Inversion of IceBridge gravity data for continental shelf bathymetry beneath the Larsen Ice Shelf, Antarctica

Abstract

A possible cause for accelerated thinning and break-up of floating marine ice shelves is warming of the water in the cavity below the ice shelf. Accurate bathymetry beneath large ice shelves is crucial for developing models of the ocean circulation in the sub-ice cavities. A grid of free-air gravity data over the floating Larsen C ice shelf collected during the IceBridge 2009 Antarctic campaign was utilized to develop the first bathymetry model of the underlying continental shelf. Independent control on the continental shelf geologic structures from marine surveys was used to constrain the inversion. Depths on the continental shelf beneath the ice shelf estimated from the inversion generally range from about 350 to 650 m, but vary from <300 to >1000 m. Localized overdeepenings, 20-30 km long and 900-1000 m deep, are located in inlets just seaward of the grounding line. Submarine valleys extending seaward from the overdeepenings coalesce into two broad troughs that extend to the seaward limit of the ice shelf and appear to extend to the edge of the continental shelf. The troughs are generally at a depth of 550-700 m although the southernmost mapped trough deepens to over 1000 m near the edge of the ice shelf just south of 68° S. The combination of the newly determined bathymetry with published ice-draft determinations based on laser altimetry and radar data defines the geometry of the water-filled cavity. These newly imaged troughs provide a conduit for water to traverse the continental shelf and interact with the overlying Larsen C ice shelf and the grounding lines of the outlet glaciers.

Seabed topography beneath Larsen C Ice Shelf from seismic soundings

“…The bathymetry model presented by Cochran and Bell (2012) exhibits over-deepening along the grounding line and also two broad troughs extending from the grounding line to the ice front. These features could provide conduits for oceanic circulation beneath the entire ice shelf.…”

“…These features could provide conduits for oceanic circulation beneath the entire ice shelf. Mueller et al (2012) modelled ice-ocean interaction for Larsen C Ice Shelf, predicting basal melt rates using a numerical model with a bathymetry that appears similar to that of Cochran and Bell (2012), and also variants of this bathymetry. They demonstrated the sensitivity of the predicted melt rates to cavity geometry: localised narrowing of the cavity results in significantly elevated melt rates because of local intensification of tidal flow.…”

“…Tinto and Bell (2011) modelled bathymetry beneath Thwaites Glacier in the Amundsen Sea region using known bathymetry to constrain inhomogeneous geology. Of direct relevance to this study, Cochran and Bell (2012) used Operation IceBridge free-air gravity anomaly (FAA) data over the Larsen Ice Shelf. The Parker-Oldenburg technique (Oldenburg, 1974) used in that study relates the observed free-air anomaly to topographic relief superimposed on a horizontal interface.…”

“…The sensitivity of the resulting bathymetry model to both these variables can be tested. Cochran and Bell (2012) used previous seismic measurements (King et al, 1993;Jarvis and King, 1995) north of Francis Island (Fig. 1) to constrain the mean depth of the bathymetry model beneath Larsen C. A density contrast of 1.67 g cm −3 was assumed based on the regional geological framework.…”

“…In order that the Cochran and Bell (2012) bathymetry map beneath Larsen C could be validated for use in oceanographic modelling, a seismic survey was undertaken in 2012/13 to obtain point measurements of ice and sub-ice shelf water cavity thickness (Fig. 2).…”