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RADIOGENIC ISOTOPE PROVENANCE STUDIES OF GLACIAL TILLS IN ROSS SEA AND VICINITY, ANTARCTICA

FARMER, G. LANG  University of Colorado, Boulder.
Licht, Kathy  Indiana University-Purdue University Indianapolis.
Andrews, John T  University of Colorado, Boulder.
Swope, R. Jeffrey  Indiana University-Purdue University Indianapolis.

Competing hypothesis exist for the relative roles of the East and West Antarctic ice sheets (EAIS, WAIS) in providing ice to the Ross Sea during the LGM. Some reconstructions of the Ross Ice Sheet during the LGM that include ice streams indicate that West Antarctica supplied most of the ice (e.g., Stuiver et al., 1981). In contrast, a numerical ice sheet model reconstruction by Licht and Fastook (1998), using geologic data as constraints, indicates that East Antarctic-derived ice extends from the western to the east-central Ross Sea and that ice streams derived from West Antarctica are not present during the majority of ice retreat. The difference between these two reconstructions has major implications for constraining the style of LGM ice retreat; ice streams are a primary mechanism by which large volumes of ice can be rapidly removed from an ice sheet, causing or contributing to unstable behavior of the ice sheet.

In this study, we use Nd and Sr isotopic data from surface tills in the source areas and Ross Sea till (Fig. 1) to attempt to distinguish between a West and East Antarctic source for Ross Sea glacial sediment. In East Antarctica, the Nd isotopic compositions of surface and core tills from eastern Ross Sea and vicinity show regular geographic variations that reflect isotopic compositions of exposed basement rocks. The tills with the lowest eNd (-15) values are from the Mt. Achernar and Darwin Glacier regions (Fig. 1) along the western limit of the Ross Ice Shelf, reflecting the presence of reworked Archean rocks in the central Antarctic Mountains (Borg et al., 1990). Tills related to the Hatherton Glacier, in contrast, have isotopic compositions similar to those determined for Cambro-Ordovian granitic rocks in the Transantarctic Mountain (eNd =-6 to 9; Borg et al., 1990). Further to the north, surface tills at the western margin of the Ross Sea have the highest measured eNd (-3.8) of any of the analyzed tills and were likely derived at least in part from adjacent Cenozoic volcanic rocks.

In West Antarctica, tills related to ice streams B, C, D all have similar Nd (eNd =-6 to 9) and 87Sr/86Sr ranging from 0.719-0.731. None of these are dominated by detritus derived from higher eNd West Antarctic volcanic rocks thought to underlie much of the WAIS, but are isotopically similar to the Antarctic Cambro-Ordovian igneous rocks. These data reveal that there is significant overlap between the isotopic compositions of sediment provided to the Ross Sea by the WAIS and EAIS, but only the EAIS delivered detritus with eNd <-10, or >-5.

Tills beneath the Ross Ice Sheet (RISP site) are isotopically indistinguishable from the West Antarctic tills and suggest that these sediments were derived exclusively from the Western Antarctic Ice Sheet (Fig. 2). Surface tills from the central Ross Sea have eNd values ranging from -7 to 11, but tills from the western margin of the central trough (Fig. 1) have distinctly lower eNd values than those from the eastern margin (-9 to 11 vs. 7 to 8; Fig. 2). These data indicate that tills from the eastern trough margin may contain a component of low eNd (<-10) sediment delivered to the Ross Sea by the EAIS. Tills along the western trough margin have isotopic compositions similar to both the central Ross Ice Sheet and Western Antarctic tills, and indicate that these sediments could have been derived exclusively from the WAIS. Although additional data from the Ross Sea are needed, our data thus far demonstrate important roles for both the WAIS and EAIS in providing glacial detritus to this region during the LGM.

REFERENCES
Borg, S.G., DePaolo, D.J., and Smith, B.M., 1990, Isotopic structure and tectonics of the central Transantarctic Mountains: Journal of Geophysical Research, v.95, no.B5, p.6647-6667.

Licht, K.J., and Fastook, J., 1998, Constraining a numerical ice sheet model with geologic data over one ice sheet advance/retreat cycle in the Ross Sea: Chapman Conference on the West Antarctic Ice Sheet, University of Maine, p.25-26.

Stuiver, M., Denton, G. H., Hughes, T. J., and Fastook, J. L., 1981, History of the marine ice sheet in West Antarctica during the last glaciation: A working hypothesis: In Denton, G. H., and Hughes, T. J., eds., The Last Great Ice Sheets: New York, Wiley-Interscience, p. 319-439.



Figure 1. Sample Locations


Figure 2. Measured Nd and Sr isotopic compositions of Antarctic tills. Note the distinctly lower eNd values for central Ross Sea sediments on the western margin of the central trough.


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