Long-term sunspot forcing of savanna structure inferred from carbon and oxygen isotopes

Abstract

We used the δ18O and δ13C isotopes from a speleothem and Δ14C values from known age tree rings as proxies for vegetation structure, rainfall and sunspot activity over the last 10,000 years. The δ18O and δ13C signals are significantly correlated and wavelet coherence analysis shows that a phase locked responses is only found to occur over periods less than 150 years. The δ13C and Δ14C signals are also significantly correlated but the wavelet coherence analysis shows phase locked responses at periods in the order of 320 and 1100 years at various times in the past. We conclude that savanna vegetation structure responds to rainfall over periods of decades to about 150 years, and to solar forcing over periods greater than 150 years. We propose that elevated UV-B radiation at the earth surface during periods of low sunspot activity favours the productivity of C4 plants over C3 plants possibly because of the metabolic cost of structural adaptive responses and of producing UV-B absorbing compounds in the latter.