SUMMARYThe operational and scientific objectives of Leg 185 were achieved. Two sites were drilled in deep water and into the oldest crust of the Pacific Ocean (Fig. 18). Hole 801C in the Pigafetta Basin was re-entered and deepened in order to drill the upper oxidative alteration zone in basement. Four holes at Site 1149 in the Nadezhda Basin were drilled through sediment and into basement in order to characterize seafloor subducting at the Izu-Bonin Trench. In addition to satisfying these basic drilling objectives, the Leg 185 scientific party made a number of scientific achievements.
A Legacy Site into Pacific Jurassic Crust
Hole 801C was deepened by 340 m into basement, providing a total basement section of 470 m, making it the sixth deepest ODP or DSDP site into normal oceanic crust. Our conservative drilling objectives, to deepen the hole by 250 m, were exceeded. Recovery was very good (47%), and a high quality set of logs were run to 388 m in basement. Hole 801C is the only site to drill into Jurassic Pacific oceanic crust. The hole is in good condition, and it remains a legacy site into the Earth's oldest oceanic crust. Hole 801C is an important geochemical, geophysical, and biological reference site into old (~165 Ma), ultrafast spreading crust being subducted into the Mariana subduction zone. The basalts from Hole 801C have been sampled using a coordinated strategy to develop a common set of samples for all geochemical investigators, as well as composite samples, which will be another legacy of the site. This is a novel approach, which will lead to an unprecedented geochemical data set for this unique section of oceanic crust.
A Complete Sedimentary Sequence in the Nadezhda Basin, Western
Drilling at Site 1149 satisfied a primary objective of providing the first complete section through the pelagic sediments (~400 m) of the Nadezhda Basin, a ~1000 km x 1000 km region in the western Pacific. More than 90% of the sedimentary section was either recovered or logged, and sedimentary units at Site 1149 can be traced seismically across the basin. Thus, Site 1149 is an important reference site for Mesozoic equatorial sedimentation from the upper Valanginian and for sediment that is being subducted along the entire 1000 km Izu Trench.
Early Cretaceous Seafloor Subducting at the Izu-Bonin Trench
Basement drilling at Site 1149 achieved significant (133 m) penetration into Anomaly M11 (132-Ma oceanic crust), ranking this as one of the few ODP sites to drill >100 m into Mesozoic oceanic crust. Thus, Site 1149 will serve as an important reference site for fast-spreading, Mesozoic Pacific crust (102 mm/yr full rate) and its associated alteration and igneous composition as it subducts at the Izu-Bonin Trench.
A Mass Balance Equation for Crustal Recycling at the Mariana Arc
After drilling Hole 801C, the remaining piece of the crustal input inventory is complete for the Mariana subduction factory. Shore-based geochemical analyses of the basement section in Hole 801C will provide the first robust estimates for subducting oceanic crust with which to compare to volcanic outputs at the Mariana backarc. The basaltic inventory for K, U, Ba, CO2, and H2O will provide not only seawater-basalt fluxes but also crust-mantle fluxes for these key tracers and volatiles.
Comparisons of the Input and the Output at the Mariana and Izu
Having provided the first continuous sedimentary section to basement of sediments subducting along the Izu-Bonin margin, Leg 185 data enables comparison of the inputs to the Mariana and Izu arcs. In contrast to the East Mariana and Pigafetta Basin sediments subducting at the Mariana Trench, the Nadezhda Basin sediments subducting at the Izu-Bonin Trench lack a mid-Cretaceous volcaniclastic section and contain more siliceous and carbonate rich biogenic material because of its longer passage beneath zones of high biological productivity. Shore-based geochemical studies will demonstrate the extent to which these different sedimentary histories can be traced to the volcanic output from the two arc systems. For example, does the sedimentary and basaltic input on the incoming plate provide suitable Pb isotope mixing end-members for the Izu arc volcanics, or are other mantle and upper plate sources required? Does the extensive biogenic section in the lower half of Site 1149, which is highly depleted in alkali elements, contribute to the low alkali content of the Izu arc? The coordinated shipboard sampling and analytical effort organized by Leg 185 scientists will provide an unprecedented geochemical data set (major elements, trace elements, and Pb, Nb, Sr, Os, Hf, Li, B, Be, Cl, S, Se, C, N, O, H, and S isotopes) of crustal inputs to the two subduction factories.
The Jurassic Quiet Zone
Hole 801C Jurassic basement records up to six geomagnetic reversals. Not only are there several reversals, but some sections preserve gradual changes in the magnetic field direction from one polarity interval to the other. Thus, igneous basement at Hole 801C was extruded at a time of rapid polarity alternations of the geomagnetic field. Hence, these data may provide an explanation for the JQZ in a series of superposed flows with opposite polarity, essentially canceling out one another. The presence of fresh basaltic glass at depth in Hole 801C will also provide suitable material for paleointensity studies, to test the hypothesis that the JQZ was a time of low geomagnetic field intensity.
Leg 185 was the first ODP leg to invest a significant effort in conducting microbial contaminant tests, equipping a microbiology laboratory, and establishing techniques for core handling of biological samples. Contaminant tests using perfluorocarbon and fluorescent microsphere tracers demonstrated that sediments cored with the APC showed less susceptibility to contamination than RCB coring. Several APC core interiors were entirely free of contaminants. These tests, which demonstrate that biological contamination can be assessed and surmounted, pave the way for establishing ODP as a new platform for microbiological studies. Leg 185 samples were used to start culturing experiments in various media at both atmospheric and in situ pressure and for shore-based DNA extraction and community characterization. Several glass samples from Hole 801C showed textural evidence for microbial alteration and leave the intriguing question of whether there is still microbiological activity in 165-Ma volcanic basement.
Calibrating Magnetic Anomaly M11
Based on a re-evaluation of existing seafloor magnetic anomaly lineations, Site 1149 lies in crust of Anomaly M11, which is consistent with the presence of T. verenae found in the basal core in Hole 1149B. Obtaining a radiometric date on the basement at Site 1149 could provide a reasonably precise date of Anomaly M11 and help to refine the time scale during this age near the breakup of Gondwana.
Mesozoic and Cenozoic Pelagic Sequences
The equatorial paleolatitude history of Site 1149 during the mid-Cretaceous, combined with a predictable subsidence history, is ideal for testing variations in the Cretaceous CCD. Site 1149 sediments record a well-developed metalliferous sedimentary profile, which clearly documents the decreasing influence of plume precipitation with lateral distance from the ridge. Very high sediment accumulation rates (~30 m/m.y.) and the mineral composition of the youngest sediments suggest that Site 1149 was in the reach of the Asian dust plumes after the early Pleistocene.
Petrology of Mesozoic Crust
Fresh basaltic glass was recovered from both Site 1149 and Site 801, providing pristine samples of the igneous liquid that forms Mesozoic Pacific crust. These are valuable samples that record mid-ocean ridge processes, mantle composition, and mantle temperature at a time preceding the Cretaceous superplume event in the Pacific.
Architecture of Fast-Spreading Crust
Sites 801 and 1149 provide the first sections into Mesozoic fast-spreading crust, Layer 2A. Geochemical alteration of the volcanic section in Hole 801C is found in several discrete zones associated with ocherous Si-Fe-hydrothermal deposits and thick massive flows. These zones control the alteration pattern of crust and contrast with "accepted" models for a gradual decrease downhole in the alteration of oceanic basement. The pattern of alteration at Site 801, controlled by local pathways for hydrothermal fluids, may be a feature of fast-spreading crust.
Continued Diffusive Exchange between Basement and Sediments
Although it is generally accepted that there is diffusive exchange between interstitial waters in the volcanic section of oceanic crust and the overlying sediments, the organic-poor nature of the sediments at Site 1149 allows modeling of S and metal budgets between basement and sediments, as well as assessing the potential for bacteria in the basement to affect the redox state of the overlying sediments.
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