171B Scientific Prospectus


Drilling Strategy
Triple coring with the advanced hydraulic piston corer (APC) technique will be done at five sites in a transect from the margin of the Blake Plateau to the edge of the Blake Escarpment (Figs. 2, 3). Proposed Sites BN-1, BN-2Alt, BN-3, BN-4Alt, and BN-5Alt are primary sites and proposed Sites BN-1Alt and BN-4 are alternates. Use of the extended core barrel (XCB) is planned for all sites except for proposed Site BN-1. Double XCB coring is planned for proposed Sites BN-3 and BN-5Alt, whereas time limitations force us to plan drilling only single XCB holes at proposed Sites BN-2Alt and BN-4Alt. The safety panel has mandated that sites on Blake Nose be drilled up the depth transect from proposed Site BN-1 to proposed Site BN-5Alt. If time permits, we have permission to deepen Sites BN-2, BN-3, and BN-4Alt XCB holes to 700 mbsf. In addition, proposed Site BN-5Alt may also be deepened to 700 mbsf should time permit, and there is a consensus to attempt to recover more of the Aptian-Alpian section. Alternatively, we may choose to drill an alternate site (BN-4 or BN-1Alt) should time permit at the end of the leg.

Boreholes will be located along an existing, high-quality multichannel line that is crossed by a dense web of single-channel and 3.5-kHz lines. Seismic interpretations are supported by a series of JOIDES, USGS, and DSDP boreholes as well as observations from submersibles. The boreholes are intended to penetrate between 170 to 600 m of nannofossil ooze of Eocene, Paleocene, Maastrichtian, Campanian, and Aptian-Albian age. All holes will extend below Reflector Purple, which is identified as basal Campanian in Hole 390A. All holes should penetrate middle to lower Eocene oozes and Paleocene and Maastrichtian-Albian strata. Drilling at proposed Sites BN-2Alt and BN-3 has a high probability of recovering a complete K/T boundary sequence because these boreholes will penetrate the most stratigraphically complete sequences, where the upper Paleocene to Maastrichtian section is about 300 m thick.

The drilling strategy is intended to recover a depth transect in pre-Eocene strata that approaches the depth-resolution possible in upper Pleistocene piston core transects (e.g., Slowey and Curry, 1987, 1992; Lynch-Stieglitz et al., 1994). This will ensure a complete >1300-m depth transect from 1200 to 2500 mbsl in the Paleocene-middle Eocene and bathyal Barremian-basal Albian. The K/P boundary will be recovered in at least four of the five sites and will permit studies of sedimentation of the boundary beds across a depth transect. Triple APC coring will ensure nearly 100% recovery of the sedimentary section in the uppermost 160 m of each site.

There is no evidence from previous drilling or from existing seismic records for appreciable hydrocarbons in the slope sediments proposed for drilling. It is unlikely that there are mobile hydrocarbons of Early Cretaceous or younger age on the Blake Nose. The sediments have never been deeply buried to depths sufficient for hydrocarbon maturation. It is possible that hydrocarbons may exist in the limestones below the pelagic drape of Barremian to Eocene sediments, but the proposed drilling will not penetrate these limestones except during redrilling of Site 390 (proposed Site BN-1). Truncation of the Albian-Barremian clinoforms by the middle Cretaceous unconformity (Reflector Purple) could represent a stratigraphic trap. However, there is no evidence of hydrocarbons below this unconformity at Site 390. In addition, there are no obvious cross-strata reflectors that could represent trapped hydrocarbons within the sedimentary section proposed for drilling.

A complication to drilling in this area is the existence of cherts in the lower Eocene and upper Paleocene. Seven partly lithified layers of limestone and chert were encountered in Hole 390A near the edge of the Blake Plateau. The layers are all about 5 cm thick or less and consist of irregularly replaced carbonates in a 30-m-thick interval of nannofossil ooze. Once through this section, the lower Paleocene through upper Aptian section is entirely ooze at Site 390.

The Blake Nose is covered with a thin layer of phosphorite sand that rests on an erosion surface of late Paleogene or early Neogene age. Erosion has truncated the Paleogene section to varying degrees, particularly at the landward end of the transect (toward proposed Site BN-5Alt). Therefore, it is not necessary to recover the core top in good condition to meet the paleoceanographic objectives of the leg. However, we will attempt to piston core surficial sediments, if possible. In the event that the phosphorite damages the drilling tools or slows drilling operations, we will wash the phosphorite away prior to cutting our first APC core.

Logging Plan
Logging data will be important to achieve the objectives of this proposed leg. Logging will enhance site-to-site correlation, which is critical for reconstructing hydrography. Unfortunately, logging is typically restricted to parts of the hole more than 75-100 m below seafloor (mbsf). Consequently, intersite correlation in the Eocene portion of the record will rely on logs generated from the recovered cores.

Only holes deeper than 400 m will be logged, and these holes will be logged with the triple combination (triple-combo) tool string, Formation MicroScanner (FMS), and geological high-sensitivity magnetic tool (GHMT) strings. The triple-combo provides measurements of porosity, density, and electrical resistivity that will be used to describe the lithology, sedimentary fabric, degree of lithification, and fluid composition. The FMS tool string produces electrical resistivity images of the wall of the hole that can be used to study the orientation and structure of bedding, diagenetic features, fractures and sonic velocity. The GHMT provides continuous measurements of magnetic susceptibility and may provide a magnetic reversal stratigraphy if the magnetization of the sediments is sufficiently strong.

Sampling Strategy
One of the primary objectives of Leg 171B is to reconstruct water column structure, sea surface temperature, and bottom water chemistry at high temporal resolution. We hope to sample at ~3-5 ky resolution to obtain records of biological and physical oceanography at temporal resolutions used routinely in studies of the late Neogene. High resolution sampling in the Cretaceous and Paleogene will provide some of the first histories of the ancient oceans at a resolution similar to that frequently obtained from Pleistocene sediments and will facilitate comparison of the oceanographic dynamics of these time periods. In contrast to Neogene sections in which multiple APC cores are easily obtained because they are close to the seafloor, at some sites we may only be able to retrieve one copy of these Paleogene/Cretaceous sections since we must obtain these sections in a transect across a range of water depths.

Given low regional sedimentation rates in the Cretaceous and Paleogene (1-3 cm/ky), the high temporal-resolution sampling required, and the operational reality of obtaining these deeply- buried sections across a transect of water depths indicates that in special circumstances we may need to exceed JOIDES sampling policy both in sample density and volume to achieve the primary paleoceanography, paleoclimatic, and paloemagnetic cruise objectives. Sampling of ¼ or more of the core volume at specific levels may occasionally be required to achieve cruise objectives in instances even where only one hole or interval exists; however, no such samples will be taken without the express consent of both co-chiefs and the curatorial representative. This may be necessary even in instances where we are able to produce composite sections from multiple holes (to obtain the most continuous and undisturbed sequence). Sample volumes must be large enough to ensure adequate numbers of planktic and benthic foraminifers for biostratigraphic and stable isotope/trace element studies and will depend on exact nature of sediment recovered. This is not meant as a blanket statement for heavy sampling of all cores. In fact, every effort will be made to stay within sample guidelines by first attempting to assemble consortia of scientists to develop plans to attack these problems with the most efficient use of samples and/or by encouraging scientists to work on time slices or a subset of sites (as high resolution sampling of all core at all sites will result in an unreasonably large number of samples to be analyzed by any individual scientist for the scientific results). These sample will be taken during a post-cruise sampling party.

Low resolution sampling will be carried out on board ship to conduct shipboard description/characterization, to facilitate pilot studies, to identify sections requiring high-resolution post-cruise sampling, and to provide samples for studies which do not require a high temporal resolution.

Critical boundary sampling: We expect that critical boundaries will be recovered in most or all of the holes drilled during this leg. All K-P and P-E boundary cores will be treated as critical intervals, subject to appropriate ODP policies and procedures. No samples will be removed from an interval within 3 m above or below the boundary until a consortium of interested scientists has formulated a detailed sampling plan, except for studies aimed at characterizing the interval (including smear slides and toothpick paleontological samples), or for pre-approved studies of the ephemeral properties deemed essential to K-P investigation.

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