Speleothem carbon-isotope response to an explosive volcanic eruption ~12 ka ago near Liang Bua, Flores, Indonesia

Michael K. Gagan¹, Heather Scott-Gagan¹, Joan A. Cowley¹, Jian-xin Zhao², Linda K. Ayliffe¹, Wahyoe S. Hantoro³ and Bambang W. Suwargadi³

1 Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
2 Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, Qld. 4072, Australia
3 Research and Development Center for Geotechnology, Indonesian Institute of Sciences, Jalan Cisitsu No. 21/154 D, Bandung 40135, Indonesia.

Knowing what caused the surprisingly recent extinction of the dwarf hominin Homo floresiensis ("the Hobbit") ~18-12 ka (thousand years ago) on the island of Flores in eastern Indonesia is an intriguing question of great international interest.  At present, we do not know if predation by modern humans or severe climate change pushed the Hobbit beyond its adaptive capability.  However, a prominent volcanic ash layer overlies remains of the Hobbit recovered from Late Pleistocene sediments in Liang Bua cave, suggesting that an explosive volcanic eruption could have altered the local ecosystem at ~12 ka, and played a role in the Hobbit’s demise (Morwood et al., 2004).

Interestingly, out of the ~850 volcanic sulfate signals recorded by the GISP2 (Greenland) ice core over the past 100 kyr, including the Toba super-eruption ~73 ka (Zielinski et al., 1997), the largest and most abundant volcanic signals occur between 17 ka and 6 ka, when the Hobbit became extinct (Fig. 1a).  It is thought that crustal stresses associated with post-glacial sea-level rise may have significantly increased explosive volcanic activity in island arc systems, such as Indonesia, during this period.

In 2006, our ARC Discovery grant team (Gagan et al., 2006) collected several speleothems (cave calcite deposits) from Liang Luar cave (located ~1 km from Liang Bua) that show clear dark laminae at ~12 ka, which may be indicative of volcanic ash.  High-resolution analysis of carbon-isotope ratios (¹³C/¹²C) in the speleothem calcite shows a sharp increase in ¹³C at ~12 ka, suggesting that vegetation cover may have been substantially reduced for ~300 years (Fig. 1b). 

On tropical islands, like Flores, isotopically light carbon derived from oxidation of abundant soil organic matter dominates speleothem ¹³C/¹²C because forested tropical soils have CO₂ partial pressures 1-2 orders of magnitude greater than that of the overlying atmosphere (Kessler and Harvey, 1999).  Therefore, an abrupt reduction in vegetation cover, soil productivity, and soil CO₂ production following local deposition of volcanic ash would reduce the supply of isotopically light carbon to the cave drip-water, thus raising speleothem ¹³C/¹²C.

Explosive island arc volcanic eruptions are rich in sulphur, so our follow-up approach will be to measure S concentrations in speleothem calcite (using SHRIMPII at RSES) as an indicator of sulfate rain-out.  It is also possible that fresh volcanic ash above caves could be detected by LA-ICP-MS measurements of leachable metals (e.g. Ni, Cu, Zn, Mo, Ti, Co, Rb) and rare earth elements in speleothem calcite.  Precise U-series dating of these multi-proxy records will shed light on the timing of large volcanic eruptions and the innate ability of humans to adapt to natural catastrophes and environmental change.

 

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Gagan MK, Zhao J-x, Drysdale RN, Hantoro WS, Schmidt GA, ARC Discovery Grant DP0663274 (2006-2008): Monsoon extremes, environmental shifts, and catastrophic volcanic eruptions: Quantifying impacts on the human history of southern Australasia.
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