USING STABLE CARBON ISOTOPES TO ANALYZE SPATIAL CHANGES IN VEGETATION OF THE MISSOURI AND MISSISSIPPI RIVERS

Previously, stable carbon isotopes from speleothems and soil/alluvial sediment of small drainage basins have provided information on the presence of vegetation groups western Missouri), have d13C that are climate sensitive in a local area. Alluvial carbon bulk d13C values of -26 to -30 from headwater stream sediment of the Missouri River basin in the mountains of southwest Montana are typical of forested environments. Values downstream of the confluence of the three Missouri River tributaries are -27 to -28, intermediate among the values of three tributaries and most similar to the d13C values of -27 in the Jefferson River, largest of the three tributaries, evincing mixing of those sources. In contrast, the Missouri River in the lower quarter of its drainage basin (eastern Nebraska, northern Kansas and western Missouri) and small tributaries to this section of the Missouri River (Peters Creek in eastern Kansas and Sni-a-Bar Creek in values of -19 to -22, typical of more arid grasslands. In the Mississippi River, downstream of the Missouri and Ohio River confluences, the d13C values of alluvial C are -19 to -24, intermediate between values from the Missouri River headwaters and the Nebraska/Kansas/Missouri localities, but are more similar to lower Missouri River d13C values. This is to be expected because the Missouri River provides 10 times the amount of sediment as the forested Ohio River drainage basin historically and an estimated 20 times the Ohio River sediment load prior to European settlement. These data support the hypothesis that watersheds integrate allochthonous carbon from throughout the drainage basin, approximately proportional to the sediment load of the tributaries. Our preliminary results of the study validate the methodology by discriminating differing inputs of allochthonous and autochthonous carbon in small watersheds and adjacent large watersheds and characterize spatial change in vegetation and climate across ecoregions in the Mississippi River drainage.