B105-06
Calcium complexation by organic matter in calcareous sediments
Tuesday, 15 December 2020: 19:20
Virtual
Sharon Bone1, Niranjan Govind2, Amity Andersen2, John B Cliff3, Christopher J Takacs4, Karrie L Weaver5, Scott James Roycroft5, Scott E Fendorf6 and John Bargar7, (1)SLAC National Accelerator Laboratory, Menlo Park, CA, United States, (2)Pacific Northwest National Laboratory, Richland, WA, United States, (3)Pacific Northwest National Laboratory, Environmental Molecular Sciences Laboratory, Richland, WA, United States, (4)SLAC National Accelerator Laboratory, Menlo Park, United States, (5)Stanford University, Stanford, CA, United States, (6)Stanford University, Department of Earth System Science, Stanford, CA, United States, (7)SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource, Menlo Park, CA, United States
Abstract:
Soils and sediments host large stores of organic carbon which can be released to the atmosphere upon mineralization. Ca may play a key role in preventing mineralization of this organic carbon by promoting mineral protection. It has long been recognized that Ca forms cation bridges that link together negatively charged functional groups from organics and mineral surfaces, stabilizing organic carbon via mineral sorption. However, there is little direct experimental observation of the coordination between Ca, organic matter (OM), and minerals in sediments, which is necessary to determine the molecular-scale mechanism by which Ca bridges OM moieties to one another or to mineral surfaces.
We examined whether and how Ca was associated with OM in calcareous sediments. We sought to determine whether Ca was co-associated with OM and specific minerals, such as iron oxides or clays; as well as what types of OM Ca was associated with. Finally, we sought to determine the local coordination environment of Ca complexed by OM to assess how it participated in briding interactions.
Calcareous sediments were subjected to density fractionation to separate organic and mineral components of the sediments, yielding a particulate organic matter fraction, a fraction containing organic-mineral aggregates, and a fraction that was mostly mineral, with a small amount of organics. Calcium carbonate minerals were separated into the “mineral” fraction. In each fraction, the co-localization of Ca with different organic species and with Al, Si and Fe minerals was probed using nano secondary ion mass spectrometry (NanoSIMS) and scanning transmission X-ray microscopy (STXM) at the scale of ca. 100 nm. Ca speciation was probed directly using (micro-) X-ray absorption near edge structure (XANES) spectroscopy and density functional theory. Our analysis elucidates the role that Ca plays in stabilizing OM in calcareous sediments, thereby limiting its mineralization and release as CO2 into the atmosphere.
