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Applying two-dimensional correlation spectroscopy to (13)C NMR and FTIR spectra of the high molecular-weight dissolved organic matter (HMW-DOM) isolated along an Elizabeth River/Chesapeake Bay salinity transect shows that HMW-DOM consists of three major components that have different biogeochemical reactivities. The first appears to be a heteropolysaccharide (HPS) component and its contribution to carbon increases as we approach the marine offshore. The second appears to be composed of carboxyl-rich compounds (CRC); its carbon percentage decreases. The third component contains the major functional group of amide/amino sugar (AMS) and its carbon percentage stays almost constant along the salinity transect. It seems that the HPS and CRC are present in many aquatic environments at different relative ratios. The 2D-correlation maps reveal that each of these components is composed of dynamic mixtures of compounds that share similar backbone structures but have significant functional group differences. Two-dimensional (2D) correlation spectroscopy is a powerful new biogeochemical tool to track the changes in complex organic matter as a function of space, time, or environmental effects.

Photochemical processing of dissolved organic matter (DOM) in natural waters can alter its composition and structure, supply particulate organic matter (POM) to sediments, and deliver modified terrestrial DOM to the ocean. Our studies show that terrestrial DOM exposed to simulated sunlight is altered to produce POM with a markedly different molecular composition enriched with newly formed aliphatic and condensed aromatic molecules. This process is closely tied to the chemistry of iron, which primarily exists as dissolved Fe(II) and Fe(III)–organic complexes in initial DOM and photochemically matures to Fe(III) oxyhydroxides before coprecipitating out with POM. The newly formed condensed aromatic compounds resemble black carbon, which until now was thought to be produced by only combustion. These new molecules contribute a pool of Fe-rich, aliphatic, and black carbon-like organic matter to sediments as the terrestrial DOM is transported through rivers. We estimate that the annual global flux of this photoproduced black carbon, most of which may be preserved in sediments, is nearly equivalent to the estimated flux of dissolved black carbon to the ocean from all other sources.