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Century-Scale Discharge Stagnation and Reactivation of the Ross Ice Streams, West Antarctica C

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Century-Scale Discharge Stagnation and Reactivation of the Ross Ice Streams, West Antarctica C JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, F03S27, doi:10.1029/2006JF000603, 2007 Click Here for Full Article Century-scale discharge stagnation and reactivation of the Ross ice streams, West Antarctica C. Hulbe1 and M. Fahnestock2 Received 21 June 2006; revised 31 October 2006; accepted 10 January 2007; published 23 May 2007. [1] Flow features on the surface of the Ross Ice Shelf, West Antarctica, record two episodes of ice stream stagnation and reactivation within the last 1000 years. We document these events using maps of streaklines emerging from individual ice streams made using visible band imagery, together with numerical models of ice shelf flow. Forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration. According to our analysis, Whillans Ice Stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and MacAyeal Ice Stream either stopped or slowed significantly between 800 and 700 years ago, restarting about 150 years later. Until now, ice-stream scenarios emphasized runaway retreat or stagnation on millennial timescales. Here we identify a new scenario: century-scale stagnation and reactivation cycles, as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains. This introduces uncertainty into predictions for future sea-level withdrawals by the West Antarctic Ice Sheet, which are based in part on recent slowing of Whillans Ice Stream and the stagnant condition of Kamb Ice Stream. Citation: Hulbe, C., and M. A. Fahnestock (2007), Century-scale discharge stagnation and reactivation of the Ross ice streams, West Antarctica, J. Geophys. Res., 112, F03S27, doi:10.1029/2006JF000603. 1. Introduction [4] The temporal character of grounding line retreat since the LGM is also of interest. Seismically imaged sedimentary [2] The West Antarctic Ice Sheet (WAIS) has the poten- wedges on the sea floor seaward of the RIS front have been tial for rapid and significant change in grounding line interpreted as grounding line features deposited at sites position due to its marine character and fast-flowing ice where a retreating grounding line paused and held a steady streams [Alley and Whillans, 1991]. Such change is of position before resuming retreat [Mosola and Anderson, interest in part because it would affect sea level immedi- 2003]. Deformed ice–internal stratigraphy upstream of the ately. The Ross Ice Shelf (RIS) grounding line has retreated present-day Kamb Ice Stream (KIS, formerly named C) more than 1000 km, from the continental shelf edge to its grounding line suggests that the grounding line of this present location, in the time since the Last Glacial Maximum currently quiescent outlet was upstream of its present (LGM) [Conway et al., 1999; Shipp et al., 1999]. Of interest location within the last few hundred years [Catania et al., now is whether or not that retreat will continue, and if so, at 2006]. The implication of these observations is that ground- what rate. ing line retreat since the LGM has been episodic, with [3] Timescales for grounding line migration and varia- transient stable stands and local readvance events. This tions in ice-stream discharge in the Ross sea sector of the behavior must have been tied to variability in the volume WAIS are not well known. Observational data are rare but of ice discharged by ice streams into the shelf. some general statements may be made. Ice streams were [5] The relationship between ice stream discharge and present when the grounding line retreated from the conti- grounding line position is complicated. In simplest terms, nental shelf edge [Mosola and Anderson, 2003]; thus it the location of the grounding line depends on sea level, bed seems likely that ice streams have persisted since that time. elevation, and ice thickness (or the volume of ice dis- The broad pattern of retreat since the LGM is early retreat to charge). All else being equal, an increase in volume flux Roosevelt Island in the east and then a gradual ‘‘swinging would lead to grounding line advance while a decrease gate’’ retreat to the present location with the island as a would lead to grounding line retreat. Changes in strain rates hinge point [Conway et al., 1999]. that may accompany changes in volume flux modify this simple relationship. Acceleration of the flow accompanied by increased longitudinal strain rates across an ice stream to 1Department of Geology, Portland State University, Portland, Oregon, USA. ice shelf transition would thin the ice and perhaps yield 2Institute for the Study of Earth, Oceans, and Space, University of New grounding line retreat, as in the classic marine ice sheet Hampshire, Durham, New Hampshire, USA. instability hypothesis [Thomas and Bentley, 1978; Hulbe and Payne, 2001]. Complications may arise owing to the Copyright 2007 by the American Geophysical Union. 0148-0227/07/2006JF000603$09.00 F03S27 1 of 11 F03S27 HULBE AND FAHNESTOCK: ICE STREAMS STOP AND START F03S27 Figure 1. Composite MODIS image of the Ross Ice Shelf. development of sticky spots in the discharge path [Hulbe 2001]. Models designed to study ice stream discharge and Fahnestock, 2004]. cycles (slowing, stagnation, and reactivation) produce mil- [6] Evidence for both past and ongoing variation in ice lennial or longer timescales [MacAyeal, 1992; Payne, 1995; stream discharge is found in many locations in the Ross Sea Parizek et al., 2003]. Quantitative interpretations of present- sector of the WAIS, for example, the stagnation of KIS day WAIS conditions [Bougamont et al., 2003; Joughin and about 150 years ago [Retzlaff and Bentley, 1993], the Tulaczyk, 2002; Vogel et al., 2005] generally favor ice stagnation of Siple Ice Stream (a distributary of Kamb) stream deceleration and relatively long timescales for reac- 420 ± 60 years ago (Ben Smith, personal communication tivation. The prediction by Vogel et al. [2005] that the now- 2005), and recent slowing at the downstream end of quiescent KIS may soon reactivate, based on borehole Whillans Ice Stream (WIS, formerly B) [Joughin and observations of subglacial conditions, is an exception to Tulaczyk, 2002]. Together, these observations suggest a conventional views. negative trend in the volume of ice being discharged from [8] Flow features in the surface of the RIS provide an the WAIS. One conclusion that may be drawn from the opportunity to extend the high-resolution observational recent trend is that the ice sheet is nearing the end of a long record of ice stream and grounding line variability beyond downwasting phase and the future contribution to sea level the instrumental era [Fahnestock et al., 2000]. Because the will be negative. However, these are as yet short-lived flow of the shelf must adjust to the volume flux of ice events in a system that has persisted for tens of thousands discharged into it, physical tracers within the ice form a of years. Quantification of variability in ice stream dis- time-integrated record of past events [MacAyeal et al., charge will help place the recent events in a useful predic- 1988]. The RIS record extends back about 1000 years, the tive context. approximate age of ice now at the shelf front. Hulbe and [7] Numerical models of the coupled ice sheet and ice Fahnestock [2004] studied features associated with the shelf system have been used to study variability in ice sheet lightly grounded, low-slope ice plains across which active discharge. Unfortunately, models of the whole ice sheet ice streams make the transition from a grounded to floating system are in general too coarse to resolve many details of condition, and concluded that grounding line migration is grounding line migration and often do not include realistic episodic and driven by thermal processes near the base of transitions from grounded to floating ice [Hulbe and Payne, the fast-flowing ice. Here we direct our attention to recon- 2 of 11 F03S27 HULBE AND FAHNESTOCK: ICE STREAMS STOP AND START F03S27 Figure 2. Ice provenance map developed by tracing streaklines and relict ice stream margins upstream to the grounding line. Digitizations of major streakline packages are drawn on the map. The dashed line on Kamb Ice Stream is the trace to which the stream’s northern margin jumped to create the Duckfoot feature. structing the history of ice stream discharge implied by flow [10] The subtle surface morphology present on the RIS features now visible in the ice shelf. can be observed, to a limited extent, in many types of visible-band satellite imagery [Bindschadler, 1993; Casassa 2. Flow Features in the Ice Shelf Surface et al., 1991; Williams and Ferrigno, 1988]. Enhancements made using multiple images can greatly improve our ability [9] Flow features observed on the surfaces of floating ice to see such features [Scambos and Fahnestock, 1999; shelves chronicle kinematic and dynamic events over the Fahnestock et al., 2000]. Here we use a multiple-image lifetimes of the features [MacAyeal et al., 1988]. Three composite from 250 m pixel size red-light (band 1) images broad classes of flow feature are observed throughout the from the MODIS sensor on NASA’s Terra platform. The RIS (Figures 1 and 2): subtle ridges in surface elevation that MODIS imagery was obtained from the NASA Goddard in gross scale trend downstream (streaklines); trains of Space Flight Center Earth Sciences Distributed Active shear-margin crevasses that advect downstream from their Archive Center. At any location, the composite includes a sites of formation; and transverse crevasses of multiple few to ten images spanning a few-week period in November possible origins. Streaklines originate in the grounded ice and December 2001, using multiple local solar times from sheet, where ice flows over a basal disturbance such as a 1000 GMT to 2200 GMT. This range of image acquisition sticky spot, roches moutonnee, or other obstacle at the bed times provides a range of sun azimuths in the images used.
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