|
James A. Smith, British Antarctic Survey (United Kingdom)
Claus-Dieter Hillenbrand, British Antarctic Survey (United Kingdom)
Robert Larter, British Antarctic Survey (United Kingdom)
Alastair Graham, British Antarctic Survey (United Kingdom)
Gerhard Kuhn, Alfred Wegener Institute for Polar and Marine Research (Germany)
|
|
Several recent studies have demonstrated the movement of considerable volumes of water beneath today's Antarctic ice sheets, potentially in well-organised channel systems. In addition, the flow of water is also thought to be sporadic and inherently unstable. Since basal meltwater facilitates ice motion through increased sliding or sediment deformation a clear understanding of the magnitude and rates of subglacial meltwater movement is important for our understanding of ice stream dynamics ice sheet stability. Here we present new swath bathymetric data from the western Amundsen Sea Embayment (ASE), West Antarctica, showing a network of anastomosing channels eroded into granitic bedrock. The features are similar in scale to those described elsewhere in Antarctica (e.g., Labyrinth, Dry Valleys) with some channels measuring up to 25 km long, 4.5 km wide and 400 m deep. Their morphological characteristics (reverse gradient, undulating thalweg) and size are consistent with incision by subglacial meltwater. In order to obtain a better understanding of how and when these channels formed we have performed detailed sedimentological analyses on three cores recovered from within the channels. Diamictons deposited beneath or proximal to an expanded grounded West Antarctic Ice Sheet (WAIS) are present in two of the channels and these are overlain at both sites by glaciomarine sediments deposited when the ice sheet retreated from the continental shelf. The sediment core from the third channel recovered a turbidite sequence that was probably deposited after the last deglaciation. The presence of deformation till at one core site suggests that channel incision pre-dates overriding by fast flowing grounded ice during the last glacial period and thus provides a minimum age for when the channel was last active. Moreover, the presence of the deformation till within one of the channels is consistent with findings from the Dry Valleys which indicates that the channels are preserved beneath a wet-based eroding ice sheet and hence, it is possible that they survived numerous advances and retreats of grounded ice. Given the overall scale of the channels and their incision into bedrock it is likely that the channels probably formed over multiple glaciations, possibly since the Miocene, and have been reoccupied on several occasions. In addition our sedimentological data differs from one previous study in the eastern ASE (Pine Island Bay) which reported graded gravels and sands interpreted as meltwater deposits associated with channel incision. We explore several possibilities for this regional variability.
|
