Geophysical Research Letters, 2008,
35 (2), p. NIL_7-NIL_10 ISSN 0094-8276
We used the delta(18) O and delta(13) C isotopes from a speleothem and Delta C-14 values from known age tree rings as proxies for vegetation structure, rainfall and sunspot activity over the last 10,000 years. The delta O-18 and delta C-13 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 delta C-13 and Delta C-14 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 C-4 plants over C-3 plants possibly because of the metabolic cost of structural adaptive responses and of producing UV-B absorbing compounds in the latter.
