An improved method for estimating water-mass ventilation age from radiocarbon data

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, F04023, doi:10.1029/2012JF002371, 2012 Ice flow sensitivity to geothermal heat flux of Pine Island Glacier, Antarctica E. Larour, 1 M. Morlighem, 2 H. Seroussi, 1 J. Schiermeier, 1 and E. Rignot 1,2 Received 9 February 2012; revised 20 September 2012; accepted 27 September 2012; published 16 November 2012. [ 1 ] Model projections of ice flow in a changing climate are dependent on model inputs such as surface elevation, bedrock position or surface temperatures, among others. Of all these inputs, geothermal heat flux is the one for which uncertainty is greatest. In the area of Pine Island Glacier, Antarctica, available data sets differ by up to a factor of 2.5. Here, we evaluate the impact of such uncertainty on ice flow, using sampling analyses based on the Latin-Hypercube method. First, we quantify the impact of geothermal heat flux errors on ice hardness, a thermal parameter that critically controls the magnitude of ice flow. Second, we quantify the impact of the same errors on mass balance, specifically on the mass flux advecting through thirteen fluxgates distributed across Pine Island Glacier. We contrast our results with similar uncertainties generated by errors in the specification of ice thickness. Model outputs indicate that geothermal heat flux errors yield uncertainties on ice hardness on the order of 5–7%, with maximum uncertainty reaching 15%. Resulting uncertainties in mass balance remain however below 1%. We discuss the uncertainty distribution and its relationship to the amount of heat available at the base of the ice sheet from friction, viscous and geothermal heating. We also show that comparatively, errors in ice thickness contribute more to model uncertainty than errors in geothermal heat flux, especially for fast-flowing ice streams. Citation: Larour, E., M. Morlighem, H. Seroussi, J. Schiermeier, and E. Rignot (2012), Ice flow sensitivity to geothermal heat flux of Pine Island Glacier, Antarctica, J. Geophys. Res., 117, F04023, doi:10.1029/2012JF002371. 1. Introduction [ 2 ] Ice flow models used to project the mass balance of Antarctica and Greenland on decadal timescales are limited by uncertainties in model inputs. Such inputs include the ice sheet geometry (surface elevation, bedrock position or ice thickness, and grounding-line position) and boundary con- ditions (geothermal heat flux and basal drag at the ice/bed interface as well as temperature at the surface). Traditionally, the view was that ice thickness and surface elevation are critical because of their role in the stress-balance equations [MacAyeal, 1989; Blatter, 1995; Pattyn, 2003]. However, some model inputs, such as geothermal heat flux at the base of the ice sheet are so ill-constrained as to justify the need to assess the impact such lack of knowledge might have on model outputs. [ 3 ] To specify geothermal heat flux in Antarctica, mode- lers rely on several data sets, among them: 1) the data set by Shapiro and Ritzwoller [2004], inferred using a global Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. Department of Earth System Science, University of California, Irvine, California, USA. Corresponding author: E. Larour, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109-8099, USA. (eric.larour@jpl.nasa.gov) ©2012. American Geophysical Union. All Rights Reserved. 0148-0227/12/2012JF002371 seismic model and 2) the data set by Maule et al. [2005], generated using satellite magnetic data. In the first data set, uncertainty is high, with a standard deviation ranging from 20 mW/m 2 in East-Antarctica to 90 mW/m 2 in West- Antarctica, particularly in the area of Pine Island Glacier (Figure 1). In some areas, the uncertainty is as high as the value of the geothermal heat flux itself. The second data set by Maule et al. [2005] is consistently lower across Antarctica, with relative differences to Shapiro and Ritzwoller [2004] ranging from 30% to 140%. [ 4 ] Other types of data sets have also been used, especially for long reconstructions of ice flow, such as in Ritz et al. [1997, 2001], Kerr and Huybrechts [1999], Hansen and Greve [1996], Takeda et al. [2002], or Pollard et al. [2005]. In these simulations however, geothermal heat flux is spatially uniform over long distances. In Antarctica, it ranges between 42 and 60 mW/m 2 in Ritz et al. [1997], 42 and 105 mW/m 2 in Hansen and Greve [1996], and 54.6 mW/m 2 (for the East Antarctic Ice Sheet, EAIS) and 65 mW/m 2 (for the West Antarctic Ice Sheet, WAIS) in Kerr and Huybrechts [1999]. For million-year paleo- reconstructions, Pollard et al. [2005] used among others a data set ranging from 55 mW/m 2 in the EAIS (except West of Victoria Land, where values reached 41 mW/m 2 ) to 60 mW/m 2 in the Trans-Antarctic mountains and 70 mW/m 2 in the WAIS. For almost all of the studies cited, one of the common findings was that geothermal heat flux does not have a strong influence on global ice sheet volume and extent, but rather on the pattern of basal melting rate. F04023 1 of 12