Claudine H. Stirling

Techniques for Measuring Uranium-series Nuclides: 1992–2002

Advances in geochemistry and geochronology are often closely linked to development of new technologies for improved measurement of elemental and isotopic abundance. At the beginning of the past decade, thermal ionization mass spectrometric (TIMS) methods were just beginning to be applied for long-lived uranium-series nuclide measurement (Edwards et al. 1987; Goldstein et al. 1989; Bard et al. 1990), with considerable advances in measurement speed, precision, and sensitivity over decay-counting methods. This opened up a vast number of applications in uranium-series geochronology and geochemistry of young sediments, volcanic rocks, and aqueous systems. Over the past decade there have continued to be advances in thermal ionization techniques, and the advent of alternative mass spectrometric methods, particularly multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS), has continued to improve the quality of uranium-series studies. So the past decade has been a particularly dynamic time for not only development of mass spectrometric techniques, but initiation of other methods related to long-lived uranium-series nuclide measurement. In the area of sample preparation, further development of microwave digestion methods had led to advances in speed and cost of analysis. In chemical separations, development of extraction chromatographic resins for isolating specific elements have simplified many separation problems and consequently improved analytical characteristics including sensitivity, speed of analysis, waste generation, and cost. With regard to instrumental analysis, advances in both decay-counting and mass spectrometry instrumentation have improved either measurement sensitivity or precision, speed of analysis, or analytical cost. One could argue that instrumental developments will continue to drive scientific breakthroughs in the application of uranium-series nuclides as tracers and chronometers in the earth and other sciences. In this chapter we discuss improvements documented in the literature over the past decade in these areas and others. Chemical procedures, decay-counting spectroscopy, and mass spectrometric techniques published prior to 1992 were …

Precise determination of the open ocean 234U/238U composition

Uranium has a long residence time in the open oceans, and therefore, its salinity-normalized U concentration and 234U/238U activity ratio (expressed herein as δ234U, the ‰ deviation from secular equilibrium) are assumed to be uniform. The marine 234U/238U activity ratio is currently in radioactive disequilibrium and shows a ∼15% excess of 234U with respect to the secular equilibrium value due to continuous input from riverine sources. Knowledge of the marine δ234U, and how it has evolved through the Quaternary, is important for validating age accuracy in the U series dating of marine carbonates, which is increasingly relied upon for providing a chronological basis in paleoclimate research. However, accurate and precise measurements of δ234U are technically difficult. Thus, existing compilations of the open ocean δ234U value vary by up to ∼10‰, and the assumed uniformity in the oceanic δ234U remains to be confirmed. Using MC-ICPMS techniques and a suite of multiple Faraday cups instead of the typical configurations based on a combined Faraday cup–multiplier array, a long-term reproducibility of better than ±0.3‰ (2σ) is achieved for δ234U measurements. Applying these very high precision techniques to open ocean seawater samples, an average δ234U of 146.8 ± 0.1‰ (2σm, n = 19) is obtained. These high-precision seawater measurements yield an external reproducibility of better than ±0.4‰ (2σ) and show that the open oceans have a uniform δ234U on the sub-‰ level. These new data constrain the vertical mixing time of the open oceans to less than 1000 years.

The First New Zealanders: Patterns of Diet and Mobility Revealed through Isotope Analysis

Direct evidence of the environmental impact of human colonization and subsequent human adaptational responses to new environments is extremely rare anywhere in the world. New Zealand was the last Polynesian island group to be settled by humans, who arrived around the end of the 13th century AD. Little is known about the nature of human adaptation and mobility during the initial phase of colonization. We report the results of the isotopic analysis (carbon, nitrogen and strontium) of the oldest prehistoric skeletons discovered in New Zealand to assess diet and migration patterns. The isotope data show that the culturally distinctive burials, Group 1, had similar diets and childhood origins, supporting the assertion that this group was distinct from Group 2/3 and may have been part of the initial colonizing population at the site. The Group 2/3 individuals displayed highly variable diets and likely lived in different regions of the country before their burial at Wairau Bar, supporting the archaeological evidence that people were highly mobile in New Zealand since the initial phase of human settlement.

Carbon cycling and burial in New Zealand's fjords

Understanding carbon cycling in continental margin settings is critical for constraining the global carbon cycle. Here we apply a multiproxy geochemical approach to evaluate regional carbon cycle dynamics in six New Zealand fjords. Using carbon and nitrogen concentrations and isotopes, lipid biomarkers, and redox-sensitive element concentrations, we show that the New Zealand fjords have carbon-rich surface sediments in basins that promote long-term storage (i.e., semirestricted basins with sediment accumulation rates of up to 4 mm yr−1). Using δ13C distributions to develop a mixing model, we find that organic carbon in fjord sediments is well-mixed from marine and terrestrial sources in down-fjord gradients. This is driven by high regional precipitation rates of >6 m yr−1, which promote carbon accumulation in fjord basins through terrestrial runoff. In addition, we have identified at least two euxinic subbasins, based on uranium, molybdenum, iron, and cadmium enrichment, that contain >7% organic carbon. Because the strength and position of the Southern Hemisphere westerly winds control precipitation and fjord circulation, carbon delivery and storage in the region are intimately linked to westerly wind variability. We estimate that the fjord region (759 km2) may be exporting up to 1.4 × 107 kgC yr−1, outpacing other types of continental margins in rates of carbon burial by up to 3 orders of magnitude.

Anomalous isotopic shifts associated with organic resin residues during cadmium isotopic analysis by double spike MC-ICPMS

Previous studies have reported that matrix effects during analysis by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS), leading to highly anomalous isotopic results, can be derived from ion exchange separation procedures, as resin-derived organics are stripped from the column together with the element of interest. In low concentration samples, where matrix to analyte ratios are high, these artifacts cannot be corrected for using sample-standard bracketing or external normalization techniques to monitor instrumental mass fractionation. It has been suggested that such artifacts can be overcome by implementing a double spike for the correction of instrumental mass fractionation. We present new Cd isotopic results showing that although a double spike reliably corrects for instrumental mass fractionation during sample analysis, the effects of these resin-derived organics introduce very anomalous shifts in bracketing standards that have not been chemically processed using ion exchange resins. These inaccurate results appear to be the consequence of one or more polyatomic interference/s or mass independent isotopic fractionation as opposed to the previously proposed matrix effects. These analytical artifacts can be completely alleviated by the oxidation of samples with solutions of either H2O2/HNO3 or HClO4/HNO3 prior to analysis. Although the present results have direct relevance for Cd analysis, it is probable that other isotopic systems are affected by similar resin-derived organics. The increased sensitivity of the next generation of MC-ICPMS instruments means that progressively smaller sample sizes are accessible for analysis, leading to a larger proportion of resin-derived matrix in many samples, such that oxidation of samples will prove to be essential to circumvent the issue of inaccurate isotopic measurement.

Does the trace element composition of brown trout Salmo trutta eggs remain unchanged in spawning redds

The temporal stability of trace element concentrations in fertilized, artificially incubated anadromous brown trout Salmo trutta eggs and newly hatched fry was investigated. The anadromous status of the parental fish was confirmed using strontium isotopic analysis of otoliths. Whilst manganese concentrations in eggs varied over time, concentrations of aluminium, potassium, magnesium, strontium, barium and calcium were all unchanged 1 week and 6 weeks post-fertilization as well as in recently hatched larvae. The results clearly suggest that the distinctive trace element signature present in the eggs and newly hatched larvae of anadromous S. trutta (typically characterized by high strontium, low barium) is stable over time. Therefore analysis of the trace element composition of eggs is concluded to be a cost-effective and reliable method for determining the spatial and temporal extent of upstream spawning migration by anadromous salmonids. The temporal variability of at least one element in this study suggests the stability of untested multi-element signatures cannot automatically be assumed.

Uranium isotope evidence for two episodes of deoxygenation during Oceanic Anoxic Event 2

Significance Past “Oceanic Anoxic Events” (OAEs) represent important carbon cycle perturbations that offer the opportunity to study Earth’s response to extreme climate warming. A fundamental limitation for understanding OAEs is quantifying the timing and total extent of ocean anoxia. We present a quantitative account of global redox conditions for OAE 2 (∼94 million years ago), using a high-resolution record of uranium isotopes combined with a biogeochemical model. We present new evidence for two discrete intervals of globally extensive anoxia that were coupled to enhanced terrestrial weathering, within the typically defined OAE interval. These anoxic intervals were separated by ocean reoxygenation and the temporary recovery of the carbon cycle. Oceanic Anoxic Event 2 (OAE 2), occurring ∼94 million years ago, was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. It was typified by a rapid rise in atmospheric CO2, global warming, and marine anoxia, leading to the widespread devastation of marine ecosystems. However, the precise timing and extent to which oceanic anoxic conditions expanded during OAE 2 remains unresolved. We present a record of global ocean redox changes during OAE 2 using a combined geochemical and carbon cycle modeling approach. We utilize a continuous, high-resolution record of uranium isotopes in pelagic and platform carbonate sediments to quantify the global extent of seafloor anoxia during OAE 2. This dataset is then compared with a dynamic model of the coupled global carbon, phosphorus, and uranium cycles to test hypotheses for OAE 2 initiation. This unique approach highlights an intra-OAE complexity that has previously been underconstrained, characterized by two expansions of anoxia separated by an episode of globally significant reoxygenation coincident with the “Plenus Cold Event.” Each anoxic expansion event was likely driven by rapid atmospheric CO2 injections from multiphase Large Igneous Province activity.

Isotope Tracing of Long-Term Cadmium Fluxes in an Agricultural Soil

Globally widespread phosphate fertilizer applications have resulted in long-term increases in the concentration of cadmium (Cd) in soils. The accumulation of this biotoxic, and bioaccumulative metal presents problems for the management of soil-plant-animal systems, because the magnitude and direction of removal fluxes (e.g., crop uptake, leaching) have been difficult to estimate. Here, Cd isotopic compositions (δ114/110Cd) of archived fertilizer and soil samples from a 66 year-long agricultural field trial in Winchmore, New Zealand, were used to constrain the Cd soil mass balance between 1959 and 2015 AD, informing future soil Cd accumulation trajectories. The isotopic partitioning of soil Cd sources in this system was aided by a change in phosphate source rocks in 1998 AD, and a corresponding shift in fertilizer isotope composition. The dominant influence of mixing between isotopically distinct Cd end-members was confirmed by a Bayesian modeling approach. Furthermore, isotope mass balance modeling revealed that Cd removal processes most likely increased in magnitude substantially between 2000 and 2015 AD, implying an increase in Cd bioaccumulation and/or leaching over that interval. Natural-abundance stable isotopes are introduced here as a powerful tool for tracing the fate of Cd in agricultural soils, and potentially the wider environment.

The evolution of hydrous magmas in the Tongariro Volcanic Centre: the 10 ka Pahoka-Mangamate eruptions

The majority of arc-type andesites in the Tongariro Volcanic Centre are highly porphyritic, hornblende-free, two-pyroxene andesites. An exception is tephras from the c. 10,000 ka Pahoka-Mangamate event. Magmas of these Plinian eruptions bypassed the extensive crustal mush columns under the central volcanoes and sequentially derived a series of almost aphyric rocks spanning a compositional range from dacite to basaltic andesite. Mineral composition, trace element and isotopic data suggest that this eruptive series tapped a mid-crustal magma reservoir, resulting in the initial eruption of an hydrous dacitic magma and several following eruptions characterised by less-evolved and less-hydrous compositions at progressively higher temperatures and substantially lower 87Sr/86Sr ratios. Systematic changes in magma chemistry are also reflected in a sequential change in phenocryst content starting with an early hornblende–plagioclase-dominated assemblage to a late olivine–plagioclase-dominated assemblage.

Keeping Time with Earth's Heaviest Element

New 238U/235U ratios for uranium-bearing minerals provide a better chronometer for dating geological processes. Uranium is the heaviest naturally occurring element on Earth. It has three natural isotopes (238U, 235U, and 234U), of which 238U and 235U are the parent nuclides of the 238U- and 235U-decay series chains, which ultimately decay to stable isotopes of lead (Pb), thereby forming the basis of the U-Pb chronometer. Conventional theories of stable isotope fractionation have dictated that uranium is too heavy to display resolvable mass-dependent isotope effects. The expectation was that Earth would display homogeneous 238U/235U isotopic compositions. The convention has been to adopt an invariant present-day 238U/235U ratio equal to 137.88 throughout the solar system, on the basis of early studies of uranium ore deposits. This critical assumption, which underpinned the veracity of the U-Pb chronometer for the past 30 years, was overturned by the discovery of surprisingly large 238U/235U variations in Earths surface environments (1, 2). On page 1610 of this issue, Hiess et al. (3) report the 238U/235U composition of a large suite of U-bearing accessory minerals to facilitate a more accurate U-Pb geochronometer. These new results also provide fundamental but unexpected insights into the mechanisms controlling 238U/235U fractionation.