GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L20503, doi:10.1029/2011GL049026, 2011 Progressive unpinning of Thwaites Glacier from newly identified offshore ridge: Constraints from aerogravity K. J. Tinto1 and R. E. Bell1 Received 22 July 2011; revised 6 September 2011; accepted 21 September 2011; published 26 October 2011. [1] A new bathymetric model from the Thwaites Glacier bathymetry to be modeled from gravity observations for the region based on IceBridge airborne gravity data defines first time. morphologic features that exert key controls on the evolu- tion of the ice flow. A prominent ridge with two distinct 2. Methods peaks has been identified 40 km in front of the present‐ day grounding line, undulating between 300–700 m below [4] Operation IceBridge is a multiyear NASA project sea level with an average relief of 700 m. Presently, the bridging the gap between ICESat missions by conducting Thwaites ice shelf is pinned on the eastern peak. More airborne geophysical surveys in Antarctica and Greenland. extensive pinning in the past would have restricted flow One objective of IceBridge is to map previously unsurveyed of floating ice across the full width of the Thwaites Glacier regions using ice penetrating radar and gravity measure- ments. The data presented here were acquired by Operation system. At present thinning rates, ice would have lost contact – with the western part of the ridge between 55–150 years ago, IceBridge during October November 2009. allowing unconfined flow of floating ice and contributing [5] Data were acquired using the Sander Geophysics, to the present‐day mass imbalance of Thwaites Glacier. Airborne Inertially Referenced Gravimeter (AIRGrav) on board NASA’s DC8 aircraft, flying a draped survey at The bathymetric model also reveals a 1200 m deep trough ∼ beneath a bight in the grounding line where the glacier is 500 m above ground level [Cochran and Bell, 2009]. The moving the fastest. This newly defined trough marks the main survey over Thwaites Glacier was flown as a series of lowest topographic pathway to the Byrd Subglacial Basin, parallel lines, 10 km apart, approximately perpendicular to and the most likely path for future grounding line retreat. the grounding line (Figure 2). Data with high horizontal Citation: Tinto, K. J., and R. E. Bell (2011), Progressive unpin- accelerations due to aircraft maneuvers were excluded from the dataset. Free‐air anomalies were filtered with a 70 s full ning of Thwaites Glacier from newly identified offshore ridge: ∼ ‐ Constraints from aerogravity, Geophys. Res. Lett., 38, L20503, wavelength filter, resulting in 4.9 km half wavelength doi:10.1029/2011GL049026. resolution for a typical flying speed of 140 m/s. [6] The ice surface elevation was measured with the NASA Airborne Topographic Mapper (ATM) laser [Krabill, 1. Introduction 2009], which is capable of measuring elevations to deci- [2] The glaciers flowing into Pine Island Bay drain ∼20% meter accuracy [Krabill et al., 2002]. Ice thickness was of the West Antarctic Ice Sheet, and are collectively losing provided by a radar from the University of Kansas Center mass at a rate of 90 Gt/yr [Rignot et al., 2008]. Because their for Remote Sensing of Ice Sheets (CReSIS), the Multi- beds slope downward inland [Holt et al., 2006; Vaughan channel Coherent Radar Depth Sounder (MCoRDS) [Allen, et al., 2006] (Figure 1) these glaciers are considered to be at 2009]. Average crossover error for ice thickness measure- risk of unstable grounding line retreat and potential sites of ments near the grounding line was 25 m (J. Paden, personal ice sheet collapse [Hughes, 1981]. communication, 2011). A 1% error in the dielectric constant [3] Satellite observations since 1992 show that Pine Island of ice of contributes an uncertainty of 0.5% of the total ice Glacier has been accelerating, while Thwaites Glacier has thickness, which is 1000 m at the grounding line. Combined maintained a consistently high velocity and negative mass radar errors across the survey region are ∼30 m. As the laser balance, with some widening of the fast flow area [Rignot, and radar data were acquired on the same survey flights as 2006]. The event that triggered the Thwaites Glacier nega- the gravity data, these data are coincident in time and space, tive mass balance must have occurred prior to 1992 [Rignot with along‐track resolution equal to or better than that of the et al., 2008]. This difference in behavior points to the gravity survey. importance of local morphological controls on the glacial [7] Bathymetry modeling from gravity was conducted in history of the Amundsen Sea. Knowledge of the bathymetry 2D along individual flight lines with Geosoft GMSys soft- in front of the present‐day grounding line is critical to ware using methods of Talwani et al. [1959]. A four‐body 3 3 understanding the retreat of the grounding line in the past. forward model, of air (0 g/cm ), ice (0.915 g/cm ), seawater 3 3 Operation IceBridge surveys over the area allow this (1.028 g/cm ) and rock (2.67 g/cm ), was used (Figure 2c). Rock density was constrained by local geology, which comprises a crystalline basement of granodiorites and gneisses. Where homogeneous geology did not fit known 3 1 bathymetric constraints, denser rock (3.0 g/cm ) was mod- Lamont-Doherty Earth Observatory, Earth Institute at Columbia University, Palisades, New York, USA. eled. Discrete volcanic centers that outcrop above the ice sheet in the survey area have a bimodal petrology of tra- 3 3 Copyright 2011 by the American Geophysical Union. chytes (2.73 g/cm ) and basalts (∼3.0 g/cm )[LeMasurier 0094‐8276/11/2011GL049026 L20503 1of6 L20503 TINTO AND BELL: THWAITES OFFSHORE RIDGE L20503 Figure 1. (a) Location of profile lines, superimposed on MODIS Mosaic of Antarctica image [Haran et al., 2006], red line is 1996 grounding line of Rignot et al. [2011], black box shows survey area. (b) Cross section line across the grounding zone of Thwaites Glacier (x‐x′). Upper profile shows velocity [Rignot et al., 2008], lower profile shows ice surface, base, and bed surface. (c) Cross section line along flow line from Thwaites Glacier (y‐y′). and Thompson, 1990]. Sedimentary rocks were not included model was pinned to bathymetry known from marine surveys in these models, as supported by the absence of sedimentary at the northeastern end of line 1021.16, which was used as a sequences in marine seismic sections from the proximal tie line to ensure a self‐consistent model. portions of the Amundsen Sea [e.g., Lowe and Anderson, 2002]. 3. Results [8] Ice surface and base were established by laser and radar data, respectively, each filtered to the same 70 s 3.1. Bathymetry Model and Errors wavelength as the gravity data. Elevations and modeled [9] The observed gravity anomaly (Figure 2b) ranges depths are reported with respect to the WGS84 ellipsoid. The between −53 and 13 mGal. The largest anomaly is an Figure 2. (a) Position of IceBridge 2009 Thwaites Glacier survey lines, superimposed on interpolated bathymetry of Nitsche et al. [2007]. Black line is 1996 grounding line from Rignot et al. [2011]. (b) Grid of free air gravity anomaly from survey. (c) The 2D model along survey line 1021.7. Calculated gravity is shown for homogeneous bedrock of density 2.67 g/cm3. Dotted line on model shows distribution of 3.0 g/cm3 rock required to fill residuals between calculated and observed gravity anomaly. 2of6 L20503 TINTO AND BELL: THWAITES OFFSHORE RIDGE L20503 Figure 3. (a) The new bathymetry model from IceBridge data combined with bathymetry from Nitsche et al. [2007], Jenkins et al. [2010], Holt et al. [2006] and Vaughan et al. [2006]. (b) Close up of model with flow vectors from Rignot [2001] superimposed. (c) Cartoon of key features of the bathymetric model and flow regime. Ridge defined by 800 m depth contour, trough defined by 1000 m depth contour, 1 and 2 mark eastern and western peaks of ridge respectively. Black line is 1996 grounding line from Rignot et al. [2011], red line is 2009 grounding line estimated from IceBridge data. (d) Close up of grounding line at the bathymetric trough. Red zone shows error envelope of grounding line estimates, blue shows ice that is ungrounded in all estimates. elongate gravity high with values of −16 to 13 mgal, ∼40 km 2001]. A residual gravity anomaly over the eastern peak offshore from the present‐day grounding line. A second can be accounted for by the presence of denser rock on the high, ranging from −10 to 10 mgal, exists onshore of the ridge (Figure 2c), indicating a likely volcanic origin of these grounding line, at the southernmost end of the survey lines. ridges. The bathymetry model also shows a trough, on the [10] The gravity‐based bathymetric model (Figure 3) has northern flank of the ridge, reaching depths of 1200 m. a similar shape to the observed gravity except where density [12] The southern gravity high was not reflected in bed differences have been included in order to fit known con- topography from laser and radar on grounded ice, indicating straints. The bathymetric surface was constrained to always a dense body in the bedrock along the present‐day be below the base of the ice, and bed geology was consid- grounding line. Gravity lows at the southern ends of lines ered homogeneous (density 2.67 g/cm3) unless denser rock 1021.8, 1021.9 and 1021.10 (Figure 2b) are modeled as a was required to account for residual anomalies. deep (1200 m) trough (labeled on Figure 3c), connected to a [11] The prominent feature of the gravity‐based bathy- 1000 m deep, ice filled channel leading toward the Byrd metric model is a 15 km wide, 700 m relief ridge trending Subglacial Basin.