Evidence of long-term seasonal climate forcing in rhizolith isotopes during the last glaciation

High density carbonate rhizoliths were found from a loess-paleosol succession from the late Wisconsin period (21-11 ka) in Illinois. Their morphology shows that they formed in a close contact with living and decomposing roots, suggesting a root/microbial respiration origin. Carbon (δ¹³C) and oxygen (δ¹⁸O) isotopic analyses were performed on 36 and 37 individual rhizoliths of two separate 10 cm intervals and 98 bulk rhizoliths of all 10 cm intervals. The results of the individual rhizolith δ¹³C and δ¹⁸O analyses suggest that the carbon source was largely derived from respiring C₃, C₄ and microbial biomass, and that meteoric water was controlled mainly by warm-season precipitation. The results of bulk rhizolith δ¹³C and δ¹⁸O analyses show that warm-season proxies varied in phase with glacial fluctuations at submillennial scales, suggesting long-term seasonal forcing may have played an important role on climate change during the late Wisconsin glaciation in North America.