Cosmogenic Nuclide Research at RSES

The earth is continually bombarded with extremely high-energy (GeV) particles originating from outside our solar system. These ‘cosmic rays’ produce a variety of nuclear reactions in the atmosphere and within the Earth’s surface, resulting in the creation of ‘cosmogenic’ nuclides. These nuclides can be used to directly date ‘new’ rock surfaces and trace sediment and water through the landscape.

Concentrations of cosmogenic isotopes in typical earth materials are incredibly low, being less than one in a million million (10^-12 ) relative to their stable counterparts. Hence, the ultrasensitive techniques of accelerator mass spectrometry and noble gas mass spectrometry are required. The long half lives of ¹⁰Be, ²⁶Al, and ³⁶Cl and stable ³He and ²¹Ne make these cosmogenic nuclides useful for studying landscape evolution on geological timescales (10³ -10⁶ years).

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The following staff members are involved in developing Cosmogenic Nuclide research at RSES.				Other Links:
	Dr Timothy T. Barrows	Email		Cosmogenic Nuclide Laboratory
	Dr Masahiko Honda	Email		AQUA

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Current Research projects

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Glacial geology and climate change

Exposure dating has revolutionised the study of the history of glaciers and ice sheets. By directly dating glacial debris and eroded bedrock, the timing of the advance and retreat of the ice (a sensitive indicator of climate) can be determined with unprecedented reliability. Dr Timothy T. Barrows is using this technique to take a fresh look at the history of glaciation and climate change in Australia, New Zealand, Papua New Guinea and Spain.

Studies of relics of the last Ice Age in the Snowy Mountains and Tasmania have led to a complete revision of the glacial history of Australia. Hypothetical ideas about glacier extent and its timing that stood for nearly a century have been replaced with a robust chronology placing Australia into a global context. It transpires that there was not just one but at least four major advances of glacier ice during the last 70,000 years. The coldest part of the last ice age was about 20,000 years ago and only lasted a few thousand years. The ensuing global warming is the greatest in recent geological history. On the basis of the altitude of the ice age landforms, mean temperatures around Canberra are about 9°C warmer today. This research provides an important baseline from which to assess climate variability and future climate change.

[Read the press release here.]

Younger Dryas event in New Zealand Science paper

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Geochronology of volcanic landscapes

Cosmogenic nuclides have become indispensable for dating lava flows. Drs Masahiko Honda and Timothy Barrows are dating lavas that have proven too young to date using traditional K/Ar and Ar/Ar techniques. Volcanism has occurred in western Victoria from the Pliocene throughout the Pleistocene. The style of eruption has varied from lava fountains through to explosive maar eruptions. It is likely that volcanism has continued through into the Holocene and will resume again in the future. Overall, the chronology of volcanism is poorly known within the Pleistocene and is being improved by exposure dating.

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Cosmogenic nuclide production rates

Exposure dating is based on the principle that cosmogenic nuclides accumulate in surface rocks as a function of time. After a geological process freshly exposes a rock surface, these cosmogenic nuclides build up at a known rate. Measurement of their present-day abundance, in conjunction with knowledge of the rate at which they are produced, allows an ‘exposure age’ of the surface to be determined. The accuracy of exposure dating is ultimately limited by the accuracy of known production rates.

To improve our knowledge of production rates, Drs Timothy Barrows and Masahiko Honda are calibrating a variety of cosmogenic nuclide production channels using surfaces of known age. The main calibration site is Blue Lake in the Snowy Mountains of Australia, where fresh surfaces of granodiorite have been created by glacial activity and the age of retreat can be independently determined by radiocarbon dating. The rock type here allows all major production channels for ¹⁰Be, ²¹Ne,  ²⁶Al, and ³⁶Cl to be determined.