Sources of particulate organic matter in rivers from the continental USA: Lignin phenol and stable carbon isotope compositions

The origins and forms of particulate organic matter (POM) suspended in rivers are major considerations in assessing how fluvial materials integrate landscape and climate properties and ultimately record these characteristics in marine sedimentary deposits. We examined the elemental, stable carbon isotope and lignin-phenol compositions of well-characterized samples of suspended POM collected from rivers draining the south central United States. The atomic C/N ratios of 17 fine-grained samples (similar to 11 +/- 2, n = 17) are similar to those reported worldwide for texturally similar soil and riverine POM. The corresponding delta(13)C values vary from -18.5 to -26.4 parts per thousand, in patterns that correspond to the relative abundances of C3 and C4 plants in the drainage basins, and hence to temperature and hydrologic patterns. Lignin-phenol yields from 12 samples indicate input of angiosperm-rich plant materials in distributions that also reflect upstream climate and vegetation. The lignin associated with these plant remains has been substantially degraded, as is typical of soil organic matter. The mass- and surface-area-normalized organic carbon contents of the river-borne POM also correspond to drainage basin properties in patterns characteristic of soils. These results indicate that highly degraded soil organic matter is a major component of fine-grained POM transported by rivers of the central US. The Mississippi River currently discharges heavy (delta(13)C approximate to -20 parts per thousand) lignin-poor POM. The isotopic composition of this organic matter is difficult to distinguish from marine plankton remains in fine-grained sediments of the northern Gulf of Mexico, whose isotopic compositions have been previously interpreted to broadly indicate limited offshore transport of terrigenous organic matter. Copyright (C) 2000 Elsevier Science Ltd.