Composition of size-fractioned sedimentary organic matter in coastal environments is affected by difference in physical forcing strength

The diagenesis of sedimentary organic matter (SOM) in three different depositional environments in south Texas and dominated by vascular plant input was investigated. Sediments were collected from McGuill Lake, a small freshwater lake, and two salt marsh sites in Nueces Delta and Copano Bay, and size-fractioned by wet sieving. Physical forcing or energy, indicated by tides, tidal current, and wind and wind fetch area, is weakest in the lake, intermediate strength at the Nueces Delta, and strongest at Copano Bay. Results from total hydrolyzable amino acid (THAA) and n-alkane analysis, and 13C solid-state nuclear magnetic resonance spectroscopy (NMR) showed that the SOM from all three sites became more degraded towards finer size fractions, from > 300 μm, 300–125 μm, 125–63 μm, 63–32 μm to < 32 μm. Relative to the coarse fractions, the fine fractions were enriched in the degradation indicator γ-aminobutyric acid, and in aliphatic and proteinaceous components, as shown from NMR spectra and lower C/N ratio values. This degradation pattern of SOM with size is typical for sediments from coastal and lacustrine environments. However, the contents of organic carbon (OC), THAAs and n-alkanes were higher in sandy fractions than in silt and clay fractions from McGuill Lake sediment, suggesting that the SOM was relatively undegraded in coarse fractions. The size distribution of OM in the lake could be explained by the weak physical forcing, which may not easily break down coarser plant detritus into smaller pieces. The trend contrasted with degradation patterns for sediments from Nueces Delta and Copano Bay, where SOM was concentrated in the < 32 μm fraction, which may be due to the strong physical forcing. The physical forcing effects could also explain the attachment of clay minerals to sand grains in both the lake and Nueces Delta because of the relatively weak physical forcing effects, while no clay attachment to sand grains was found at the Copano Bay site because of the strong physical forcing effects. Physical forcing may be an important mechanism in the processing and transport of OM from vascular plants and other organic sources in estuaries and bays before the SOM is exported to coastal oceans.