Uranium-series chronology and cosmogenic 10Be– 36Cl record of Antarctic ice

Abstract

We present new uranium-series and cosmogenic nuclide age data obtained by sensitive thermal ionization and accelerator mass spectrometric techniques for samples of dusty polar ice from Allan Hills, Antarctica. These data are used to further evaluate the applicability of these nuclides for direct radiometric dating of polar ice. In order to minimize artifacts during sample processing we developed an improved method utilizing ethylenediaminetetraacetic acid (EDTA) to stabilize uranium-series nuclides dissolved in polar ice during ice melting and filtration. Our measurements revealed low uranium-series daughter abundances and large uranium-series ( 234U/ 238U, 230Th/ 234U, 226Ra/ 230Th, 231Pa/ 235U) disequilibria in the filtered <0.05-μm fraction of these ice samples. Both the long-lived uranium-series disequilibria and 10Be/ 36Cl ratios are most likely recently inherited from precipitation. The short-lived daughter 226Ra is the exception and is enhanced in the <0.05-μm fraction, presumably due to recoil effects from dust bands present in the ice. Based on a simplified recoil model, ice ages of approximately 30 ka are obtained from our data. Absence of measurable 238U– 234U– 230Th– 226Ra disequilibria in the volcanic dust bands also suggests that ice ages are greater than 10 ka, consistent with recoil-based ages. Our relatively young ice ages can be compared with previously published relatively old terrestrial ages of meteorites from this area and suggests that meteorites at Allan Hills have experienced a multi-stage history of ice accumulation and/or ice flow. Our results also suggest that large uranium-series disequilibria can be inherited from precipitation, which may provide a method for dating clear ice samples.