STABLE-ISOTOPE COMPOSITION OF SOIL ORGANIC-MATTER AND PHYTOLITHS AS PALEOENVIRONMENTAL INDICATORS

The stable C isotope composition of organic matter and opal phytoliths in diverse ecosystems demonstrate that soils carry a C isotopic signa ture that reflects long-term inputs of above-and below-ground C-3 or C -4 biomass. The utility of these isotopic characterization data for pa leoenvironmental study is based on a knowledge of dominant soil formin g processes as well as geomorphic and climatic conditions. This paper reports on both the theory and applicability of isotopic characterizat ion of soil organic C and opal phytoliths. The theoretical perspective s are discussed in light of broad environmental applications. Details of the sample collection and preparation an provided with insights int o quantities and pretreatment required for stable C isotopic character ization. Two case studies from the central Great Plains region are pre sented. Paleoclimatic interpretations are discussed for a portion of t he central Great Plains. These interpretations are based on the stable C isotope data recovered from Holocene paleosols. These data indicate higher proportions of C-3 vegetation persisted during the early Holoc ene. The concordance in the C isotopic signatures of soil organic matt er and phytoliths provide strong biological evidence of regionally coo ler conditions. This C isotopic concordance also appears during the mi d-Holocene; C isotope values indicate an increase in the proportion of C-4 vegetation, which reflects regionally warmer climatic conditions than present. Isotopic discordance in soil organic C and phytoliths ca n indicate the degree of diagenesis resulting from pedological alterat ion. Geomorphic and pedologic evidence indicate that the C isotope dis cordance between soil organic matter and opal phytoliths is the result of local topographic variations and spatial heterogeneity associated with plant distribution. (C) 1998 Elsevier Science B.V.