Collaborative Research: Development of an In Situ Porewater Sampler Coupled to an Underwater Mass Spectrometer for High-Resolution Biogenic Gas Measurements in Permeable Sediments

大陆架被认为是丰富的渔场，但它们也提供了一套较少受限制的、有价值的生态系统服务。特别是，这些近岸环境和开阔海洋之间的边界地带，是生物地球化学的“热点”，调解了向深海的碳(C)和氮(N)通量。碳和氮之间的紧密耦合对海洋生产力、大气二氧化碳（CO2）水平以及最终的全球气候具有重要意义。由于大陆架又宽又浅，大部分的碳氮循环发生在沉积物中——大约70%的沉积物被可渗透的沙子覆盖。传统上，人们认为砂质沉积物在碳和氮循环中不重要，但在过去20年的研究表明，砂质沉积物可能是海洋生物地球化学循环的关键参与者。然而，由于缺乏在适当时间对这些环境进行采样的有效方法，对砂质陆架环境的贡献进行量化的能力受到限制。在这项研究中，研究人员将开发一种与水下质谱仪耦合的新型现场可部署孔隙水采样装置。该系统将沿垂直梯度测量全套生物气体（如O2、CO2、N2、Ar、CH4、H2S），并根据同时估计的气体浓度和垂直平流孔隙水交换计算现场反应速率。随着该仪器的开发，研究人员还将创建一个在线指导手册，其中包含视频，说明如何构建孔隙水采样系统，如何校准和测试质谱仪等，目的是将这项技术推广到更广泛的社区。这项工作的成功完成将产生一个灵活的孔隙水采样和分析平台，科学家和工程师可以使用现成的组件来复制。本研究采用膜入口质谱仪对孔隙水进行分析，但该采样平台也可以与其他现场分析仪器一起使用。重要的是，该仪器将广泛扩展我们测量可渗透海洋沉积物中C和N循环的能力，并在此过程中为生物地球化学和全球气候模型提供相关数据。主要研究人员（pi）将设计、建造并现场测试一种新型孔隙水采样接口，用于集成水下膜引入质谱仪。该设备将允许高分辨率、实时测量生物地球化学重要气体（如O2、CH4、N2、H2S）以及渗透性沉积物孔隙水中的总DIC（如CO2），采样误差最小。系统中包含的传感器允许从热传输推断孔隙水中的垂直平流，从而允许实时计算现场气体浓度和反应速率。该系统在测量可渗透沉积物中耦合生物地球化学过程的能力方面取得了重大进展，并将有助于更好地了解大陆架的生物地球化学。大陆架沉积物处理了大部分从陆地输出或固定在上覆水域的碳和氮。大约70%的陆架海底以可渗透的砂质沉积物为特征。实验室规模的实验表明，渗透性沉积物中的有机质处理是快速有效的。此外，成岩作用和养分循环似乎是由底部水通过这些可渗透的表层沉积物孔隙空间的平流控制的。平流决定颗粒捕获的速率（通过床层过滤），矿化的速率和途径（通过氧化剂通量），以及再矿化营养物质返回水柱的速率。目前，我们对渗透性沉积物生物地球化学的理解与我们如何在建模框架中表示大陆架生物地球化学之间存在脱节。“缺失的一环”是一套可靠的工具来测量现场的动态过程，从而为适当的参数化建模提供良好的数据和基础。由pi和他们的学生建造的仪器将克服目前在渗透性沉积物环境中孔隙水采样的几个限制。该系统被设计为在没有封闭工件的情况下在现场操作。它的设计目的是在较长的时间内取样，使其更有可能捕捉到可能主导沉积物-水交换的短暂事件。由于采样接口与质谱仪耦合，该系统可以同时采集多种化学物质的精确数据。提出的质谱仪和孔隙水采样界面将代表着在可渗透沉积物环境中测量生物地球化学特性的能力向前迈出了重要一步。来自该仪器系统的数据将使研究人员能够解决砂质沉积物在全球生物地球化学中的作用的基本不确定性，例如反硝化的效率或沉积物中甲烷和氧化二氮等温室气体的产生和消耗。为了帮助将在这里获得的知识转移到更广泛的社区，建立了一个附有零件清单的仪器网站，以及一个博客，感兴趣的科学家可以在这里讨论该仪器和该技术的潜在更广泛应用。此外，有关相关主题（例如，施工、校准、故障排除、现场部署）的教学视频将通过专门用于该项目的YouTube频道托管。一名博士生将接受工程学、海洋学和生物地球化学交叉学科的研究训练。本科生将接受实地和实验室方法方面的培训，并将利用波士顿大学现有的资源（例如，波士顿大学夏季本科生研究奖学金招收少数民族学生）和斯基德韦海洋学研究所，共同努力吸引妇女和少数民族。

Continental shelves are known as rich fishing grounds but they also provide a suite of less constrained, valuable ecosystem services. In particular, these borderlands between the near shore environment and open ocean, are biogeochemical "hot spots" mediating fluxes carbon (C) and nitrogen (N) to the deep sea. The tight coupling between C and N has important implications for ocean productivity, atmospheric carbon dioxide (CO2) levels, and ultimately global climate. Because continental shelves are broad and shallow, much of the C and N cycling occurs within their sediments - approximately 70% of which are covered with permeable sands. Traditionally, it was thought the sandy sediments were unimportant in C and N cycling but, over the last two decades research has revealed that sandy sediments may be key players in marine biogeochemical cycles. However, the ability to quantify the contributions of sandy shelf environments has been limited by the absence of effective methods for sampling these environments on appropriate time. In this research the investigators will develop a novel field-deployable porewater sampling device coupled to an underwater mass spectrometer. This system will measure a full suite of biogenic gases (e.g., O2, CO2, N2, Ar, CH4, H2S) across a vertical gradient and calculate in situ reaction rates based on simultaneous estimates of gas concentrations and vertical advective porewater exchange. Along with the development of this instrument the investigators will also create an online instruction manual with videos explaining how to build the porewater sampling system, how to calibrate and test the mass spectrometer, etc., with the goal of propagating this technology the wider community. Successful completion of this work will produce a flexible porewater sampling and analysis platform that can be reproduced by scientists and engineers using off the shelf components. The research proposed here employs membrane inlet mass spectrometer analysis of the porewater, but this sampling platform could be used with other in situ analytical instrumentation as well. Importantly, this instrumentation will broadly expand our capabilities to measure C and N cycling in permeable marine sediments, and, in doing so, will provide relevant data for biogeochemical and global climate models.The principal investigators (PIs) will design, build, and field test a novel porewater sampling interface for integration with an underwater membrane introduction mass spectrometer. The device will allow highly resolved, real-time measurements, with minimal sampling artifacts, of biogeochemically important gases (e.g., O2, CH4, N2, H2S) as well as total DIC (as CO2) in porewater of permeable sediments. Included in the system are sensors that allow vertical advection within porewater to be inferred from heat transport, thus permitting both in situ gas concentrations and reaction rates to be calculated in real time. This system will represent a major advance in the ability to measure coupled biogeochemical processes within permeable sediments and will contribute to a better understanding of the biogeochemistry of continental shelves. Continental shelf sediments process much of the carbon and nitrogen exported from land or fixed in overlying waters. Approximately, 70% of the shelf seafloor is characterized by permeable sandy sediments. Lab-scale experiments indicate that organic matter processing within permeable sediments is rapid and efficient. Additionally, it appears that diagenesis and nutrient recycling are controlled by advection of bottom water through the pore space of these permeable surficial sediments. Advection determines the rate of particle capture (by bed filtration), the rate and pathways of mineralization (by oxidant flux), and the rate of return of remineralized nutrients to the water column. At present there is a disconnect between what we understand about permeable sediment biogeochemistry from small-scale manipulations and how we represent continental shelf biogeochemistry in a modeling framework. The "missing link" is a set of reliable tools to measure dynamic processes in the field, and thus to provide good data and a basis for appropriate parameterization for modeling. The instrumentation to be built by the PIs and their students will overcome several of the present limitations of porewater sampling in permeable sediment environments. The system is designed to operate in situ without enclosure artifacts. It is designed to sample over extended time periods, making it more likely to capture transient events that may dominate sediment-water exchange. Because the sampling interface is coupled to a mass spectrometer, the system can collect accurate and precise data for multiple chemical species simultaneously. The proposed mass spectrometer and porewater sampling interface will represent a major step forward in the ability to measure biogeochemical properties in permeable sediment environments. Data from this instrument system will allow researchers to address fundamental uncertainties about the roles of sandy sediments in global biogeochemistry such as the efficiency of denitrification or the production and consumption of greenhouse gases such as CH4 and N2O within sediments. To assist the transfer of knowledge gained here to the wider community, a website for this instrument with part lists, and a blog where interested scientists can hold discussions on the instrument and potential broader applications of the technology. Additionally, instructional videos on relevant topics (e.g., construction, calibration, troubleshooting, field deployment) will be hosted via a YouTube channel dedicated to this project. One Ph.D. student will receive interdisciplinary research training at the junctures between engineering, oceanography, and biogeochemistry. Undergraduate students will be trained in field and laboratory methods and a concerted effort to attract women and minorities will be made using resources available at BU (e.g., BU Summer Undergraduate Research Fellowship recruits minority students) and at the Skidaway Institute of Oceanography.