The ability to make in situ measurements of analytes on relevant spatial and temporal scales and the need for robust porewater transport characterizations have limited measurements of chemical fluxes within permeable sediments. Two scientists from the University of Hawaii will combine in situ electrochemistry, modeling, traditional geochemistry, and an eddy correlation oxygen flux measuring system to address the biogeochemical fluxes from nearshore permeable sediments. The research will be carried out at the Kilo Nalu Cabled Nearshore Reef Observatory which will provide (1) data acquisition and data dissemination systems; (2) accessible power and high bandwidth communications for in situ instruments; (3) a wide range of physical oceanographic measurements; and (4) Oahu?s south shore has a wide and predictable variety of surface wave conditions and land-based inputs that impact the redox-sensitive biogeochemistry of its permeable sediments. Results from this study will be used to (1) improve our understanding of the interaction between these active, carbon recycling sediments and the overlying water column; (2) examine in detail the temporal and spatial variability of key redox-reactive chemical species; (3) quantify the relative contributions of benthic photosynthesis, sand ripple position, currents and waves, and episodic organic loading events to redox oscillations; and 4) integrate fine-scale chemical measurements with porewater velocity modeling to calculate biogeochemical fluxes.
As regards broader impacts, this project will significantly further our knowledge of the solute fluxes across the permeable sediment/seawater interface, as well as improve our understanding of biogeochemical processes within these types of sediments. In addition, this research would make novel in situ sensor technology available to the ocean sciences community. Outreach activities include education of the public via the Bishop Museum and providing information on their research for websites and educational videos displayed in Waikiki hotels. One postdoc, one graduate student and one undergraduate student would be supported and trained as part of this project.
Permeable sandy sediments act as a filter for bottom seawater in the nearshore environment, and they exhibit high rates of biogeochemical exchange between bottom waters and the sediment porewater. However, the conventional methods for sampling muddy sediments typically either do not work in permeable sediments because of the physical nature of sandy sediments, or they interfere with the hydrodynamics that make permeable sediments so reactive. Advances in instrumentation that can provide the capability for making more frequent measurements of important chemical parameters (like dissolved oxygen, iron, sulfide, pH) in such environments, as opposed to only making laboratory measurements upon samples collected in the field and returned to a lab provides researchers with indispensible information about actual processes occurring in the environment. Through this NSF award, we modified proven technology to design a coastal seafloor instrument package capable of accurately measuring chemical fluxes across the sediment-water interface and within the upper sediments using high-resolution temperature measurements as a proxy for porewater movement. At the heart of the package is a miniature thermistor chain that can be buried in the upper sediments. As daily temperature oscillations propagate from the bottom water to the sediment porewater, we use the signal to accurately constrain our porewater velocity calculations. Once we know the rate of movement of porewaters, and collect fluid samples to measure various biogeochemical parameters, we can accurately calculate a flux for the sediments. The new instrument package is capable of making autonomous, preprogrammed deployments, or can be configured to run under real-time control over Ethernet communication. Our field deployments have demonstrated that the new instrument’s performance and reliability result in an innovative new approach for measuring porewater velocities, which when coupled to measurements of dissolved oxygen or nutrients, can provide rigorous and site-specific flux calculations.
