Ice-sheet grounding-zone wedges (GZWs) on high-latitude continental margins

Abstract

Please cite this article as: Batchelor, C.L., Dowdeswell, J.A., Ice-sheet groundingzone wedges (GZWs) on high-latitude continental margins, Marine Geology (2015), ABSTRACT Grounding-zone wedges (GZWs) are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line-source at the grounding zone of marine-terminating ice sheets during still-stands in ice-sheet retreat. GZWs form largely through the delivery of deforming subglacial sediments. The presence of GZWs in the geological record indicates an episodic style of ice retreat punctuated by still-stands in grounding-zone position. Moraine ridges and ice-proximal fans may also build up at the grounding zone during still-stands of the ice margin, but these require either considerable vertical accommodation space or sediment derived from point-sourced subglacial meltwater streams. By contrast, GZWs form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding-zone. An inventory of GZWs is compiled from available studies of bathymetric and acoustic data from high-latitude continental margins. The locations and dimensions of GZWs from the Arctic and Antarctic, alongside a synthesis of their key architectural and geomorphic characteristics, is presented. GZWs are only observed within cross-shelf troughs and major fjord systems, which are the A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT former locations of ice streams and fast-flowing outlet glaciers. Typical high-latitude GZWs are less than 15 km in along-flow direction and 15 to 100 m thick. GZWs possess a transparent to chaotic acoustic character, which reflects the delivery of diamictic subglacial debris. Many GZWs contain seaward-dipping reflections, which indicate sediment progradation and wedge-growth through continued delivery of basal sediments. GZWformation is inferred to require high rates of sediment delivery to a fast-flowing ice margin that is relatively stable for probably decades to centuries. Although the long-term stability of the grounding zone is controlled by ice-sheet mass balance, the precise location of any stillstands is influenced strongly by the geometry of the continental shelf. The majority of highlatitude GZWs occur at vertical or lateral pinning points, which encourage grounding-zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone.

How dynamic are ice-stream beds?

“…of the order of m a -1 , beneath thick, warmbased ice as manifested by polar ice streams. Observations of substantial till deposition at the grounding lines of contemporary and palaeo West Antarctic ice streams support this view (Anandakrishnan et al, 2007;Batchelor and Dowdeswell, 2015). The precision of our measurements, essentially defined by the vertical range resolution of the radar, means that the maximum 20 possible erosion rate that could go undetected along our profiles is 500 mm a -1.…”

Palaeo-ice stream pathways and retreat style in the easternmost Amundsen Sea Embayment, West Antarctica, revealed by combined multibeam bathymetric and seismic data

“…3), which add to the inventory of Antarctic GZWs recently compiled (Batchelor and Dowdeswell, 2015). The northernmost GZW (GZWa) is located WNW of Thurston Island directly north of 72°S, the central GZW (GZWb) is located west of the Demas Ice Tongue, and the southernmost GZW (GZWc) is located between King Peninsula and Burke Island at 73°S (Fig.…”

Sedimentology and chronology of the advance and retreat of the last British-Irish Ice Sheet on the continental shelf west of Ireland

“…3b, c) suggests deposition below a floating ice mass that 405 constrained vertical accretion (Dowdeswell et al, 2008;Batchelor and Dowdeswell, 2015). Thus, we 406 interpret this landform as a grounding-zone wedge that formed against an ice lobe with a floating ice 407 shelf.…”