Evolution of Mineral-Organic Matter Associations in Sediments: From (Bio)mineralization to Burial

Abstract:

Physical and chemical associations with mineral surfaces may protect organic matter (OM) from oxidative degradation and allow its preservation in soils and sediments. This study evaluates the mechanism of mineral-based preservation (MBP) and the time scale on which MBP is operative by tracking the co-evolution of oxide minerals and associated OM during mineral precipitation and ripening. Scanning transmission X-ray microscopy coupled to near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS) as well as bulk NEXAFS demonstrate that, in laboratory systems using cell-free filtrate from pure bacterial cultures, an association between OM and biogenic manganese oxides is rapidly established. OM associated with freshly precipitated biominerals consists of proteinaceous carbon and nitrogen consistent with a microbial origin; this composition remains constant over the course of 96 hours, despite mineral aggregation and structural evolution from hexagonal to triclinic birnessite. We predict that, in natural systems, oxide minerals simultaneously drive remineralization and offer MBP. Different minerals will promote a different balance between the two, imparting a mineral-specific signature on the concentration and composition of preserved OM. We test this idea by conducting incubations of natural estuary waters spiked with compositionally and structurally diverse synthetic oxide minerals. The concentration and composition of mineral-associated OM were tracked by element analyzer-isotope ratio mass spectrometry (EA-IRMS) and STXM-NEXAFS in multiple experiments lasting between 4 weeks and 1 year. Results from incubation experiments are contrasted with natural sediment samples from a range of depositional environments in order to evaluate the potential for long-term sequestration of organic carbon in sediments facilitated by minerals.