The primary objective of Leg 208 drilling is to recover sediments representing each of the target intervals over a paleowater depth range of 2 km. The recovered sequences should be stratigraphically continuous and have sedimentation rates sufficient to resolve at least 41-k.y. orbital cycles. In addition, the sediments should be relatively unlifithified to allow for easy extraction of calcareous microfossils for geochemical analyses. Preservation need not be perfect, but good enough so that relative changes in isotopic and trace metal ratios are preserved.
To meet the objectives listed above, we have designed a transect with 5 primary and 12 alternate sites (Tables T1, T2). Several of the existing Paleocene–Eocene sections recovered from the Atlantic are riddled with unconformities. Some unconformities are clearly erosional (nondepositional) in nature, whereas others may be due to intense dissolution. Evidence suggests that the unconformities around the primary target, the P/E boundary, on Walvis Ridge, are highly localized. The new site survey data demonstrate that more expanded (and possibly continuous) sequences can be recovered nearby. We have tried to maximize our opportunities to recover the P/E boundary and other key target intervals by selecting sites that appear to have a relatively thick Paleogene sequences. We also have included at least one proximal alternate site for each of the primary sites, which will give us flexibility to move to a new site should we discover a local unconformity at the P/E boundary.
At present, the primary sites for Leg 208 are WALV-8B, 9A, 10C, 11B, and 12A (WALV-9A will most likely be replaced by WALV-9B once it is approved by the PPSP). They span a water depth range from 2557 to 4726 m. The primary stratigraphic targets for Sites WALV-8B and WALV-9A include expanded PETM and EOGM intervals in upper Paleocene to lower Oligocene chalks and oozes (Fig. F5). We are avoiding areas proximal to DSDP Site 525, where the upper Eocene and lower Oligocene are absent, and Site 529, where several slumps are present. Proposed Site WALV-10C, located to the north of Site 528 (Fig. F14) at an intermediate depth of 3842 m, offers a potentially more expanded upper Paleocene to upper Oligocene section. The deepest site, WALV-12B, is located well north of DSDP Site 527. Proposed Site WALV-11B is located near Site 527 but at slightly shallower water depth of 4375 m. The primary target is a complete Paleogene sequence that includes the PETM and EOGM excursions, as well as the K/T boundary. Drilling will proceed to lower Paleocene sediments at three sites (WALV-8B, WALV-9A, and WALV-10C) and the upper Maastrichtian at two (WALV-11B and WALV-12A).
Each site will be double APC and/or XCB cored. Third holes will be drilled if it can be established through near–real time stratigraphic correlation that primary target horizons fall within coring gaps in the first two holes. Multiple hole coring is intended to fill coring gaps from one hole with recovered sequences from another hole.
The current time estimate for coring is ~33 days, with ~6 days available for logging. Total transit time is estimated to be ~18 days (Table T2).
An important part of the shipboard measurements at each site will be the near–real time construction of a composite depth scale from hole-to-hole correlation of cores and spliced sections representing complete stratigraphic intervals. The primary splice is used for later sampling, and an alternate spliced section, as complete as possible, will be used for ODP archiving and also for additional sampling if warranted. Composite sections will be constructed using core logging data (magnetic susceptibility, natural gamma radiation, color reflectance, and gamma ray density). Two dedicated stratigraphic correlators will operate the whole-core multisensor track (MST), retrieve the relevant data from the database, and correlate cores from multiple holes by vertical whole-core depth shifting. The MST sampling program will be set to optimize measurement time (within constraints of adequate data quality and depth resolution) for the overall time available. In typical operations, ~60–90 min will be available to process one core (~10 m), which will also set the core processing rate for all other laboratory stations. Correlation should occur as rapidly as possible such that the stratigraphic correlator can inform the driller (or core technician) at what depth the drill string should be placed for shooting cores in the third hole based on the correlation of the cores from the first two holes.
After MST measurements are completed, the cores will be split. Working halves will be available for moisture and density sampling and other physical properties measurements. The archive halves will undergo digital imaging with the new digital imaging system, visual core description, diffuse color reflectance measurements with the automated Minolta CM2002, and natural remanent magnetization analysis in the paleomagnetism laboratory. The imaging and color reflectance devices will be operated by the lithostratigraphy (sedimentology) group.
Sample Allocation CommitteeFor each drilling leg, a sample allocation committee (SAC) is constituted, comprised of the Co-Chief Scientists, the ODP Staff Scientist, and the ODP Curator. During the leg, the Curator's authority and responsibilities to the SAC may be ceded to the shipboard Curatorial Representative.
Because the SAC best understands the scientific needs of their leg, this group establishes a leg-specific sampling strategy and makes decisions on leg-specific sample requests received before the leg sails, during the leg, and within (but not after) the 1-yr postcruise moratorium. The sampling strategy outlined here was agreed upon by the Leg 208 SAC in September 2002.
Samples for routine shipboard analyses of ephemeral properties and for measurements essential to safety monitoring and initial shipboard interpretations will be taken during the cruise by the shipboard participants. Samples for shipboard analyses are taken by the appropriate laboratory representatives. Shipboard samples include the following:
Whenever possible, samples for postcruise research will be taken from the splice constructed by hole-to-hole correlation of cores during the cruise. Such sampling will maximize integration and impact of scientific results. Because it takes a few days to complete the composite depth section and splice (and more time to prepare adequate sampling templates), high-resolution sampling will not be possible during the cruise. This is also true for low- and high-resolution sampling of "critical intervals," which include the K/T, P/E, and E/O boundaries. All sampling for postcruise research, except for special cases mentioned above, will therefore be deferred to one or more postcruise sampling party(ies) ~4–5 months postcruise in Bremen, Germany.
It is the cruise participants' responsibility to read and understand the ODP Sample Distribution, Data Distribution, and Publications Policy, which can be found at http://www-odp.tamu.edu/publications/policy.html.
Sample requests can be submitted by shipboard and shore-based participants at any time. Participants are asked to submit a request no later than 3 months precruise. Requests received precruise may have precedence over competing requests made during the cruise. All precruise requests will be posted on the ODP web page and the URL will be made available to the Leg 208 participants only. The SAC encourages participants to contact each other and coordinate sample requests and proposed work before the cruise to minimize this effort during the precious time available during the cruise. A draft sampling plan will be prepared at the early in the cruise by the scientific party, under the leadership of the SAC, and refined during the cruise based on the actual coring results. The final sampling plan will be highly integrated, ensuring that essential work will be completed on all intervals and that duplication of efforts will be avoided.
Downhole logging will provide physical and chemical proxy data for interpreting some of the fundamental questions Leg 208 is addressing. Where core recovery is poor, downhole logs may present the most reliable source of information; where core recovery is good, log data can be correlated with core data to produce more detailed and emphatic results. All sites deeper than 300 mbsf and all sites using XCB coring in the lower interval will be logged with the two standard logging tool strings, the triple combination (triple combo) and the Formation MicroScanner (FMS)/sonic string. The Well Seismic Tool (WST) will be deployed at a minimum of two sites. The characteristics of these logging tools are summarized below (additional information can be found at http://www.ldeo.columbia.edu/BRG).
The triple combo tool string includes the Dual Induction Tool (DIT), which measures indirect resistivity at three invasion depths, the Accelerator Porosity Sonde (APS), which measures porosity from epithermal neutron measurements, and the Hostile Environment Litho-Density Sonde (HLDS), which measures bulk density from Compton scattering and provides an indication of general lithology from the photoelectric effect. Commonly, the Hostile Environment Gamma Ray Sonde (HNGS) is added to this tool string. Following its past successes, the LDEO Multi-Sensor Spectral Gamma Ray Tool (MGT) will be added to the triple combo tool string, providing a vertical resolution three times higher than the standard gamma ray sonde. Density measurements will be crucial for calculating acoustic impedance values, and neutron porosity data will be particularly valuable for identifying intervals of high-porosity radiolarian ooze. The downhole natural gamma radiation and density values will be correlated with comparable analyses from the MST, which will enable the precise depth matching of cored sections. MGT logs will be useful for cyclostratigraphic analysis of Upper Cretaceous and lower Cenozoic sequences.
The FMS/sonic tool string includes the Formation MicroScanner, which measures resistivity at centimeter resolution on four pads moving along the borehole, the General Purpose Inclinometer Tool (GPIT), and the Dipole Sonic Imager (DSI), which measures compressional and shear wave velocity, as well as cross-dipole and Stoneley waveforms. Chert horizons likely to present in some intervals at Leg 208 sites show up exceptionally well as resistive stripes on FMS images.
The WST is a single-axis checkshot tool used for zero-offset vertical seismic profiles (VSPs). It consists of a single geophone that is used to record acoustic waves generated by a water or air gun located near the sea surface. Downhole sonic data in conjunction with the density results, and checkshot surveys where necessary, allow generation of a velocity profile, a time/depth model, and synthetic seismograms. These results will be compared with the regional seismic sections to interpret the origin and geological significance of the major reflectors at a regional scale. WST checkshot surveys should be measured at 30- to 50-m spacing.