Speleothem isotopic evidence for rapid human-induced expansion of grasslands in Madagascar at 890 CE

Abstract:

The degree to which human activity impacted the landscape, vegetation and fauna of Madagascar remains under debate. Since the early 1920's, the prevailing hypothesis has been that the savannah grasslands that now cover 70% of Madagascar were the result of deforestation, which has also been tied to the disappearance of much of the island’s endemic megafauna. Other studies suggest that Madagascar's grasslands are largely natural and that megafaunal extinctions may be climatically induced, leading some authors to question the entire narrative of extensive alteration of the landscape by early human activity. We collected two stalagmites, M14-AB2 and M14-AB3, from Anjohibe Cave in northwestern Madagascar (15.55°S, 46.89°E, 100 masl). Age models were constructed using 8 U/Th age determinations from AB2 and 10 from AB3. The samples began to grow at ~500 CE and were active at the time of collection. Carbon and oxygen stable isotope ratios were measured on 266 samples from AB2 and 173 samples from AB3, yielding sub-decadal temporal resolution. A rapid, more than 10 per mil increase in stalagmite carbon stable isotope ratios documents an almost complete transformation of the landscape from one with a flora dominated by C₃ plants to a C₄ grassland system. This transformation, well replicated in both stalagmites, occurred at approximately 890 +/- 20 CE and was complete in 100 years. Further, relatively constant oxygen isotope ratios across the carbon isotope transition demonstrate that landscape alteration was not related to changes in climate. We hypothesize that the transformation was caused primarily by expansion of the use of fire by early inhabitants of Madagascar to promote agriculture and the growth of grass as fodder for cattle. The resulting loss of forest habitat very likely increased environmental pressures on Madagascar's megafauna and accelerated their disappearance.