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| Titel |
450,000 years of groundwater (234U/238U)0 variations in SW Nevada, USA |
| VerfasserIn |
Kathleen A. Wendt, Gina E. Moseley, Mathieu Pythoud, Yuri Dublyansky, R. Lawrence Edwards, Christoph Spötl |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250147299
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| Publikation (Nr.) |
 EGU/EGU2017-11447.pdf |
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| Zusammenfassung |
| Subaqueous speleothems represent a unique archive for geochemical variations in regional
groundwater systems. Devils Hole 2 cave, located in SW Nevada, USA, is an open fault zone
intersecting the Ash Meadows groundwater flow system. Speleothem layers that coat the
submerged walls of Devils Hole 2 cave record the isotopic composition of groundwater
uranium at the time of precipitation. Past variations of 234U/238U initial activity ratios
in groundwater may provide insight into paleohydrological conditions, such as
changes to groundwater flow rates or source inputs. We aim to reconstruct 450 ka of
groundwater (234U/238U)0 variations at Devils Hole 2 cave. To do so, an 80 cm-long
core was drilled from the cave wall. Over 100 (234U/238U) and U-Th ages were
measured in order to calculate initial activity ratios. Despite relatively constant uranium
concentrations and growth rates throughout the core, preliminary results show a range
in values (2.851 -2.616) deviating from modern day groundwater (234U/238U)0
which we measured to 2.762 (±0.002). (234U/238U)0 variations appear to follow
interglacial-glacial cycles from 450 ka to present day, such that maximum (234U/238U)0
ratios identified at roughly 43, 185, 289, 374, and 449 ka correspond to glacial
periods, while minimum (234U/238U)0 ratios at roughly 5, 121, 239, 336 and 422 ka
correspond to interglacial periods. Focusing on the last 200 ka, we observe increasing
(234U/238U)0 ratios coupled with depleted Devils Hole 2 δ18O values and water table
high-stands (Moseley et al. 2016, Science 2016). We suggest that (234U/238U)0
variations are positively correlated to precipitation amount, contrary to dripstone
speleothem records in the Great Basin region. Mechanisms driving the fluctuation in
(234U/238U)0 values are still uncertain, but may be due to increased inputs of additional
minor groundwater sources to the Ash Meadows flow system during pluvial periods. |
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