Geoarchaeological finds below Liang Bua (Flores, Indonesia): A split-level cave system for Homo floresiensis?

We report on new geoarchaeological finds in a recently discovered cave-chamber (Liang Bawah, "Cave Underneath") positioned below Liang Bua on the island of Flores, Indonesia, where the type specimen for Homo floresiensis was recovered from Late Pleistocene sediment. At the rear of Liang Bua, a 23-m-long shaft, inclined at 60°, leads to a lower chamber measuring 23m ×24m. ×5m high (about half the size of Liang Bua). Stone artefacts and bones were found shallowly buried in rubble at the base of the shaft, and around a 5-m-high mud mound that fills the northwest sector of Liang Bawah. We recovered 17 stone artefacts made from chert and volcanics, and more than 220 well-preserved bone elements belonging to endemic giant rats, pigs, primates, small murid rodents, bats and introduced species. Multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) analysis of uranium and thorium in carbonate coatings on four bones yielded ages of ~240-180ka (endemic giant rat femur), ~110-60ka (unidentified phalanx), ~33-23ka (pig skull fragment), and ~7-3ka (giant rat femur), which overlap with the ~95 to 17ka occupation of Liang Bua by H. floresiensis. The ~33-23ka age of the pig skull fragment indicates that Sus sp. may have dispersed into island Southeast Asia earlier than previously recognised. The passageway at the rear of Liang Bawah, and a currently buried front entrance, represent two possible transport paths for cultural and faunal material to the cave-chamber. Analysis of the geomorphic evolution of Liang Bawah shows that it may have been a Late Pleistocene depocentre for material transported from the occupation chamber of Liang Bua, and a repository for human subsistence refuse, or pit-fall trap, via the rear passage. These physical attributes, and the antiquity of the faunal remains found thus far, indicate that Liang Bawah could contain an archive of the Late Pleistocene and, potentially, remains of H. floresiensis.