Uranium thorium dating on corals

Posted by / 27-Sep-2015 16:14

Uranium thorium dating on corals

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Where the motion of the land can be constrained, measurements of past sea level markers allow local sea level changes to be determined.

Determining changes in sea level, their magnitude and timing is important for understanding the relationship between climate and continental ice volume and for discriminating between potential drivers of past climate change.

The use of different standard materials and laser systems had no significant effect on method accuracy.

Uncertainty on the measured (Carbonate materials such as corals and speleothems are utilised in a wide variety of ways to investigate past ocean and climate states such as sea-level determination through glacial/interglacial cycles (Broecker et al., 1968, Fairbanks, 1989, Gallup, 1997, Potter et al., 2004, Richards et al., 1994 and Thompson et al., 2011), tracking changes in Asian monsoon intensity (Cheng et al., 2006, Dykoski et al., 2005 and Wang et al., 2001) and assessing ventilation rates of the deep ocean (Adkins et al., 1998, Burke and Robinson, 2012, Mangini et al., 1998 and Robinson et al., 2005).

Higher precision, and longer ranging, chronology can, however, be determined through uranium-thorium dating.

When the coral precipitates its skeleton the calcium carbonate incorporates small quantities of natural uranium from the surrounding seawater (typically 3 parts per million).

Two IODP expeditions to Tahiti, French Polynesia, and the Great Barrier Reef, Australia recovered such material.

This uranium is locked into the skeleton and as the fossil coral gets older the uranium decays to thorium.

Using trace metal free sample preparation, chemically isolating the uranium and thorium, measuring the thorium/uranium ratio by mass spectrometry allows the age of the fossil to be determined.

Laser ablation (LA) methods are not as precise as most isotope dilution methods, but can be used to generate calendar ages rapidly, expanding the range of dating tools that can be applied to late Pleistocene carbonates.

Here, existing LA methods are revisited for corals (cold- and warm-water) and speleothems spanning the last 343 thousand years (ka).

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Underpinning these studies is the ability to assign an age to the carbonate material using the decay of uranium-series (U-series) isotopes.

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