Leg 172 Scientific Report


Leg 172 succeeded in recovering complete and expanded sequences of slope and drift sediments that are well suited for high-resolution paleoceanographic studies, particularly those concentrating on the past 1 m.y. of Earth's history. The cored locations span a wide range of water depths, from 1306 m at Site 1054 on the Carolina Slope to the 4786 m at Site 1062 on the Bahama Outer Ridge, and sample virtually all the various components of the North Atlantic Deep Water. Besides providing the raw material for shore-based studies that will address the long-range leg objectives (as given in the Leg 172 Scientific Prospectus and outlined below in italics), Leg 172 accomplished many of the objectives through shipboard studies. These results are described briefly below.

1. Obtain a detailed history of late Neogene paleoceanography and paleoclimate in the North Atlantic by investigating: (a) millennial scale oscillations of stable isotopes (C and O), faunal and floral abundance, percent carbonate and other lithologic components, and trace metals in drift deposits; (b) the nature of cyclicity of these oscillations; and (c) how these cycles are related to the history of Northern Hemisphere glaciations during the late Neogene.
Through coring multiple holes at each site along the Blake-Bahama Outer Ridge and at the Bermuda Rise, we obtained thick, complete Pleistocene sedimentary sequences. Shore-based studies of stable isotope, chemical, and sedimentological paleotracers from these sequences 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. The expanded sections recovered will offer the opportunity of an accurate calibration of late Pliocene to late Pleistocene calcareous plankton horizons and an evaluation of their interregional 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.

2. Investigate sediment wave migration and drift sedimentation processes.
Mud-wave dynamics have been a long-standing interest of ODP, but had not been successfully studied by ODP until this leg. On Leg 172, we cored a transect of eight holes across a single mud wave. The coring results revealed variations in the sedimentation rates on opposite flanks of the mud wave, indicating a net eastward migratration of the mud wave. Our pre-drilling seismic survey also revealed, with great clarity, structures within the mud wave, which will be combined with the coring results to give a detailed dynamical history of this mud wave and to document The changes in paleocirculation that have occurred during the late Pliocene and Pleistocene.

3. Investigate the detailed variations of the Earth's magnetic field (secular variations and reversals).
Most sites proved to be excellent magnetic recorders, and given the high sedimentation rates, they will yield some of the most detailed records of the behavior of the Earth's magnetic field obtained to date. Highlights from shipboard analyses include exceptional magnetostratigraphic records that agree well with biostratigraphic constraints, transitional field behavior at the Brunhes-Matuyama reversal that can be correlated at sites over 1600 km apart, and excursions and secular variation events that can be correlated between multiple holes and sites, and that were documented in measurements on discrete samples as well as split cores. Shore-based studies at higher resolution will further refine the paleomagnetic direction and paleointensity record, which will then be used to test and constrain models that attempt to explain how the geomagnetic field is generated and maintained.

4. Investigate geotechnical/acoustic properties of deep-sea sediments.
Physical properties showed variations that appear to reflect both the 41-k.y. cycles of the early Pleistocene and the onset of the 100-k.y. glacial/interglacial cycles of the middle and late Pleistocene. The orbitally driven variations in acoustic properties were so dominant that they were even visible in seismic records collected during the cruise, an observation that could prove to be very significant for seismic stratigraphy studies in the region.

5. Investigate geochemical signals associated with the formation, dissociation, and distribution of gas hydrate.
On Leg 172, seismic data collected showed a probable bottom simulating reflector under all the sites drilled along the Blake-Bahama Outer Ridge. Moreover, the pore-water samples taken from cores give chloride concentrations that strongly suggest the presence of underlying gas hydrate. These data are critical to improve estimates of the size of the gas hydrate reservoir in the Blake Outer Ridge area (and elsewhere), and to understand the geochemical processes involved in the development of extensive gas hydrate fields.

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