MUDROCK GEOCHEMISTRY AS INDICATORS OF SEDIMENT PROVENANCE AND CENOZOIC PALEOCLIMATES, CAPE ROBERTS PROJECT DRILLCORES, MCMURDO SOUND, ANTARCTICA

The three Cape Roberts Project (CRP) cores, drilled in the austral summers of 1997-99, penetrated a composite section of ~1500 m of Quaternary through lower Oligocene (possibly uppermost Eocene) sediments, at least half of which contains evidence of significant glacial influence. Evidence of significant glacial influence is less obvious, however, in the lower Oligocene/upper Eocene sediments that form the basal ~350 m of the section. Sediment provenance has been evaluated using bulk mudrock Al/Ti ratios and trace element abundances (particularly Nb and Sr), whereas the paleoclimatic record has been examined using the Chemical Index of Alteration (CIA) of Nesbitt & Young (1982). The CIA is calculated from the relative abundances of Al, K, Ca, and Na, and its magnitude increases as the effects of chemical weathering increase.

Maximum Al/Ti ratios and minimum Sr contents in the basal ~500 m indicate limited input from the Ferrar Dolerite (FD) relative to input from the Beacon Supergroup (BSG) and local basement (BSM) during the early Oligocene/?late Eocene?. Higher Al/Ti ratios and intermediate Sr contents indicate a mixed input of FD, BSG, and BSM detritus from the early Oligocene to the late(?) Oligocene. High Nb contents and low Al/Ti ratios record significant addition of McMurdo Volcanic Group (MVG) detritus since the late(?) Oligocene.

These provenance changes dominate the mudrock CIA record, so interpretations of the CRP paleoclimatic record remain preliminary until the source area contributions can be quantified and a "provenance-normalized" CIA record can be calculated. However, a preliminary qualitative analysis suggests that chemical weathering intensity increased briefly during the early Oligocene, interpreted as a climatic moderation that ended at approximately the time when significant glacial influence began to be recorded in these sediments.