EVOLUTION OF TILL DURING TRANSPORT DOWN THE BYRD AND NIMROD GLACIERS, ANTARCTICA

Samples were collected from sixteen moraines at the head and along the length of the Byrd and Nimrod Glaciers, which drain from the East Antarctic ice sheet, cut across the Transantarctic Mountains and discharge into the Ross Embayment. Bulk sediment (till) and >300 pebbles were collected at each site in order to assess changes in particle size and composition during transport in a polar environment. Sites at the head of both glaciers contain more abundant silt and clay (fines) than downstream sites. In particular till from the Lonewolf Nunataks at the head of the Byrd Glacier contains >50% fines. Till collected from active lateral moraines along the trunk of both glaciers typically has <10% fines. The exception is where limestone bedrock erodes to produce ~35% fines. A comparison of two Nimrod Glacier moraine sites, both adjacent to exposed Beardmore Group rocks, do not have similar particle size distributions. The KonTiki Nunatak site, which outcrops in the midstream of Nimrod Glacier and forms a medial moraine, is significantly finer-grained than its lateral moraine counterpart. A simple proportional model of size distributions from surrounding upstream sites shows that the KonTiki till appears to be a mixture of sediment eroded from the Geologists Range metamorphic rocks, Beacon Supergroup volcanic and sedimentary rocks (25-90km upstream) plus a locally-derived component from Beardmore Group rocks. This three-component mixture accounts for the details observed in the KonTiki particle size distribution but slightly overestimates the coarse sand fraction. The presence of abundant fine sediment in upstream and midstream Byrd and Nimrod Glacier till indicates a subglacial component whereas lateral moraines lack fines and are dominated by locally eroded bedrock. This observation is supported by preliminary analysis of the pebble composition. Pebble composition reflects local bedrock outcrops in all lateral moraines from the trunk of both glaciers, whereas upstream sites contain a component of non-locally derived material. Understanding glacial transport is essential to interpreting Ross Sea provenance data used to provide constraints on past Antarctic ice sheet flow paths.