Pleistocene climatic change in Southern Australia and its effect on speleothem deposition in some Nullarbor caves

Activity ratios of ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U, and ²³⁰Th/²³²Th have been determined for calcite, gypsum and halite speleothems from caves of the Nullarbor Plain, mostly in the area N and NW of Mundrabilla Station, for the purpose of U-series dating. All calcite speleothems contain adequate amounts of uranium for dating, but some show an excess of ²³⁰Th. Stratigraphic relationships indicate that there were at least three phases of calcium carbonate deposition in the Nullarbor caves. The calcite samples, with one possible exception, have ages in excess of ca. 400000 yrs BP. This suggests that no significant amounts of calcium carbonate deposition have taken place during the last 400ka. At present, active deposition of speleothems is restricted almost entirely to gypsum and halite. The only gypsum speleothem dated was found to have a finite age of ca. 185 ka. Six dates on a small halite speleothem containing insect and arachnid remains indicate that it formed rapidly during Holocene time.     

洞穴地点骨化石铀系年龄可信度的讨论*

骨化石是铀不平衡系测年法广泛应用而又颇有争议的研究对象。文章通过对典型铀加入模式的计算,指出以α能谱的精度,二种铀系年龄差异的显著性表明近期内有较大量铀的迁移,但其在误差范围内的一致不能保证样品构成封闭体系。以往积累的数据表明,大多数洞穴地点骨化石给出了二法一致的铀系年龄,但钙板与下伏骨化石铀系年龄大多差异显著且与地层顺序矛盾,即使被次生碳酸盐岩包裹,多半骨化石的铀系年龄仍显著偏低于其包裹体。此外,相当一部分样品的U-Th同位素比难以用简单的铀后期加入或淋失来解释。基于对次生碳酸盐岩铀系年龄可信度的认识,骨化石总体上不构成封闭体系,所载铀系年代信息只具有限的分辨率。     

Uranium-series dating of pedogenic silica and carbonate, Crater Flat, Nevada

A ²³⁰Th-²³⁴U-²³⁸U dating study on pedogenic silica-carbonate clast rinds and matrix laminae from alluvium in Crater Flat, Nevada was conducted using small-sample thermal-ionization mass spectrometry (TIMS) analyses on a large suite of samples. Though the ²³²Th content of these soils is not particularly low (mostly 0.1-9 ppm), the high U content of the silica component (mostly 4-26 ppm) makes them particularly suitable for ²³⁰Th/U dating on single, 10 to 200 mg totally-digested samples using TIMS. We observed that (1) both micro- (within-rind) and macro-stratigraphic (mappable deposit) order of the ²³⁰Th/U ages were preserved in all cases; (2) back-calculated initial ²³⁴U/²³⁸U fall in a restricted range (typically 1.67±0.19), so that ²³⁴U/²³⁸U ages with errors of about 100 kyr (2σ) could be reliably determined for the oldest, 400 to 1000 ka rinds; and (3) though 13 of the samples were >350 ka, only three showed evidence for an open-system history, even though the sensitivity of such old samples to isotopic disruption is very high. An attempt to use leach-residue techniques to separate pedogenic from detrital U and Th failed, yielding corrupt ²³⁰Th/U ages. We conclude that ²³⁰Th/U ages determined from totally dissolved, multiple sub-mm size subsamples provide more reliable estimates of soil chronology than methods employing larger samples, chemical enhancement of ²³⁸U/²³²Th, or isochrons.     

Th/U/U and Pa/U ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry

The ²³⁰Th/²³⁴U/²³⁸U age dating of corals via alpha counting or mass spectrometry has significantly contributed to our understanding of sea level, radiocarbon calibration, rates of ocean and climate change, and timing of El Nino, among many applications. Age dating of corals by mass spectrometry is remarkably precise, but many samples exposed to freshwater yield inaccurate ages. The first indication of open-system ²³⁰Th/²³⁴U/²³⁸U ages is elevated ²³⁴U/²³⁸U_initial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have ²³⁴U/²³⁸U_initial values close to seawater, there is a need for age validation. Redundant ²³⁰Th/²³⁴U/²³⁸U and ²³¹Pa/²³⁵U ages in a single fossil coral fragment are possible by Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (MC-MS-ICPMS) and standard anion exchange column chemistry, modified to permit the separation of uranium, thorium, and protactinium isotopes from a single solution. A high-efficiency nebulizer employed for sample introduction permits the determination of both ²³⁰Th/²³⁴U/²³⁸U and ²³¹Pa/²³⁵U ages in fragments as small as 500 mg. We have obtained excellent agreement between ²³⁰Th/²³⁴U/²³⁸U and ²³¹Pa/²³⁵U ages in Barbados corals (30 ka) and suggest that the methods described in this paper can be used to test the ²³⁰Th/²³⁴U/²³⁸U age accuracy.Separate fractions of U, Th, and Pa are measured by employing a multi-dynamic procedure, whereby ²³⁸U is measured on a Faraday cup simultaneously with all minor isotopes measured with a Daly ion counting detector. The multi-dynamic procedure also permits correcting for both the Daly to Faraday gain and for mass discrimination during sample analyses. The analytical precision of ²³⁰Th/²³⁴U/²³⁸U and ²³¹Pa/²³⁵U dates is generally better than ±0.3% and ±1.5%, respectively (2 Relative Standard deviation [RSD]). Additional errors resulting from uncertainties in the decay constant for ²³¹Pa and from undetermined sources currently limit the ²³¹Pa/²³⁵U age uncertainty to about ±2.5%. U isotope data and ²³⁰Th/²³⁴U/²³⁸U ages agree with National Institute of Standards and Technology (NIST) reference materials and with measurements made by Thermal Ionization Mass Spectrometry (TIMS) in our laboratory.     

U/Th dating of the travertine deposited at transfer zone between two normal faults and their neotectonic significance: Cambazli fissure ridge travertines (the Gediz Graben-Turkey)

Abstract

The neotectonic characteristics of the travertines that outcrop near Cambazli Village to the west of the Gediz Graben in the Western Anatolia and the age determination of the travertine were carried out using the ²³⁰Th/^234,238U disequilibrium method. The Cambazli fissure ridge travertines represent the travertine depositions that develop at a transfer zone. The extent of these travertines is at NW-SE and NE-SW orientations and the ridge crest-trend of these travertines range between approximately 55° and 82° and they are located at an intersecting position. The evaluation of the fissure ridge travertine directions indicated that the compression stress that was responsible for the deposition of the Cambazli travertine was determined to be in the N-S orientation and the extensional stress was determined to be in the E-W orientation. The orientation of the dominant extension in Western Anatolia during the neotectonic period was N-S and this orientation is not in accordance with the directions of stress for the travertines. This situation indicates that the travertines were deposited along a transfer zone in N10W orientation between two normal faults. The travertines were determined to be active since the Upper Pleistocene as indicated by the age determination conducted using the ²³⁰Th/^234,238U disequilibrium method. The dilation rate of the travertines during dilation and the post-dilation period and the average dilation rate of the Cambazli travertines to the north of the Gediz Graben were calculated as 0.01–0.02 mm yr^?1 during deposition and as 0.05 mm yr^?1 during the post-dilation period. These dilation rates indicate lower tectonic activity along the northern ridge of the Gediz Graben than along the southern ridge.

© 2011 Lavoisier SAS. All rights reserved     

南海珊瑚礁铀系年龄及其地质意义

珊瑚礁是铀系方法测定年龄相当理想的对象,在国外已获得十分有价值的成果。主要用于讨论更新世以来海平面变化和构造运动上升速率,研究反映环境的珊瑚生长速度。本文珊瑚礁样品选取于海南岛鹿回头、雷州半岛、西沙和中沙群岛等地。成功地测试了铀系年龄,揭示出与¹⁴C 数据的不一致,初步讨论了构造上升速率和南海海面的变化。     

Can 234U–230Th dating be used to date large semi‐arid tufas? Challenges from a study in the Naukluft Mountains,Namibia

The large, extensive tufa deposits of the semi‐arid Naukluft Mountains, Namibia are potentially important palaeoenvironmental indicators in an area with few proxy records. Tufas are reliable indicators of increased moisture availability, and have been shown to be amenable to ²³⁴U–²³⁰Th dating, although two challenges are detrital contamination and open‐system behaviour. Densely cemented tufa facies are good candidates for dating, minimising these problems. We report attempts to date five densely‐cemented units, which are only found rarely within the Naukluft deposits. We applied a detailed methodology using multiple subsample analysis, measurement of insoluble residues, application of ‘isochron’ mixing lines, and attempted open‐systems modelling, alongside observations of micromorphology and cathodoluminescence in order to assess the validity of any obtained dates. Surprisingly, densely cemented tufas were found not always to be suitable for dating. Two units contained detrital contamination, which could not be corrected for using a single leachate correction or ‘isochron’ methods. Two units contained ‘excess ²³⁰Th’. This could result under a closed‐system if initial (²³⁴U/²³⁸U) was sufficiently high. Alternatively this may be the result of open‐system behaviour, and loss of uranium, or incorporation of initial unsupported ²³⁰Th, which render samples unsuitable for ²³⁴U–²³⁰Th dating. Micromorphological appearance and cathodoluminescence behaviour are used to explore these possibilities. This study exemplifies the need for careful sample selection, and highlights the importance of analysing multiple subsamples from any tufa sample. The detailed methodology applied proves to be a powerful tool for identifying the range of problems that can be encountered when selecting suitable candidate samples for successful dating. It also shows that semi‐arid tufa sequences may contain very little material suitable for dating. A reliable age of c 80 ka was obtained for a banded unit within a large fluvial barrage, with less reliable dates suggesting tufa deposition during times since >350 ka through to the late Holocene. Copyright © 2010 John Wiley & Sons, Ltd.     

U—Series Dating of Fossil Bones：Results from Chinese Sites and Discussions on Its Reliability

Calculations,according to some open-system models,point out that while a statistically significant discrepancy between the results of two U-series methods,^230Th/^234U and ^227Th/^220Th(or ^231Pa/^235U),attests a relatively recent and important uranium migration,concordant dates cannot guarantee closes-system behavior of sample.The results of 20 fossil bones from 10 Chinese sites,19 of which are determined by two U-series methods,are given,Judging from independent age controls,8 out of the 11 concordant age sets are unacceptable,The results in this paper suggest that uranium may cycle into or out of fossil bones,such geochemical events may take place at any time and no known preserving condition may securely protect them from being affected.So for the sitew we have studied,the U-series dating of fossil bones is of limited reliability.     

In situ U-series dating by laser-ablation multi-collector ICPMS: new prospects for Quaternary geochronology

The capabilities and potential applications of in situ dating of Quaternary materials using laser ablation-MC-ICPMS are explored. ²³⁴U/²³⁸U and ²³⁰Th/²³⁴U can be measured with precision sufficient for dating at a spatial resolution of 100 μm or better in samples that contain as a little as 1 ppm uranium. Moreover, U and Th concentrations and U-series isotope ratios can be continuously profiled to determine changes in age that occur with sample growth (e.g. in speleothems). These capabilities additionally permit the dating of bones, teeth and possibly molluscs, which are subject to post-mortem open-system behaviour of U-series isotopes, and can be employed to elucidate processes of U-series migration during weathering and diagenesis. A drawback of laser ablation-MC-ICPMS is that it cannot in general provide U-series age estimates with the high precision and accuracy of conventional TIMS or solution MC-ICPMS methods. However, sample preparation is straightforward, the amount of sample consumed negligible, and it can be used to rapidly characterise or screen and select samples from which more precise and accurate dates can be obtained using conventional methods. Given further instrumental developments and the establishment of suitable matrix-matched standards for carbonates and other materials, we foresee that laser ablation-MC-ICPMS will play an increasingly important role in Quaternary dating research.     

Electron spin resonance (ESR) and thermal ionization mass spectrometric (TIMS) 230Th/234U dating of teeth in Middle Paleolithic layers at Amud Cave,Israel

Electron spin resonance (ESR) dating and thermal ionization mass spectrometric ²³⁰Th/²³⁴U dating was conducted on six teeth from the prehistoric site of Amud Cave. By combining the ESR and ²³⁰Th/²³⁴U analyses, we obtained burial ages for teeth in various layers of the site. Layer B₁/6–7, from which the Amud I Neanderthal skeleton was recovered, is dated to 53 ± 8 ka. Layer B₂/8, which yielded other important human remains including the Amud 7 skeleton, gives a mean burial age of 61 ± 9 ka. One tooth from the lowest layer (B₄) yielded a date of 70 ± 11 ka, but another tooth from this layer gave an 113 ± 18 ka. Despite this discrepancy, these ages agree with previously published TL ages on heated flints for the corresponding layers. This agreement between ESR on tooth enamel and TL on burned flint is also seen at all other sites studied with both methods in Israel. © 2001 John Wiley & Sons, Inc.     

Thorium-uranium disequilibrium in a geothermal discharge zone at yellowstone

Whole rock samples of hydrothermally-altered Biscuit Basin rhyolite from Yellowstone drill cores Y-7 and Y-8 were analyzed for ²³⁰Th, ²³⁴U, ²³⁸U, and ²³²Th. Extreme disequilibrium was found, with (²³⁰Th/ ²³⁴U) ranging from 0.30 to 1.27. Values of (²³⁰Th/²³²Th) and (²³⁴U/²³²Th) define a linear correlation with a slope of 0.16 ± 0.01, which corresponds to a (²³⁰Th/²³⁴U) age of approximately 19 ka. The (²³⁰Th/²³⁴U) disequilibrium was apparently caused by U redistribution which occurred mostly at about 19 ka, and is not related simply to the relative degree of hydrothermal alteration and self-sealing of the rhyolite. Mass balance of U requires a large flux of U-bearing groundwater through the rhyolite at the time of U redistribution; rough estimates of minimum water/rock ratio range from 10² to 10⁴, for a range of possible groundwater U concentrations. Conservative hydraulic calculations indicate that the required groundwater flux could have occurred within a period of hundreds of years prior to self-sealing. The disequilibrium data are consistent with a model involving U redistribution during the initial stages of development of a geothermal discharge zone that formed in response to the hydrogeologic effects of glacial melting and unloading during the decline of the Pinedale Glaciation.     

A new technique for precise uranium-series dating of travertine micro-samples

Secondary carbonate formations, such as travertine and calcareous tufa deposits, are important archives for quaternary continental climate studies and archaeology. The extremely complex growth mechanisms result in some serious problems for precise mass spectrometric uranium-series dating. Often, detrital and organic particles contaminate the carbonate and large pore volumes yield a great potential for open system behavior. We utilized microscopic, mineralogical and geochemical methods prior to sample selection to determine the abundance of primary calcite, i.e. micrite and spar. Furthermore, the state of alteration was characterized by cathodoluminescence and trace-element analysis. We conclude that travertine and calcareous tufa are appropriate for precise U-series age determination if a) micrite and/or spar are the dominant phases; b) cathodoluminescence of both phases is weak or absent; c) Fe and Al levels are low; and d) Sr concentrations are close to the average of the studied site. We mapped and sampled solely areas of major micrite/spar abundance having minor alteration for accurate U-series dating. When this new method was applied, travertines located in eastern Germany (sites Bad Langensalza, Burgtonna and Weimar-Ehringsdorf) gave single ²³⁰Th/²³⁸U-ages consistent with the lithological growth sequence and greatly improved compared to previously published chronologies. In addition, we determined ²³⁰Th/U isochron ages on bulk samples that confirm our single ages. In contrast to primary calcite, pore cements are homogeneously distributed throughout the travertine fabric and reflect early diagenetic processes and/or weathering.     

Natural radionuclide mobility and its influence on U-Th-Pb dating of secondary minerals from the unsaturated zone at Yucca Mountain, Nevada

Extreme U and Pb isotope variations produced by disequilibrium in decay chains of ²³⁸U and ²³²Th are found in calcite, opal/chalcedony, and Mn-oxides occurring as secondary mineral coatings in the unsaturated zone at Yucca Mountain, Nevada. These very slowly growing minerals (mm my⁻¹) contain excess ²⁰⁶Pb and ²⁰⁸Pb formed from excesses of intermediate daughter isotopes and cannot be used as reliable ²⁰⁶Pb/²³⁸U geochronometers. The presence of excess intermediate daughter isotopes does not appreciably affect ²⁰⁷Pb/²³⁵U ages of U-enriched opal/chalcedony, which are interpreted as mineral formation ages.Opal and calcite from outer (younger) portions of coatings have ²³⁰Th/U ages from 94.6 ± 3.7 to 361.3 ± 9.8 ka and initial ²³⁴U/²³⁸U activity ratios (AR) from 4.351 ± 0.070 to 7.02 ± 0.12, which indicate ²³⁴U enrichment from percolating water. Present-day ²³⁴U/²³⁸U AR is ∼1 in opal/chalcedony from older portions of the coatings. The ²⁰⁷Pb/²³⁵U ages of opal/chalcedony samples range from 0.1329 ± 0.0080 to 9.10 ± 0.21 Ma, increase with microstratigraphic depth, and define slow long-term average growth rates of about 1.2-2.0 mm my⁻¹, in good agreement with previous results. Measured ²³⁴U/²³⁸U AR in Mn-oxides, which pre-date the oldest calcite and opal/chalcedony, range from 0.939 ± 0.006 to 2.091 ± 0.006 and are >1 in most samples. The range of ⁸⁷Sr/⁸⁶Sr ratios (0.71156-0.71280) in Mn-oxides overlaps that in the late calcite. These data indicate that Mn-oxides exchange U and Sr with percolating water and cannot be used as a reliable dating tool.In the U-poor calcite samples, measured ²⁰⁶Pb/²⁰⁷Pb ratios have a wide range, do not correlate with Ba concentration as would be expected if excess Ra was present, and reach a value of about 1400, the highest ever reported for natural Pb. Calcite intergrown with opal contains excesses of both ²⁰⁶Pb and ²⁰⁷Pb derived from Rn diffusion and from direct α-recoil from U-rich opal. Calcite from coatings devoid of opal/chalcedony contains ²⁰⁶Pb and ²⁰⁸Pb excesses, but no appreciable ²⁰⁷Pb excesses. Observed Pb isotope anomalies in calcite are explained by Rn-produced excess Pb. The Rn emanation may strongly affect ²⁰⁶Pb-²³⁸U ages of slow-growing U-poor calcite, but should be negligible for dating fast-growing U-enriched speleothem calcite.     

Uranium-series disequilibrium in tuffs from Yucca Mountain,Nevada, as evidence of pore-fluid flow over the last million years

Samples of tuff from boreholes drilled into fault zones in the Exploratory Studies Facility (ESF) and relatively unfractured rock of the Cross Drift tunnels, at Yucca Mountain, Nevada, have been analysed by U-series methods. This work is part of a project to verify the finding of fast flow-paths through the tuff to ESF level, indicated by the presence of ‘bomb’ ³⁶Cl in pore fluids. Secular radioactive equilibrium in the U decay series, (i.e. when the radioactivity ratios ²³⁴U/²³⁸U, ²³⁰Th /²³⁴U and ²²⁶Ra/²³⁰Th all equal 1.00) might be expected if the tuff samples have not experienced radionuclide loss due to rock-water interaction occurring within the last million years. However, most fractured and unfractured samples were found to have a small deficiency of ²³⁴U (weighted mean ²³⁴U/²³⁸U=0.95±0.01) and a small excess of ²³⁰Th (weighted mean ²³⁰Th/²³⁴U 1.10±0.02). The ²²⁶Ra/²³⁰Th ratios are close to secular equilibrium (weighted mean=0.94±0.07). These data indicate that ²³⁴U has been removed from the rock samples in the last ∼350 ka, probably by pore fluids. Within the precision of the measurement, it would appear that ²²⁶Ra has not been mobilized and removed from the tuff, although there may be some localised ²²⁶Ra redistribution as suggested by a few ratio values that are significantly different from 1.0. Because both fractured and unfractured tuffs show approximately the same deficiency of ²³⁴U, this indicates that pore fluids are moving equally through fractured and unfractured rock. More importantly, fractured rock appears not to be a dominant pathway for groundwater flow (otherwise the ratio would be more strongly affected and the Th and Ra isotopic ratios would likely also show disequilibrium). Application of a simple mass-balance model suggests that surface infiltration rate is over an order of magnitude greater than the rate indicated by other infiltration models and that residence time of pore fluids at ESF level is about 400 a. Processes of U sorption, precipitation and re-solution are believed to be occurring and would account for these anomalous results but have not been included in the model. Despite the difficulties, the U-series data suggest that fractured rock, specifically the Sundance and Drill Hole Wash faults, are not preferred flow paths for groundwater flowing through the Topopah Spring tuff and, by implication, rapid-flow, within 50 a, from the surface to the level of the ESF is improbable.     

Timing and intensity of groundwater movement during Egyptian Sahara pluvial periods by U-series analysis of secondary U in ores and carbonates

Wet climatic episodes are known to have prevailed in the Egyptian Sahara several times during the late Quaternary, most recently during the Holocene 8000 yr ago. Earlier wet episodes have been recognized as having occurred during the past 300,000 yr and have been dated by U-series methods in speleothems and in lake travertines. We show here that the times of enhanced groundwater movement can also be determined by ²³⁰Th/²³⁴U dating of secondary U in ores of uranium, iron, and phosphate. We also present evidence that such acceleration of groundwater movements is indicated by relatively low ²³⁴U/²³⁸U activity ratios in the secondary uranium. Our new data show that pluvial periods in Egypt occurred during marine oxygen isotope stages 4, 5, 6, and 7 and therefore are consistent with the view that the wet episodes are the results of migration of the tropical monsoonal belt driven primarily by the 23,000-yr precession cycle of the Milankovich curve, modulated by the 100,000-yr eccentricity cycle.     

Precise two chronometer dating of Pleistocene travertine: The Th/U and Raex/Ra(0) approach

In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the ²³⁰Th/²³⁴U ingrowth system. Additional analyses of further radionuclides within the ²³⁸U decay chain, i.e. ²²⁶Ra and ²¹⁰Pb, showed that the Th/U chronometer started with insignificant inherited ²³⁰Th over the entire formation period of the travertine setting (i.e. ²³⁰Th(0)=0). A contribution from detrital impurities with ²³⁰Th/²³⁴U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise ²³⁰Th/²³⁴U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the ²³⁸U-decay series, based on decay of excess ²²⁶Ra normalized to the initial, i.e.²²⁶Ra_ex/²²⁶Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the ²³⁰Th/²³⁴U chronometer. Analytical solutions of the coupled ²³⁴U/²³⁰Th/²²⁶Ra radionuclide system predicted that the ²²⁶Ra_ex/²²⁶Ra(0) chronometer is independent of the actual ²³⁰Th activity build up from decay of ²³⁴U, if the systems starts with zero inherited ²³⁰Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated ²²⁶Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of ²²⁶Ra. This is concluded because (i) the ²²⁶Ra_ex/²²⁶Ra(0) ages agreed well with those derived from ²³⁰Th/²³⁴U, (ii) all data plot within uncertainty on the ²²⁶Ra_ex/²²⁶Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, ²²⁶Ra_ex/²²⁶Ra(0) should be applicable to materials which are suitable for ²³⁰Th/²³⁴U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support ²³⁰Th/²³⁴U for samples that have been formed a few thousand years ago.     

Mixing and cycling of uranium,thorium and 210Pb in Puget Sound sediments

Activity profiles of excess ²³⁴Th, excess ²¹⁰Pb, ²³²Th, ²³⁰Th, ²³⁴U and ²³⁸U, and ^228/232Th ratios determined in eight box cores of sediment from six sites in central Puget Sound provide new insights into the dynamic nature of solid phase mixing in surface sediments, the exchange of ²²⁸Ra and other soluble species across the sediment-water interface, and the cycling of U, Th and ²¹⁰Pb in this coastal zone.Comparison of excess ²³⁴Th inventories in sediments with its production rate in the overlying water column indicates a mean residence time of at most 14 days for particles in the central Puget Sound water column.Surface sediment horizons with excess ²³⁴Th have no excess ²²⁸Th which might be used to ascertain sediment accumulation rates over the past decade. Instead, deficiencies of ²²⁸Th due to loss of soluble ²²⁸Ra from pore water to the overlying water persist to 20–30 cm, revealing that exchange of soluble chemicals between pore and overlying waters reaches these depths in the extensively bioturbated sediments of Puget Sound.Solid phase U isotope concentrations tend to increase by up to a factor of two with depth in sediments, as a result of dissolved U being biologically pumped down into sediments where it is partially removed when conditions become mildly reducing. ²³²Th and ²³⁰Th activities and ^230/232Th ratios are constant with depth in sediments, indicating constant detrital phase compositions and essentially no authigenic ²³⁰Th. Steady state ²¹⁰Pb depositional activities in and fluxes to Puget Sound sediments average only about onehalf those for sediments of the open Washington coast north of the Columbia River mouth, primarily because of a much lower supply of dissolved ²¹⁰Pb in sea waters adverting into Puget Sound.Excess ²³⁴Th profiles in sediments reveal much more detail about the depth dependency, dynamic nature and recent history of solid phase mixing processes than excess ²¹⁰Pb profiles. At least six of eight ²³⁴Th profiles show that mixing within the ²¹⁰Pb-defined surface mixed layer is depth dependent. In three profiles, ²³⁴Th-derived mixing rates are fastest several centimeters below the sediment-water interface, indicating greater macro-benthic activity at these depths. Depth dependent mixing coefficients derived from the best fit of a four layer, advection-diffusion-decay model to the ²³⁴Th data are consistent with ²¹⁰Pb profiles determined for the same sediments, strongly suggesting that ²³⁴Th and ²¹⁰Pb are mixed equivalently and in a multilayered manner.     

Transport of radionuclides in an unconfined chalk aquifer inferred from U-series disequilibria

U-series disequilibria measured in waters and rocks from a chalk aquifer in France have been used as an analog for long-term radionuclide migration. Drill core samples from a range of depths in the vadose zone and in the saturated zone, as well as groundwater samples were analyzed for ²³⁸U, ²³⁴U, ²³²Th and ²³⁰Th to determine transport mechanisms at the water/rock interface and to quantify parameters controlling the migration of radionuclides. Isotope measurements in rocks were done by TIMS, whereas (²³⁴U/²³⁸U) and (²³⁰Th/²³²Th) activity ratios in water samples were measured by multi-collector-ICP-MS. Both depletion and enrichment in ²³⁴U relative to ²³⁸U were observed in carbonate rock samples resulting from chemical weathering in the unsaturated zone and calcite precipitation in the zone of water-table oscillation, respectively. The correlation between (²³⁰Th/²³²Th) activity ratios and ⁸⁷Sr/⁸⁶Sr ratios found in the chalk samples indicates that thorium is mainly contained in a minor silicate phase whose abundance is variable in chalk samples. Water samples are all characterized by (²³⁴U/²³⁸U) > 1 resulting from α-recoil effect of ²³⁴Th. Groundwaters are characterized by a more radiogenic signature in ⁸⁷Sr/⁸⁶Sr than the rocks. Moreover, (²³⁰Th/²³²Th) activity ratios in the waters are lower than in the rocks, and increase with distance from the water divide, which suggests that Th transport is controlled by colloids formed during water infiltration in the soil. A 1-D transport model has been developed in order to constrain the U-series nuclide transport considering a transient behavior of radionuclides in the aquifer and a time-dependent composition for the solid phase. This model permits a prediction of the time scale of equilibration of the system, and an estimation of parameters such as weathering rate, distribution coefficients and α-recoil fractions. Retardation factors of 10-35 and from 1 × 10⁴ to 2 × 10⁵ were predicted for U and Th, respectively, and can be used to predict the migration of radionuclides released as contaminants in the environment. At the scale of our watershed (∼32 km²), a characteristic migration time from recharge to riverine discharge of 200-600 yr for U and 0.2-3.7 Myr for Th was obtained.     

The first case study of 230Th/U and 14C dating of mid-valdai organic deposits

From the viewpoint of precision and reliability of radioisotopic dating, deposits whose quantitative age can be determined through several methods of geochronometry are of special interest. The mutually conforming finite ¹⁴C and ²³⁰Th/U dates of buried Neopleistocene organic deposits, taken from the Tolokonka section by the North Dvina River (100 km downstream from the city of Kotlas), have been obtained for the first time in Russia. The stratigraphical reference of these results to those obtained via the optically induced luminescence for upper and lower bedding layers has been established. The presented geochronometric data have allowed us to consider the age of oxbow lake organic deposits completely reliable and refer the time of their formation to the Tyrbei warming within the MIS-3. The applicability of the new version of the ²³⁰Th/U method for dating of interglacial and interstadial deposits, for the purpose of solving the Middle and Late Neopleistocene chronostratigraphy issues, is confirmed.     

Dating fluvial terraces by Th/U on pedogenic carbonate, Wind River Basin, Wyoming

Reliable and precise ages of Quaternary pedogenic carbonate can be obtained with ²³⁰Th/U dating by thermal ionization mass spectrometry applied to carefully selected milligram-size samples. Datable carbonate can form within a few thousand years of surface stabilization allowing ages of Quaternary deposits and surfaces to be closely estimated. Pedogenic carbonate clast-rinds from gravels of glacio-fluvial terraces in the Wind River Basin have median concentrations of 14 ppm U and 0.07 ppm ²³²Th, with median (²³⁰Th/²³²Th) = 270, making them well suited for ²³⁰Th/U dating. Horizons as thin as 0.5 mm were sampled from polished slabs to reduce averaging of long (≥10⁵ yr), and sometimes visibly discontinuous, depositional histories. Dense, translucent samples with finite ²³⁰Th/U ages preserve within-rind stratigraphic order in all cases. Ages for terraces WR4 (167,000 ± 6,400 yr) and WR2 (55,000 ± 8600 yr) indicate a mean incision rate of 0.26 ± 0.05 m per thousand years for the Wind River over the past glacial cycle, slower than inferred from cosmogenic-nuclide dating. Terrace WR3, which formed penecontemporaneously with the final maximum glacial advance of the penultimate Rocky Mountain (Bull Lake) glaciation, has an age of 150,000 ± 8300 yr indicating that it is broadly synchronous with the penultimate global ice volume maximum.