SHORT-TERM VARIATION OF C3/C4 PLANTS AND GROWING-SEASON CLIMATE IN CARBON AND OXYGEN ISOTOPES OF CARBONATE ROOTLET CASTS

Records of terrestrial paleoclimate with annual to decadal resolutions are difficult to assemble. Unlike the marine environment, there is a lack of quantifiable proxies for temperature, humidity, and plant biomass in the terrestrial environment, and implications are often made based on modeling and vague indices rather than accurately measurable parameters. This problem is keen in arid and semiarid regions where pollen records are rare, making the reconstruction of past climate and habitat particularly challenging. One promising approach for determining short-term variations of C3/C4 plants and growing-season climate in arid-semiarid regions is to analyze carbon and oxygen isotopic compositions of small carbonate rootlet casts. Micro and macro morphologies of these carbonate rootlet casts reveal that they formed around living and respiring roots. The nucleation center of calcite precipitation is within root epidermal and cortex cell walls. Microorganisms such as fungal hyphae associated with the living plant roots are also seen calcified inside and outside the root cells, suggesting that root respiration provides the primary carbon source. Rootlets have short life spans and root respiration occurs only during growing seasons. Thus, carbon and oxygen isotopic compositions of carbonate rootlet casts should provide evidence for short-term variations in C3/C4 plants and growing-season climates. This approach has demonstrated that during the last glaciation, seasonal climate events in terrestrial environments such as monsoonal seasonalities and El Niño/Southern Oscillation abnormal patterns could be imprinted in the carbon and oxygen isotope signatures of carbonate rootlet casts. Furthermore, the distribution of global arid-semiarid soils expanded significantly during the last glaciation and the formation of soil carbonates may have been an important process in the sequestration of atmospheric CO2. Hence, measuring the thickness and the completeness of rootlet cell walls replaced by micritic calcite crystals may provide valuable information for modeling the contribution of soil carbonates to the atmospheric CO2 concentration during that time.