Collaborative Research: Pythia's Oasis - Access to Deep Subduction Zone Fluids

合作研究：皮提亚的绿洲 - 获取深俯冲带流体

• 批准号: 1657084
• 负责人: Marta Torres
• 金额: $ 12.2万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-09-15 至 2022-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1657084&HistoricalAwards=false
• 关键词: Collaborative; Research; Pythia; Oasis; Access

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.

由于流体和矿物蚀变过程对俯冲带环境中的地震活动和地震的影响，人们对了解活动大陆边缘深处流体流动的性质、强度和条件产生了浓厚的兴趣。对于俄勒冈州、华盛顿州和加拿大南部的大陆边缘（称为卡斯卡迪亚边缘）尤其如此。这是因为，这条约 1000 公里长的俯冲带断层自 1700 年上次断裂以来一直被锁定，那次断裂引发了 9 级地震，引发了巨大海啸，影响了太平洋西北地区和日本，并引发了严重的水下滑坡。然而，获取深俯冲带流体具有挑战性，因为它们基本上无法进入，发生在海底以下很深的地方。然而，2015 年，在俄勒冈州海岸附近发现了一个渗漏点，一股温暖、富含碳氢化合物、低盐度的流体从海底喷出。这个地点被称为皮提亚绿洲，与迄今为止发现的任何渗漏地点不同，它首次提供了进入卡斯卡迪亚俯冲带深处形成的流体的可能。第二个相邻地点包括一个广泛的塌陷区，其中有多个渗漏点，这些渗漏点发出温暖和/或低盐度、富含甲烷的流体的扩散流，支持密集的微生物和大型动物群落。这个为期两年的合作研究项目的目标是描述这个特殊环境的地质和化学特征，以测试有关这些流体起源的假设。这将使用伍兹霍尔海洋研究所国家深潜设施的遥控潜水器和自动水下潜水器、热流探测器、取芯装置以及地球化学示踪剂和分析来完成。如果这些流体来自俯冲带深处，那么该地点将为研究卡斯卡迪亚俯冲带唯一被推断为地震活跃的部分的高温流体源和反应提供前所未有的机会。这项工作的结果对于理解其他俯冲带的深层流体和地震过程也具有重要意义。更广泛的影响包括广泛的海上学生培训计划，其中至少 10 名本科生和研究生将参加为期 13 天的海上实地考察，在那里他们将了解并参与海上活动和发现过程。学生们将与经验丰富的科学家以及船舶和深潜车辆船员一起工作，利用在巡航中收集的数据以及随后的岸上分析来进行自己的研究。公共宣传将由学生进行，学生将通过邮轮网站以及正式和非正式的演示来交流他们的经验和发现，重点是综合海洋学工程项目中的 K-20 机会。这项研究将描述位于卡斯卡迪亚俯冲带的皮提亚绿洲这一非凡的新发现渗漏点的地质和化学特征。将测试关于这些流体来源的三个假设：（1）这些流体是大气流体，对边缘的地震行为没有作用，但可能有助于陆地和海洋之间的元素传输，影响目前对沿海水文地质学的理解； (2) 流体源自增生沉积物中的蒙皂石-伊利石脱水，而不是板块边界，这为板块边界上方的增生棱柱内的流体产生和超压发展提供了重要信息； (3)流体起源于CSZ地震区深处的高温变质反应，导致板块边界出现极端超压。 在后一种情况下，脱水反应可以解释大陆边缘这部分断层的部分锁定行为，或者表明锁定的发震带可能不对应于南开等其他俯冲带区域所假设的中度超压。日本附近和哥斯达黎加附近。将通过渗漏地点的详细地质特征来检验假设；使用主量元素和微量元素地球化学彻底分析渗漏液和孔隙流体；通过检查气体化学和同位素分析；以及热流和建模研究。该现场项目将利用伍兹霍尔海洋国家深潜设施的远程操作潜水器 Jason 和自主水下潜水器 Sentry 来完成海底照片马赛克以及渗透点周围 1 米分辨率的测深图。将使用等压气密流体挥发性采样器收集渗漏流体，并进行实时温度测量以确定端元流体成分。所有孔隙水样品将在船上进行盐度、pH 和碱度分析，部分样品将通过船上离子色谱法进行硫酸盐分析。将分析大约 300 个样品的 Cl、SO4、Br、Ca、Mg、Na、K 和碱度。将分析选定样品的 Li、Rb、Cs、Ba、DIC 和 O/H 同位素。将分析孔隙水和喷口流体中的 Si、NH4 以及甲烷到戊烷的浓度。 C、D、H、CH4、He、Li 和 Cl 的稳定同位素比率也将被确定。将获得通风部位周围的详细热流测量值，以限制热梯度的相对变化。将收集和分析精确定位的推芯，用于沉积物化学和物理性质以及孔隙水分析。 Sentry 上的水柱传感器还将提供近底部流体的 eH 和 CTD-O2 浊度特征。重力取芯将允许对与海水混合区域下方的孔隙流体进行直接采样，并确保捕获平流流体信号。将使用新的非稳态反应传输模型根据溶质分布来估计向上平流速率。由杰森推芯围绕渗流和船载重力核心构建的流动空间分布将用于估计流体和溶质的输出通量。 R/V Thompson 上的 EM302 多波束系统将用于对气泡羽流进行成像，以确定对上部水柱生物活动产生影响的空间范围。为了确定当地的水流状况以及水柱中最强烈的羽流位置，安装在船体上的 75 kHz ADCP 将用于记录所有调查期间的水流速度。