Application of Plutonium Isotopes to the Sediment Geochronology of Coarse-Grained Sediments from Englebright Lake, California (USA)
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The determination of sediment accumulation rates in environments with temporal variations in texture is challenging using traditional radioisotope methods, largely due to low activities associated with coarse sediments. This study used Englebright Lake, an impoundment in northern California, as a model system to examine the application of plutonium isotopes in lacustrine environments where the interlayering of coarse and fine sediments complicates the geochronology. Inductively coupled plasma mass spectrometry was used to quantify plutonium isotopes and low limits of detection allowed for the measurement of plutonium in sand, clay, and silt fractions. Although measurable levels of plutonium were found in sand fractions, over 75 % of the total plutonium activity was found in fine-grain-size fractions (<63 μm). Correlations between cesium-137 and plutonium activities in fine-grained sediments (r = 0.81–0.98, p < 0.005) suggest that plutonium isotopes may be substituted for cesium isotopes in coarse-grained sediments where cesium is typically below detectable levels. Sediment accumulation rates calculated from grain-size normalized plutonium activity profiles ranged from 6 to 145 cm year−1 in Englebright Lake and identified a sediment depocenter at the delta front upstream of Englebright Dam. Progradation of the delta front reflected changes in sediment supply from the watershed in response to flood events, whereas average annual accumulation responded to human impacts. This study extends the application of plutonium isotopes for sediment geochronology to aquatic environments dominated by coarse sediments and provides new information that contributes to a better understanding of the processes influencing sediment deposition in Englebright Lake.
KeywordsPlutonium isotopes Cesium isotopes Coarse sediments Sediment accumulation Impoundment Englebright Lake
We thank the US Geological Survey for allowing us the opportunity to sample the cores used in this study. We also thank Mary Goodwyn (VIMS) and Micheal Ketterer (NAU) for laboratory and analytical assistance. This study was supported by NSF GK-12 (Divison of Graduate Education 0840804), VIMS GSA Mini-Grant, and VIMS Maury fellowship. This paper is contribution 3497 of the Virginia Institute of Marine Science, College of William and Mary.
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Conflict of interest
The authors declare that they have no conflict of interest.
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