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.
Organic matter remineralization in marine sediments proceeds in a continuous redox sequence from an oxygen-supported reaction to methane generation. This classic redox ladder, where oxidants are derived from seawater, has been known since the early 1970’s, and results in formation of minerals within the sediment such as carbonates and barite. Results from analyses of sediment and pore fluids recovered by drilling at IODP Sites C0011 and C0012 on the Shikoku Basin document two significant changes in this paradigm: 1) The effect of a deep basalt aquifer, that acts as a diffusional source of oxidants and significantly impacts the deep biosphere of basal sediments; and 2) Formation of authigenic carbonates throughout the entire sediment column driven not only by carbon cycling reactions, but also by alteration of volcanic ash and igneous basalt that act as a supply of calcium. Drilling on the Shikuko Basin was aimed at documenting changes in properties of the incoming section to the Nankai margin on its pathway to lithification and incorporation into the accretionary prism. Sediment inputs to the Nankai subduction zone consist of ash-bearing hemipelagic muds, a bioturbated mudstone interbedded with tuffaceous/volcaniclastic sandstone, siliciclastic turbidites and a basal volcaniclastic-rich facies. Igneous basement contains four types of lava: pillow basalt, massive basalt, breccia, and mixed pieces of rubble. The presence of oxidant supply from fluids circulating within the buried upper oceanic crust, was first evidenced by profiles of interstitial water chemistry that show an increase in sulfate toward the sediment-basalt interface. Analyses of barium and manganese provide additional evidence for to the development of a redox system similar to, but inverted relative to the classic depletion gradient observed in shallow sediments that are in contact with bottom seawater (Figure 1). Microbiology data further support the presence of an anaerobic oxidation of methane at depths of ~400 mbsf. These data complement earlier reports of a deep biosphere sustained by supply of oxygen and nitrate from buried young oceanic crust, and completes the redox system supported by a continuum supply of oxidant as that begins with formation near the ridge flank and progresses either to subduction or a loss of permeability as igneous crust ages. The active biosphere in the transitional environment between sediment and basalt operational in the Shikoku basin sediments is likely a common phenomenon in deeply buried sediments along transects of young oceanic crust. Authigenic carbonate is known to form in marine sediments when enough alkalinity is produced from diagenetic reactions associated with organic matter decomposition via sulfate and iron reduction, and with anaerobic oxidation of methane (AOM). These carbonates form in shallow sediments where carbon cycling reactions are more active. In the Shikoku basin, alteration of ash within the sediments, and an additional calcium supply from the basement aquifer is enough to maintain calcite supersaturation throughout the entire sediment column. Strontium and stable isotope data, including carbonate clumped isotope thermometry, provide constraints on the mechanism and timing of carbonate formation in these sediments. While carbonate cements begin to form very early in the burial history, strontium isotopes and element ratios in the carbonates reveal that calcite is forming deep in the section as the sediments lithify. The clumped isotope temperatures derived from samples ranging from 350 to 755 mbsf are in excellent agreement with current temperature gradient estimates, documenting that the main fraction of the carbonate at depth formed in situ, and demonstrate that carbonate clumped isotope thermometry is a useful tool in constraining geothermal gradients in subduction systems. This project funded two graduate students (Craig Joseph, WeiLi Hong), who used data collected by this project as part of their dissertation. Both graduate students have presented their work (from this and other projects) at national (AGU) and international (New Zealand, Germany) venues. One undergraduate student (Lisa Tedder) received training in strontium isotopic analyses as part of this project. With her assistance we have established an efficient and accurate procedure for strontium isotope analyses, which we use to train undergraduate students on a continuing basins.
