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WHOI geologist Liviu Giosan and his colleagues determined the age of ancient river sediments using both radiocarbon dating of organic material and a technique called optically-stimulated luminescence using grains of quartz sand from the sediment. Quartz crystals absorb ionizing radiation that can be released as light. Exposure to sunlight “bleaches” the crystal, halting the emission of light. If a crystal is buried, such as by being covered by flood-borne sediment, the ionizing radiation can build up again and restore the crystal’s ability to luminesce. Scientists can tell how long ago the crystals were buried by unearthing them and exposing them to specific wavelengths of light that cause them to luminesce. The longer the crystal has been in the dark, the more light it emits. Each new exposure to sunlight resets the crystal’s “clock” to zero, allowing researchers to discover when the mineral was last on the surface. (Public domain)[back]
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