Randy Dirszowsky and Robert Gilbert, Department of Geography, Queen's University, Kingston ON K7L 3N6, Canada

Contact: dirszows@post.queensu.ca

This study seeks to identify variations in chemical signature that indicate temporal variations in the depositional environment of the Larsen-A Ice Shelf, as well as changes in sedimentary provenance which may be used to clarify glacial and oceanic sedimentary processes. A two step Instrumental Neutron Activation Analysis was used to determine the concentrations of seven major elements (Al, Ca, Fe, K, Mg, Na and Cl) and 26 minor and trace elements from core KC23 recovered from the Greenpeace Trough during the U.S. Antarctic Program cruise NBP0103 on board RVIP N.B. Palmer in May 2000.

Background levels of NaCl, ranging from ~1-3%, are correlated with pore water content but distinct peaks up to 10 times this level occur in the upper core. Silica content determined indirectly from the other major elements is estimated to range from 67-87%. Aluminium/silicon ratios suggest that the majority of silica is terrigenous except in diatomaceous ooze layers where up to 83% of the silica is biogenic. Carbonate, if present, occurs in concentrations of less than ~3% based on Ca content.

Among the measured elements, correlation and factor analysis identify up to seven geochemical assemblages (NaCl-free basis) that may have mineralogical, lithological and potential source area significance. Al-related elements, for example, appear to correspond to clay minerals derived locally from sedimentary or metasedimentary rocks.

In general, the geochemistry confirms the previous assessment of KC23. A lower unit (150-250cm) associated with a grounded ice sheet contains elevated Al evidently correlated with clay content and the presence of shale in the sand fraction. An overlying unit (130-150cm), thought to represent floating of the sheet to form an ice shelf, is coarse-grained and contains less Al but elevated As and related elements. Upper core zones inferred to represent shelf ice (85-130cm and 1-10cm) and open water/sea ice (10-85cm) marine conditions are more variable chemically and difficult to distinguish without resorting to sensitive elemental ratios such as La/Lu. The open water/sea ice zone appears to be characterized by elevated I, Sr and La related elements. Conspicuous diatom ooze layers in this zone correspond to concentrated NaCl, As, Br and Co, as well as to elevated silica which tends to dilute the remaining elements. The significance of these and other short-term chemical variations is the subject of ongoing work.