CAN WE EXTRACT INFORMATION FROM ANTARCTIC MARINE DIAMICTONS THAT IS USEFUL IN INTERPRETING PAST ICE-STREAM AND ICE-SHEET ACTIVITY IN THE ROSS SEA?

We address this question by (1) analysis of diatoms (abundance, stratigraphic age, and preservation), bulk chemistry, and Be-10 on selected piston cores from the Central Ross Sea, and from beneath the Ross Ice Shelf, and (2) controlled shearing experiments on diatomaceous mud, at pressures similar to that beneath ice streams.

The stratigraphic sequence described from the piston cores is post-glacial diatomaceous mud, overlaying mud and/or granulated sediment, capping a diamicton. The sub ice-shelf sediment cores consist of diamicton. The results show a wide range, within defined sediment sequences, in the measured parameters. True diatomaceous mud is dominated by a typical post-glacial diatom assemblage with high diatom abundance, excess of barium (Ba) and a 10-Be activity reflecting present day values. However some sediments classified as diatomaceous mud contain relatively low diatom abundance, with an assemblage dominated by reworked Tertiary diatoms. The sediment sequences containing abundant reworked diatoms are depleted in Ba and 10-Be. Despite evidence of a high proportion of reworked components, the 14-C dates of these sediment units indicate a Holocene age, suggesting that reworked carbon is relatively minor. Mud and granulated sediment sequences are variable in all parameters measured, but most mud samples are typically low in diatom abundance and characterized by excess zinc (Zn). Diamicton sequences are similar between sites with regard to bulk chemistry, and show a reduced 10-Be activity. The diamictons range from very poor to extremely rich in diatom valves and diatom debris, all indicating a stratigraphic mixing. Diamicton with high diatom abundance is dominated by a Miocene diatom flora, whereas diamicton with lower diatom abundance show a more intense stratigraphic mixing.

The variability in diatom signature, bulk chemistry and cosmogenic isotopes within these defined sediment sequences clearly show different depositional processes and sediment sources, both at present and in the past. In addition to illustrating how fine-grained diatomaceous sediment is altered by shearing, analyses of diatom comminution will give insight into mode of sediment transport. Initial results show distinct differences between diatom taxa broken by shearing versus breakage by normal load.