Intellectual merit: NanTroSEIZE, a central part of the Seismogenic Zone Initiative, is aimed at unraveling hydrologic processes at subduction megathrusts. A key component of the project, and the target of IODP Exp. 322, is the characterization of the incoming sedimentary strata and the top of igneous basement prior to the plate entering the subduction zone. Two reference sites (Sites C0012 and C0011) were drilled in the Shikoku Basin, which, when analyzed in the context of other sites previously drilled seaward of the deformation front (ODP Legs 131 and 190), provide an ideal database to characterize subduction inputs. Though still unaffected by subduction processes, these inputs are influenced by lateral fluid migration from zones of deeperseated dehydration reactions. Constraining this hydrologic regime is key to unraveling the effect of fluids and diagenetic reactions on the geomechanical properties of the plate interface at depth. Shipboard results point to the intriguing possibility of two fluids from different sources migrating through the sedimentary strata seaward of the trench. One regime (characterized by fluid freshening and the presence of methane and higher hydrocarbons) is driven by expulsion of fluids from the subducting sediment and up-dip migration through high permeability horizons in the subduction inputs. The other flow regime (characterized by the presence of sulfate at depth) is driven by migration of a seawater-like fluid through the upper oceanic crust into the sandstone turbidites of the overlying sediment. At the interface of both fluid regimes, microbial activity appears to be stimulated; with a newly discovered deep zone of active anaerobic oxidation of methane (AOM) at approximately 417 mbsf, where peak methane concentrations coincide with the complete consumption of sulfate and a marked increase in pore-water sulfide. Here we propose to: 1) characterize the nature of these flow regimes using pore water data in a collaborative program with other shipboard scientists; and 2) extrapolate these findings through time by analyses of authigenic phases (carbonates and barites), which contain a record of fluid sources (strontium isotopes), metabolic processes (carbon and sulfur isotopes) and formation temperatures (clumped oxygen isotopes). The recently developed clumped oxygen isotope (i.e. multiply-substituted isotopologues) analysis has been shown to effectively constrain temperatures of crystallization to ± 2.4 °C. Measurement on select carbonate veins and cements recovered near the bottom of the holes will provide key data on fluid temperatures needed to characterize: the state properties of the incoming sediments; temperature-dependent diagenetic reactions; and subsurface microbial communities. No in situ temperature data was collected during Exp. 322. In addition to providing information on basic geochemistry in the lower strata of the incoming plate (a key objective of Exp. 322), the data sets generated here will be used to further our understanding of newly postulated and highly significant topics: 1) history and nature of flow in the lower strata of the incoming plate, which may include an -as of yet undocumented- fluid flow regime in the upper oceanic crust; and 2) deep biosphere processes, which include a newly discovered AOM zone that is sustained by the electron acceptors supplied by an upper basement flow of sulfate bearing fluids. Broader impact: The data set we will assemble will allow us to provide fundamental information on the role of fluids on the geochemical, geomechanical and geobiological processes in the subducting strata, a key objective of the expedition. Our data will be shared with other NantroSeize scientists, in a fully integrated and cooperative effort. In addition to these collaborations, Prof. Peckman (U. of Bremen) has agreed to analyze biomarkers in the carbonates at no cost to this proposal. The project will directly support the thesis research of two graduate students, and an undergraduate research fellow. Outreach efforts include dissemination of Seismogenic zone processes through The SMILE program at Oregon State University (http://smile.oregonstate.edu/), Adult Education across the State of Oregon (http://literacyworks.org/ocean/) and professional development courses for in-service teachers through the Math-Science Partnership Program of the Arizona Department of Education.
This project is part of the larger Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). The main goals of NanTroSeize are to sample and install instruments in a seismogenic zone at a subduction margin that has produced large earthquakes impacting Japan in the historical past. The project funded by this grant was designed to help characterize the incoming reference section of sediment before it enters the subduction zone. This starting material will then be subducted or accreted and take part in deformation that can produce large earthquakes and tsunamis. We collected sediment cores and interstitial waters to examine processes of fluid and rock interaction that might change the physical properties of the incoming sediment during during its burial on the subducting plate. As part of this collaborative research project between Northern Arizona University and Oregon State University, we collected interstitial waters from sediment cores for chemical analysis, in order to used chemical tracers of fluid flow and fluid-rock interaction. Sulfur isotopes of dissolved sulfates provided evidence for fluids entering the sedimentary section from below. This confirms previous evidence from other kinds of data that at Nankai and other plate boundaries, fluid circulation through basalt in the subducting plate is an important mechanism of transfer of heat and mass. In addition, this dissolved sulfate can provide an important chemical constituent for microorganisms that oxidize methane as an energy source. This process is part of the deep biosphere, which is increasingly being recognized as major part of the biosphere on Earth, and might have played an important role in the origin and evolution of life on the planet. The combined process of sulfate reduction and methane oxidation can also lead to carbonate cementation of sediments, because the reaction increases carbonate alkalinity in the pore waters. This kind of cementation is one example of how the strength of sediment can be increased before subduction, which might lead to an increase in seismogenic failure during deformation. Along with interstitial waters, we sampled carbonate cements that are locally abundant in certain sedimentary horizons. Combined results from dissolved barium in pore waters and barium in carbonates, along with strontium, carbon, and oxygen isotopes, suggest much of the carbonate forms by a process of chemical alteration of volcanic matter in the sediment, which consumes dissolved carbon dioxide and increases carbonate alkalinity. The relatively new method of carbonate clumped isotope geothermometry shows that much of the carbonate is forming in equilibrium with current temperatures measured in the borehole by other means. This first application of carbonate clumped isotope methods to an active subduction system shows the potential for using this method to determine the modern geothermal gradient in sediments. This knowledge is important for modeling the evolution of sediment properties before and during subduction. The successful application of carbonate clumped thermometry will be important for future research drilling programs when temperatures cannot be measured directly due to time and budget constraints. Knowledge of temperature and evolution of physical properties can be of starting materials can be important in modeling earthquake and related tsunamigenic behavior of faults during deformation in the shallow portion of subduction zones. This is important in planning for future earthquake hazards in Japan, and can be applicable to similar hazards in the Pacific northwest of the US.
