The effects of late Quaternary climate and pCO2 change on C4 plant abundance in the south-central United States

The late Quaternary was a time of substantial environmental change, with the past 70,000 years exhibiting global changes in climate, atmospheric composition, and terrestrial floral and faunal assemblages. We use isotopic data and couple climate and vegetation models to assess the balance between C3 and C4 vegetation in Texas during this period. The carbon isotope composition of fossil bison, mammoth, and horse tooth enamel is used as a proxy for C3 versus C4 plant consumption, and indicates that C4 plant biomass remained above 55% through most of Texas from prior to the Last Glacial Maximum (LGM) into the Holocene. These data also reveal that horses did not feed exclusively on herbaceous plants, consequently isotopic data from horses are not reliable indicators of the C3–C4 balance in grassland biomes. Estimates of C4 percentages from coupled climate–vegetation models illuminate the relative roles of climate and atmospheric carbon dioxide (CO2) concentrations in shaping the regional C4 signal. C4 percentages estimated using observed modern climate–vegetation relationships and late Quaternary climate variables (simulated by a global climate model) are much lower than those indicated by carbon isotope values from fossils. When the effect of atmospheric CO2 concentration on the competitive balance between C3 and C4 plants is included in the numerical experiment, however, estimated C4 percentages show better agreement with isotopic estimates from late Quaternary mammals and soils. This result suggests that low atmospheric CO2 levels played a role in the observed persistence of C4 plants throughout the late Quaternary.