5. Site 10901

Shipboard Scientific Party2

BACKGROUND AND OBJECTIVES

Site 1090 (proposed site TSO-3C) is located in the central part of the Subantarctic Zone on the southern flank of the Agulhas Ridge, where the topography of the ridge is more intricate than near Site 1088 to the northeast (see Figs. F1, F5; both in the "Leg 177 Summary" chapter). The water depth of Site 1090 (3702 m) places it near the boundary between North Atlantic Deep Water (NADW) and underlying lower Circumpolar Deep Water (CDW) (see Fig. F2, in the "Leg 177 Summary" chapter). The water depth is above the calcium carbonate compensation depth and ensures preservation of calcareous microfossils during glacial and interglacial periods. Together with Site 1088 (2082 m) on the Agulhas Ridge to the northeast and Site 1089 (4620 m) in the Cape Basin, Site 1090 (3702 m) forms a depth transect that intersects most of the major water masses of the South Atlantic Ocean (see Fig. F2 in the "Leg 177 Summary" chapter).

A geophysical survey of the area surrounding Site 1090 was conducted during Thompson Cruise TTN057 (Fig. F1). The seismic profile indicates a very thick (>1000 m) package of sediment overlying basement near magnetic Anomaly 34 (Upper Cretaceous) (Fig. F2). Two unconformities are evident in the seismic section at ~120 and 300 ms. The sedimentary succession bounded by these unconformities thickens to the southwest, and Site 1090 was selected to take advantage of maximum uniform thickness of these seismic stratigraphic units. Piston cores TTN057-6-PC4 and PS2489-2 from the vicinity of Site 1091 indicate sedimentation rates of 20-30 m/m.y. during the late Pleistocene. On the basis of this estimate, we predicted that a 200-m-deep hole at Site 1090 should penetrate late Miocene sediment. This prediction turned out to be a gross underestimate because of an unanticipated hiatus spanning ~15 m.y.

The objective of Site 1090 was to recover a late Miocene-Pleistocene section at moderate temporal resolution to study (1) past migrations in the position of the Polar Front Zone; (2) changes in the mixing ratios of lower NADW and CDW in the Southern Ocean, and its relation to high-latitude climate change; and (3) the response of the Southern Ocean to orbital forcing, and the phase relationships (leads and lags) to climatic changes in the low and high northern latitudes. These objectives apply to the sedimentary record of the upper 70 meters composite depth (mcd) at Site 1090, which ranges from Holocene to late Pliocene in age. At ~70 mcd, a hiatus was encountered that documents the erosion and/or nondeposition of late Pliocene-early Miocene sediments. This unexpected hiatus presented the opportunity to recover older sediments than expected and to address the Cenozoic objectives that were not realized at Site 1088, where the recovered sedimentary section only documents a Pleistocene through middle Miocene interval and lacks a preserved signal of remanent magnetization. To recover the shallow-buried Paleogene record, we requested and received permission to deepen the holes at Site 1090 to a maximum depth of 400 meters below seafloor (mbsf).

The objectives for the early Miocene to middle Eocene part of the section below 70 mcd were to

  1. Calibrate Paleogene and early Neogene biostratigraphic datums to the geomagnetic polarity time scale (GPTS) and to construct an astronomically tuned time scale for the late Paleogene-early Neogene based on the lithologic cycles observed at Site 1090. Such tuned time scales were previously obtained from sedimentary records of the Ceara Rise (Weedon et al., 1997);
  2. Study marine proxies for the formation and stability of the Antarctic cryosphere during the Paleogene and early Neogene, where major differences exist in the interpretation of the extent and volume of the Antarctic ice sheet;
  3. Investigate the establishment and expansion of the Antarctic Circumpolar Current during the late Paleogene and early Neogene and its implications for thermal isolation and cryospheric development on Antarctica;
  4. Document changes in the production rates and chemistry of deep water in the Southern Ocean, and clarify the hypothesis of warm, saline deep-water production during the Paleogene (Kennett and Stott, 1990); and
  5. Reconstruct the vertical movements of the Agulhas Ridge.

1Examples of how to reference the whole or part of this volume.
2Shipboard Scientific Party addresses.

Ms 177IR-105