THE WEDDELL SEA IN THE PLIOCENE: INTERPRETING THE SEDIMENT RECORD FROM ODP SITES 693 AND 697

Sediment from East and West Antarctica drain into the Weddell Sea. Sites 693 and 697 record Pliocene cryospheric history through ice rafted detritus (IRD), biosilica production and sediment composition. Age resolution at both sites is based on shipboard biostratigraphy and paleomagnetics. We have interpreted this data in light of the LR04 benthic stack to provide a context with global oceanography. Site 697 lies in the Jane Basin, adjacent to the South Orkney Islands at 3480 meters below sea level (mbsl). The terrigeneous sediment is sourced from the West Antarctic Ice Sheet. We examined the interval between 3.0 and 3.8 Ma. IRD (fraction >63um). varies between 1 and 13%. Obliquity pacing, correlateable with the LR04 stack, is observed. However during the Pliocene warm period (3.0-3.3Ma), wt% IRD is less than 2% and biosilica increases to its highest values. This suggests conditions that are amenable to high productivity and that few icebergs are reach this far north.

In contrast, Site 693, 2480 mbsl, is adjacent to Dronning Maud Land on a shelf bench. Its terrigeneous sediments are sourced exclusively by the East Antarctic Ice Sheet (EAIS). Core 8R, the focus of this study, was deposited during Chron 2Ar (3.58-4.18 my). Most of the closely-spaced samples (roughly 1 sample/2-3 kyr) show little variation in wt% biosilica and wt% IRD. The exception is around 4 Ma where biosilica ranges between 6 and 14%, while IRD is relatively constant at 2 wt%. These changes do not parallel the LR04 stack, suggesting limited interaction at this site with the global ocean. Nevertheless, the limited age range 400-650 Ma (Pan African orogeny) and heavy mineral assemblages dominated by clinopyroxenes and calcium-rich amphiboles, suggest a single source and support the interpretation that this portion of the EAIS was relatively stable at this time. At both sites the highest biosilica content is where the wt% IRD is the lowest, suggesting no or limited ice and high productivity.