The Flores speleothem carbon isotope record: vegetation, volcanism and the demise of Homo floresiensis
Abstract:The last surviving non-human member of the Homo genus: Homo floresiensis, disappeared from the stratigraphic record in Liang Bua cave, Flores, Indonesia, between 17 and 10 kyr BP (Roberts et al. 2009, J. Hum. Evol.). The cause of the disappearance (e.g. climate change, volcanic catastrophe or human competition) has not been established. Here, we present a new 92,000-year long speleothem δ13C record for Liang Luar cave, 800m South of Liang Bua. Our record acts as a proxy for local environmental and vegetation health throughout H. floresiensis’ occupation of the area.
The Liang Luar speleothem δ13C record is primarily a record of vegetation productivity and soil respiration rates, highlighting local ecological responses to changing regional and global climate forcings such as temperature, atmospheric pCO2 and precipitation amount. Changes in speleothem δ13C can largely be considered an environmental response to climate change.
Events that caused significant harm to the local environment and H. floresiensis are likely to be outside the natural range of variability: being quick enough or extreme enough that adaptation to new conditions is not possible. We identify disturbances to the vegetation system using two indicators in the speleothem record. 1) Abrupt positive δ13C excursions which indicate periods of reduced vegetative activity. 2) A loss of correlation between the δ13C and δ18O records, indicating that precipitation is no longer a dominant control on vegetation productivity. The largest (8‰) and longest (7 kyr) abrupt positive excursion, the 68 kyr event, sees positive speleothem δ13C values, due to increased bedrock contribution and/or C4 vegetation at this time. Crucially a H. floresiensis occupation interval dates to this period.
The largest abrupt deterioration in vegetation (positive δ13C excursion) between 17 and 10 kyr BP is a 1 in 5kyr occurrence - An event of a magnitude that was likely encountered and survived by H. floresiensis multiple times during the last 90kyr. The mechanism/s that led to the disappearance of H. floresiensis must have been highly selective, and did not have a major impact on the local environment. This seemingly rules out volcanism and climate change as potential agents, leaving us with an intriguing puzzle as to what caused the recent extinction of Homo floresiensis.