The Antarctic Peninsula inner shelf has been deeply scoured by glacial erosion resulting in numerous deep troughs that funnelled ice flow out across the shelf. Palmer Deep is one of these erosional troughs located at the convergence of three distinct ice accumulation centres: Anvers Island, Bruce Plateau and the Graham Coast (Fig. 1a). A distinctive channelized morphology observed in swath-bathymetric data from the trough-outlet sill reflects a combination of subglacial meltwater scour and underlying structural weakness within the bedrock. The origin of these features can be linked to the development of a subglacial lake basin within Palmer Deep during or prior to the Last Glacial Maximum (LGM), its subsequent drainage and the recession of the Palmer Deep ice-stream system (Domack et al. 2006).
Multibeam bathymetry of Palmer Deep collected during NB Palmer cruises NBP9903 and NBP0107 (US Antarctic Program). Aquisition system Seabeam 2100. Frequency 12 kHz. Grid-cell size 50 m. (a) Multibeam image. VE×3. Land area from LandSat Image Mosaic of Antarctica (US Geological Survey). (b) Location of study area (red box; map from IBCSO v. 1.0). (c) Enlarged image of Palmer Deep. (d) Detail of Palmer Deep Outlet Sill. (e) Gradient map of Palmer Deep Outlet Sill showing steepness of bedrock meltwater channels.
Description
The broad-scale morphology of the Palmer Deep basin is dominated by streamlined relief with mega- to mesoscale features including slope terraces, channels and prograded slopes (Fig. 1c). The basin floor of Palmer Deep lies below 1400 m water depth and shows a flat bottom made of ponded unconsolidated sediments in excess of 200 m thick, as observed in seismic profiles (Rebesco et al. 1998) and in core material from Ocean Drilling Program (ODP) Site 1099 (Barker et al. 1999).
The western slope of Palmer Deep is gentler than the steeper eastern slope and connects to a shallow silled region dissected by a series of tributary channels (Fig. 1c). Domack et al. (2006) refer to this region as the Palmer Deep Outlet Sill (Fig. 1c) due to the glacially streamlined topography that clearly indicates ice flow out of the basin following this constricted landscape.
The NW–SE-orientated channels conform to the overall glacial streamlined seafloor and follow a curvilinear pattern towards the open shelf (Fig. 1c). The channels in this region are broad (200–500 m wide), deep (100–300 m) and intersect at distinct 75–50° angles (Fig. 1d, e). Channel sides vary in relief with steep slopes above 45° in broad and overdeepened U-shaped channels, whereas more narrow interconnecting channels are typically V-shaped. Individual channels demonstrate ubiquitous reversals in longitudinal profile. The broad series of channels end westwards by opening into a series of dispersed and shallow incisions at 500–800 m water depth. The western end of Palmer Deep is characterized by a flat area interpreted as a possible relict sedimentary delta (Fig. 1c; Domack et al. 2006).
Interpretation
High-resolution swath bathymetry, together with seismic and sedimentologic data, indicates the former presence of a subglacial lake within the Palmer Deep basin beneath the head of the Palmer Deep palaeo-ice-stream system (Rebesco et al. 1998; Domack et al. 2006). According to the stratigraphic observations from ODP Sites 1098 and 1099 (Fig. 1c), the formation of the subglacial lake within the Palmer Deep basin took place just prior to the LGM after a period of ice dome growth and flow convergence into Palmer Deep. The complex channel systems located in the outlet sill of the basin (Fig. 1d, e) are indicative of the subglacial meltwater flow that fed the subglacial lake (Domack et al. 2006).
Meltwater channels represent organization of subglacial meltwater flowing along a pressure gradient controlled primarily by the thickness of the ice and ice-surface slope, but also by the underlying geology (Shreve 1972; Lowe & Anderson 2003). The distribution of the Palmer Deep meltwater channels, which were incised into crystalline bedrock, reflects the underlying structural control, with faulting or joint sets striking 35–140° azimuth (Hawkes 1981; Domack et al. 2006).
Subglacial meltwater could have enhanced ice flow out through the rough bedrock of Palmer Deep topography and the development of mega-scale glacial lineations (e.g. Clark 1993; Ó Cofaigh et al. 2002) beyond the basin outlet sill by increasing the ice-flow velocity (Domack et al. 2006). Similar meltwater-related features have been observed in other parts of the inner Antarctic continental shelf including Pine Island Bay (Lowe & Anderson 2003; Nitsche et al. 2013), Marguerite Bay (Ó Cofaigh et al. 2005; Hogan et al. 2016) and the Amundsen Sea embayment (Larter et al. 2009; Smith et al. 2009). The increasing number of high-resolution multibeam-bathymetric surveys performed in the Antarctic inner continental shelves highlights the importance of subglacial meltwater drainage to understand the past ice-sheet dynamics.
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
