COMPARISON OF STABLE CARBON ISOTOPE SIGNATURES IN CHANNEL SEDIMENT VERSUS UPLAND SOILS OF THE UPPER MISSISSIPPI AND MISSOURI RIVER DRAINAGE BASINS

Alluvium sediment should include allochthonous stable carbon isotopes derived from plants within the drainage basin. Trees and relatively cool-climate grasses (C₃ plants) have a mean C13:C12 ratio (d¹³C ) of -27 and relatively warm-climate grasses (C₄ plants) have a mean C13:C12 ratio (d¹³C) of -13. In small drainage basins the allochthonous carbon isotopes should reflect that of the vegetation within the basin and that of the adjacent upland. In contrast, in large drainage basins that extend across several ecoregions, the allochthonous alluvial carbon should be a mixture of the d¹³C from plants throughout the entire basin and may not be the same as that of the soil in the adjacent uplands. The relative distribution of C isotopes in alluvium should be representative of the isotopes present in the basin vegetation upstream of the sample site. Variations in sediment yield from different parts of the basin can reflect the carbon isotope signatures toward those of the vegetation present in that portion of the basin with a greater sediment yield. Stable carbon isotopes of the fine-grained channel alluvium and of the A horizons in the adjacent upland grass and/or forest areas were sampled thoughout the upper Mississippi and upper and middle Missouri drainage basins. Preliminary results indicate that the d¹³C of allochthonous carbon from the upper Missouri River drainage basin and the adjacent uplands is dominated by d¹³C values typical of C₃ signature plants in the recent past but downstream dC of allochthonous carbon includes more C₄ plants and is not necessarily the same as the adjacent upland vegetation because of C contribution throughout the entire drainage basin.