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

•We characterized size-fractioned sedimentary organic matter (SOM) from coastal wetlands.•Analysis included total hydrolyzable amino acids, alkanes, carbon isotope ratios NMR.•SOM became more degraded with decreasing particle size.•Physical forcing controls the dynamics of particle aggregation and disaggregation and is important in SOM preservation. 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 >300μm, 300–125μm, 125–63μm, 63–32μm to