A multi-century δ13C record of the C4 grass Setaria macrostachya in the US Southwest: Identifying environmental causes of variability

Annual and 5-year averages of δ13C measured on florets of the C4 grass Setaria macrostachya decreased from 10 to 13‰ between 1849 and 2016. Atmosphere-plant δ13C discrimination (Δ) changed from 3 to 4 early in this interval, to 4-5 since 1970. These century-scale changes result from decrease in atmospheric δ13C and, prior to 1970, from changes in Δ related to increase in the partial pressure of CO2 in the atmosphere, both largely caused by the burning of fossil fuels. Short-term fluctuations (interannual to decadal) in slope of the time series of averaged δ13C do not result from natural causes, but in certain cases appear to coincide with human economic activities. Annual δ13C ranges in years with sufficient measurements are consistently about 2‰. It is unlikely that useful information on past atmospheric δ13C will be obtained from S. macrostachya seeds preserved in packrat middens or other archives.