As discussed above, previous drilling has already laid the foundation to much of the crustal flux equation at the Izu and Mariana subduction systems and provided a strong rationale for continuing the effort to determine the mass balance fluxes across the subduction zones. The missing part of the flux equation is largely the input: (1) the altered oceanic crust seaward of the Mariana Trench—Site 801—and (2) both the incoming sediment and basaltic sections approaching the Izu-Bonin Trench—Site 1149. In addition, both sites are located along the same flow line in Mesozoic Pacific oceanic crust (from ~170 to 130 Ma) and provide an unparalleled opportunity to study the geochemical and physical nature of old Pacific crust and its tectonic, sedimentation, and magnetic histories.
The primary motivation for returning to Hole 801C, seaward of the Mariana Trench (Fig. F2), was 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 basement rocks from 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 (e.g., Alt et al., 1986; Gillis et al., 1992). The objectives of coring and logging at this site were to
The primary motivation for drilling at Site 1149, a site ~100 km seaward of the Izu-Bonin Trench, was to provide the first complete section of sediment and altered oceanic crust entering this subduction zone. Previous drilling in the Nadezhda Basin failed to penetrate resistant cherts, so most of the sediment column was unsampled. Only 1 m of basalt had been previously recovered from basement in this vast area. The objectives of coring and logging at this site were to
In addition to serving as an important reference site for crustal inputs to the Izu-Bonin Trench, Site 1149 can also address additional paleomagnetic and paleoceanographic problems (objectives 4 and 5 above).
According to Nakanishi et al. (1992), Site 1149 is approximately on magnetic Anomaly M12. Its basement age should be ~133 Ma and 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 constrained but may be tested by drilling. Specifically, a reasonably precise date on the magnetic Anomaly M12 at Site 1149 could test the proposed new time scale of Channell et al. (1995) and our proposed magnetic anomaly interpretation (see "Geophysics").
Based on its theoretical Cretaceous paleolatitude history, Site 1149 may have formed at ~5°S, migrated south to 10°S in its early history, and then gradually moved north, crossing the paleoequator as the Pacific plate accelerated its northward motion ~85-90 Ma (Fig. F7). A site such as Site 1149 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 (Thierstein, 1979; Arthur et al., 1985). In addition, Erba (1992), following Roth (1981), has shown that certain species of nannofossils can be characterized as "high fertility indices" and used as approximate indicators of the paleoequatorial upwelling zone. Using these high fertility indices, potential fluctuations in the equatorial circulation system could be studied at Site 1149 for the mid-Cretaceous, when it was located near the paleoequator (especially from 115 to 95 Ma).
The deep water (~6000 m) and proposed penetration into old oceanic basement provided an intriguing target in the search for hidden bacterial life forms. Leg 185 was the first ODP leg to incorporate microbiology as a major new initiative. The microbiology objectives for Leg 185 were to