Biogeochemical controls exerted by organic-rich sediments on carbon cycling and uranium mobility
EMSL Project ID
49333
Abstract
Floodplains are hydrologically and biogeochemically dynamic environments that store extensive quantities of carbon, host diverse ecosystems, and serve as interfaces for nutrient and contaminant exchange between surface/subsurface and terrestrial/aquatic environments. Our work focuses on resolving key biogeochemical molecular mechanisms controlling carbon cycling, nutrient dynamics, and contaminant mobility in the floodplain environment. Our field sites in the upper Colorado River Basin formerly hosted U-ore processing facilities and currently exhibit persistent groundwater plumes despite initial predictions of dilution within a few decades. Our field research has shown that organic-rich sulfidic sediment lenses are ubiquitous throughout this region, acting as biogeochemical hotspots that store large inventories of carbon, nutrients, and contaminants. These lenses lie within or below the zone of groundwater fluctuations, and thus undergo radical hydrologic changes from saturation to highly unsaturated. Drainage of reduced sediments and evapotranspiration drives aeration and concomitant oxidation of nutrients and contaminants, with profound implications for carbon fate, dissolved nutrient loads, and U plume persistence. The exact mechanism(s) driving carbon cycling and the retention/release of nutrients and U are unknown, but involve intense coupling to redox cycles of S, N, and Fe.
To address the mechanistic linkages between these cycles, the molecular speciation of organic C, Fe, and S will be correlated with that of U (i.e. U(IV)-mineral and -NOM associations) in samples harvested throughout seasonal fluctuations in water saturation. Thus, we will evaluate the coevolution of oxidation states and molecular structures for these elements over the full range of typical ecosystem conditions. Achieving this goal will require integration of EMSL expertise in Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS; C speciation and oxidation state), nuclear magnetic resonance spectrometry (NMR; C speciation), Mossbauer spectroscopy (Fe, S mineralogy, Fe oxidation state), nano-scale secondary ion mass spectrometry (Nano-SIMS; imaging U co-localization with C, Fe, S), and transmission electron microscopy (phase identification). Oxidation states and molecular structures of Fe, S, and U will also be assessed using synchrotron based x-ray absorption (micro)spectroscopy through our active proposals at SSRL and CLS. In addition to improving BER's ability to understand and predict ecosystem function, carbon cycling, and response to perturbations (temperature and precipitation), this project will help to address the mechanisms behind the major problem of U plume persistence regionally.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2016-10-01
End Date
2018-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Co-Investigator(s)
Team Members
Related Publications
Discerning Microbially Mediated Processes During Redox Transitions in Flooded Soils Using Carbon and Energy Balances
Boye K.E., A.M. Herrmann, M.V. Schaefer, M.M. Tfaily, and S. Fendorf. 2018. "Discerning Microbially Mediated Processes During Redox Transitions in Flooded Soils Using Carbon and Energy Balances." Frontiers in Environmental Science. PNNL-SA-134588. doi:10.3389/fenvs.2018.00015
Thermodynamically controlled preservation of organic carbon in floodplains
Boye K.E., V. Noel, M.M. Tfaily, S.E. Bone, K.H. Williams, J. Bargar, and S. Fendorf. 2017. "Thermodynamically controlled preservation of organic carbon in floodplains." Nature Geoscience 10. PNNL-SA-126035. doi:10.1038/NGEO2940