Leg 185 Preliminary Report

Subduction zones are the primary regions on Earth today where crust recycling takes place, and through geological time they have been the sites of continent formation. Many of the key elements (e.g., Th, rare earth elements, Ba, and Be) that are important in understanding crustal growth are sequestered in the sedimentary column and in the uppermost oxidized portions of the volcanic section of oceanic basement (K, B, U, CO2, H2O). The principal objective of Leg 185 was to core two sites in Mesozoic crust in the west Pacific, which is being subducted into the Mariana and Izu-Bonin subduction systems, in order to determine the inputs into the “west Pacific subduction factory.” Hole 801C was first drilled in the oldest (~165 Ma) crust in the Pacific Ocean during Ocean Drilling Program (ODP) Leg 129. During Leg 185 the hole was deepened to nearly 500 meters below seafloor (mbsf), and at Site 1149, located on magnetic Anomaly M11 (~132 Ma) 100 km east of the Izu-Bonin Trench, the entire sedimentary sequence (410 m) and an additional 133 m of highly altered volcanic basement was drilled. Using the recovered core and the logging results, it is possible to reconstruct the volcanic section for Hole 801C. Seven volcanic sequences have been defined, some with massive lava flows up to 20 m thick and others with thin pillows and sheet flows of <1 m. The uppermost unit is a series of alkali basalts drilled during Leg 129 and dated at ~155 Ma. These are separated from the underlying tholeiites of normal oceanic crust by an ocherous Si-Fe–rich hydrothermal deposit. A similar deposit is 100 m lower in the hole. These hydrothermal deposits and numerous interpillow sediments observed in the upper volcanic sequences define the alteration character of the basement, which is confined to three zones downcore and appears to be controlled by local permeability structures. The pattern of alteration for basement at Site 801 contrasts with that from other deeply drilled sections in oceanic crust where oxidative alteration decreases continuously with depth. The estimated seafloor spreading rate for Site 801 is 160 km/m.y. Thus, both the alteration and lava sequences may be typical of fast-spreading environments, such as the present-day East Pacific Rise. Site 1149 basement is dramatically different in character. It is pervasively altered at low temperatures to red dusky brown and preserves multicolored halos around veins and fractures. The volcanic facies are dominated by thin flows, hyaloclastite, and flow breccia. Preliminary estimates of the geochemical budget for K were made for Site 801 volcanic sections of ocean basement using gamma-ray intensities from downhole logs and multisensor track (MST) measurements, in addition to chemical analyses of core samples and estimates of the volume percentages of veins and alteration types. The K content of the entire core indicates a threeto fourfold enrichment as a result of low-temperature alteration. Similar estimations will be possible for other key elements following shore-based analyses. Leg 185 Preliminary Report 8 The deep basement penetration in Hole 801C provided ideal samples for probing the causes of the Jurassic Quiet Zone (JQZ). From paleomagnetic measurements on cores and geophysical logs we discovered a series of reversals downhole. Given the spreading rates estimated for the region, the reversals must relate to rapid fluctuations in field polarity. Thus, at Site 801 the JQZ may represent a canceling out of normal and reversed polarities associated with an unstable and relatively weak magnetic field. The sediments being subducted into trenches must, in part, control geochemical differences in the composition of arc magmas. Both the Mariana and Izu-Bonin margins are characterized by complete subduction of the sedimentary section on the downgoing plate, thus simplifying the dynamics of the subduction problem. Although the subducting sediments have been reasonably well sampled in the Pigafetta Basin (Mariana region), earlier drilling attempts to recover the sedimentary section in the Nadezhda Basin, seaward of the Izu-Bonin Trench, had largely been thwarted by difficult drilling conditions. Thus, an important objective of Leg 185 was met by continuously coring and logging the ~400 m sedimentary section at Site 1149. The uppermost sediments consist of pelagic clays with admixtures of volcanic ash and biosiliceous material for which paleomagnetic data define an excellent record of 6 m.y. of sedimentation in the west Pacific. These are underlain by barren pelagic clays characterized by low sedimentation rates, which overlie radiolarian cherts and clays and a lower unit of chert intercalated with marl and chalk above basement. The basal sediments have been dated from nannofossil assemblages (Tubodiscus verenae and Rucinolithus wesei) as lowermost Hauterivian to uppermost Valanginian, in accord with the assigned M-11 magnetic lineation. The lower sedimentary units preserve a record of high rates of biogenic sedimentation (~18 m/m.y.) as the site passed beneath equatorial zones of high biological productivity. The sedimentary sequence at Site 1149 is substantially different from that being subducted at the Mariana Trench, the latter being characterized by an extensive mid-Cretaceous volcaniclastic sequence derived from the local seamounts and as being carbonate free, which may explain some of the geochemical differences between the two arc systems. Leg 185 was the first ODP leg to conduct a series of in-hole contamination tests while undertaking a systematic study of the deep biosphere in oceanic sediments and basement in an attempt to establish the JOIDES Resolution as a platform for microbiological studies. Deepbiosphere contamination tests involved adding highly sensitive tracers (i.e., perfluorocarbons and fluorescent microspheres) to the drilling fluids and the core barrel to evaluate the extent of contamination of the cores by microbes introduced by the drilling process. Results of the tests revealed that the centers of advanced hydraulic piston corer (APC) cores are essentially uncontaminated during coring, whereas rotary core barrel (RCB) cores in sediment and basement contain variable amounts of introduced tracer. In addition, samples of sediments and basalts were placed in cultures aboard ship for shore-based study. Nonetheless, possible microbial tracks observed in 170-Ma volcanic glass are intriguing evidence for a deep biosphere still active at the extreme depths (>930 mbsf) sampled during Leg 185. Leg 185 Preliminary Report 9 INTRODUCTION The primary objective of Leg 185 was to determine the geochemical composition of the sediments and upper volcanic section of oceanic crust being subducted into the western Pacific arc system. These data are required as part of the subduction equation, which involves quantifying the inputs and outputs, both into the arc and back into the mantle, of the subduction factory (Fig. 1). These processes are important, as it is in the subduction factory that the majority of chemical recycling is currently taking place on Earth. These “factories” were probably the main sites of crustal production through geological time (Armstrong, 1968; Karig and Kay, 1981; Reymer and Schubert, 1984; McLennan, 1988), and despite the fact that there is good evidence for transport of fluid and melt from the subducted plate to the arc system (Morris et al., 1990; Hawkesworth et al., 1997; Elliott et al., 1997), there are few quantitative constraints on the recycling equation and its effect on the dynamics of crust formation and destruction. The ODP program since the late 1980s has, as a part of several drilling legs, tackled this problem (see “Historical Perspectives” section), but Leg 185 was the first ODP leg for which the objectives were specifically applied to coring oceanic crust and sedimentary sections representative of the different inputs into the subduction factory; in this case the Mariana and Izu-Bonin arcs of the west Pacific ocean (Fig. 2). Many of the key elements that are important in understanding crustal growth (e.g. Th, rare earth elements [REEs], Ba, and Be) are sequestered in the sedimentary column and in the uppermost oxidized portions of the volcanic section of oceanic basement (K, B, U, CO2, and H2O). The input of these and other elements may vary as a function of sediment composition (Plank and Langmuir, 1998) or the nature of the volcanic basement (Staudigel et al., 1986 ; Alt and Teagle, 1999). For example, the absence of significant carbonate in the sediments may influence the CO2 content of an arc. The presence of organic-rich sediments or hydrothermal sediment may influence the input of metals in different arcs. Alkaline off-axis volcanics and associated volcaniclastic sediments, when subducted, may significantly affect the alkali inventory to the subduction factory. The igneous section of ocean crust inherits many of its physical and geochemical characteristics at the spreading ridge. Significant differences in eruption style, ridge morphology, and structure occur depending on the rate of spreading (see review in Perfit and Chadwick, 1998). Similarly, the hydrothermal systems vary as a function of the longevity and depth of the magma chamber (e.g., Gillis, 1995; Haymon et al., 1991), and, on average, the alteration characteristics and, therefore, the geochemical inventory of crust, must vary as a function of spreading rate. As crust ages and moves away from the spreading axis, it is initially cooled by hydrothermal activity and later warms again as it equilibrates with the geothermal gradient. The chemical changes occurring during this transition are important, not only in controlling the compositions of the oceans (Staudigel et al., 1986; Alt and Teagle, 1999), including the retroactions between continental erosion and oceanic composition, but also in fixing key elements Leg 185 Preliminary Report 10 that will later be recycled into the subduction factory. Some of these elements will migrate into the arc crust, whereas others will be recycled into the mantle, possibly to return to the oceanic crust as hot-spot magma. Although the chemical maturation of crust must continue for several tens of millions of years after its formation (Stein and Stein, 1994) and probably throughout its history, the most significant alteration is at the ridge axis and for about 10–30 m.y. following crustal accretion (Staudigel et al., 1986; Alt et al., 1986; Alt et al., 1992). In an analogous fashion, the history of sedimentation on the oceanic crust as it transits different oceanic regimes will influence the composition of the input into the subduction factory (Plank and Langmuir, 1998). The sedimentary sequence in subduction regimes in nonequatorial zones will differ significantly in composition from those where the oceanic crust has “resided” mainly in equatorial regimes. The presence of intraplate volcanoes may result in a significant flux of volcaniclastic material into the sedimentary sequence. This may have very different characteristics in key isotope ratios, especially Pb, that the arc may inherit or that may be recycled back into the mantle. When proximal to active margins, the upper sediments may contain significant quantities of terrigenous turbidites. As the oceanic plate approaches the trench, the final contribution to the sedimentary pile will include ash from the volcanic arc. For margins that are not accreting sediments, this component will be recycled into the mantle or created beneath the forearc. The oldest oceanic crust on Earth is subducting into the Izu-Mariana arc system, and in addition to providing geochemical data to input into the subduction equation, the two sites studied provide important geochemical constraints on the nature and history of Mesozoic ocean crust. Both sites are shown in Figure 2. Site 801 is in the Pigafetta Basin, which is in the Jurassic Quiet Zone (JQZ) and is dated as ~170 Ma (Pringle, 1992). It is the oldest crust drilled by ODP or the Deep Sea Drilling Project (DSDP). The second site, Site 1149 in the Nadezhda Basin, is on the same flow line as Site 801 but is on magnetic Anomaly M11 and, as such, has an estimated age of ~135 Ma. Both sites originated at spreading centers in the Southern Hemisphere and then migrated northward, but at different times and durations. Thus, in addition to the “Subduction Factory experiment,” Leg 185 scientists had an unparalleled opportunity to (1) assess the paleoequatorial sedimentation history of the Pacific Ocean since Mesozoic time, (2) place limits on the ages of the oldest magnetic anomalies in the ocean basins, and (3) study the nature of the JQZ. With the exception of relatively soft oozes and clays, drilling in oceanic crust rarely recovers the entire sedimentary and igneous section; thus, calculating the geochemical inventory is problematic. The gaps in the data have to be filled in by combining detailed core description and logging the drill hole both for physical parameters, such as resistivity, porosity, and velocity, and for geochemical composition. In addition to the regular inventory of ODP logs, the geochemical logging tool was used during Leg 185. The ultimate long-term goal of studies of the subduction factory is to create a complete geochemical mass balance of the inputs, outputs, and residues lost from the system. Geochemists Leg 185 Preliminary Report 11 and geophysicists argue strongly for the recycling of oceanic crust and sediments to the mantle (Hofmann, 1997; Van der Hilst et al., 1997). Given adequate control on the subduction equation, it may ultimately be possible to identify the recycled products of the factory, not only in the arc volcanoes, but as they reappear as mantle plumes on the Earth’s surface after being recycled into the mantle. The Izu-Mariana system was chosen as the first of these studies because it is relatively simple: 1. It is characterized only by limited sediment accretion. 2. It has a well-defined subduction geometry, which is relatively steep in the Mariana arc, and penetrates the 670 km discontinuity and is shallower in the Izu-Bonin arc (Van der Hilst et al. (1997). 3. It has a wide aperture from forearc across the arc to the backarc. 4. The region has already been the subject of ODP drilling in serpentinite mounds associated with forearc dewatering (Leg 125; Fryer, 1992) and has a well-studied deepsea ash and volcanic record (Legs 125 and 126) Too often postcruise research on ODP samples produces a data set dispersed among many individual investigators. During Leg 185 a novel approach was that the investigators chose to work on a common set of samples. The geochemical database thus developed for Leg 185 will be a unique contribution to the Geochemical Earth Reference Model (GERM) and to the MARGINS Program initiative, and the communal samples will be a legacy of the leg. The two sites drilled during Leg 185 provide critical information on the input to the “subduction factory” system. In particular, Hole 801C, now at a depth of 934 mbsf remains as an ODP legacy hole in the oldest ocean crust on Earth. An important objective not directly related to the problem of geochemical recycling involved the study of the deep biosphere at both sites. Bacteria have been located in association with ridge axis hydrothermal systems within the sediment column as deep as 800 m (Parkes et al., 1994). In addition, textural evidence suggests that bacteria living off nutrients associated with basaltic glass alteration may thrive in the basaltic crust (Thorseth et al., 1995; Fisk et al., 1998; Furnes and Staudigel, 1999). The fascinating possibility that bacterial activity may persist in oceanic crust as old and as deep as that at Sites 801 and 1149 provided the motivation for sampling the basement for bacteria culturing and DNA extraction in the search for extremophile life. To control the extent of contamination from surface waters, drilling mud, and drilling tools, a series of tests for contaminants were undertaken as part of the operations at Sites 801 and 1149. Leg 185 Preliminary Report 12 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. Leg 185 Preliminary Report 13 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 grass—different 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 milk—whether 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. Leg 185 Preliminary Report 14 IZU-MARIANA ARC The Izu-Mariana arc system involves subduction of the ancient Pacific plate beneath the relatively young Philippine Sea plate with the resulting production of a classic island arc chain of volcanoes and marginal backarc basins (Fig. 2). There are several reasons why the Izu-Mariana margin is favorable for studying material recycling in subduction zones. The first is that significant progress has already been made on many parts of the flux equation. Serpentine seamounts, which represent forearc sites of fluid outflow, have already been drilled (Leg 125; Fryer, 1992), as have most of the sedimentary components being subducted at the Mariana Trench (Leg 129; Lancelot, Larson, et al. 1990; see below). The Izu and Mariana volcanic arcs and the Mariana Trough and Sumisu Rift backarcs are among the best characterized intraoceanic convergent margins, both in space and time (Legs 125 and 126; Gill et al., 1994; Arculus et al., 1995; Elliott, et al., 1997; Ikeda and Yuasa, 1989; Stern et al., 1990; Tatsumi et al., 1992; Woodhead and Fraser, 1985). Thus, major parts of the forearc, arc, and backarc output, as well as the sedimentary input have already been characterized. The other advantage to the Izu-Mariana system is that the problem is simplified here because the upper plate is oceanic—therefore, upper crustal contamination is minimized, and sediment accretion in the forearc is nonexistent (Taylor, 1992)—so sediment subduction is complete. Despite the simple oceanic setting and the shared plate margin, there are clear geochemical differences between the Izu and Mariana arcs. The Mariana arc erupts basalts in which both subducted sedimentary and altered oceanic crustal components can be identified (e.g., Elliott et al., 1997), and the arc conforms well to the global trend in Ba sediment input vs. Ba arc output (Fig. 4A). On the other hand, the Izu arc erupts basalts that are among the most depleted of any global arc in trace element concentrations (e.g., REEs, Ba, and Sr). In addition to the contrast in elemental concentrations, there are also clear differences in the isotopic composition of Mariana and Izu basalts, such as 207/204 and 206/204, which may derive from isotopic differences in the input to the two trenches (Fig. 4B). The divergence of compositions between the volcanics of these two oceanic arcs provides the simplest test for how the composition of the subducting crust affects them. The key missing information is the composition of the incoming crustal sections, specifically the basaltic basement subducting at the Mariana Trench, and the sediment and basement sections subducting at the Izu Trench. The low trace element concentrations of Izu volcanics may derive from a lower flux of these elements at the trench, and their distinctive isotopic composition may be inherited from the composition of the sediments subducting there. These hypotheses can be tested by drilling the subducting sediment and basement sections feeding the two arc systems. Alternatively, differences in the fluxes cycled to the arcs may derive from different operations of the subduction factory in the two areas. For example, along-strike changes (e.g., dip, age, and depth) in the subducting slab could affect where material exits the slab and enters the arc melting regime. The changes in the geometry of the slab, and its relationship to the volcanic arc, may Leg 185 Preliminary Report 15 signal a change in where the volcanoes are sampling the slab fluids. Distinguishing between these two models—the input model vs. the slab model—requires good control on the subducted inputs, which was the primary objective of Leg 185. WESTERN PACIFIC STRATIGRAPHY The ancient crust of the western Pacific plate is the primary input to the >2000-km-long IzuMariana subducting margin. Ideally, this input would be constrained by drilling several holes through the sedimentary section and deeply into oceanic crust along the length of the trench. Because of the great expense and time it takes to drill in ~6000-m water depth, we are limited in practice to a few drill holes and extending this information regionally using sedimentation and plate-motion models, along with seismic stratigraphy. It is thus important to understand the context of sedimentation and plate history in the western Pacific in order to maximize the information gained from a small number of reference sites, such as Leg 185 Sites 801 and 1149. The crust subducting into the Mariana Trench includes Jurassic seafloor of the East Mariana and Pigafetta Basins (Fig. 5). Based on magnetic anomaly lineations, this region was thought to contain the Earth’s oldest in situ oceanic crust formed at ultra fast spreading rates (160 km/m.y. at Site 801). The basic goal of Leg 129 was to sample Jurassic oceanic crust. Earlier attempts to recover Jurassic sediments and basement in the western Pacific had been thwarted by extensive mid-Cretaceous volcanics and sills and by problems with the drill string sticking in chert horizons. While drilling two holes (ODP Sites 800 and 801) that also bottomed in Cretaceous basalt, Leg 129 was the first to succeed in recovering Jurassic oceanic basement in the Pacific Ocean, and Hole 801C rocks are still the oldest sampled in the ocean basins, at 167 ± 5 Ma (Pringle, 1992). During Leg 129 entire sedimentary columns at three sites in the East Mariana and Pigafetta Basins (ODP Sites 800–802; Fig. 5) were also successfully sampled. Before this leg, the recovery from nine DSDP and ODP sites averaged <50 m each. Taking Hole 801C as typical of the region, the sedimentary stratigraphy consists of Cenozoic brown pelagic clay overlying Coniacian to Campanian cherts and porcellanite, Albian seamount volcaniclastics, and Bajocian to Valanginian radiolarites (Fig. 6; Lancelot, Larson, et al., 1990). This sedimentary history reflects the plate history, which begins in the Southern Hemisphere in a zone of high biological productivity, as recorded by the Jurassic radiolarites (Fig. 7; Lancelot, Larson, et al., 1990). The plate then moved southward until the Early Cretaceous when it began to move northward again, collecting volcaniclastics from the nearby Albian Magellan Seamounts and then more siliceous sediments as it again crossed the high-productivity zone 5E–10E south of the paleoequator. The Cenozoic was characterized by very slow accumulation of deep brown pelagic clays, very depleted in biogenic, terrigenous, or eolian input, as is expected for the open-ocean environment. This history is typical for the East Mariana and Pigafetta Basins, and this stratigraphy, Leg 185 Preliminary Report 16 particularly the clay/chert and volcaniclastic intervals, can be traced regionally from seismic records (Abrams, et al., 1992). Although recovery was generally low (<30%), Leg 129 provided adequate sampling of the different sedimentary components to characterize the sedimentary geochemical flux into the Mariana Trench. This contrasts with the existing information to the north, along the entire 1000 km of the Izu margin. Previous drilling in the Nadezhda Basin, seaward of the Izu-Bonin Trench, was about as successful as drilling to the south prior to Leg 129. The chert horizons plagued drilling during Leg 20, which placed five holes in the region, none of which was to hit basement except DSDP Hole 197, where only 1 m of undatable tholeiite was recovered. Thus, the M-series magnetic anomaly ages have never been tested in this region. Along the Izu Trench, magnetic anomalies predict that the oceanic crust decreases in age from Jurassic (>M18) in the south to Early Cretaceous (M11) at Site 1149 (Fig. 5). It was unknown if the extensive mid-Cretaceous volcanism that took place in the south extended north into the Nadezhda Basin. Average recovery of sediments in the Nadezhda Basin was extremely low (<15 m) for previous DSDP sites, again because of sticking problems and spot coring. Leg 20 cores indicate an upper ashand diatom-rich clay unit overlying a brown pelagic clay and Cretaceous chert and chalk. The paleolatitude history for Site 1149 predicts a longer duration beneath equatorial zones of high biological productivity, and thus, extensive chert and chalk sequences (Fig. 7). Watergun seismic profiles collected during a presite seismic survey show a prominent reflection at ~0.2 s two-way traveltime (TWT) that corresponds to the chert horizon and another prominent reflection at 0.42 s TWT (Fig. 8) that corresponds with probable basement. These two reflections/horizons are prominent features, which are well correlated across the Nadezhda Basin. Thus, a primary objective of Leg 185 was to drill the missing inputs (i.e., subducting sediments and oceanic crust) to the Izu-Mariana recycling equation. Mariana sediments had already been adequately sampled; the remaining component was altered oceanic crust. For the Izu margin, both sediment and oceanic crust were virtual unknowns. Thus, the goal of Leg 185 was to drill a basement site seaward of the Mariana Trench and a sediment and basement site seaward of the Izu Trench. Hole 801C was chosen for the Mariana site because it is the only certain window into Jurassic basement in the western Pacific with a re-entry cone set and ~60 m already drilled into the upper mid-ocean-ridge basalt (MORB) tholeiites. Other sites could have been chosen closer to the trench, but this would likely require drilling through a significant section of Cretaceous volcanics or intrusives above Jurassic basement. Deepening the MORB section at Hole 801C would help to characterize the chemical fluxes into the Mariana subduction zone as well as define the aging and architecture of Layer 2 in fast-spreading crust. Site 1149 lies within the same spreading compartment as Site 801, along a flow line in crust ~30 m.y. younger (Fig. 5), formed at a spreading rate slower than Site 801 (100 mm/yr full rate). Roughly 100 km from the trench, Site 1149 lies seaward of the main faulting of the plate as it bends into the subduction zone. The main objective at Site 1149 was to drill through the inferred Leg 185 Preliminary Report 17 400-m-thick sedimentary sequence and into basement subducting along the Izu margin, which would enable a comparison with the fluxes to the south into the Mariana Trench. SCIENTIFIC OBJECTIVES 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. Site 801 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 5. Characterizing the geochemical fluxes and geophysical aging attending the upper oxidative alteration of the oceanic crust in Hole 801C; 6. Comparing igneous compositions, structure, and alteration with other drilled sections of in situ oceanic crust (in particular Hole 504B, contrasting a young site in Pacific crust with the oldest site in Pacific crust); 7. Helping to constrain general models for seafloor alteration that depend on spreading rate and age (Hole 801C is in the world’s oldest oceanic crust that was formed at a fastspreading ridge, so it embodies several end-member characteristics); and 8. Testing models for the magnetic Jurassic Quiet Zone. Site 801 is located in an area of very low amplitude magnetic anomalies, the JQZ. This quiet zone has been suggested to result from (1) oceanic crust of a single polarity with only small anomalies from field 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 Leg 185 Preliminary Report 18 with a more normal frequency of magnetic reversals acquired when the dipole field intensity was anomalously low. Deepening Hole 801C permitted testing of the above hypotheses, and in particular, the third hypothesis of magnetic reversals during a period of anomalously low field intensity as fresh, unaltered volcanic glass was obtained. Such material can yield reliable paleointensity information (Pick and Tauxe, 1993) on the very fine, single-domain grains of titanium-free magnetite within the volcanic glass. Site 1149 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 ~5ES, drifted south to 10ES in its early history, and then gradually drifted north, crossing the Leg 185 Preliminary Report 19 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.