21.11.2017 - 01:27
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U-Series Dating

Relevant isotopes of the 238U decay chain
Temporal evolution of isotope ratios relevant for U-Th dating
U-Th dates for a stalagmite from a submerged cave on the Bahamas

The U-series laboratory focuses on development and application of U-series dating techniques to provide a robust chronological framework for palaeoclimatology, archaeology and human evolution. The U-series disequilibrium method is based on the radioactive decay of radionuclides within the naturally occurring decay chains. There are three such decay chains, each starts with an actinide nuclide (238U, 235U, 232Th) having a long half live and ultimately ends with different a stable isotope of lead. U-Th dating can be applied to secondary calcium carbonate formations like speleothems, travertine or corals. For dating e.g. speleothems, we make use of an initial elemental fractionation between Th and U when carbonate bedrock is dissolved by percolating water. Differential solubility between uranium and its long lived daughter isotope 230Th means that drip water in caves and calcite precipitates from this water (e.g. stalagmites, stalagtites and flowstones) contain traces of uranium but essentially no 230Th. Over time, there is ingrowth of 230Th from the radioactive decay of uranium until radioactive equilibrium is reached where all isotopes in the series are decaying at the same rate. It is the degree of disequilibrium (measured as 230Th/238U activity ratio) that can be used together with the activity ratio of the two U isotopes 234U/238U to calculate the age of the calcite precipitation. Natural processes usually also cause an initial disequilibrium between 238U and 234U so the age since formation of a calcite sample is calculated iteratively from measurements of 234U/238U and 230Th/238U. The ratios between 238U and the daughter isotopes 234U and 230Th is measured using MC-ICPMS. This method has a dating range up to about 600.000 years.

We use U-Th dating to obtain a chronology for stalagmites which are used for palaeoclimate research (pdf), dating carbonate crusts on cave art to derive minimum age constraints for underlying art

(paper Hoffmann et al. (2016) (DOI: 10.1016/j.quageo.2016.07.004,

Pike et al. (2012) (DOI: 10.1126/science.1219957),

speleothem formations in stratigraphic relationship with archaeological artefacts, travertine formation in relation with archaeology.