Site 1092 (proposed site SubSAT-3B) is located on the northern Meteor Rise at a water depth of 1974 m (Fig. F1 in the "Leg 177 Summary" chapter). The Meteor Rise is one of the dominant topographic features in the southeast Atlantic and marks the westward limit of the Agulhas Basin (Fig. F5 in the "Leg 177 Summary" chapter). This oval-shaped, aseismic plateau consists of basement highs alternating with depositional basins and is the conjugate feature of the Islas Orcadas Rise in the western South Atlantic.
Much of the impetus for drilling Site 1092 came from a late Neogene sedimentary record obtained during Ocean Drilling Program (ODP) Leg 114 at Site 704, located only 34 nmi to the southeast. Site 704 was drilled to 671.7 meters below seafloor (mbsf) and recovered sediment of lower Oligocene to Quaternary age that was characterized by (1) the presence of carbonate throughout, (2) minor proportions of terrigenous matter that can be related to individual ice-rafting episodes, (3) the continuous presence of all calcareous and siliceous microfossil groups, and (4) a relatively good paleomagnetic stratigraphy. Until Leg 177, Site 704 was one of the few sites available from the Southern Ocean that had sufficient stratigraphic continuity and carbonate content to be suitable for Pliocene-Pleistocene paleoclimatic studies (Hodell and Venz, 1992; Hodell, 1993). One of the shortcomings of Site 704, however, is that it was drilled in a small sedimentary basin on the southern Meteor Rise that is surrounded by three basement highs that have provided material for downslope transport. For example, Parasound profiles near Site 704 revealed clear evidence for the top of a turbidite sequence beginning at ~25 mbsf. In addition, the soft, water-saturated sediment of Site 704 was easily disturbed during coring and handling in the exceptionally high seas experienced during Leg 114. We targeted Site 1092 in the hope that many of the shortcomings of Site 704 could be overcome by our recent ODP drilling initiative on Meteor Rise.
A detailed geophysical survey was conducted of the northern Meteor Rise with efforts focused on a broad shallow area that is protected from the undesirable effects of downslope transport (Fig. F1). Site 1092 was selected on the basis of the uniformity and thickness of the upper seismic reflectors (Figs. F2, F3). Core PS2083-3 (Bathmann et al., 1992), located approximately 1.4 nmi to the northwest of Site 1092, obtained 10.61 m of sediment consisting of alternating diatom-bearing foraminifer mud, foraminifer ooze, and diatom mud deposited with an average sedimentation rate of 10 to 15 m/m.y.
Site 1092 is located in the Polar Front Zone (PFZ), ~3º north of the present-day position of the Polar Front. The shallow water depth (1974 m) places the site above the regional carbonate lysocline and calcium carbonate compensation depth and within a mixing zone between North Atlantic Deep Water (NADW) and Circumpolar Deep Water (CDW) (Fig. F2 in the "Leg 177 Summary" chapter). Together with Sites 704 (2532 m) and 1091 (4363 m), Site 1092 (1974 m) forms a depth transect on the Meteor Rise that can be used to study changes in the flux of NADW and its effect on the chemistry of CDW.
The primary objective at Site 1092 was to recover a carbonate-bearing, upper Miocene-Pleistocene sedimentary sequence that could be used to study (1) past migrations in the position of the PFZ, (2) changes in the mixing ratios of upper NADW and CDW in the Southern Ocean and its relation to high-latitude climate change, (3) the response of the Southern Ocean environment to the initiation of Northern Hemisphere glaciation, (4) marine proxies indicating the changes in the Antarctic cryosphere during the early and early late Pliocene when the climate was warmer than present, and (5) paleoceanographic changes in the subantarctic South Atlantic during the late Miocene (Müller et al., 1991; Wright et al., 1991).
1Examples of how to reference the
whole or part of this volume.
2Shipboard
Scientific Party addresses.
Ms 177IR-107