Isotopic constraints on the provenance of fine-grained sediment in LGM tills from the Ross Embayment, Antarctica

The major element, trace element and Nd, Sr, and Pb isotopic compositions were determined for the < 63 mu m sediment fractions of fourteen onshore tills and twenty-one offshore tills in the Ross Sea and surrounding areas, Antarctica. The purpose was to determine the provenance of tills deposited on the continental shelf of the Ross Sea during the Last Glacial Maximum (LGM) and to assess the relative importance of the West and East Antarctic ice sheets in providing ice to the Ross Embayment during this time. Onshore tills exposed around the margins of the Ross Embayment associated with specific ice streams and outlet glaciers have Nd and Pb compositions that vary regularly with geographic position, reflecting changes in the age and isotopic compositions of basement rocks along the Ross Sea margin. Specifically, till epsilon(Nd) values range from -5.6 to -9.9 for the West Antarctic (Siple Coast) ice streams and the southern Transantarctic Mountains, while the epsilon(Nd) in the central portions of the mountains range from -11.9 to -14.9. Uranogenic Pb becomes less radiogenic in the same direction. The composition of Late Quaternary tills from the Ross Sea also varies geographically, with the < 63 mu m fraction from both the western and eastern Ross Sea tills having higher epsilon(Nd) values (-5.8 to -7.5 and -3.8 to -6.9, respectively) than the till in the central Ross Sea (-7.1 to -12.5). Tills in the eastern Ross Sea (most proximal to West Antarctica) have isotopic and chemical compositions identical to onshore till from West Antarctica and the southern Transantarctic Mountains and were most likely derived from these areas. Tills in the western Ross Sea with the highest epsilon(Nd) values have low wt-% SiO2 (46%), and high Ni, Nb, Ta and Sr contents requiring a large component of material eroded from extrusive igneous rocks from the adjacent Late Cenozoic McMurdo Volcanic Field. In contrast, tills in the central Ross Sea were most likely related to ice emanating from the central Transantarctic Mountains that flowed north across the Ross Embayment to the continental shelf margin. Overall, our data provide further direct evidence for the model described in a previously published companion paper [K.J. Licht, JR. Lederer, J.R. Swope, Provenance of LGM glacial till (sand fraction) across the Ross Embayment, Antarctica, Quat. Sci. Rev. 24 (2005) 1499-1520.] that ice derived from the East Antarctic ice sheet was active and responsible for LGM till deposition throughout the western half of the Ross Sea (including the central Ross Sea). As a result, models for the past and future dynamics of the Antarctic ice sheets must account for a major East Antarctic contribution to the Ross ice sheet. (c) 2006 Elsevier B.V. All rights reserved.