Validation of a New Geochemical Approach to Constrain Deep Sea Porewater Residence Times and Advection Rates: Applications to Biogeochemical Cycling at Guaymas Basin

Constraining flow characteristics (direction, velocity, and residence time) of porewaters through deep sea sediments remains a challenge to hydrothermal and cold seep studies, due in large part to the lack of a robust measurement approach that can work across all types of subsurface discharge features.  Heat and cation/anion profiles through the sediment column have traditionally been the only available conservative tracers, but these are limited in application to discrete geologic settings.  This modest proposal seeks to validate a new approach to constrain deep sea porewater flow dynamics, using a ubiquitous and naturally-occurring isotope of radium (Ra-224; half-life: 3.6 days).  This approach compares in-situ porewater Ra-224 activities to Ra-224 production rates in the sediments (via sediment-bound Th-228 decay) to estimate a layer-specific porewater residence time.  This approach has been used on two previous cruises of opportunity, but never in an area that allowed comparison of the derived rates with an independent, conservative tracer.  The main objective of this proposal is to validate the 224Ra approach in the hydrothermal flow fields at Guaymas Basin in the Gulf of California using independent measures of heat and Mg flux through the sediments.  This project has been invited to participate in an NSF-funded cruise to the Guaymas Basin (Collaborative Research: Microbial Carbon Cycling and its Interactions with Sulfur and Nitrogen Transformations in Guaymas Basin Hydrothermal Sediments).

Once validated, this method has the potential to provide critical information on porewater flow characteristics across practically all oceanic settings.  It will not be limited in application to specific areas, such as heat and cations/anions typically are, because it is based on simple radioactive ingrowth principles from decay of Th-228, an isotope nearly ubiquitously distributed among all oceanic sediments.  A secondary objective of this proposal is to demonstrate the importance of deriving porewater flow characteristics via this method by collaborating with biogeochemical and microbiological efforts aboard the funded cruise.  Porewater velocities will be coupled with ongoing measures of geochemical parameters (e.g., methane, sulfide, ammonia) to determine solute fluxes through the sediments.  Such fluxes will help those researchers determine rates and mechanisms of anaerobic oxidation of methane (AOM), the reliance of this process upon other constituents (e.g., sulfide and ammonia), and the efficiency of this process to consume the methane flux to the shallow sediments.  Once validated, this new method has the potential to provide greater insight into a wide range of studies, including those on diagenetic processes, microbial community cycling, global heat flow, and ocean chemical budgets.