Collaborative Research:Expedition 322 Objective Research on the nature and history of subsurface flow within the Nankai sediments prior to subduction

Oregon State University, Corvallis OR

Investigators

Abstract

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.