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CC Attribution 3.0 License. Paleo ice flow and subglacial meltwater dynamics in Pine Island Bay, West Antarctica F. O. Nitsche1, K. Gohl2, R. D. Larter3, C.-D. Hillenbrand3, G. Kuhn2, J. A. Smith3, S. Jacobs1, J. B. Anderson4, and M. Jakobsson5 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA 2Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany 3British Antarctic Survey, Cambridge, UK 4Department of Earth Sciences, Rice University, Houston, Texas, USA 5Department of Geological Sciences, Stockholm University, Stockholm, Sweden Correspondence to: F. O. Nitsche ([email protected]) Received: 9 August 2012 – Published in The Cryosphere Discuss.: 4 October 2012 Revised: 7 January 2013 – Accepted: 11 January 2013 – Published: 8 February 2013 Abstract. Increasing evidence for an elaborate subglacial 1 Introduction drainage network underneath modern Antarctic ice sheets suggests that basal meltwater has an important influence on Response of the Antarctic ice sheets to changing climate con- ice stream flow. Swath bathymetry surveys from previously ditions is one of the largest uncertainties in the prediction of glaciated continental margins display morphological features future sea-level (IPCC, 2007). Much of that response will de- indicative of subglacial meltwater flow in inner shelf areas of pend on the behaviour of large ice streams, the main conduits some paleo ice stream troughs. Over the last few years sev- of ice flux from the inner portions of the ice sheets to the eral expeditions to the eastern Amundsen Sea embayment coast (Bentley, 1987; Bennett, 2003). Understanding how ice (West Antarctica) have investigated the paleo ice streams streams behaved in the past can improve predictions of future that extended from the Pine Island and Thwaites glaciers. A changes (e.g., Stokes and Clark, 2001; Anderson et al., 2002; compilation of high-resolution swath bathymetry data from Vaughan and Arthern, 2007; Livingstone et al., 2012). inner Pine Island Bay reveals details of a rough seabed to- The West Antarctic Ice Sheet (WAIS) is considered espe- pography including several deep channels that connect a se- cially vulnerable, because the WAIS is mainly grounded be- ries of basins. This complex basin and channel network is low sea level (Hughes, 1973), with ice shelf margins exposed indicative of meltwater flow beneath the paleo-Pine Island to “warm” Southern Ocean water masses (Joughin and Al- and Thwaites ice streams, along with substantial subglacial ley, 2011). About 25–35 % of the WAIS is currently draining water inflow from the east. This meltwater could have en- into the Amundsen Sea, mostly through the Pine Island and hanced ice flow over the rough bedrock topography. Meltwa- Thwaites glaciers (Drewry et al., 1982; Rignot et al., 2008). ter features diminish with the onset of linear features north of These ice streams are potential weak points in the ice sheet the basins. Similar features have previously been observed in because they occupy troughs that become steadily deeper several other areas, including the Dotson-Getz Trough (west- towards the WAIS interior and are buttressed by relatively ern Amundsen Sea embayment) and Marguerite Bay (SW small ice shelves (Hughes, 1973; Vaughan et al., 2006). The- Antarctic Peninsula), suggesting that these features may be oretical studies have concluded that ice grounding lines are widespread around the Antarctic margin and that subglacial unstable on such reverse gradients and, therefore, once re- meltwater drainage played a major role in past ice-sheet dy- treat starts it may proceed rapidly (Weertman, 1974; Schoof, namics. 2007), as recently observed under the Pine Island Ice Shelf (Jenkins et al., 2010). Published by Copernicus Publications on behalf of the European Geosciences Union. 250 F. O. Nitsche et al.: Paleo ice flow and subglacial meltwater dynamics Interest in the Amundsen Sea sector has increased since studies showed that Pine Island and Thwaites Glaciers are presently thinning significantly (Wingham et al., 1998; Shep- herd et al., 2001) and that their flow velocity has increased up to 4000 m yr−1 (Rignot and Thomas, 2002; Joughin et al., 2010). These changes appear to be caused by strong melt- ing under their floating extensions, driven by the intrusion of relatively warm Circumpolar Deep Water (CDW) onto the continental shelf (Jacobs et al., 1996; Jenkins et al., 1997; Hellmer et al., 1998; Rignot and Jacobs, 2002; Shepherd et al., 2004; Jacobs et al., 2011; Pritchard et al., 2012). As a consequence, these ice streams now account for an ice loss of ∼ 50–85 Gt yr−1 (Rignot et al., 2008; Pritchard et al., 2009) and already contribute to current sea-level rise (Shepherd and Wingham, 2007; Rignot et al., 2011). While those observations document ice stream change over the last few decades, detailed marine geological stud- Fig. 1. Bathymetry map of the Amundsen Sea shelf with the red rectangle marking the location of the Pine Island Bay study area in ies of the previously glaciated seafloor have established a Figs. 2 and 4 (based on Nitsche et al., 2007). PIT marks the Pine framework of past ice sheet extent, flow pattern, and ground- Island Trough system and GT the Dotson-Getz Trough System. ing line retreat in the Amundsen Sea sector since the Last Glacial Maximum (LGM), defined as the time interval ∼ 23– 19 kyr before present (BP) in the Southern Hemisphere (Liv- Published bathymetry data from Pine Island Bay revealed a ingstone et al., 2012). The regional bathymetry shows a complex seafloor dominated by deep bedrock basins with lo- large cross-shelf trough system that extends from the present cal patches of a generally thin sedimentary cover (Fig. 2b) Thwaites and Pine Island ice shelves to the outer continen- (Kellogg and Kellogg, 1987b; Lowe and Anderson, 2002). tal shelf (Fig. 1; Nitsche et al., 2007). Detailed studies of Lowe and Anderson (2002) identified a series of detailed this trough system revealed that it was occupied by a paleo morphological features, which they interpreted as gouges, p- ice stream and documented an episodic retreat soon after the forms and drumlins formed by glacial erosion (their zones LGM on the outer and middle continental shelf (Graham et 1, 2; see Fig. 2b) and mega-scale glacial lineations formed al., 2010; Jakobsson et al., 2011, 2012). by ice streams overriding sediment deposits (zone 3). The The grounding line of the paleo-Pine Island Ice Stream change in seafloor morphology was linked to changes in shifted from the outer shelf to the central shelf by subglacial substrate from crystalline bedrock to sediment −1 ∼ 16 400 cal yr BP, followed by another landward shift that near the inner to middle shelf transition (Wellner et al., left the central shelf covered by an ice shelf from ∼ 12 300 2001; Lowe and Anderson, 2002). These data also led to −1 to ∼ 10 600 cal yr BP (Lowe and Anderson, 2002; Kirsh- the first identification of subglacial meltwater channels on ner et al., 2012). An episode of ice shelf collapse believed to the Antarctic continental shelf (Lowe and Anderson, 2003). have been triggered by warm deep water incursion onto the Both the presence of subglacial meltwater and the variability shelf prompted rapid grounding line retreat towards the inner of subglacial substrate could have significantly influenced ice shelf (Kirshner et al., 2012). stream behaviour, as lubricated beds and sedimentary sub- The retreat history and dynamics of the paleo ice stream strate tend to allow faster ice flow, whereas dryer beds and in inner Pine Island Bay is less well understood. New radio- exposed bedrock may result in higher bed friction (Bennett, carbon dates from sediment cores located ∼ 93 km from the 2003). A detailed understanding of the origin of the sub- modern grounding line of Thwaites Glacier and ∼ 112 km glacial features, configurations and substrates could, thus, from that of Pine Island Glacier suggest minimum ages for provide critical information on the dynamics of these ice −1 grounded ice retreat of ∼ 10 350 and ∼ 11 660 cal yr BP, streams. respectively (Hillenbrand et al., 2013). Cosmogenic surface Here we present extensive new high-resolution swath exposure ages on erratics from the flanks of Pine Island bathymetry with almost complete coverage of inner Pine Is- Glacier in the Hudson Mountains yielded a record of pro- land Bay (Figs. 1, 2). These data reveal networks of channels gressive ice thinning for the last ∼ 14.5 ka BP that is consis- eroded by subglacial meltwater, with possible implications tent with the marine record, although the few available dates for ice flow mechanisms and subglacial water sources of the do not provide much detail about variations in thinning rates Pine Island and Thwaites Ice Streams.