Researcher unearths more layers of cave history

Samples of cave breccia, which are broken rock fragments cemented by fine-grained sediment.

A Griffith University PhD candidateis the first touse a novelvirtualmethodtoexaminefossil-bearingdepositsinSumatrancave sitesto form a chronology of fossil burial and cave formation.

PhD candidate Holly Smith examines the cave interior.

Published inScientific Reportsfor a special edition entitledQuaternary Taphonomy, PhD candidate HollySmith from the Australian Research Centre for Human Evolution examined three different cave sites on the Indonesian island of Sumatra using rapid thermal neutron tomographic imaging to determine the composition of breccia and fossil distributions within.

The study disproves previous hypotheses that fossil remains entered the caves as one rapid debris flow, and insteadare a result of multipledeposits laid down by water and gravity flow over a prolonged period.

The sites -NgalauGupin,NgalauSampitandLidaAjer- in the Padang Highlands of western Sumatra, contained a diverse range of vertebrate fossils dating around 70,000 years old that were buried in ancient sediments.

The results suggestthatthe timelines ofbothhuman and animal arrivals, migrations, settlements, and extinction patterns previously formed by dating thecavefossils could be inaccurate,asnatural mixing processes may have occurred during this more complexdepositionalhistory.

Thus,several different ecosystems separated by tens of thousands of years may appear to have co-existed.

Traditional methods of fossil excavation from cave breccia(broken rock fragments cemented by a fine-grained sediment)and subsequent preparation risks damage to the specimens and can destroy contextual evidence in the surrounding sediments.

Previous studies had mainly been restricted to observations as to surface colour and composition of the breccia while the researcherwas in the cave, which obscured and disregarded a vast amount of important information.

Holly Smith prepares the sample for imaging.

Neutron tomographic imaging had previously been considered expensive and time-consuming, but the method is becoming increasingly affordable and obtainable for scientific research.

This method had not been used earlier as tropical cave brecciaweretoo dense for traditional CT imaging to penetrate, andit was considered that theinternal characteristics of the deposits could not be observed. Neutrons can, however,pass through these samples much more easilyand allow for the virtual reconstruction of the breccia interior.

“This is the first time that neutron tomography has been used to conduct a survey of geological samples to extract prehistoric data,”MsSmithsaid.

Neutron imaging is a relatively new methodinpalaeontology andis usedhere torevealthe sedimentsthat form tropical cave brecciaand fossils (and the interrelationships between both) within.

Thenovel creation of rapid neutronimagingby Holly’s colleagueand co-author,DrJosephBevitt, at the Australian Nuclear Science and TechnologyOrganisation in Sydney,is much faster than previous neutron studiesandincurs less residual radioactivity, so samples are readymorequickly for further analysis.

The Sumatran cave site.

The results of thisstudyrevealnew evidence as to the dominant factors responsible for the accumulation and preservation of cave fossil remains.Itmay refine our understanding of the timing and modification of tropical cave fossil accumulations and thus improve the accuracy of the historical record of biodiversity, climate, and environmental changes in Southeast Asia,”MsSmith said.

These data suggest the original theories of site formation have previously been oversimplified. This neutron study proves the mixing and modifying processes in the Sumatran caves are significant and may have serious implications on the validity of fossil dating in the region.

Rapidthermal neutron tomography may develop into a key conventional methodto studyfossil and sediment evidencepreserved in tropical cave brecciaand informthe validity of previous timelines established in the region.

I hopethis neutron imaging study could be replicated, and integrated with further new methods, to form detailed histories oflife, death and burial of ancient faunapreserved intropical caves.”

The research ‘High-resolution rapid thermal neutron tomographic imaging of fossiliferous cave breccias from Sumatra’ has been published inScientific Reportsfor a special edition entitledQuaternary Taphonomy.