting the Byrd Subglacial Basin (Jankowski and Drewry, 1981). More recent work indicated that the ridge is actually a sequence of disconnected volcanic highlands possibly characterized by recent volcanism (Behrendt et al., 1998; Morse et al., 2001). Flanking the rift to the south is the Ellsworth-Whitmore crustal block (EWB) with Marie Byrd Land (MBL) forming the rift flank on the north and west beneath the TG interior drainage. The Thurston Island (TI) crustal block borders the BSB to the north and east where it forms part of the geologic framework for the PIG drainage. 10th International Symposium on Antarctic Earth Sciences Figure 1. Coverage of UTIG-BAS aerogeophysical surveys superimposed on (a) balance velocity of Bamber et al. (2000) with surface elevation contours, (b) tectonic setting indicated with BEDMAP bed topography (Lythe et al., 2000) and crustal boundaries (Dalziel and Lawver, 2001) (c) pre-existing data coverage. Pine Island Glacier basin (PIG) and Thwaites Glacier Basin (TGB) indicate the glacier catchments, outlined in yellow., BSB = Byrd Subglacial Basin; MBL = Marie Byrd Land block; TI = Thurston Island block; EWB = Ellsworth Whitmore block. Red triangles denote identified volcanoes. Aerogeophysical survey Data were acquired on a 15 x 15 km grid over most of the Thwaites, Smith, Kohler, Pope and Haynes Glacier catchments, augmented by seven along-flow profiles of the major trunks and tributaries (Fig. 2). Over 43,500 line-km of geophysical data were collected by UTIG in 77 survey flights over 7 weeks, using Thwaites Camp (78.5ºS, 118ºW) as the primary base and Pine Island Camp (77.57ºS, 95.93ºW) as a secondary base (Fig. 1). Data were also acquired over the TG catchment on five flights by the British Antarctic Survey. Due to the need for minimal altitude changes which can disrupt airborne gravity measurements, the Thwaites survey area was subdivided into blocks of constant elevation. Typical survey clearance was 500 meters above the surface. Geophysical instrumentation included UTIG's 8-kilowatt High- Capability Radar Sounder (HICARS) system (Peters et al., 2005) which was configured for a chirped (52.5 – 67.5 MHz) pulse, a Reigl laser altimeter, a LaCoste & Romberg Air-Sea II gravity meter, and a Geometrics 823 airborne magnetometer. Data processing Radar data were processed and interpreted according to the method described in Blankenship et al. (2001) for bed and surface returns. A 5-km gridded data set compilation for the ASE including both the UTIG and BAS results were released in 2006 (Holt et al., 2006; Vaughan et al., 2006) and are available at the NSIDC website (http://nsidc.org/data/nsidc-0292.html). Airborne gravity results from Figure 2. Flight lines over the ASE from the the Thwaites catchment are described in Diehl et al. (this volume) and UTIG-BAS joint survey of 2004-05. are used in conjunction with the aeromagnetic data to constrain the Previous UTIG surveys shown in light blue. distribution of sedimentary basins. The aeromagnetic data is undergoing standard data reduction 2 Holt et al. : New aeromagnetic results from the Thwaites glacier catchment, West Antarctica processing. In order to correct for diurnal variations of the geomagnetic field, magnetometers were operated at each of the two base stations. Data quality appears to be excellent. Initial results Subglacial topography results (Holt et al., 2006) show that overall, the Thwaites Glacier catchment is underlain by a broad basin fed by a set of large valleys in a radiating pattern. This extends into the "sinuous ridge" in the divide region which is heavily dissected by glacial valleys. This indicates the dominance of erosional and depositional processes over tectonic features in the recent evolution of the landscape. This morphology contrasts with that of Pine Island Glacier whose main trunk is underlain by a narrow, deep, and linear trough suggesting tectonic control. However, certain features in the magnetic anomaly data over the Thwaites catchment show a strong tectonic influence, including the Marie Byrd Land crustal block boundary to the west of the main Thwaites trunk. Within the broad zone of the Thwaites Glacier trunk, correspondence of magnetic anomalies with topographic features oriented transverse to glacier flow, including a bedrock sill near the grounding line, indicates that there may be important structural controls on future grounding line retreat dictated by geological processes. Acknowledgments. This work was supported by National Science Foundation grant OPP-0230197, the Jackson School of Geosciences at the University of Texas, the G. Unger Vetlesen Foundation, and the British Antarctic Survey. F. Ferraccioli acknowledges support and funding from the Long-Term Survey and Monitoring Programme of the Geological Sciences Division (BAS). References Bamber, J.L., D.G. Vaughan, and I. Joughin (2000), Widesread complex flow in the interior of the Antarctic ice sheet, Science, 287, 1248-1250. Behrendt, J.C., C.A. Finn, D.D. Blankenship, and R.E. Bell (1998), Aeromagnetic evidence for a volcanic caldera(?) complex beneath the divide of the West Antarctic Ice Sheet, Geophysical Research Letters, 25 (23), 4385-4388. Behrendt, J.C., W. LeMaasurier, and A.K. Cooper (1992), The West Antarctic rift system-a propagating rift "captured" by a mantle plume?, in Recent Progress in Antarctic Earth Science, edited by Y.Y.e.