Energy-filtering transmission electron microscopy (EFTEM) and electron energy-loss spectroscopy (EELS) investigation of clay–organic matter aggregates in aquatic sediments

High resolution (<10 nm) transmission electron microscopy (TEM), energy-filtering TEM (EFTEM), and electron energy loss spectroscopy (EELS) have been used for the direct microstructural imaging and analysis of clay-organic matter aggregates in fine-grained aquatic sediments from Jourdan River Estuary, MS, USA. EFTEM and EELS allow rapid, high-resolution spatial mapping and analysis of light elements such as carbon. The study area sediments are comprised of discrete organic matter masses and aggregates of clay plates. Clay aggregates often include organic matter. The comparison of clay aggregate images obtained by the TEM bright-field technique and the EFTEM carbon mapping technique shows that carbon within clay domains has spatial features that are in the same size scale as the features of individual clay plates within the aggregates (< 20 nm). These intimate spatial associations suggest that organic matter in clay aggregates is either closely attached to the surfaces of individual clay plates or structurally incorporated into clay crystals. Organic matter within clay aggregates does not appear to exist as discrete or massive masses that fill the pore spaces within the clay aggregates. These intimate associations, whether they are due to chemical interaction or physical sequestration, should affect the reactivity of organic matter during early diagenesis. The EELS spectra of clay aggregates show that organic matter always coexists with calcium, suggesting that Ca-containing smectite, rather than Ca-poor clay minerals such as kaolinite or illite, is preferentially associated with organic matter in the study area sediments. Further research is required to determine whether this is due to the sources and depositional history or physico-chemical properties of different clay minerals.