Sorting Out Mixed Messages from Till Provenance: Bulk Sediment Isotopic Compositions vs. U-Pb Detrital Zircon Ages from Antarctica
Abstract
Understanding how basal erosion by ice sheets takes place as a function of time and position is critical to models of sub-ice landscape evolution and to determining whether isotopic and chemical data from subglacial deposits and ice rafted detritus can be expected to fingerprint the activity of specific ice outlets during ice sheet expansion and/or deglaciation. To investigate erosion by an active ice sheet we analyzed the mineralogy, chemical, isotopic and detrital zircon U-Pb ages of till deposited from the head to the mouth of the Byrd and Nimrod Glaciers in the Transantarctic Mountains (TAM) in East Antarctica. We then compared the isotopic compositions and detrital zircon ages compositions of Ross sea tills downstream of the Byrd and Nimrod glaciers to assess the sources of those glacial sediments deposited on the Ross Sea shelf and slope. In general, lateral and mid-stream ice cored moraines along the entire lengths of both glaciers largely contain locally derived material in both sand and pebble size fractions. The primary exception is Lonewolf Nunatak at the head of Byrd Glacier, which contains Precambrian igneous and metamorphic erratics, similar to what outcrops and is found in moraines in the Miller Range at Nimrod Glacier. Detrital zircons from all moraines analyzed are dominated by grains 520-600 Ma associated with the Ross/Pan-African Orogeny. Ages of 580-600 Ma are only found in sites at the head of the glaciers. The Nd, Sr and Pb isotopic compositions of <63micron sediments are consistent with their origin as comminution of the coarser sediments found at each sample location, with lowest ɛNd(0) sediments (~-20) restricted to upstream tills. Central and western Ross Sea LGM tills generally match the petrography, detrital zircon, and isotopic compositions expected for detritus delivered by central TAM glaciers. However, detrital zircon populations include a significant 580 Ma-600 Ma age population as well as Precambrian zircon only found in 'upstream' tills, suggesting that crustal material eroded and the heads of the Byrd and Nimrod glaciers, and even detritus derived from the East Antarctic craton, were delivered to the continental margin during expansion of the East Antarctic ice sheet. The <63 micron fraction from the central Ross Sea tills have isotopic compositions that are also similar to some upstream tills in the Byrd and Nimrod glaciers, but can also be interpreted as being principally derived from Cambrian sedimentary and granitic rocks, and late Cenozoic volcanic rocks, found at and near the outlets of the Byrd and Nimrod glaciers. Additional tills and bedrock chemical, isotopic and zircon U-Pb data will be necessary to reconcile the available data and better assess the relative proportions of detritus derived from valleys cut through the Transantarctic Mountains and from locations further inboard.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP41B0802F
- Keywords:
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- 0720 CRYOSPHERE Glaciers;
- 1040 GEOCHEMISTRY Radiogenic isotope geochemistry