Leg 172 Scientific Report


Ocean Drilling Program Leg 172 drilled 11 sites in the westernmost North Atlantic Ocean: two on the Carolina Slope (Sites 1054 and 1055), seven on the Blake-Bahama Outer Ridge (Sites 1056 to 1062), one on the Bermuda Rise (Site 1063), and one on the Sohm Abyssal Plain (Site 1064). More than 5700 m of sediments were cored that range in age from the early Pliocene to the Holocene. The westernmost North Atlantic Ocean is presently the location of an important exchange of heat, salt, and water with other ocean basins and the location of huge sediment accumulations (sediment drifts) related to deep- and intermediate-water oceanic circulation. Sediment drifts are characterized by high accumulation rates, and have recently proven to be excellent recorders of past climate variability on orbital to centennial time scales. The 11 sites cored during Leg 172 succeeded in recovering complete and expanded sequences of slope and drift sediments well suited for high-resolution paleoceanographic studies, particularly for the past 1 m.y. The cored locations span a wide range of water depths (from 1306 m at Site 1054 on the Carolina Slope to 4786 m at Site 1062 on the Bahama Outer Ridge), "sampling" virtually all the various components of the North Atlantic Deep Water. Shore-based studies of stable isotope, chemical, and sedimentological paleotracers should allow a detailed three dimensional reconstruction of circulation changes related to climatic evolution and variability since the inception of Northern Hemisphere Glaciation. Shipboard results show striking evidence of orbital-scale climatic changes, but millennial and perhaps centennial scale changes should be resolvable at some sites. Sedimentation patterns are surprisingly uniform in the Carolina Slope and Blake-Bahama Outer Ridge region. Every location has an upper unit characterized by cyclic alternation between nannofossil-rich and clay-rich beds, the base of which appears at most sites to be about 0.8 m.y. old. Hence, this lithologic change seems to reflect the dramatic onset of the 100-k.y. glacial/interglacial cycles of the middle and late Pleistocene and attests to the strong influence of climate on sedimentation and circulation patterns. Below this upper lithologic unit, clayey sedimentation prevails. The stratigraphy at Bermuda Rise Site 1063 is largely similar to that detected in the Blake-Bahama Outer Ridge and Carolina Slope region, thus suggesting basinwide sedimentation patterns.

Primary age control was provided by calcareous planktonic biostratigraphy, integrated at most sites with magnetostratigraphy. Paleontologists noted that some planktonic foraminifer datums do not seem to be applicable in the western North Atlantic region. The expanded sections recovered will, however, offer the opportunity of an accurate calibration of late Pliocene to late Pleistocene calcareous plankton horizons and an evaluation of their inter-regional synchroneity. Furthermore, at all sites, except the shallow-water Site 1054, it has been possible to establish a highly resolved age model for the past 0.9 m.y. based on magnetic susceptibility, which correlates well with the standard marine oxygen isotope scale. This age model surprisingly suggests no major change in the sediment accumulation rates between glacial and interglacial cycles.

Most sites contain a detailed record of the behavior of the Earth's magnetic field, providing unprecedented documentation of secular variation (e.g., excursions) during the Brunhes Chron and the Matuyama-Brunhes transition. Finally, shipboard geochemical analyses have documented the regional extension of gas hydrate and the important role of organic and inorganic diagenesis in the sediment-drift environment.

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