Cementation affects the mechanical properties that control sediment strength and deformation. A small volume of grain coating cement can greatly increase sediment strength. Therefore, even minor cementation may affect consolidation in basins and control deformation in accretionary margins. The effects of grain-coating silica cement on the physical properties of sediment on the Philippine Sea plate as it approaches the Nankai Trough subduction zone in the central and southwestern portions of the Shikoku Basin were documented by previous work on samples from Deep Sea Drilling Project Site 297 and Ocean Drilling Program Sites 1173 and 1177. At these sites, a small amount of glass disseminated throughout hemipelagic sediment is altered to a silica gel upon burial. The gel coats grain contacts, and inhibits sediment consolidation. The cemented sediment has anomalous porosity, seismic velocity, and rigidity. With further burial, onset of tectonic deformation, and increasing temperature, cement dissolution and mechanical breakdown leads to dramatic reduction in rigidity and collapse of the sediment framework (i.e. porosity loss). How do differences in sediment thermal history, fluid flow, and pore water chemistry between sites control shifts in the location and extent of the cemented zone? How does the incorporation of cemented units with transient properties into the margin wedge influence the nature and distribution of deformation? Toward answers to these questions, the silica cement distribution at IODP Sites C0011 and C0012 will be determined, and multicomponent reactive transport modeling for Sites C0011, C0012, 1173, and 1177 will be performed. The shear-wave velocity of samples from NanTroSEIZE drilling sites C0011 and C0012 will be determined to locate regions of anomalous strengthening. The four sites selected for examining sediment-pore water interactions provide a range of sediment accumulation and thermal histories. The results will allow examination of the effects of fluid flow rate and thermal state on the vertical location and extent of silica cementation in the Shikoku Basin sediments. The proposed investigation addresses one of the main goals of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE); to determine how geologic differences affect mechanical properties, permeability, fluid flow, pore pressure, shear strength, and earthquake rupture processes within the Nankai margin. The proposed study may transform our understanding of accretionary margin processes by shedding light on a previously underappreciated control on wedge deformation. By examining processes that control cementation of sediment entering a subduction zone, the proposed research has societal relevance as it advances understanding of the mechanisms at play in potentially hazardous seismogenic margins. Deformation features control fluid drainage through a margin wedge. Therefore, sediment cementation and deformation impact margin hydrogeology and fluid pressure, which are related to strain accumulation and seismicity on the plate interface. The proposed research will be of interest to seismologists, geochemists, and hydrogeologists. This work enhances human resources by funding a graduate student at New Mexico Tech (NMT), a Hispanic-Serving Institution. The proposed project will enhance facilities used for both research and teaching at NMT. Outreach efforts include dissemination of seismogenic zone processes through The SMILE program at Oregon State University (http://smile.oregonstate.edu/), and through an ongoing Adult Education program at COAS (http://literacyworks.org/ocean/). In addition, the Pi?fs will work with the COSEE?]Pacific Partnerships (www.coseepacificpartnerships.org) by participating in one of their ?gscience pub nights?h in Newport, OR discussing earthquake related processes around the Pacific Rim.
Quantifying the mass transport during flow through marine sediments, and the geochemical response to such flow with numerical model approaches has become a common and powerful tool for geochemists to interpret their data. In particular, a mathematical description of diagenetic processes that is based on physical, chemical, and geological principles is critical to any robust assessment and hypothesis testing that involve complex geochemical systems. Any synthetic consideration that includes all the system components is usually impossible without quantification of all processes involved and a proper tool to evaluate different scenarios that can be constrained by data. In addition, numerical modeling can simulate the temporal evolution of a system and provide insights on how a system may respond to perturbations. It is not always possible to directly observe the temporal evolution or the response of a system to various forcing mechanisms (.e.g temperature changes, variations in flow, sediment slumping, volcanic eruptions). Lastly, numerical modeling can be used to evaluate laboratory observations, which very often disturb the system as the experiments are being performed. . We developed a numerical model approach called transport-reaction model to highlight its two most important components: transport behavior and reactivity of fluid and solid. Through this grant we modified the software CrunchFlow (Steefel, 2009), a FORTRAN-based model routine developed by Dr. Carl Steefel from Lawrence Berkeley National Laboratory to investigate the critical reactions that control the silica diagenesis in the incoming sediments to the Nankai Trough. We establised a silica reaction network and validated our results against existing pore water and sediment geochemical data at four sites of different sedimentary and thermal conditions, drilled during ODP and IODP expeditions. Our model reproduces a silica diagenetic boundary (SDB) at each site, which is defined by marked decreases in reactive volcanic ash, pore water silica and potassium. Volcanic ash alteration, was constrained by modeling pore water 87Sr/86Sr profiles, and the derived reaction rate of 10-12.13 to 10-11.75 mol of Si/m2/sec provides the SiO2(aq) at rates required to form authigenic amorphous silica phase above the SDB. Below the SDB, formation of clinoptilolite consumes potassium and regulates the extension of amorphous silica by consuming SiO2(aq). The observed low SiO2(aq) and dissolved potassium in these deep sequences require continuous precipitation of clinoptilolite; however in order to maintain oversaturation of this mineral at the low SiO2(aq) in sediments below the SDB, an increase in pH is required, consistent with pore water observations. Thermal history, rather than temperature alone, controls the inferred reaction network as shown by the convergence of the thermal maturity of sediments at the SDB from all sites (STTI ranges from 0.0053-0.0124), consistent with other locations documented onshore Japan. Our study results, in particular the realistic estimates of ash alteration rates, silica reaction networks and controlling parameters on these reactions (pH, temperature and sedimentation rate) constitute a framework against which to build upon, as we move forward in understanding the mechanisms and consequences of ash alteration in convergent margins worldwide, which may include sediment mechanical and hydrogeologic properties, development of fault zones and seismogenic behavior. This project funded doctoral student WeiLi Hong, who received training in numerical model approaches and implemented the transport reaction model to address silica diagenesis, as well other biogeochemical processes in marine sediment as the main topic of his dissertation. He completed his graduate program in march 2014, and has presented his work at national (AGU) and international (Germany) venues. Because of his modeling expertise, Dr. Hong contributed to several research efforts by providing key modeling results, which are reflected in the many papers he has co-authored during the last few years. To disseminate results and train other researchers interested in this modeling approach, a short course (OC699: Special Topics- Multicomponent Reactive Flow and Transport Modeling) was offered both using local and on-line delivery methods to introduce transport-reaction modeling starting from the basic math and chemistry background, and then introduce the full capability of CrunchFlow. OSU students from CEOAS, Math and Engineering took this course, which was simultaneously be transmitted via webex to participants at University of Washington, University of Florida, Carnagie Mellon University and the NETL labs in Albany and Pittsburgh.
