A multi-century δ 13 C record of the C 4 grass Setaria macrostachya in the US Southwest: Identifying environmental causes of variability
Abstract Annual and 5-year averages of δ 13 C measured on florets of the C 4 grass Setaria macrostachya decreased from 10 to 13‰ between 1849 and 2016. Atmosphere-plant δ 13 C 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 δ 13 C and, prior to 1970, from changes in Δ related to increase in the partial pressure of CO 2 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 δ 13 C do not result from natural causes, but in certain cases appear to coincide with human economic activities. Annual δ 13 C ranges in years with sufficient measurements are consistently about 2‰. It is unlikely that useful information on past atmospheric δ 13 C will be obtained from S. macrostachya seeds preserved in packrat middens or other archives.