xamining marine particulate organic matter at sub-micron scales using scanning transmissieon X-ray microscopy and carbon X-ray absorption near edge structure spectroscopy

Marine sinking particulate organic matter (POM) represents the link between surface primary production and burial of organic matter in marine sediments. As such, the nature of this material has been the subject of numerous studies attempting to characterize its composition. The results of these studies have shown that a significant proportion of POM is not recognizable as known compounds, and that the proportion of uncharacterized material increases with age/depth/diagenesis. However, few studies have examined the spatial heterogeneity of this material. This study uses a new tool, scanning transmission X-ray microscopy (STXM), together with carbon X-ray absorption near edge structure (C-XANES), to examine POM collected from sediment traps deployed at one location in the Arabian Sea as part of the JGOFS program. The results indicate that POM is composed primarily of four distinct phases: protein, an aliphatic rich phase, a carboxylic-acid-rich phase, and a phase with complex unsaturated and quinone character. This last phase may be a condensation product between carbohydrates and proteins, or from degraded plant pigments. Many particles consisted of a single chemical phase; however, in particles with mixed compositions, individual domains retained distinctive chemical signatures at the instrumentʹs resolution limit (50 nm). All major chemical phases were observed in sediment trap particles from 531 to 3369 m depth, supporting the hypothesis that non-selective degradation dominates particle remineralization, and that overall particle compositions are determined by near surface processes. Only one particle, out of more than 60 examined, exhibited soot-like composition. The lack of a significant black carbon/soot component may be attributable to sampling during the winter monsoon period.