Challenges Of Interpreting Antarctic Pliocene Climate From The Sediment Record At ODP Sites 693 And 697 In The Weddell Sea

Abstract:

The Weddell Sea receives sediment from both East and West Antarctica and records cryospheric changes in both ice sheets. Examination of Pliocene sediment from two ODP Sites (693 and 697) provides a history of ice rafted detritus (IRD), biosilica production and orbital pacing. Age dating at both sites is limited to shipboard biostratigraphy and paleomagnetics, yielding low age resolution. Using this age data we have attempted to correlate sedimentary changes with the LR04 benthic stack to provide a context with global oceanographic changes.

Site 693 is adjacent to Dronning Maud Land on a shelf bench in 2480 m water and receives sediment exclusively from the East Antarctic Ice Sheet (EAIS). Detailed analysis of Core 8R, (approximately 1 sample/2-3 kyr) within Chron 2Ar (3.58-4.18 my) shows little variation in wt% biosilica and wt% IRD (fraction >63um). The exception is around 4 Ma when IRD is relatively constant at 2 wt%, while biosilica ranges 6 to 14%. These changes do not parallel the LR04 stack, suggesting limited interaction at this site with the global ocean. ⁴⁰Ar/³⁹Ar dating of hornblendes and biotites give a consistent age between 400-650 Ma (Pan African orogeny). The limited age range and composition, as well as small variation in % of IRD and biosilica suggest a single source and support the interpretation that this portion of the EAIS was relatively stable at this time.

Site 697, in 3480 m water, lies in the Jane Basin, adjacent to the South Orkney Islands. The interval between 3.0 and 3.8 Ma has higher variation in both IRD (1-13%) and biosilica (2-12%) than Site 693, an expected outcome because its terrigeneous sediment source is dominated by the less stable West Antarctic Ice Sheet. Clear obliquity pacing, correlateable with part of the LR04 stack, is observed. However after 3.3 Ma, near the beginning of the Pliocene warm period, wt% IRD is less than 2% and biosilica increases to it’s highest values. This suggests that few icebergs are reaching this far north and that conditions are amenable to high productivity. At both sites the highest biosilica content is where the wt% IRD is the lowest, suggesting no or limited ice and high productivity.