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Planet Earth

A Wasp's-Nest Clock

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The rock outcroppings of the Kimberley region of northwestern Australia are painted, pecked, and engraved with vestiges of aboriginal art. Unfortunately, except for the odd charcoal sketch, most aboriginal rock art is nearly impossible to date--it is often colored with ocher, a mineral pigment that lacks the organic carbon compounds required by radioactive-dating techniques. Without an absolute scale, archeologists have had to rely on informed guesswork to date most aboriginal art. But that may soon change. Some Australian researchers have found a way to use fossilized wasps’ nests to determine the age of ancient art.

Grahame Walsh, a rock-art specialist at the Takarakka Rock Art Research Center at Carnarvon Gorge, was studying the Kimberley paintings when he noticed that a nearby wasp’s nest he had assumed to be of recent origin was in fact fossilized. Wasps’ nests--made of a loosely packed fabric of sand, silt, and pollen grains--are not generally durable. But Walsh found that silica carried by water apparently seeped through the sandstone overlying the rock shelters and filled in the pores in the nest, reinforcing it to withstand the ravages of time.

Walsh realized that the sand grains worked into the nest would make it possible to date the nest. He contacted Richard Roberts, a geologist at La Trobe University in Melbourne who specializes in reading these grainy timepieces using a method known as optical luminescence dating. Radiation from radioactive trace elements in the sand bombards the grains. The radiation causes atoms in the grains to spit out electrons that become trapped in imperfections in the grains’ crystalline structure. Here the displaced electrons remain until ultraviolet radiation in sunlight frees them from their crystal prisons. As the electrons return to more stable positions, each emits a photon of light. By exposing sand grains to light and measuring the intensity of the light emitted by the grains, geologists can estimate how long ago they were last exposed to sunlight.

Two years ago Roberts joined Walsh on an expedition to Kimberley in search of petrified wasps’ nests built on top of rock art. While clambering around the rocky outcroppings, the researchers came across two fossilized nests overlying a mulberry-colored painting of a human figure whose elongated body, narrow head, and semicircular headdress suggested it belonged to a style believed by most archeologists to date back some 5,000 years. The researchers pried the fossilized nests free and extracted sand grains from their cores.

Using luminescence dating, Roberts discovered that the nests were more than 17,000 years old. That makes the underlying aboriginal rock painting the oldest depiction of a human figure in the world. Roberts believes the figure may actually be much older than the nest, since the Ice Age had reached its height at that time and the Kimberley region would thus have been arid and inhospitable to humans.

Presumably the paintings were made during a previous, wetter period, says Roberts, perhaps 25,000 to 30,000 years ago or even earlier, before the peak of the last glacial maximum. If he’s right, the mulberry- colored figure may rival the oldest known paintings of animals--from the Chauvet cave in France--thought to be about 30,000 years old.

Although too sparse to date, the pollen found in the petrified nests can be used to identify the plants the wasps visited while building their homes many thousands of years ago. The nests Walsh and Roberts found contain mostly eucalyptus pollen and smatterings of pollen from an array of flowering plants and grasses. For Roberts, that makes the nests a still more important record, enabling a detailed reconstruction of the environment in which the ancient artists lived. My feeling is that the greatest global application of the approach will be to examine past vegetation histories and infer past climate from preserved nests, says Roberts. That wasn’t the main aim of our project--the presence of pollen was pure luck; but the combination of being able to date the rock art and reconstruct past environments makes wasp nests extremely versatile time capsules.

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