Search Results

Advanced search parameters have been applied.
open access

Sediment transport time measured with U-Series isotopes: Resultsfrom ODP North Atlantic Drill Site 984

Description: High precision uranium isotope measurements of marineclastic sediments are used to measure the transport and storage time ofsediment from source to site of deposition. The approach is demonstratedon fine-grained, late Pleistocene deep-sea sediments from Ocean DrillingProgram Site 984A on the Bjorn Drift in the North Atlantic. The sedimentsare siliciclastic with up to 30 percent carbonate, and dated by sigma 18Oof benthic foraminifera. Nd and Sr isotopes indicate that provenance hasoscillated be… more
Date: June 5, 2006
Creator: DePaolo, Donald J.; Maher, Kate; Christensen, John N. & McManus,Jerry
Partner: UNT Libraries Government Documents Department
open access

U-Sr isotopic speedometer: Fluid flow and chemical weatheringrates inaquifers

Description: Both chemical weathering rates and fluid flow are difficultto measure in natural systems. However, these parameters are critical forunderstanding the hydrochemical evolution of aquifers, predicting thefate and transport of contaminants, and for water resources/water qualityconsiderations. 87Sr/86Sr and (234U/238U) activity ratios are sensitiveindicators of water-rock interaction, and thus provide a means ofquantifying both flow and reactivity. The 87Sr/86Sr values in groundwaters are controlled… more
Date: December 27, 2005
Creator: Maher, Kate; DePaolo, Donald J. & Christensen, John N.
Partner: UNT Libraries Government Documents Department
open access

Laboratory Experiments Bearing on the Origin and Evolution of Olivine-rich Chondrules

Description: Evaporation rates of K2O, Na2O, and FeO from chondrule-like liquids and the associated potassium isotopic fractionation of the evaporation residues were measured to help understand the processes and conditions that affected the chemical and isotopic compositions of olivine-rich Type IA and Type IIA chondrules from Semarkona. Both types of chondrules show evidence of having been significantly or totally molten. However, these chondrules do not have large or systematic potassium isotopic fraction… more
Date: June 24, 2011
Creator: Richter, Frank M.; Mendybaev, Ruslan A.; Christensen, John N.; Ebel, Denton & Gaffney, Amy
Partner: UNT Libraries Government Documents Department
open access

Isotopic Tracking of Hanford 300 Area Derived Uranium in the Columbia River

Description: Our objectives in this study are to quantify the discharge rate of uranium (U) to the Columbia River from the Hanford Site's 300 Area, and to follow that U down river to constrain its fate. Uranium from the Hanford Site has variable isotopic composition due to nuclear industrial processes carried out at the site. This characteristic makes it possible to use high-precision isotopic measurements of U in environmental samples to identify even trace levels of contaminant U, determine its sources, a… more
Date: October 31, 2010
Creator: Christensen, John N.; Dresel, P. Evan; Conrad, Mark E.; Patton, Gregory W. & DePaolo, Donald J.
Partner: UNT Libraries Government Documents Department
open access

Uranium-series comminution ages of continental sediments: Case study of a Pleistocene alluvial fan

Description: Obtaining quantitative information about the timescales associated with sediment transport, storage, and deposition in continental settings is important but challenging. The uranium-series comminution age method potentially provides a universal approach for direct dating of Quaternary detrital sediments, and can also provide estimates of the sediment transport and storage timescales. (The word"comminution" means"to reduce to powder," reflecting the start of the comminution a… more
Date: April 30, 2010
Creator: Lee, Victoria E.; DePaolo, Donald J. & Christensen, John N.
Partner: UNT Libraries Government Documents Department
open access

Identifying the sources of subsurface contamination at the Hanford site in Washington using high-precision uranium isotopic measurements

Description: In the mid-1990s, a groundwater plume of uranium (U) was detected in monitoring wells in the B-BX-BY Waste Management Area (WMA) at the Hanford Site in Washington. This area has been used since the late 1940s to store high-level radioactive waste and other products of U fuel-rod processing. Using multiple collector ICP source magnetic sector mass spectrometry (MC ICPMS) high precision uranium isotopic analyses were conducted of samples of vadose zone contamination and of groundwater. The ratios… more
Date: March 30, 2004
Creator: Christensen, John N.; Dresel, P. Evan; Conrad, Mark E.; Maher, Kate & DePaolo, Donald J.
Partner: UNT Libraries Government Documents Department
open access

Pb Isotopes as an Indicator of the Asian Contribution to Particulate Air Pollution in Urban California

Description: During the last two decades, expanding industrial activity in east Asia has led to increased production of airborne pollutants that can be transported to North America. Previous efforts to detect this trans-Pacific pollution have relied upon remote sensing and remote sample locations. We tested whether Pb isotope ratios in airborne particles can be used to directly evaluate the Asian contribution to airborne particles of anthropogenic origin in western North America, using a time series of samp… more
Date: October 25, 2010
Creator: Ewing, Stephanie A.; Christensen, John N.; Brown, Shaun T.; Vancuren, Richard A.; Cliff, Steven S. & DePaolo, Donald J.
Partner: UNT Libraries Government Documents Department
open access

Isotopic mass-dependence of metal cation diffusion coefficients in liquid water

Description: Isotope distributions in natural systems can be highly sensitive to the mass (m) dependence of solute diffusion coefficients (D) in liquid water. Isotope geochemistry studies routinely have assumed that this mass dependence either is negligible (as predicted by hydrodynamic theories) or follows a kinetic-theory-like inverse square root relationship (D {proportional_to} m{sup -0.5}). However, our recent experimental results and molecular dynamics (MD) simulations showed that the mass dependence … more
Date: January 11, 2009
Creator: Bourg, Ian C.; Richter, Frank M.; Christensen, John N. & Sposito, Garrison
Partner: UNT Libraries Government Documents Department
Back to Top of Screen