Stable isotope data from nankai sites C0011B and C0012A constrain fluid processes in the pre-subduction inputs for the nantroseize project, Japan

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Author(s): Sample, James C.; Torres, M. E.; Joseph, C.
Author Affiliation(s): Primary:
Northern Arizona University, School of Earth Sciences and Environmental Sustainability, Flagstaff, AZ, United States
Oregon State University, United States
Volume Title: AGU 2011 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 fall meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: We report data from drill sites C0011B and C0012A from Expedition 322 along the Nankai margin. This research is part of a larger effort designed to investigate the pre-subduction inputs of sediment and oceanic basement for the NanTroSEIZE Project. C0011B is entirely in the Shikoku Basin section on the flank of the Kashinosaki Knoll basement high, and C0012A is in a thinner equivalent section from the top of the knoll. Previous work suggests there may have been recent lateral flow of fluids from the accretionary prism into the section penetrated by C0011B. At C0012A, however, there is less evidence for lateral flow, however, there are indications for the presence of a seawater-like fluid flowing through the upper basaltic crust. We analyzed S isotopes from dissolved sulfate in interstitial waters from C0012A, and carbon and oxygen isotopes of carbonate minerals from both sites as proxies for fluids during carbonate formation. The sulfate profile differs from other sites along the Nankai Trough in that it never drops to zero, but decreases to a minimum at 244 CSF (core depth below seafloor in meters), and then fluctuates in value. S isotopes range from +6.96 to +34.40 ppm relative to the VCDT standard. The data can be divided into two distinct groups. An upper group above ∼250 CSF shows a concave-upward profile with increasing δ34S values consistent with a zone of sulfate reduction, leaving residual sulfate increasingly enriched in the heavy isotope 34S. Below this depth the δ34S values drop back to lower values similar, but slightly higher than, global seawater. Carbonates are relatively widespread at both sites, and may be an important diagenetic phase altering sediment physical properties before subduction. Average values are 2.3 wt% and 3.3 wt% at C0011B and C0012A, respectively. Carbonates are mainly low-Mg calcite, dolomite, and siderite. A plot of δ13C vs. δ18O (VPDB) of carbonates in C0011B shows a range of δ13C = -23.36 to -2.58ppm and δ18O = -11.86 to +1.86ppm. Generally the lowest δ18O values occur deepest in the sediment section, but the pattern is complicated. C isotope values are scattered with depth. In C0012A there is a strong covariance between C and O isotopes (R2 > 0.65), δ18O values range from -12.23 to +3.98ppm, and δ13C values from -22.48 to -6.39ppm. Both C and O isotopes values generally decrease with depth. At both sites the C isotopes are consistent with inputs from a combination of methane and dissolved inorganic carbon (DIC), though we cannot discern whether methane has a bacterial or thermogenic source. The C isotopes of carbonates and DIC, however, are consistent with deep-seated zone of AMO, where methane is oxidized by sulfate supplied via seawater-like fluids flowing through basement. The O isotope data show that many of the samples are out of equilibrium with pore waters. The lowest δ18O values suggest warm fluids may have circulated upward through the sediments during carbonate formation. This is most pronounced for C0012A, where the lowest δ18O value of -12.23ppm occurs at a depth of only 430 CSF. Both O and S isotope data at C0012A are consistent with some upward flow of fluids from basaltic basement, with a seawater sulfate signature and elevated temperatures.
Year of Publication: 2011
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 02 Geochemistry; 07 Marine Geology and Oceanography; Accretion; Basaltic composition; Calcite; Carbonates; Crust; Dolomite; Expedition 322; IODP Site C0011; IODP Site C0012; Integrated Ocean Drilling Program; Isotope ratios; Isotopes; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; O-18/O-16; Oceanic crust; Oxygen; Pacific Ocean; S-34/S-32; Sea water; Sediments; Shikoku Basin; Siderite; Stable isotopes; Sulfur; West Pacific
Coordinates: N324400 N325000 E1365600 E1365200
N324944 N324945 E1365256 E1365254
N324453 N324453 E1365501 E1365501
Record ID: 2015099563
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