Samarium-146 in the early solar system: evidence from neodymium in the allende meteorite

Determination of Nd isotopic composition in seawater using newly developed solid phase extraction and MC-ICP-MS

Determination of neodymium (Nd) isotopic composition in seawater is useful for tracing water masses and geochemical cycles for lithogenic elements in the ocean. A new separation procedure for determination of the Nd isotopic composition of in seawater samples was developed that offers enhanced sample throughput and improved measurement reliability. The procedure consists of conventional Fe hydroxide coprecipitation, solid phase extraction using DGA chelating resin column chromatography, and Ln Resin column chromatography to preconcentrate samples. High selectivity in HNO₃ medium and elution by low concentration HCl medium for Nd are characteristics of extraction using DGA Resin®, and they allowed an evaporation step to be omitted between the chromatographic steps. These chromatographic steps, using DGA Resin to separate REEs and Ln Resin® to remove Sm, were refined from a previous study. The procedural blank value of Nd was obtained as 2 pg (n = 6) from 3 L of water samples. Chemical yield of Nd from 3 L of seawater ranged within 90-95%. The developed procedure was combined with multiple collector-ICP-MS and applied to analysis of vertical seawater samples obtained from the western subarctic gyre of the North Pacific Ocean, where ε_Nd ranged from -1.29 ± 0.42 at the surface to -3.80 ± 0.41 at 4000 m depth. These results were validated by comparing them with results obtained by the conventional method verified in the GEOTRACES inter-calibration program.

Changes in Circulation and Particle Scavenging in the Amerasian Basin of the Arctic Ocean over the Last Three Decades Inferred from the Water Column Distribution of Geochemical Tracers

Since the 1980-1990s, international research efforts have augmented our knowledge of the physical and chemical properties of the Arctic Ocean water masses, and recent studies have documented changes. Understanding the processes responsible for these changes is necessary to be able to forecast the local and global consequences of these property evolutions on climate. The present work investigates the distributions of geochemical tracers of particle fluxes and circulation in the Amerasian Basin and their temporal evolution over the last three decades (from stations visited between 1983 and 2015). Profiles of 230-thorium (230Th) and 231-protactinium (231Pa) concentrations and neodymium isotopes (expressed as εNd) measured in the Amerasian Basin prior to 2000 are compared to a new, post-2000s data set. The comparison shows a large scale decrease in dissolved 230Th and 231Pa concentrations, suggesting intensification of scavenging by particle flux, especially in coastal areas. Higher productivity and sediment resuspension from the shelves appear responsible for the concentration decrease along the margins. In the basin interior, increased lateral exchanges with the boundary circulation also contribute to the decrease in concentration. This study illustrates how dissolved 230Th and 231Pa, with εNd support, can provide unique insights not only into changes in particle flux but also into the evolution of ocean circulation and mixing.

Ultra-high-precision Nd-isotope measurements of geological materials by MC-ICPMS

We report novel techniques allowing the measurement of Nd-isotope ratios with unprecedented accuracy and precision by multi-collector inductively coupled plasma mass spectrometry. Using the new protocol, we have measured the Nd-isotopic composition of rock and synthetic Nd standards as well as that of the Allende carbonaceous chondrite. Analyses of BCR-2, BHVO-2 and GSP-2 rock standards yield mass-independent compositions identical to the JNdi-1 Nd-reference standard, with an external reproducibility of 2.4, 1.6, 1.6 and 3.5 ppm respectively, on μ142Nd, μ145Nd, μ146Nd and μ150Nd (μ representing the ppm-deviation of the ratios from JNdi-1) using 148Nd/144Nd for internal normalization. This represents an improvement in precision by a factor of 2, 7 and 9 respectively for μ142Nd, μ145Nd and μ150Nd. Near-quantitative recovery from purification chemistry and sample-standard bracketing allow for the determination of mass-dependent Nd-isotopic composition of samples. Synthetic standards, namely La Jolla and AMES, record mass-dependent variability of up to 1.2 ε per atomic mass unit and mass-independent compositions resolvable by up to 3 ppm for μ142Nd and 8 ppm for μ150Nd, relative to JNdi-1. The mass-independent compositions are consistent with equilibrium mass fractionation during purification. The terrestrial rock standards define a uniform stable ε145Nd of -0.24 ± 0.19 (2SD) relative to JNdi-1, indistinguishable from the mean Allende ε145Nd of -0.19 ± 0.09. We consider this value to represent the mass-dependent Nd-isotope composition of Bulk Silicate Earth (BSE). The modest mass-dependent fractionation of JNdi-1 relative to BSE results in potential effects on mass-independent composition that cannot be resolved within the reproducibility of our analyses when correcting for natural and instrumental mass fractionation by kinetic law, making it a suitable reference standard for analysis of unknowns. Analysis of Allende (CV3) carbonaceous chondrite returns an average μ142Nd deficit of -30.1 ± 3.7 ppm in agreement with previous studies. The apparent deficit is, however, lowered to -23.8 ± 4.0 ppm while normalizing to 148Nd/144Nd instead of 146Nd/144Nd. We interpret this as the effect of a possible nucleosynthetic anomaly of -6.3 ± 0.5 ppm in μ146Nd. As 142Nd and 146Nd are both s-process-dominated nuclides, this hints at the possibility that terrestrial μ142Nd excess may not reflect 146Sm decay as widely accepted.

Obliquity pacing of the western Pacific Intertropical Convergence Zone over the past 282,000 years

The Intertropical Convergence Zone (ITCZ) encompasses the heaviest rain belt on the Earth. Few direct long-term records, especially in the Pacific, limit our understanding of long-term natural variability for predicting future ITCZ migration. Here we present a tropical precipitation record from the Southern Hemisphere covering the past 282,000 years, inferred from a marine sedimentary sequence collected off the eastern coast of Papua New Guinea. Unlike the precession paradigm expressed in its East Asian counterpart, our record shows that the western Pacific ITCZ migration was influenced by combined precession and obliquity changes. The obliquity forcing could be primarily delivered by a cross-hemispherical thermal/pressure contrast, resulting from the asymmetric continental configuration between Asia and Australia in a coupled East Asian–Australian circulation system. Our finding suggests that the obliquity forcing may play a more important role in global hydroclimate cycles than previously thought.

Predicting future migrations in the Intertropical Convergence Zone—Earth's heaviest rain belt—is limited by a lack of long-term records. Here, the authors present a 282 kyr precipitation record from the Papua New Guinea coast and show that obliquity forcing plays a more important role than previously recognized.

Rapid neodymium release to marine waters from lithogenic sediments in the Amazon estuary

Rare earth element (REE) concentrations and neodymium isotopic composition (ɛNd) are tracers for ocean circulation and biogeochemistry. Although models suggest that REE release from lithogenic sediment in river discharge may dominate all other REE inputs to the oceans, the occurrence, mechanisms and magnitude of such a source are still debated. Here we present the first simultaneous observations of dissolved (<0.45 μm), colloidal and particulate REE and ɛNd in the Amazon estuary. A sharp drop in dissolved REE in the low-salinity zone is driven by coagulation of colloidal matter. At mid-salinities, total dissolved REE levels slightly increase, while ɛNd values are shifted from the dissolved Nd river endmember (−8.9) to values typical of river suspended matter (−10.6). Combining a Nd isotope mass balance with apparent radium isotope ages of estuarine waters suggests a rapid (3 weeks) and globally significant Nd release by dissolution of lithogenic suspended sediments.

Neodymium isotopes are tracers for past and present ocean circulation and biogeochemistry. Here, the authors combine observations of neodymium and radium isotopes in the Amazon estuary and show that the rapid release of neodymium from river suspended sediments leaves a strong imprint on coastal sea water.

Late-glacial elevated dust deposition linked to westerly wind shifts in southern South America

Atmospheric dust loadings play a crucial role in the global climate system. Southern South America is a key dust source, however, dust deposition rates remain poorly quantified since the last glacial termination (~17 kyr ago), an important timeframe to anticipate future climate changes. Here we use isotope and element geochemistry in a peat archive from Tierra del Fuego, to reconstruct atmospheric dust fluxes and associated environmental and westerly wind changes for the past 16.2 kyr. Dust depositions were elevated during the Antarctic Cold Reversal (ACR) and second half of the Younger Dryas (YD) stadial, originating from the glacial Beagle Channel valley. This increase was most probably associated with a strengthening of the westerlies during both periods as dust source areas were already available before the onset of the dust peaks and remained present throughout. Congruent with glacier advances across Patagonia, this dust record indicates an overall strengthening of the wind belt during the ACR. On the other hand, we argue that the YD dust peak is linked to strong and poleward shifted westerlies. The close interplay between dust fluxes and climatic changes demonstrates that atmospheric circulation was essential in generating and sustaining present-day interglacial conditions.

146Sm-142Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth's mantle

Application of the 147Sm-143Nd chronometer (half-life of 106 Gyr) suggests that large-scale differentiation of the Earth's mantle may have occurred during the first few hundred million years of its history. However, the signature of mantle depletion found in early Archaean rocks is often obscured by uncertainties resulting from open-system behaviour of the rocks during later high-grade metamorphic events. Hence, although strong hints exist regarding the presence of differentiated silicate reservoirs before 4.0 Gyr ago, both the nature and age of early mantle differentiation processes remain largely speculative. Here we apply short-lived 146Sm-142Nd chronometry (half-life of 103 Myr) to early Archaean rocks using ultraprecise measurement of Nd isotope ratios. The analysed samples are well-preserved metamorphosed sedimentary rocks from the 3.7-3.8-Gyr Isua greenstone belt of West Greenland. Our coupled isotopic calculations, combined with an initial epsilon 143Nd value from ref. 6, constrain the mean age of mantle differentiation to 4,460 +/- 115 Myr. This early Sm/Nd fractionation probably reflects differentiation of the Earth's mantle during the final stage of terrestrial accretion.

The role of off-line mass spectrometry in nuclear fission

The role of mass spectrometry in nuclear fission has been invaluable since 1940, when A. O. C. Nier separated microgram quantities of (235) U from (238) U, using a gas source mass spectrometer. This experiment enabled the fissionable nature of (235) U to be established. During the Manhattan Project, the mass spectrometer was used to measure the isotope abundances of uranium after processing in various separation systems, in monitoring the composition of the gaseous products in the Oak Ridge Diffusion Plant, and as a helium leak detector. Following the construction of the first reactor at the University of Chicago, it was necessary to unravel the nuclear systematics of the various fission products produced in the fission process. Off-line mass spectrometry was able to identify stable and long-lived isotopes produced in fission, but more importantly, was used in numerous studies of the distribution of mass of the cumulative fission yields. Improvements in sensitivity enabled off-line mass spectrometric studies to identify fine structure in the mass-yield curve and, hence, demonstrate the importance of shell structure in nuclear fission. Solid-source mass spectrometry was also able to measure the cumulative fission yields in the valley of symmetry in the mass-yield curve, and enabled spontaneous fission yields to be quantified. Apart from the accurate measurement of abundances, the stable isotope mass spectrometric technique has been invaluable in establishing absolute cumulative fission yields for many isotopes making up the mass-yield distribution curve for a variety of fissile nuclides. Extensive mass spectrometric studies of noble gases in primitive meteorites revealed the presence of fission products from the now extinct nuclide (244) Pu, and have eliminated the possibility of fission products from a super-heavy nuclide contributing to isotopic anomalies in meteoritic material. Numerous mass spectrometric studies of the isotopic and elemental abundances of samples from the Oklo Natural Reactor have enabled the nuclear parameters of the various reactor zones to be calculated, and the mobility/retentivity of a number of elements to be established in the reactor zones and the surrounding rocks. These isotopic studies have given valuable information on the geochemical behavior of natural geological repositories for radioactive waste containment. © 1997 John Wiley & Sons, Inc.

Live Iron-60 in the Early Solar System

Isotopic analyses of nickel in samples from the differentiated meteorite Chervony Kut revealed the presence of relative excesses of (60)Ni ranging from 2.4 up to 50 parts per 10(4). These isotopic excesses are from the decay of the now extinct short-lived nuclide (60)Fe and provide clear evidence for the existence of (60)Fe over large scales in the early solar system. Not only was (60)Fe present at the time of melting and differentiation (that is, Fe-Ni fractionation) of the parent body of Chervony Kut but also later at the time when basaltic magma solidified at or near the surface of the planetesimal. The inferred abundance of (60)Fe suggests that its decay alone could have provided sufficient heat to melt small (diameters of several hundred kilometers) planetary bodies shortly after their accretion.

Barium Isotopes in Allende Meteorite: Evidence Against an Extinct Superheavy Element

Carbon and chromite fractions from the Allende meteorite that contain isotopically anomalous xenon-131 to xenon-136 (carbonaceous chondrite fission or CCF xenon) at up to 5 x 10(11) atoms per gram show no detectable isotopic anomalies in barium-130 to barium-138. This rules out the possibility that the CCF xenon was formed by in situ fission of an extinct superheavy element. Apparently the CCF xenon and its carbonaceous carrier are relics from stellar nucleosynthesis.