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

由于流体和矿物蚀变过程对地震活动性和俯冲带环境中的地震的影响，人们对了解活动大陆边缘深处流体流动的性质、大小和条件有着浓厚的兴趣。俄勒冈州和华盛顿州以及加拿大南部的大陆边缘，也就是所谓的卡斯卡迪亚边缘，尤其如此。这是因为这个长达1000公里的俯冲带断层自1700年最后一次破裂以来一直处于锁定状态，那次破裂产生了9级地震，导致了大海啸，影响了太平洋西北部和日本，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，因为它们大多难以接近，并且发生在海底深处。然而，2015年，在俄勒冈州海岸外发现了一个渗漏点，在那里发现了一股温暖的、富含碳氢化合物的低盐度流体从海底喷涌而出。这个地点被称为皮提亚绿洲，与迄今发现的任何一个渗漏点都不同，它首次为卡斯卡迪亚俯冲带深处形成的流体提供了可能的通道。第二个相邻的地点包括一个广泛的塌陷区，容纳多个渗漏，这些渗漏产生温暖和/或低盐度的弥漫流动，富含甲烷的流体支持密集的微生物和大型动物群落。这项为期两年的合作研究项目的目标是描述这种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。这项工作将使用Woods Hole海洋研究所国家深海潜水设施的远程操作车辆和自主水下航行器、热流探测器、取芯设备、地球化学示踪剂和分析工具来完成。如果这些流体来自于俯冲带的深处，这个地点将提供一个前所未有的机会来研究高温流体的来源和反应，在卡斯卡迪亚俯冲带的唯一一个被推断为地震活跃的部分。研究结果对理解其他俯冲带的深部流体和孕震过程也具有指导意义。更广泛的影响包括一项广泛的海上学生培训计划，其中至少有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、船舶和深潜车辆工作人员一起工作，利用在巡航中收集的数据和随后的陆上分析进行自己的研究。学生们将通过邮轮网站和正式和非正式的演讲来传达他们的经验和发现，并将信息集中在综合海洋学工程项目的K-20机会上。这项研究将描述一个非凡的新发现的渗漏地点的地质和化学特征，称为皮提亚绿洲，位于卡斯卡迪亚俯冲带。本文将对这些流体来源的三种假设进行验证：(1)这些流体是大气流体，在边缘的孕震行为中没有作用，但可能有助于陆地和海洋之间的元素运输，影响目前对沿海水文地质的认识；(2)流体来源于吸积沉积物中的蒙脱石-伊利石脱水，而不是在板块边界处，这为研究板块边界以上的吸积棱镜内流体生产和超压发展提供了重要信息；(3)这些流体来源于地壳发震区深处的高温变质反应，在板块边界处产生了极端的超压。在后一种情况下，脱水反应可以解释这部分大陆边缘断层的部分闭锁行为，或者表明闭锁的孕震带可能不对应于其他俯冲带地区（如南开）所假定的中度超压。在日本和哥斯达黎加附近假设将通过对渗漏地点的详细地质特征进行检验；利用主微量元素地球化学方法对渗流和孔隙流体进行了深入分析；通过气体化学和同位素分析；通过热流和模型研究。现场项目将利用Woods Hole海洋国家深海潜水设施的远程操作车辆Jason和自主水下航行器Sentry来完成海底的显微摄影，以及以1米分辨率绘制渗水点周围的水深图。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。所有孔隙水样品将分析盐度，pH值和碱度，并通过船上离子色谱法分析一部分样品的硫酸盐。大约300个样品将被分析Cl， SO4, Br, Ca, Mg, Na， K和碱度。选择样品将分析Li， Rb, Cs, Ba， DIC和O/H同位素。通过孔隙水和喷口流体的戊烷浓度分析Si、NH4以及甲烷。C、D、H、CH4、He、Li和Cl的稳定同位素比值也将测定。将获得排气点周围详细的热流测量，以约束热梯度的相对变化。将收集和分析精确定位的推岩心，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。重力取心将允许对与海水混合区域以下的孔隙流体进行直接采样，并确保捕获平流流体信号。向上平流率将使用一个新的非稳态反应输运模型从溶质剖面估计。由詹森推岩芯和船载重力岩芯构建的流体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，以确定空间范围以及对上层水柱生物活动的影响。为了确定当地的水流状况，从而确定水柱中最强烈的羽流位置，在所有调查期间，将使用安装在船体上的75 kHz ADCP来记录水流速度。

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.