Reconstructing the response of C 3 and C 4 plants to decadal-scale climate change during the late Pleistocene in southern Illinois using isotopic analyses of calcified rootlets

Abstract

The δ 13C and δ 18O values of well-preserved carbonate rhizoliths (CRs) provide detailed insights into changes in the abundance of C 3 and C 4 plants in response to approximately decadal-scale changes in growing-season climate. We performed stable isotope analyses on 35–40 CRs sampled at 1-cm intervals from an 18-cm-thick paleosol formed in southern Illinois during Wisconsin interstadial 2. Minimum δ 13C values show little variation with depth, whereas maximum values vary dramatically, and average values show noticeable variability; maximum δ 18O values vary less than the minimum δ 18O values. These findings indicate that a diverse and stable C 3 flora with a limited number of C 4 grass species prevailed during this interval, and suggest that the maximum growing-season temperatures were relatively stable, but minimum growing-season temperatures varied considerably. Two general patterns characterize the relationships between the δ 13C and δ 18O values obtained from the 1-cm samples. In some cases, low δ 13C values correspond to low δ 18O values and high δ 13C values correspond to high δ 18O values, suggesting that cooler growing-season temperatures favored C 3 and warmer growing-season temperatures favored C 4 plants. In other cases, low δ 13C values correspond to high δ 18O values, likely suggesting that wetter growing-season conditions were favorable to C 3 plants. The high density of well-preserved CRs in this paleosol provides a unique opportunity to study detailed ecological responses to high-resolution variability in growing-season climate.