Potential use of soil c isotope analyses to evaluate paleoclimate

The objective of this study was to evaluate potential use of the naturally occurring isotopic carbon (C) in soil organic matter to help evaluate paleoclimate in the Central United States. Sampling sites were selected on major geomorphic units with similar slope and aspect in three soil temperature regimes and three soil moisture regimes within the historic grasslands in US Great Plains and Western Corn Belt. Soil samples were collected from approximate 0- to 5- and 5- to 10-cm (A1 and A2 profile) depths and by genetic soil horizon thereafter in pits dug by backhoe or by hand to ~2 m depth. Bulk density and soil texture were determined. Sieved, handpicked, and delimed subsamples of soil from each horizon were analyzed for total soil organic carbon (SOC) and δC and C dated (as mean residence time of SOC in calendar yr B.P.). Data from some sites support that major aeolian movement of soil occurred during the Holocene, often coincidental with literature reports of long periods of drought. However, soil profiles provide a low-resolution record of drought and/or wetter climatic conditions and thus time frames are general. Measurement of SOC age and δC signature in soil horizons was related to preboreal warming during entry into the Holocene. An important question raised by our data but requiring further confirmation by other studies is the timing of the peak of the Medieval Warm Period that is reported to have occurred prior to commencement of climate cooling that preceded the Little Ice Age. Based on δC, our data indicate a change from C4 plants to increasing C3 plant dominance (as a surrogate of cooler temperature) at ~1,500 yr B.P., whereas the reported start of the Little Ice Age is ~600 yr B.P. In summary, the approach we used holds promise as an additional proxy for evaluating prior climate back thousands of years, but additional measurements besides those from the isotopes of C will be needed.