Molecular and optical characterization reveals the preservation and sulfurization of chemically diverse porewater dissolved organic matter in oligohaline and brackish Chesapeake Bay sediments

•Upper bay PDOM molecular and fluorescence signatures reflect a terrestrial source.•Mid bay PDOM is indicative of an autochthonous source.•Sulfurization of algal-derived PDOM is evident in mid bay sediments.•Upper bay and mid bay sediment porewater have similar fluorescence spectra.•Mid bay fluorescence correlates with H2S and bacterial sulfate reduction.