PRELIMINARY STUDY OF MORPHOLOGY OF ICE-RAFTED DEBRIS IN THE SOUTHEAST ATLANTIC ACROSS TERMINATION V

The mid-Brunhes climate transition comprises a marked increase in amplitude of interglacial periods and associated sea-level changes beginning with Termination V, the largest deglaciation of the late Pleistocene. We studied morphology of ice-rafted quartz and other clear minerals at Site 1094 to ascertain if and how the large amplitude temperature and sea-level changes of Termination V may have impacted ice-rafting in the South Atlantic region.

Sphericity of randomly-selected clear mineral grains (150-2000 microns) from 21 samples spanning Termination V was quantified using Image J software. Roundness was estimated using the visual comparison chart of Krumbein and Sloss (1963). Preliminary results showed high sphericity and moderate roundness early in the termination accompanied by an unusually large peak in concentration of clear minerals followed by a pronounced drop in sphericity and roundness of clear minerals at ~425ka (perhaps reflecting a change in mineralogy) accompanied by a change in the dominant lithic component to ash. This is followed by a long absence of ice-rafted materials at Site 1094.

These changes are accompanied by millennial changes in diatom-inferred summer sea-surface temperature (SSST; Kunz-Pirrung etal., 2002). The peak in clear minerals and high sphericity and roundness at ~430ka coincides with a brief but intense cold reversal (SSST ~0.5C) that interrupts increasing temperatures of the termination. The ensuing peak in ash and lower sphericity and roundness values coincide with a second cold reversal (to SSST ~2C). The shift in dominant lithology and change in grain morphology follow inferred SSST (~3.6C ) well above present values at a time that sea-level may have been rapidly rising to MIS 11 levels as much as 20m above present (Hearty et al., 1999). Causes of the shift in lithology and grain morphology remain unclear, potentially including changes in source area and/or iceberg versus sea-ice rafting.