Using ice-flow models to evaluate potential sites of million year-old ice in Antarctica

Abstract

Abstract. Finding suitable potential sites for an undisturbed record of million-year old ice in Antarctica requires slowmoving ice (preferably an ice divide) and basal conditions that are not disturbed by large topographic variations. Furthermore, ice should be thick and cold basal conditions should prevail, since basal melting would destroy the bottom layers. However, thick ice (needed to resolve the signal at sufficient high resolution) increases basal temperatures, which is a conflicting condition for finding a suitable drill site. In addition, slow moving areas in the center of ice sheets are also low-accumulation areas, and low accumulation reduces potential cooling of the ice through vertical advection. While boundary conditions such as ice thickness and accumulation rates are relatively well constrained, the major uncertainty in determining basal thermal conditions resides in the geothermal heat flow (GHF) underneath the ice sheet. We explore uncertainties in existing GHF data sets and their effect on basal temperatures of the Antarctic Ice Sheet, and propose an updated method based on Pattyn (2010) to improve existing GHF data sets in agreement with known basal temperatures and their gradients to reduce this uncertainty. Both complementary methods lead to a better comprehension of basal temperature sensitivity and a characterization of potential ice coring sites within these uncertainties. The combination of both modeling approaches show that the most likely oldest ice sites are situated near the divide areas (close to existing deep drilling sites, but in areas of smaller ice thickness) and across the Gamburtsev Subglacial Mountains.

Ice-flow reorganization within the East Antarctic Ice Sheet deep interior

“…Because direct observations of geothermal flux remain infrequent and logistically challenging, the distribution of subglacial lakes is used as a proxy for sufficiently high geothermal flux to maintain ice at the basal pressure melting point (e.g. Siegert & Dowdeswell 1996;van Liefferinge & Pattyn 2013). Here, we show that this assumption is complicated when the reorganization of ice dynamics is considered.…”

“…Conditions permitting basal melting under the present-day ice-sheet configuration were evaluated on the basis that melting may be driven by heat from either frictional sliding or basal heat flux. Present-day sliding within the SPL catchment is not expected under cold bed conditions (Price et al 2002;van Liefferinge & Pattyn 2013) and slow-flow regime (Aartsen et al 2013); basal heat flux is then the only heat source to drive melt. In the unlikely scenario that the entire present-day surface velocity was due solely to basal sliding and if basal shear stress was equal to the local driving stress (50 kPa), approximately 15 mW m −2 of frictional heating would be available at the ice-bed interface.…”

“…Identifying regions of former fast flow in the South Pole region has implications for a variety of ongoing investigations, including interpretation of ice-core climate records (Casey et al 2014), the search for recoverable million year old ice (e.g. Fischer et al 2013;van Liefferinge & Pattyn 2013) and possible expansion of the IceCube neutrino detector (Aartsen et al 2014).…”

How old is the ice beneath Dome A, Antarctica?

“…This anisotropic ice may be an order of magnitude "softer" against deformation in certain directions than ice with random fabric, and has an important influence on the age of ice in the lower 1/3 of the ice thickness (Martín and Gudmundsson, 2012;Seddik et al, 2011). Deep ice cores from sites such as Vostok and NorthGRIP in Greenland, which are not located on a true dome geometry, exhibit a girdle-type fabric pattern (Lipenkov et al, 1989;Montagnat et al, 2014), whereas deep ice in Dome C and the Greenland summit GRIP ice core exhibit single maximum; that is, the c axes are concentrated along the vertical direction (Thorsteinsson et al, 1997;Wang et al, 2003). At Dome F, which is perhaps the closest analogue to the Dome A region, a single maximum fabric dominated the bottom 1/3 of the ice core (Seddik et al, 2011).…”

“…Severinghaus, 2010;Van Liefferinge and Pattyn, 2013). The Gamburtsev subglacial mountains beneath Dome A were a major centre of ice-sheet nucleation during the Cenozoic (DeConto and Pollard, 2003;Sun et al, 2009), and hence potentially can provide ancient ice for paleoclimatic research.…”