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 rive rs are major considerations in assessing how fluvial materials integrate la ndscape and climate properties and ultimately record these characteristics in marine sedimentary deposits. We examined the elemental, stable carbon is otope and lignin-phenol compositions of well-characterized samples of suspe nded POM collected from rivers draining the south central United States. Th e 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 r iverine POM. The corresponding delta(13)C values vary from -18.5 to -26.4 p arts per thousand, in patterns that correspond to the relative abundances o f C3 and C4 plants in the drainage basins, and hence to temperature and hyd rologic patterns. Lignin-phenol yields from 12 samples indicate input of an giosperm-rich plant materials in distributions that also reflect upstream c limate and vegetation. The lignin associated with these plant remains has b een 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 approxim ate to -20 parts per thousand) lignin-poor POM. The isotopic composition of this organic matter is difficult to distinguish from marine plankton remai ns 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 Elsevi er Science Ltd.