Stable isotope composition of soil organic matter and phytoliths as paleoenvironmental indicators

Abstract

The stable C isotope composition of organic matter and opal phytoliths in diverse ecosystems demonstrate that soils carry a C isotopic signature that reflects long-term inputs of above- and below-ground C 3 or C 4 biomass. The utility of these isotopic characterization data for paleoenvironmental study is based on a knowledge of dominant soil forming processes as well as geomorphic and climatic conditions. This paper reports on both the theory and applicability of isotopic characterization of soil organic C and opal phytoliths. The theoretical perspectives are discussed in light of broad environmental applications. Details of the sample collection and preparation are provided with insights into quantities and pretreatment required for stable C isotopic characterization. Two case studies from the central Great Plains region are presented. Paleoclimatic interpretations are discussed for a portion of the 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 Holocene. The concordance in the C isotopic signatures of soil organic matter and phytoliths provide strong biological evidence of regionally cooler conditions. This C isotopic concordance also appears during the mid-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 can indicate the degree of diagenesis resulting from pedological alteration. Geomorphic and pedologic evidence indicate that the C isotope discordance between soil organic matter and opal phytoliths is the result of local topographic variations and spatial heterogeneity associated with plant distribution.