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. F1). These processes are important, as the majority of chemical recycling on Earth is currently taking place in the subduction factory. These "factories" were probably the main sites of crustal production through geologic time (Armstrong, 1968; Karig and Kay, 1981; Reymer and Schubert, 1984; McLennan, 1988). Despite the fact that there is good evidence for transport of fluid and sedimentary components 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 Ocean Drilling Program (ODP) since the late 1980s has, as a part of several drilling legs, tackled this problem (see "Historical Perspectives"), 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. F2).
Many of the key elements for understanding crustal growth are sequestered in the sedimentary column (e.g. Th, rare earth elements [REEs], Ba, and Be) and in the uppermost oxidized portions of the volcanic section of oceanic basement (e.g., 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., 1995; Alt and Teagle, 1999). For example, the absence of significant carbonate in the sediments may influence the CO2 emitted at an arc volcano. 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 add to the alkali inventory of the subduction factory.
The igneous section of oceanic crust inherits many of its physical and geochemical characteristics from 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; 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 cooled initially 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 and the retroactions with continental erosion (Staudigel et al., 1995; Alt and Teagle, 1999), but also in fixing key elements that will later be fed 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 ~10-30 m.y. following crustal accretion (Staudigel et al., 1986; Alt and Teagle, 1999).
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, 1993). Nearby intraplate volcanoes may add a significant flux of volcaniclastic material to 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 undergoing frontal accretion of sediments, this component will be recycled into the mantle or underplated beneath the forearc.
The oldest oceanic crust on Earth is subducting into the Izu-Mariana arc system. 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 oceanic crust. Site 801, in the Pigafetta Basin (Fig. F3), is within the Jurassic Quiet Zone (JQZ). It is the oldest crust drilled by ODP or the Deep Sea Drilling Project (DSDP), radiometrically dated at ~170 Ma (Pringle, 1992). The second site, Site 1149 in the Nadezhda Basin (Fig. F3), is on the same flow line as Site 801 but is on magnetic Anomaly M11 and, as such, has an estimated age of ~132 Ma. Both sites originated at fast-spreading mid-ocean ridges in the Southern Hemisphere and then migrated northward, but at different times and rates. 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 sediments, drilling rarely recovers entire sedimentary and igneous sections; 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 for both physical parameters (e.g., resistivity, porosity, velocity), and 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 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:
Too often postcruise research on ODP samples produces a data set dispersed among many individual investigators. During Leg 185 the investigators developed a novel approach and 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) (Staudigel et al., 1998) and to the NSF MARGINS Program (Plank et al., 1998), and the communal samples will be a legacy of the leg. Additionally, Hole 801C, now at a depth of 936 mbsf, remains as an ODP legacy hole in the oldest oceanic crust on Earth.
An important objective of Leg 185 involved the study of the deep biosphere at both sites. Bacteria have been located in near-ridge axis hydrothermal systems and 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 exist in oceanic crust as old and as deep as that at Sites 801 and 1149 provided the motivation for sampling the basement and sediment 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 (see "Methods for Quantifying Potential Microbial Contamination during Deep Ocean Coring" [Smith et al., 2000]).