SCIENTIFIC OBJECTIVES

Previous drilling has already provided many parts of the crustal flux equation at the Izu and Mariana Margins and provides a strong rationale for continuing the effort to mass balance fluxes across the subduction zones. The missing part of the flux equation is largely the input: (1) both the incoming sediment and basaltic sections approaching the Izu-Bonin Trench, and (2) the altered oceanic crust seaward of the Mariana Trench. In order to provide this critical information on the crustal inputs to the subduction zone, drilling is planned at two sites: one seaward of the Mariana Trench (ODP Hole 801C), and one seaward of the Izu Trench (proposed Site BON-8A).

Hole 801C
The primary motivation for returning to ODP Hole 801C, seaward of the Mariana Trench (
Fig. 2), is to sample the upper oxidative zone of alteration of this oldest in situ oceanic crust. Previous drilling during Leg 129 only penetrated 63 m into "normal" Jurassic basement. Based on Hole 504B and other basement sites with sufficient penetration, the upper oxidative zone of alteration, which contains the lion's share of some element budgets (e.g., K, B, etc.), lies in the upper 200 300 m of the basaltic crust (Alt et al., 1986; Staudigel et al., 1995). The transition from volcanics to sheeted dikes may not lie much deeper: 500-600 m at Hole 504B (Detrick et al., 1994); 450 m to Layer 2b (Carbotte et al., 1997); and only a few 100 m at Hess Deep (Francheteau et al., 1992). We propose to deepen Hole 801C an additional 250 m (~400 m total basement penetration) into basement at Hole 801C to accomplish the following scientific objectives to

  1. characterize the geochemical fluxes and geophysical aging attending the upper oxidative alteration of the oceanic crust in Hole 801C (as discussed above);
  2. compare igneous compositions, structure, and alteration with other drilled sections of in situ oceanic crust, in particular 504B, contrasting a young site in Pacific crust with the oldest site in Pacific crust;
  3. help constrain general models for seafloor alteration that depend on spreading rate and age. Hole 801C is in the world's oldest drilled oceanic crust, which is at 165 Ma and was formed at a fast-spreading ridge at ~160 mm/yr full-rate, therefore it embodies several end member characteristics; and
  4. test models for the Jurassic Magnetic "Quiet" Zone (JQZ). Hole 801C is located in an area of very low amplitude magnetic anomalies, usually called the Jurassic Magnetic "Quiet" Zone. The JQZ has been suggested to result from (1) oceanic crust of a single polarity with small anomalies due to intensity fluctuations, (2) oceanic crust with magnetic reversals so numerous as to "cancel each other out" when measured at the sea surface, or (3) oceanic crust with a more normal frequency of magnetic reversals acquired when the dipole field intensity was anomalously low. Deepening Hole 801C would allow testing of the above hypotheses, and in particular, of the third hypothesis of magnetic reversals during a period of anomalously low field intensity, if fresh unaltered volcanic glass could be obtained. Such material can yield reliable paleointensity information (Pick and Tauxe, 1993) on the very fine, single-domain grains of the titanium-free magnetite within the volcanic glass.

Site BON-8A
The primary motivation for proposed Site BON-8A, a site ~60 km seaward of the Izu Trench (
Fig. 6), is to provide the first complete section of sediment and a significant section of altered oceanic crust that enters this subduction zone. Previous drilling failed to penetrate successfully through resistant cherts, so most of the sediment column is unsampled. Only 1 m of basalt has been recovered from basement in this vast area (at DSDP Site 197). We propose to drill and core the entire sedimentary sequence (470 m) at Site BON-8A, and as far into the upper oxidative alteration zone of the basaltic basement as possible to a maximum basement penetration of 430 m. The scientific objectives are to

  1. provide estimates of the sediment inputs and altered basalt inputs (geochemical fluxes) into the Izu subduction zone (as discussed above);
  2. contrast crustal budgets here with those for the Marianas, to test whether along-strike differences in the volcanics can be explained by along-strike variations in the crustal inputs (as discussed above);
  3. compare basement alteration characteristics with those in Hole 801C (also in old Pacific crust);
  4. provide constraints on the Early Cretaceous paleomagnetic time scale. Site BON-8A is approximately on magnetic anomaly M12 (Nakanishi et al., 1988). Its basement age should be about 135 Ma and should correspond to the Valanginian Stage of the Early Cretaceous according to recent time scale calibrations (Harland et al., 1990; Gradstein et al., 1994; Channell et al., 1995). However, those age estimates are poorly known and can be tested by drilling at Site BON-8A. Specifically, a reasonably precise date on M12 at Site BON-8A could test the proposed new time scale of Channell et al. (1995); and
  5. provide constraints on mid-Cretaceous carbonate compensation depth (CCD) and equatorial circulation fluctuations. Based on its theoretical Cretaceous paleolatitude history, Site BON-8A may have formed at ~5S, drifted south to 10S in its early history and then gradually drifted north, crossing the paleoequator as the Pacific plate accelerated its northward motion about 85-90 Ma. A site such as BON-8A with an Early Cretaceous basement age (~135 Ma), an equatorial paleolatitude history during the mid-Cretaceous, and a predictable subsidence history for the Cretaceous is ideal for testing proposed CCD variations (Theirstein, 1979; Arthur et al., 1985). In addition, Erba (1992), following Roth (1981), has shown that certain species of nannoplankton can be characterized as "high fertility indices" and used as approximate indicators for the paleoequatorial upwelling zone. Using these nannoflora, potential fluctuations in the mid-Cretaceous equatorial circulation system could be studied at Site BON-8A when the site was nearly stationary near the paleoequator (especially from 115-95 Ma).

Deep Biosphere
During Leg 185, scientists will conduct contamination tests and develop a standard sampling procedure for deep biosphere research. The objective of the contamination test is to determine the amount of mixing created by the coring process by introducing a tracer (latex beads) at the mouth of the core barrel as the core is cut. Some limited shipboard microbiological analyses will be conducted during Leg 185 (i.e., direct bacterial counts using the existing epifluorescence microscope). Most of the samples, however, will be adequately stored for shore-based analysis.

Diamond Core Barrel (DCB)
Leg 185 has a total of two days allocated to conduct tests of the diamond core barrel (DCB) at both Sites 801C and Bon-8A. Comparison of DCB techniques, hardware, and results in varied lithologies has great value to longer-term ODP goals and the DCB development program. The greatest potential benefit of the DCB tests to Leg 185 is to optimize core recovery and quality.

To 185 Regional Geological Setting of Proposed Sites

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