Leg 185 synthesis: Sampling the oldest crust in the ocean basins to understand earth's geodynamic and geochemical fluxes

During Ocean Drilling Program (ODP) Leg 185, two sites were sampled in the oldest crust of the western Pacific Ocean. The scientific objectives were twofold: (1) to characterize the nature of the oldest in situ oceanic crust on Earth in terms of its formation and subsequent sedimentation and alteration and (2) to determine the bulk geochemical characteristics of crust being subducted into the Izu-Bonin and Mariana trenches, as part of a larger global experiment aimed at understanding geochemical budgets at subduction zones. This synthesis summarizes results presented in more than 35 papers published not only in this Leg 185 Scientific Results volume but also in several journal articles and, specifically, in a special thematic series of articles on Oceanic Inputs to the Subduction Factory published in Geochemistry, Geophysics, and Geosystems (G³). Leg 185 science also supported the theses of 12 students in universities around the world. The oldest of the two basement sites drilled (Hole 801C) has been dated at 167 Ma and preserves late Bajocian to middle Bathonian fauna. It is thus the oldest crust yet sampled in the oceans. Hole 801C marks the birth of the Pacific plate at a time immediately following Pangaea breakup and increased subduction magmatism. Hole 801C basement is considered to represent a section of crust formed at a fast spreading ridge, and its structure, preservation of hydrothermal zones, and lava stratigraphy are interpreted as being typical of such an environment. Site 801 also provided a key location to study the unusual nature of the Jurassic magnetic field. The basement section of Hole 801C records at least six polarity intervals over ∼50,000 yr, representing an order of magnitude higher sustained geomagnetic reversal frequency than at any other time in the past 300 m.y. of Earth history. The Jurassic "quiet" zone thus results in part from very high reversal frequency, likely coupled with a relatively low dipole field intensity. Hole 801C also permitted geochemical and mineralogical characterization of altered oceanic crust being subducted into the Mariana Izu-Bonin arc system. For the first time, ODP scientists worked on a series of composite and common samples, coupled with an extensive set of downhole logging data, to define the bulk chemical composition of altered oceanic crust. These data are compiled as part of a reference suite in the Geochemical Earth Reference Model (GERM). They provide important constraints not only on the integrated fluxes of elements during seawater-seafloor interactions but also for the bulk geochemistry for key anions in the ocean-continent geochemical flux. Some of the very first measurements of Fe isotopes have been made on hydrothermally altered crust from Hole 801C, along with a larger suite of other stable (O, C, S, and Li) and radiogenic (Sr, Nd, Pb, and Hf) isotopes. The Hole 801C composites have enabled the first quantitative constraints on the parent/daughter ratios of the U-Th-Pb isotopic system through the entire cycle from mantle melting at mid-ocean ridges, to seafloor alteration, processing through the subduction zones, and final impact on the Pb isotopic evolution of the mantle. Site 1149 is the first complete sedimentary section outboard of the Izu-Bonin Trench. Seismic stratigraphy and core-logging integration have been used to correlate sedimentary sequences and develop the sedimentation history across the western Pacific. Site 1149 sediments record the global Valanginian Weissert marine anoxic event, as well as an excellent Pliocene-Pleistocene record of eolian dust and ash deposition. Representing a classical sequence of oceanic pelagic sedimentation, the Site 1149 sedimentary section has also been the target of exploratory work on novel stable isotopes (including those of N, Se, Sb, and Fe). Extensive geochemical logs and core analyses define a detailed geochemical stratigraphy, reflecting a clear hydrothermal signal at the base of the section and Asian dust and ash sources at the top. The geochemical variations in Site 1149 sediments can be compared with regional stratigraphic variations to predict the bulk sediment flux along the entire Izu-Bonin-Mariana Trench. Most notable is the northward increase in eolian input and southward increase in Cretaceous volcaniclastic sources. These sedimentological changes are recycled through the subduction zone and emerge as sympathetic latitudinal variations in the Mariana-Izu-Honshu arcs in key geochemical tracers such as Th/La and Pb isotopes. The Izu arc, however, is impoverished in the absolute abundance of subducted sediment material, which nonetheless appears in the Izu backarc region, thus demonstrating a delayed delivery response for this cold slab. Thus, one of the principal outcomes of Leg 185, as part of the "Subduction Factory" experiment, has been to quantify better than any other convergent margin to date both the flux of altered oceanic crust and the overlying sediments into the subduction system. These data constrain the "input function" of the subduction factory regionally in the western Pacific basin and globally as part of the bulk mass balance of subduction processes on Earth. Finally, Leg 185 played a pioneering role in ODP's burgeoning study of the ocean's deepest biosphere. A series of contamination tests on sediment and basement cores demonstrated their viability in conducting biological studies. These tests and some of the preliminary results have paved the way for biological studies as part of the new Integrated Ocean Drilling Program.