Evolution of subglacial bedforms along a paleo‐ice stream, Antarctic Peninsula continental shelf

Abstract

Geophysical data from the Antarctic Peninsula continental shelf reveal streamlined subglacial bedforms in a cross‐shelf trough. Bedforms exhibit progressive elongation with distance along the trough, and record flow of a paleo‐ice stream from the Antarctic Peninsula Ice Sheet during the last glacial maximum. Downflow evolution of the bedforms indicates increasing flow velocities as the ice stream traversed the shelf. This, in turn, is related to a transition from crystalline bedrock on the inner shelf to a soft sedimentary substrate on the outer shelf. Although streaming flow operated across both substrates, the highest flow velocities occurred over the soft bed. Spatial variation in the inferred nature of fast‐flow, from sliding to subglacial sediment deformation and/or ploughing, was also lithologically controlled. These data highlight the control of subglacial geology on ice‐stream dynamics in the geological record and demonstrate a direct relationship between the formation of streamlined subglacial bedforms and paleo‐ice streams.

Fresh arguments against the Shaw megaflood hypothesis. A reply to comments by David Sharpe on “Paleohydraulics of the last outburst flood from glacial Lake Agassiz and the 8200 BP cold event”

“…Contemporary West Antarctic ice streams have width scales (Raymond et al, 2001, Fig. 3) that are comparable to that of the Livingstone Lake drumlin field, and submarine deglaciated surfaces beyond the grounding lines of currently active ice streams show drumlin-like features (Shipp et al, 1999). Acoustic imaging of the sea floor off the Antarctic Peninsula also reveals spectacular drumlin-like bedforms and megalineations that are attributed to a paleo-ice stream (Ó Cofaigh et al, 2002). Although these features cannot be linked to a currently active ice stream, the geotechnical properties (Dowdeswell et al, 2004) of the sediments are strikingly similar to those encountered beneath active West Antarctic ice streams.…”

Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

“…Similar extensive and elongated MSGLs have been observed on the beds of palaeo-ice streams on the Antarctic shelf, and ones within the former Laurentide and Fennoscandian ice sheets (e.g. Shipp et al, 1999;Stokes and Clark, 2001;Ó Cofaigh et al, 2002Ó Cofaigh et al, , 2005aÓ Cofaigh et al, , b, 2007Dowdeswell et al, 2004;Ottesen et al, 2005;Mosola and Anderson, 2006;Graham et al, 2007).…”

“…The presence of meltwater channels on the inner shelf may reflect both the impermeability of the substrate and the resistance of the bed to further erosion by other processes (i.e., preservation). The contrasting absence of meltwater features on the sedimentary substrate may further suggest that: (1) any meltwater drainage to the middle and outer shelf occurred prior to the last phase of ice streaming, which obliterated pre-streaming morphological signatures on the 'soft' substrate; (2) subglacial meltwater flow occurred at transient times, when the grounding line was located on bedrock substrate; or (3) drainage was not through a well developed channel system likely to leave a geomorphic imprint, but rather through widely-distributed shallow canals or as Darcian flow through porous sediment (Walder and Fowler, 1994;Ó Cofaigh et al, 2002;Murray et al, 2008).…”

“…In profiles, the features often form a corrugated surface to a distinct transparent acoustic facies interpreted as a till or iceberg turbate (cf. Pudsey et al, 1994;Ó Cofaigh et al, 2002). The furrows arise from iceberg calving and keel-grounding at the seabed and are interpreted as iceberg ploughmarks (after Belderson et al, 1973).…”

Chapter 5 Cenozoic Climate History from Seismic Reflection and Drilling Studies on the Antarctic Continental Margin

“…Swath bathymetric data show seafloor features of subglacial origin on the shelf (Ó Cofaigh et al, 2002;Dowdeswell et al, 2004;Amblas et al, 2006), and confirm that a grounded ice sheet with ice streams extended to the shelf edge during the LGM (Pudsey et al, 1994;Larter and Vanneste, 1995). Seafloor core data indicate that retreat of the ice sheet from the outer and middle shelf after the LGM occurred between 18,500 and 13,000 cal.…”