HR: 0830h
AN: PP31B-0260    [PDF]
TI: Ice rafting and oceanographic changes in the southern Indian Ocean during the Oi-1 transition
AU: * Bohaty, S M
EM: sbohaty@es.ucsc.edu
AF: Earth Sciences Department/ Institute of Marine Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064 United States
AU: Zachos, J C
EM: jzachos@es.ucsc.edu
AF: Earth Sciences Department/ Institute of Marine Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064 United States
AU: Delaney, M L
EM: delaney@ucsc.edu
AF: Ocean Sciences Department/ Institute of Marine Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064 United States
AB: The early Oligocene Oi-1 event marks the development of the first large ice sheets on Antarctica during the Cenozoic. We have investigated this interval through high-resolution study of the stable isotope and minor element chemistry of planktonic foraminifera and the stratigraphic distribution of ice-rafted debris at ODP Sites 738 and 748 (southern Kerguelen Plateau). At both Sites 738 and 748, a major pulse of ice rafting occurs within the positive $\delta$$^{18}$O shift during the Oi-1 transition interval at $\sim$33.6 Ma. This correlation suggests a maximum production of debris-laden icebergs in the Prydz Bay region during the initial stages of ice-sheet expansion in East Antarctica. These results are consistent with ice-sheet models in which the Gamburtsev Mountains serve as one of the nucleation points for ice-sheet development, with the Gamburtsev ice margin reaching sea level in the Lambert Graben well before smaller ice sheets coalesced and maximum ice volume was attained. The paucity of ice-rafted debris during the peak of the Oi-1 event may indicate a period of ice-sheet stabilization at maximum ice volume and, possibly, a change in the character of glaciation to cold-based conditions, with a decrease in rates of ice calving and/or a decrease in the sediment load of icebergs exiting Prydz Bay. Mg/Ca data from the surface-dweller {\it Chiloguembelina cubensis} indicate no significant change in surface-water temperatures across the Oi-1 transition. Previous Mg/Ca results from benthic foraminifera have similarly suggested no change in Southern Ocean deep-water temperatures (Lear et al., 2000). If no temperature change in surface or deep waters is assumed at Site 748, a damping of the $\delta$$^{18}$O signal in {\it C. cubensis}, relative to the 1.2$\permil$ increase recorded in {\it Subbotina angiporoides} and {\it Cibicidoides} spp., can be interpreted as a negative surface $\delta$$^{18}$O$_{SW}$ anomaly. This anomaly is greatest during the interval of intense ice rafting, providing support for a local decrease in surface-water salinity. A freshening of surface waters may have resulted in a more stratified water column during the Oi-1 interval. This physical change in water-column structure may have contributed to the success of siliceous plankton and led to the observed increase in opal accumulation during the Oi-1 event.
DE: 1635 Oceans (4203)
DE: 4267 Paleoceanography
DE: 4870 Stable isotopes
DE: 4875 Trace elements
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting