HISTORICAL PERSPECTIVES
The two sites drilled during this leg are in Mesozoic crust, in the
oldest part of the Pacific Ocean Basin and in extreme water depths, which
provided a challenge in terms of drilling technology. These sites are the
most recent part of an unfolding drama of drilling by DSDP and ODP in the
west Pacific abyssal plains over the past three decades.
It was apparent in 1968 after DSDP Leg 3 that the deep ocean basins
were formed by seafloor spreading and, thus, were very young relative to
the age of the Earth. The same evidence from rifted continental margins
that led Wegner and DuToit to propose continental drift then could be used
to infer that the Atlantic and Indian Ocean Basins were no older than ~200
m.y. and probably somewhat younger. However, no such clues applied to the
Pacific basin, because it is geologically isolated from the surrounding
continents by subduction zones. Thus, in the late 1960s it seemed possible
that the world's oldest deep ocean rocks lay somewhere in the western
Pacific more than 10,000 km away from the nearest spreading ridge.
DSDP Legs 6 and 7 in 1969 were the first to search the western Pacific
for the Earth's oldest oceanic crust and sediments. The search ultimately
took 20 yr and 10 legs of DSDP/ODP (Legs 6, 7, 17, 20, 32, 33, 60, 61, 89,
and 129) to achieve the final goal. Many people were involved; the most
persistent members of the "Old Pacific Club" include B.C. Heezen, E.L.
Winterer, S.O. Schlanger, R. Moberly, I. Premoli Silva, W. Sliter, D. Bukry,
R.G. Douglas, and H.P. Foreman. During the early legs, drilling sites were
targeted with single-channel seismic records characterized by
acoustically opaque chert layers that obscured the underlying volcanic
basement. Often the coring was frustrated by these impenetrable cherts,
as well as by volcaniclastic sediments and basalts of Cretaceous age. To
those who went out repeatedly and came back with more questions than
answers, what had started as an oceanographic exercise turned into an
ongoing adventure.
Leg 129 brought the JOIDES Resolution, with improved station keeping
and heave compensation that proved capable of penetrating the cherts,
where the Glomar Challenger would have failed. Also, preparations for Leg
129 led by Yves Lancelot and Roger Larson included four multichannel
seismic expeditions to the area searching for seismic "windows" through
the Cretaceous volcaniclastic sediments and solid basalts. This
combination of improved science and technology was finally successful 10
yr ago, in 1989, at Site 801 in the Pigafetta Basin where Jurassic
sediments of Bajocian-Bathonian age were discovered overlying ~170-Ma
oceanic crust (Lancelot, Larson, et al., 1990). If older material exists in
the area, tectonic reconstructions suggest it would not exceed that at
Site 801 by more than 10 m.y. By comparison, the next oldest deep-ocean
sites are ~5-15 m.y. younger: Site 534 near the continental margin of
North Carolina in the North Atlantic and Site 765 in the Argo Abyssal
Plain, in the Indian Ocean (Table 1). Thus, the original suggestion that the
Earth's oldest deep-ocean deposits lie in the western Pacific is correct
but, coincidently, not by much.
Just before Leg 129 in the Pacific Ocean, Plank and Ludden (1992)
completed the first attempt at quantifying the global geochemical budget
in an ODP Leg 123 drill core. Drilling at Site 765 penetrated ~1000 m of
sediment, derived from the northwestern Australian margin, and ~250 m
into the basement. The main objective was to characterize the crust
subducting into the Sunda Arc of Indonesia. The idea of characterizing the
inputs to subduction zones arose from a proposition by J. Natland and C.
Langmuir in 1987-1988. The idea was extensively debated in the ODP
Planning Committee and the Indian Ocean and West Pacific Ocean Regional
Panels. Despite the fact that Leg 123 had been successful, the idea took
several years to root itself firmly within the panel structure as a viable
scientific approach. One remark was particularly pointed: "How can you
learn something about milk by studying grass when you don't know
anything about cows?"
The "cow model" is in fact an excellent way to convey the competing
models for subduction recycling studies. The grass control model is that
the same breed of cow eating different flavors of grass will produce
different flavors of milk (Fig. 3A). The cow control model emphasizes the
cow over the grassdifferent breeds will produce different flavors of
milk, even if they eat the same kind of grass (Fig. 3B). In the context of
subduction recycling studies, the cow is the subduction factory, the grass
is the oceanic crust and sediment being subducted, and the milk is the
volcanic output at the arc. The grass control model predicts that the
"flavor" of the subducted input (carbonate-rich pelagic or volcaniclastic
sediments) has a strong effect on the flavor of the volcanic output despite
slight differences in subduction style from one margin to the next. The
cow control model would predict that given similar subducted input, the
different characteristics of the subduction zone (dip of the subducted
plate, thermal structure, convergence rate) will lead to differences in the
volcanic output. Thus, the central question is whether the grass or the
cow is the dominant control on the flavor of the milkwhether the
subducted sediments or the physics of the subduction zone are the
dominant control on the volume and composition of the volcanic output.
Subduction factory studies need excellent control on the subducted input
before we can answer this question.
Drilling into the serpentinite mounds and the ash record in the Izu
Bonin forearc (Leg 126; Leg 125; Fryer, 1992) recorded the magmatic
output of the arc and the early dewatering of the subducting plate in the
Izu-Bonin arc. Leg 170 in the Costa Rica accretionary prism (Kimura,
Silver, et al., 1997) focused on problems of sediment accretion and fluid
evolution in an accretionary prism and was a continuation of initiatives
over the past 10 yr by ODP in active margin accretionary assemblages
(e.g., Barbados, Nankai, and Cascadia)
Thus, ODP has come to embrace the idea of using drilling to understand
geochemical mass balance. Hopefully, Leg 185 will be the first of several
legs dedicated to this problem in arc systems of different tectonic regime
and sedimentary input. The U.S. MARGINS project and the international
Geochemical Earth Reference Model (GERM) have adopted this approach as
an essential part of their strategy.
To 185 Izu-Mariana Arc
To 185 Table of Contents