Iceberg Alley and Subantarctic Ice and Ocean Dynamics

International Ocean Drilling Program (IODP) Site U1538 (proposed Site SCO-11) is located 365 km north-northeast of the South Orkney Islands at 57°26.52′S, 43°21.47′W in 3131 m of water. The site lies in Pirie Basin and is situated at Shotpoint 4900 on Seismic Reflection Profile SCAN2013-13, 9 km east of Crossing Line SCAN2013-11. Site U1538 is located in the center of a small basin and provides the most undisturbed stratigraphy among the six potential drill sites in Pirie Basin. The seismic presite survey was conducted with a three-channel SPARKER system, so the depth penetration and resolution of the profile are limited compared to the multichannel seismic (MCS) records at Dove Basin Sites U1536 and U1537. Thus, it was more difficult to follow prominent reflectors and assign seismic units, although extended Pliocene–Pleistocene sections (Seismic Units I and II) were expected. Identification of Reflector d and acoustic basement is questionable.

Previous piston coring in Pirie Basin was conducted 9 km west of Site U1538 at Site MD07-3133 (alternate proposed Site SCO-01) along Seismic Line SCAN2013-13. Site MD07-3133 recovered a 36 m long sediment core that reaches back to Marine Isotope Stage (MIS) 3, which implied that the high sedimentation rates observed at Dove Basin Sites U1536 and U1537 could be doubled in Pirie Basin. However, Site MD07-3133 is located at a marginal position of the small basin in the northern Pirie Basin with potential seismic discontinuities below ~160 meters below seafloor (mbsf). Therefore, Site U1538, in the central part of that basin, was chosen for drilling to maximize undisturbed recovery and recover the thickest Pleistocene section.

Sediments in the southern Scotia Sea are primarily deposited by contourite currents along the pathway of the Antarctic Circumpolar Current (ACC). Pirie Basin is located farther north and closer to the center of the ACC than Dove Basin and is expected to be less influenced by Weddell Sea Deep Water. The contourites are generally lens shaped and have a total thickness of as much as 1 km, similar to the sediment deposition observed above basement in Dove Basin (Maldonado et al., 2006).

At Site U1538, the main objective was to obtain a complete record of ice and ocean dynamics in the center of "Iceberg Alley" since the late Neogene in the more northerly of our two drilling areas in the Scotia Sea. Specific objectives in Pirie Basin included (1) the reconstruction of past variability in Antarctic Ice Sheet mass loss and the related sea level history at a site with potentially more iceberg-rafted debris (IBRD) derived directly from the Pacific side of the West Antarctic Ice Sheet through clockwise transport in the ACC; (2) the capture of the northern, more Circumpolar Deep Water–dominated part of the Drake Passage throughflow and associated north–south shifts of frontal systems, water mass properties, ocean temperature, and sea ice extent; and (3) the reconstruction of changes in atmospheric circulation and dust-climate couplings between Patagonia and Antarctica and related atmospheric circulation changes throughout the Pliocene–Pleistocene in a more proximal location relative to the main dust source, Patagonia.