SCIENTIFIC OBJECTIVESAs 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. 5), 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. The objectives of coring and logging at this site involved
The primary motivation for drilling at Site 1149, a site ~100 km seaward of the Izu 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 is unsampled. Only 1 m of basalt has been recovered from basement in this vast area. Core and logging data from this site was to
1. Provide estimates of the sediment inputs and altered basalt inputs
(geochemical fluxes) into the Izu subduction zone;
2. Contrast crustal budgets for the Izu-Bonin arc with those for the Mariana arc, to test whether along-strike differences in the volcanics can be explained by along-strike variations in the crustal inputs;
3. Compare basement alteration characteristics with those at Hole 801C (on 170-Ma crust along the same flow line);
4. Provide constraints on the Early Cretaceous paleomagnetic time scale; and
5. Provide constraints on mid-Cretaceous carbonate compensation depth (CCD) and equatorial circulation fluctuations.
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. Because the subduction
cycling objectives have already been discussed in some detail above (see
"Introduction"), we elaborate more in the following paragraphs on the
paleomagnetic and paleoceanographic objectives.
According to Nakanishi et al. (1988), Site 1149 is approximately on magnetic Anomaly M12. Its basement age should be ~133 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 1149. Specifically, a reasonably precise date on Anomaly M12 at Site 1149 could test the proposed new time scale of Channell et al. (1995).
Based on its theoretical Cretaceous paleolatitude history, Site 1149 may have formed at ~5°S, drifted south to 10°S in its early history, and then gradually drifted north, crossing the paleoequator as the Pacific plate accelerated its northward motion ~85-90 Ma (Fig. 7). 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 (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 of the paleoequatorial upwelling zone. Using these nannoflora, potential fluctuations in the equatorial circulation system could be studied at Site 1149 for the mid-Cretaceous, when it was nearly stationary near the paleoequator (especially from 115 to 95 Ma).
Microbiology Objectives for Both Sites
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 included
1. Determining the amount of biological contamination created by the
APC, XCB (extended core barrel), and RCB coring processes;
2. Developing a sample-handling strategy for routine microbiological sampling; and
3. Conducting culturing experiments with several media at both atmospheric and in situ pressure.
To 185 Drilling Strategy
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