Collaborative Research: Pythia&#39;s Oasis - Access to Deep Subduction Zone Fluids

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项目编号	1658201
	Collaborative Research: Pythia's Oasis - Access to Deep Subduction Zone Fluids
	Deborah Kelley
主持机构	University of Washington
项目开始年	2017
	2017-09-15
项目结束日期	2019-08-31
资助机构	US-NSF
项目类别	Standard Grant
项目经费	390349(USD)
国家	美国
语种	英语
英文摘要	There is intense interest in understanding the nature, magnitude, and conditions of fluid flow deep within active continental margins due to the impact that fluids and mineral alteration processes have on seismicity and earthquakes in subduction zone environments. This is particularly true for the continental margin off Oregon and Washington State and Southern Canada, which is called the Cascadia Margin. This is because this ~1000 kilometer-long subduction zone fault has been locked since its last rupture in 1700 which produced a magnitude 9 earthquake that resulted in a large tsunami that impacted the Pacific Northwest and Japan and triggered significant underwater landslides. Gaining access to deep subduction zone fluids, however, is challenging because they are largely inaccessible, occurring at great depth below the seafloor. However, in 2015, a seep site was discovered off the Oregon coast where a jet of warm, hydrocarbon-enriched, low-salinity fluid was found venting from the seafloor. This site, called Pythia's Oasis, is unlike any seep site yet discovered, providing possible access, for the first time, to fluids formed deep within the Cascadia Subduction Zone. A second adjacent site includes an extensive collapse zone hosting multiple seeps that issue diffuse flows of warm and/or low salinity, methane-rich fluids that support dense microbial and macrofaunal communities. The goal of this two-year collaborative research project is to characterize the geology and chemistry of this extraordinary environment to test hypotheses about the origin of these fluids. This will be done using a remotely operated vehicle and an autonomous underwater vehicle from the National Deep Submergence Facility at the Woods Hole Oceanographic Institution, heat flow probes, coring devices, and geochemical tracers and analyses. If these fluids come from deep in the subduction zone, this site will provide an unprecedented opportunity to study high-temperature fluid sources and reactions in the only segment of the Cascadia Subduction Zone that is inferred to be seismically active. Results of the work have implications for understanding deep-seated fluid and seismogenic processes acting in other subduction zones as well. Broader impacts include an extensive at-sea student training program in which at least 10 undergraduate and graduate students will participate on the 13-day sea-going field expedition to the site where they will learn about and participate in seagoing activities and the discovery processes. The students will work alongside experienced scientists and ship and deep submergence vehicle crews to conducting their own research using data collected on the cruise and by later onshore analyses. Public outreach will be carried out via students who will communicate their experiences and findings through a cruise website and formal and informal presentations, with messaging focused on K-20 opportunities in integrated oceanography-engineering programs. This research will characterize the geology and chemistry of the extraordinary new discovery of a seep site, called Pythia's Oasis, that is located on the Cascadia Subduction Zone. Three hypotheses on the source of these fluids will be tested: (1) that the fluids are meteoric and play no role in the seismogenic behavior of the margin but may contribute to element transport between land and ocean, impacting the current understanding of coastal hydrogeology; (2) that the fluids originate from smectite-illite dehydration in the accreted sediments, rather than at the plate boundary, providing important information on fluid production and overpressure development within the accretionary prism above the plate boundary; and (3) that the fluids originate from high temperature metamorphic reactions at depth in the seismogenic region of the CSZ, resulting in extreme overpressures at the plate boundary. In the latter case, the dehydration reactions may explain the partially locked behavior of the fault in this part of the continental margin or suggest that the locked seismogenic zone may not correspond to moderate overpressures as postulated for other subduction zone areas such as Nankai. off Japan, and off Costa Rica. Hypotheses will be tested by detailed geological characterization of the seep site; by thoroughly analyzing the seep and pore fluids using major and trace element geochemistry; by examining the gas chemistry and by isotopic analyses; and by heat flow and modeling studies. The field program will utilize the remotely operated vehicle, Jason, and the autonomous underwater vehicle, Sentry, from the Woods Hole Oceanographic National Deep Submergence Facility to complete photomosaics of the seafloor as well as bathymetric maps around the seep sites at 1-m resolution. Seep fluids will be collected using isobaric gas-tight fluid-volatile samplers with real-time temperature measurements to determine end-member fluid compositions. All pore water samples will be analyzed for salinity, pH, and alkalinity shipboard, and a subset of samples will be analyzed for sulfate by shipboard ion chromatography. Approximately 300 samples will be analyzed for Cl, SO4, Br, Ca, Mg, Na, K, and alkalinity. Select samples will be analyzed for Li, Rb, Cs, Ba, DIC, and O/H isotopes. Si, NH4, as well as the methane through pentane concentrations in pore water and vent fluids will be analyzed. Stable isotope ratios of C, D, H, CH4, He, Li, and Cl will also be determined. Detailed heat flow measurements around the venting sites will be obtained to constrain relative changes in thermal gradients. Precisely located push cores for sediment chemical and physical property and pore water analyses will be collected and analyzed. Water column sensors on Sentry will also provide eH and CTD-O2-turbidity characterizations of the near-bottom fluids. Gravity coring will allow direct sampling of pore fluids below the zone of mixing with seawater and will ensure capture of the advecting fluid signal. Upward advection rates will be estimated from solute profiles using a new, non-steady-state, reaction-transport model. The spatial distribution of flow constructed from Jason push cores around the seep and shipboard gravity cores will be used to estimate output fluxes of fluid and solutes. The EM302 multibeam system on the R/V Thompson will be utilized for imaging of the bubble plumes to determine spatial extent with implications for biological activity in the upper water column. To determine local current regimes and hence the most intense plume locations in the water column, the hull-mounted 75 kHz ADCP will be used to log current velocities during all surveys.
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文献类型	项目
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/72004
专题	环境与发展全球科技态势
推荐引用方式 GB/T 7714	Deborah Kelley.Collaborative Research: Pythia's Oasis - Access to Deep Subduction Zone Fluids.2017.