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Geochemistry of speleothem records from southern Illinois: Development of (234U)/(238U) as a proxy for paleoprecipitation

Chemical Geology

By:
, , , , ,
DOI: 10.1016/j.chemgeo.2005.02.005

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Abstract

Natural waters universally show fractionation of uranium series (U-series) parent-daughter pairs, with the disequilibrium between 234U and 238U (234U)/(238U) commonly used as a tracer of groundwater flow. Because speleothems provide a temporal record of geochemical variations in groundwater precipitating calcite, (234U)/(238U) variations in speleothems provide a unique method of investigating water-rock interaction processes over millennium time scales. We present high precision Thermal Ionization Mass Spectrometric (TIMS) U-series analyses of speleothems and drip waters from Fogelpole Cave in southern Illinois. Data from all speleothems from the cave show an inverse correlation between (234U)/(238U) and U concentration, following the pattern observed in groundwaters globally. Within a 65-cm-long stalagmite, concordant 234U-238 U-230Th and 235U-231Pa ages for 5 samples indicate accurate chronology from 78.5 ka to 30 ka. Notably, (234U)/(238U)o which differs from most speleothems by having (234U)/(238U)o <1, positively correlates with speleothem growth rate. We generalize this to the observation that speleothems globally show (234U)/ (238U)o deviating farther from secular equilibrium at lower growth rates and approaching secular equilibrium at higher grow rates. Based on the Fogelpole observations, we suggest that groundwater (234U)/(238U) is controlled by the U oxidation state, the U concentration of the water and the fluid velocity. A transport model whereby U-series nuclides react and exchange with mineral surfaces can reproduce the observed trend between growth rate and (234U)/(238U)o. Based on this result, we suggest that (234U)/(238U)o in speleothems may record changes in hydrologic flux with time and thus could provide a useful proxy for long term records of paleoprecipitation. ?? 2005 Elsevier B.V. All rights reserved.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Geochemistry of speleothem records from southern Illinois: Development of (234U)/(238U) as a proxy for paleoprecipitation
Series title:
Chemical Geology
DOI:
10.1016/j.chemgeo.2005.02.005
Volume
221
Issue:
1-2
Year Published:
2005
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Chemical Geology
First page:
1
Last page:
20
Number of Pages:
20