HR: 14:40h
AN: PP53C-05    [Abstracts]
TI: Miocene Flood Deposits in the Dry Valleys, Antarctica Dated Using Cosmogenic $^{3}$He Isotopes.
AU: * Margerison, H
EM: hrm@geo.ed.ac.uk
AF: The University of Edinburgh, School of GeoSciences University of Edinburgh , Edinburgh, EH8 9XP United Kingdom
AU: * Margerison, H
EM: hrm@geo.ed.ac.uk
AF: Scottish Universities Environmental Research Centre, Isotope Geosciences Unit Scottish Universities Environmental Research Centre East Kilbride , Glasgow, G75 0QF United Kingdom
AU: Phillips, W
EM: phillips@uidaho.edu
AF: The University of Edinburgh, School of GeoSciences University of Edinburgh , Edinburgh, EH8 9XP United Kingdom
AU: Stuart, F
EM: F.Stuart@suerc.gla.ac.uk
AF: Scottish Universities Environmental Research Centre, Isotope Geosciences Unit Scottish Universities Environmental Research Centre East Kilbride , Glasgow, G75 0QF United Kingdom
AU: Sugden, D
EM: des@geo.ed.ac.uk
AF: The University of Edinburgh, School of GeoSciences University of Edinburgh , Edinburgh, EH8 9XP United Kingdom
AB: {\it In situ} produced cosmogenic $^{3}$He measurements on deposits in the Dry Valleys, East Antarctica, provides information on Neogene climatic variation and East Antarctic ice sheet evolution. Ferrar dolerite cobble-size boulders located in the Coombs Hills form a series of mega-ripples (wavelength approximately 50 meters) associated with scabland features and stripped, corrugated bedrock surfaces. These features, together with topographic position, indicate the boulders were deposited by subglacial floodwaters. Such outburst flooding occurred during over-riding of the northern Dry Valleys by a greatly expanded East Antarctic ice sheet. Timing of the over-riding episode has been previously assigned to between 13.6 and 14.8 Ma by correlation with volcanic ash deposits dated in the Asgard Range of the Dry Valleys. Cosmogenic $^{3}$He concentrations in clinopyroxene from Ferrar dolerite boulder samples imply a minimum of 8.6 to 10.4 Ma exposure, calculated using scaling factors appropriate to Antarctica and assuming zero erosion. These are among the oldest surface exposure dates yet measured on Earth, but are however younger than the $^{40}$Ar/$^{39}$Ar defined chronology. Erosion is an important influence on measured concentrations of cosmogenic isotopes and unconstrained erosion of samples can significantly influence the accuracy of stable cosmogenic isotope dating techniques in East Antarctica. Nearby Ferrar dolerite bedrock surfaces are used to yield an average cosmogenic $^{3}$He derived steady-state erosion rate of 0.24 m Ma$^{-1}$. Applying a conservative erosion correction, (based on $<$ 25% of this rate) to the oldest flood deposit causes apparent exposure ages to rise to over 14 Ma. These results provide independent support for the model of a stable, hyper-arid polar climate persisting in East Antarctica throughout the Neogene period and provide quantitative constraints on long term rates of erosion within the Dry Valleys.
DE: 3344 Paleoclimatology
DE: 1600 GLOBAL CHANGE (New category)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting