171B Scientific Prospectus


The objectives of Leg 171B will be to drill five shallow sites (170-600 m penetration) in a transect from the margin of the Blake Plateau to the edge of the Blake Escarpment. The proposed transect of cores will be used to

1. Interpret the vertical structure of the Paleogene oceans and test the Warm Saline Deep Water hypothesis near the proposed source areas, with a related objective to provide critically needed low-latitude sediments for interpreting tropical sea-surface temperature (SST) and climate cyclicity in the Paleogene;
2. Recover complete Paleocene/Eocene and Cretaceous/Paleogene boundaries along a depth transect to describe the events surrounding the boundaries and water depth-related changes in sedimentation of the boundary beds;
3. Interpret the thermocline and intermediate water structure of the low-latitude Cretaceous oceans and refine the biochronology and magnetochronology of this period;
4. Study Cretaceous and Paleogene climate variability by analysis of marine biota and regional sedimentological patterns;
5. Study the rate and mode of evolution of marine biota in the Cretaceous and Paleogene oceans; and
6. Investigate climate variability and plate motions using the paleomagnetic signal.

Primary Objectives

Paleogene Climate History and Paleoceanography
Paleogene strata are known to exist at shallow burial depths at all sites along the proposed drilling transect. There are two principle issues that will be addressed by recovery of cores from these areas: (1) low-latitude surface temperature history during the Paleocene and Eocene and (2) sources of deep waters during the Paleocene and Eocene.

The Eocene had the most equitable climate of the Cenozoic (Dawson et al., 1976; Shackleton and Boersma, 1981; Axelrod, 1984; Rea et al., 1990). Isotopic data suggest that both surface waters and deep waters reached their maximum temperatures in the early Eocene (Savin, 1977; Miller et al., 1987; Stott and Kennett, 1990). High latitudes were substantially warmer than present and reached temperatures of 15º-17ºC (Zachos et al., 1994). This warm interval lasted 3-4 m.y. and marked fundamentally different climate conditions than were present at any other time in the Cenozoic. Latitudinal thermal gradients were probably less than 6º-8ºC during the Eocene, about one-half the modern pole-to-equator gradient (Zachos et al., 1994). Yet the interval is poorly known both because low-latitude records are rare and those that do exist are either spot cored or disturbed by drilling through Eocene cherts (Stott and Zachos, 1991).

High global temperatures in the early Eocene promise a means to test GCMs run under conditions of increased atmospheric CO2 (Popp et al., 1989; Berner, 1990). Knowledge of low-latitude temperatures provides a major constraint on GCMs because equatorial seas play a major role in regulating heat exchange with the atmosphere (Barron and Washington, 1982). It is possible that the relatively equitable Eocene climates were promoted by higher heat transport between latitudes (Covey and Barron, 1988). Hence, low-latitude temperature data are needed to constrain Paleocene climate models. More generally, the Eocene warm interval provides a test for climate models that integrate latitudinal thermal gradients, atmospheric CO2 levels, and ocean-atmosphere heat transport.

Drilling on the Blake Plateau will provide an opportunity to study each of these issues. The plateau was located on the western gateway from the relatively restricted Tethys/Atlantic basins to the open Pacific during the early Paleogene. Comparisons between Caribbean and existing equatorial Pacific sites could show whether deep waters aged as they entered the Pacific as expected if the Tethys was a major source of saline bottom waters. Existing records from the Caribbean (DSDP Site 152) and Central Pacific (DSDP Site 577; ODP Site 865) provide some of the few low-latitude temperature estimates for the Eocene. However, more complete temperature records are needed to document surface-water temperatures in the Eocene tropics and subtropics.

Late Paleocene Event
There is increasing evidence for a short-lived increase in global temperatures in the late Paleocene. The late Paleocene thermal maximum coincides with an abrupt extinction of benthic foraminifers and a marked excursion in the isotopic chemistry of both benthic and planktonic foraminifers. There is good evidence that production of Antarctic Deep Water may have been shut down at this time and deep waters may have originated at low latitudes (Kennett and Stott, 1990; Pak and Miller, 1992).

Stable isotope evidence suggests that the latest Paleocene event was a brief exception to the general formation of deep waters adjacent to Antarctica (Miller et al., 1987). The Southern Ocean has consistently had some of the youngest waters found in the deep sea since at least the Late Cretaceous (Barrera and Huber, 1990, 1991; Pak and Miller, 1992; Zachos et al., 1992). However, the stable isotope data are strongly biased toward studies of high southern latitude cores and cannot eliminate the possibility that there may have been other sources of deep or intermediate water. Indeed, marked reductions in the 13 C latitudinal gradient suggest that there may have been either extremely low rates of surface-water production or other sources of deep waters during the late Paleocene and late early Eocene. Hence, drill sites located near the ends of the Tethys seaway are needed to monitor the possible contribution of deep waters from the low latitudes and to monitor the biological effects of the latest Paleocene thermal maximum.

Leg 171B drilling will provide the opportunity to study the latest Paleocene extinction event at relatively low latitudes as well as provide the first continuous early Eocene Atlantic record of the benthic foraminifer repopulation after this extinction. Paleocene benthic foraminifer faunas were relatively uniform across a wide paleodepth, while Eocene assemblages are much more distinct at different depths. Depth-dependent changes in benthic foraminifer assemblages will provide critical data for interpretation of changes in the structure and sources of deep waters throughout the Paleogene.

Cretaceous-Paleogene Event
The Report of the Second Conference on Scientific Ocean Drilling (COSOD II) recommends that recovery and analysis of sections penetrating the Cretaceous/Paleogene (K/P) boundary should be given high priority by the Ocean Drilling Program. Cretaceous/Paleogene boundary beds are typically thin in the deep sea, yet they contain our best evidence for the geographic distribution and magnitude of the extinctions and the subsequent recovery of the biosphere.

Recent evidence of the impact of a bolide on the Yucatan Platform has focused debate over the history and consequences of this Cretaceous-Paleogene event. Evidence of an impact in the Caribbean includes discoveries of glass spherules and shocked quartz in boundary sections at Haiti and Mimbral, as well as gravity measurements and drill core data that imply the existence of a 180-km-diameter structure of Maastrichtian-earliest Paleocene age beneath the Yucatan (Sigurdsson et al., 1991a, 1991b; Margolis et al., 1991; Alvarez et al., 1991; Hildebrand and Boynton, 1990). Reanalysis of DSDP Sites 536 and 540 in the Gulf of Mexico has led to the discovery of thick deposits of reworked carbonates of diverse ages. These deposits contain upper Maastrichtian nannofossils and occur immediately below lowest Paleocene sediments. Alvarez et al. (1991) have interpreted these K/P boundary deposits as part of the ejecta blanket.

Objectives of K/P boundary drilling on the Blake Plateau include the recovery of a detailed record of the events immediately following the impact such as the sequence of ejecta fallout and settling of the dust cloud. DSDP Hole 390A contains foraminiferal markers for the earliest Paleocene P-alpha zone and the latest Maastrichtian nannofossil zones suggesting the section may be biostratigraphically complete (Gradstein et al., 1978). Unfortunately, rotary drilling of this hole extensively mixed the soft ooze, making recovery of a detailed record of K/P boundary events impossible. Modern piston and extended core barrel (XCB) coring technology should produce better recovery and less disturbed core than were obtained from Hole 390A.

Specific Objectives

Specific objectives include the following:

1. Reconstruction of climatic and biological evolution immediately prior to the end-Cretaceous extinction.
2. Evaluation of the recovery of the oceans and biotas following the extinction. Piston-cored sections could provide evidence for the magnitude of the extinctions, their duration, and patterns of diversification following the event. Cores containing records of the lowermost Paleocene should also be suitable for studies of the geochemical history of the oceans in the absence of a diverse plankton community.
3. Depth-dependent patterns of sedimentation across the K/P boundary. Our proposed drilling would penetrate the boundary section at present water depths of about 1410-2600 m allowing us to determine whether there were depth-dependent changes in the carbonate lysocline or organic matter preservation after the extinction.
4. Depth-dependent changes in benthic foraminiferal biofacies across the extinction boundary in a low-latitude setting.

Cretaceous Paleoceanography
A depth transect in Cretaceous strata offers an unparalleled opportunity to study the hydrographic structure of the low-latitude Cretaceous oceans. There are six principle issues that could be addressed by recovery of a depth transect of cores from this region.

1. Patterns of turnover in middle Maastrichtian nannofossils, foraminifers, and mollusks. Evidence for numerous Southern Ocean sites suggests a major turnover of austral species at about 71 Ma, which coincides with a major carbon isotope excursion and extinction of inoceramid mollusks (Ehrendorfer, 1993). Low-latitude sites with good recovery and good preservation through this interval are rare, so it is still unclear whether the high-latitude turnovers are synchronous with low-latitude biotic crises.
2. Determine the response of low-latitude SST to the Maastrichtian cooling trend at high latitudes.
3. Cyclostratigraphy of the Cretaceous. Can orbital cycles be recognized in low-latitude sections and, if so, can they be used to refine Cretaceous time scales? Also, what is the frequency of Cretaceous orbital periodicity and how do patterns of orbital forcing compare with those of the Cenozoic?
4. Determine the sources of deep waters during the Barremian-Albian, Campanian, and Maastrichtian at times when the North Atlantic was a relatively enclosed basin.
5. Analyze depth dependency on the formation and expression of carbonate cycles and sediment accumulation rates.
6. Study the lithology, benthic foraminiferal biofacies, and depositional patterns across an ancient slope in relationship to sea-level changes and regional climate.

Paleogene and Cretaceous Evolutionary Dynamics
Sediments recovered at Site 390 contain an extremely well-preserved microfossil fauna and flora. We hope to use similar microfossil assemblages recovered during Leg 171B to

1. Refine and integrate microfossil biochronologies between the various groups of microorganisms as well as with cyclostratigraphy and magnetostratigraphy and
2. Investigate patterns of ecological evolution through stable isotope, morphometric, and faunal analyses. Related to this effort is the study of the timing and origin of speciation events in relation to paleoceanographic history.

Mesozoic and Paleogene Magnetic Records
The intensity of the magnetic susceptibility record has the potential to reveal variability in the climate system associated with orbital cycles and variations in the supply of magnetic minerals. In addition, a well-defined paleomagnetic record in oriented cores will be used to determine paleomagnetic poles for the North American plate during the Paleogene. This is a poorly defined part of the North American apparent polar wander path and one in which the polar motion changed from a Cretaceous standstill to a Cenozoic track leading to the current pole.

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