ROCK ART DATING

Four Scientific Dating Methods

The RAD-2 project will use new knowledge of complex processes on sandstone surfaces across the north Kimberley and an innovative combination of four scientific dating methods developed in the earlier work.

The project expects to establish a well-dated sequence for Kimberley rock art based on replication of results and confirmation across different methods, all conducted in collaboration with Traditional Owners.

 


Optically Stimulated Luminescence (OSL)

Samples: Wasp Nests, Material: Quartz grains, Dating: Time since nest was built.

OSL dating is based on the principle that quartz grains accumulate electrons over time but when exposed to sunlight all of these electrons are expelled-resetting the clock. When a mud wasp carries a mud droplet to it’s nest on a rock art panel, quartz grains held in that droplet are bleached by the light, resetting the OSL clock. This grain is then buried within the nest and allowed to accumulate electrons again- if we measure how many electrons there is we know how long that nest has been there and that the painting underneath must be older.

 

Wasp gathering mud droplet (left) and building it’s nest on a rock art panel (right).

Wasp nest stubs under pigment of a ‘Tassel’ figure motif (left), Wasp nest over pigment of a ‘Sash’ figure motif (right).


Uranium Series (U-series)

Samples: Mineral Crusts, Material: Phosphate, Dating: Time since mineral formed.

U-series is usually applied to calcite in limestone environments. The remarkable finding of the last 12 months has been that it can be applied to phosphate minerals in the exposed sandstone environments of the Kimberley- a world first. These minerals precipitate out from water which contains a certain level of Uranium but no Thorium. Over time the U decays to Th and by measuring the amount of each we can work out how long that mineral layer has been over the art surface-and that the painting must be older.

The mineral accretion is a mounted and polished piece that gives a cross-section through the detailed internal stratigraphy (layers). The overlaid scans are laser ablation trace element maps (the laser vaporises a bit of material in each layer and sends it to the mass spectrometer which analyses the concentration of certain elements)- in this case they are showing us where Uranium is focussed. ‘PPM’ on the scale stands for parts per million (so its a tiny amount). Yellow is high, scaling down to black at 0.

 

Layered mineral accretions collected from mineral fringe (left), Laser ablation trace element scans showing where the Uranium is focused (yellow) (right).

 

 


Radiocarbon

Samples: Wasp Nests, Material: Organic Material, Dating: Time since organism died.

Mud wasps also furnish their nests with paralyzed spiders or caterpillars. When an organism is living it is constantly exchanging the radioactive isotope carbon 14 with the atmosphere. When that organism dies the exchange stops and the 14C starts to decay to the non-radioactive isotope, carbon 12. The radioactive decay occurs at a fixed rate so by measuring the ratios of the different carbon isotopes we can work out how long it has been since the organism died. That will tell us how old the carbon in the nest or beeswax is likely to be.

Beeswax figure (left), Modern nest over hand stencil motif (centre), nest stumps under pigment of motif (right).


Cosmogenic Nuclides

Samples: Sandstone fragments, Material: Cosmogenic Nuclides, Dating: Time since slab fall.

Cosmic rays are constantly bombarding earth. The slabs shown in the examples below are protected from these cosmic rays until it falls and becomes exposed. Once exposed the absorption of these rays generates ‘cosmogenic isotopes’ that we can measure to tell us how long it has been exposed for. If there is art on that slab we know the art must be younger than the fall.

‘Tassel’ figure panel (left), the legs of figure can be seen on the face of the fallen rock. Fallen ceiling slab with art painted before the fall on the underside of the exposed rock.


VIDEO CLIPS FROM THE ROCK ART DATING TEAM
  • 'Hot Rock' person. Prof. Janet Hergt Deputy Dean Science, University of Melbourne

  • Traditional Owner, Ian Waina explains his role in the research.

  • “We’ve now got the technology to answer the age-old questions..." Prof Richard ‘Bert’ Roberts

  • KFA Science Advisory Council member Cecilia Myers tells us about the first time she saw Kimberley rock art.

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Rock art research is central to answering some of the big questions about human migration. The impact made through KFA rock art research has the potential to rewrite the history of human migration. Recent research in Sulawesi has uncovered prehistoric stone tools thought to be 118,000 years old and nearby rock art at 35,500 years old.

It bears a close resemblance to one of the earliest Kimberley rock art styles. This research including KFA-backed research is the latest in a string of findings that is re-shaping ideas about human migration. It has shifted the focus of early archaeological research from traditional Western hotspots to Australia’s doorstep.

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You can support the Kimberley Foundation Australia by donating shares as well as cash. ShareGift Australia is a not-for-profit organisation that makes it easy for you to support us. Through ShareGift Australia you can ‘convert’ your shares into a charitable donation without paying brokerage fees. If the value of the share sale exceeds $50, you have the option to recommend Kimberley Foundation Australia as beneficiary.

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