MINERALOGY OF NEOGENE MUDROCKS FROM ANDRILL AND-1B: A COMBINED SIGNAL OF LONG-TERM PROVENANCE VARIATIONS AND ORBITAL-SCALE CLIMATIC EFFECTS IN SEDIMENTS BENEATH THE MCMURDO ICE SHELF

The ANDRILL McMurdo Ice Shelf (MIS) Project is investigating the Late Neogene geologic and paleoclimatic/paleoenvironmental histories of the NW sector of the Ross Ice Shelf, using a 1284.87 m-long drillcore (AND-1B) recovered from beneath the present-day MIS during October – December 2006.

The core contains a range of lithologies, including diamictites, sandstones, mudstones (some clast-bearing), and diatomites. The mineralogy of fine-grained terrigenous sediments in AND-1B was analyzed by x-ray diffractometry (XRD) of pressed powders to investigate sediment provenance and terrestrial weathering conditions. Data from 86 samples define large-scale compositional patterns through AND-1B, as well as detailed variations within obliquity-paced Pliocene glacial/interglacial cycles.

The large-scale compositional pattern in AND-1B is dominated by sediment provenance – older volcanics vs. ``basement’’ vs. contemporaneous volcanics. Quartz/feldspar ratios are consistent with the relative importance of older mafic volcanic vs. ``basement’’ sources identified from sandstone and clast analyses. Calcite, dolomite, and other secondary phases are present irregularly, reflecting the heterogeneity of cements and other diagenetic features observed during visual core description. The relative abundance of illite generally follows the pattern of the quartz/feldspar ratio, whereas the relative abundance of expandable clays (``smectites’’) follows the abundance of the ``contemporaneous’’ volcanic component identified in smear slides.

Illite/chlorite (I/C) abundance ratios within individual obliquity-paced Pliocene glacial/interglacial cycles show several different patterns of variation. Within a relatively thick cycle (i.e., Cycle 35 of Naish et al., 2009), the I/C ratio fluctuates repeatedly, recording changes in environmental conditions at suborbital timescales; these changes also are recorded by lithofacies variations. In thinner cycles, changes in the I/C ratio are more variable; some thin cycles show an upcore I/C increase, as expected for a deglaciation. Other thin cycles show an upcore decrease or have a central I/C minimum or maximum; this variability suggests the influence of sediment recycling and of erosional truncation at cycle tops.