PALEOENVIRONMENTAL INFERENCES FROM d13C OF SOIL ORGANIC CARBON IN 14C-DATED PROFILES IN THE U.S. GREAT PLAINS

The historic grasslands of North America stretch from Texas to North Dakota within the United States, and are a principal agricultural small grain-producing region over the last Century, i.e., America’s "breadbasket". Across this region, areas of original grassland still exist, undisturbed by plowing and cultivation, which offer us profiles developed in response to the plant cover and climate of the past several thousand years. We sampled 14 of these native grassland sites over a wide area extending over 13° of latitude and 10° of longitude. After removal of soil carbonates, we obtained radiocarbon ages on soil organic carbon (SOC) by soil profile horizon (ranging from 5 to 40 cm in thickness) to an average depth of 1.9± 0.3 m and analyzed the stable-carbon isotope composition (d¹³C) of each of these samples. In most cases, SOC in surface soils is modern or post-bomb with respect to radiocarbon content, and the corresponding d¹³C is highly correlated to the d¹³C signature of live vegetation at the site. Usually radiocarbon age of SOC increased with depth, in some cases with ages below 1.5 m in excess of 10,000 years, although occasionally a reversal occurred perhaps related to erosion-deposition effects and paleodrainage. The SOC d¹³C generally became more negative with depth. We have examined the patterns of d¹³C variation with depth (age) to infer changes in proportions of past C3 and C4 vegetation, which can shift in response to past climate conditions. A warming climate is probably indicated from ~12,000 yBP to ~2000 yBP.