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 of 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 scales. 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 to 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 will employ 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 dissolved inorganic carbon (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. 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 is understood about permeable sediment biogeochemistry from small-scale manipulations and how continental shelf biogeochemistry is represented 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 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 will be developed 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 Boston University (e.g., BU Summer Undergraduate Research Fellowship recruits minority students) and at the Skidaway Institute of Oceanography.
