DECADAL-SCALE CLIMATE VARIATIONS IN CARBONATE RHIZOLITH: MOST RELIABLE SOIL CARBONATES FOR HIGH RESOLUTION STABLE ISOTOPE ANALYSIS

Carbon and oxygen isotope ratios (δ¹³C and δ¹⁸O) of soil carbonates (SCs) have been proven to preserve aspects of paleoclimate and past environments. However, large accumulations of SCs such as nodules and concretions need centuries to millennia to form, and during the process of formation they may reach a partial isotopic equilibrium with more recent soil CO₂. Moreover, because CO₂ respired by living plants and microbes often dominates soil CO₂ in the rhizosphere during the peak of growing seasons, the δ¹³C values of SC nodules and concretions reflect the composite of the long-term and seasonal variations.

Small calcified rootlets known as carbonate rhizoliths (CRs) are important forms of SCs. Because plant rootlets have a short life span, usually ranging from one to a few years, δ¹³C and δ¹⁸O analyses of small CRs yield higher-resolution records of ecological response to growing-season climate than is possible with larger SC nodules and concretions.

Large numbers of CRs were sampled and analyzed isotopically from 1-cm intervals within 18-cm-thick paleosol-A horizons developed on Peoria Loess formed during the Wisconsin glaciation. Examination of maximum, minimum, and average δ¹³C and δ¹⁸O values revealed detailed information about vegetation and environment. On the one hand, average δ¹³C and δ¹⁸O values demonstrated changes in relative abundance of C₃ and C₄ plants in response to high-resolution variations in growing-season climate. On the other hand, assessment of variation in maximum and minimum δ¹³C and δ¹⁸O values provided insights into stability of the C₃ and C₄ floras and the range of growing-season temperatures. In some intervals, cooler growing-season conditions favored C₃ plants and warmer conditions favored C₄ plants, while in other intervals wet growing-season conditions were generally more favorable to C₃ plants.