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
- characterize
the geochemical fluxes and geophysical aging attending
the upper oxidative alteration of the oceanic crust in
Hole 801C (as discussed above);
- 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;
- 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
- 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
- provide
estimates of the sediment inputs and altered basalt
inputs (geochemical fluxes) into the Izu subduction zone
(as discussed above);
- 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);
- compare
basement alteration characteristics with those in Hole
801C (also in old Pacific crust);
- 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
- 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 ~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 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
To 185 Table of
Contents