Corey Archer

Mass discrimination correction in multiple-collector plasma source mass spectrometry: an example using Cu and Zn isotopes

Multiple-collector magnetic sector ICP-MS affords the possibility of applying instrumental mass discrimination corrections using a reference isotope ratio of an element (dopant) other than the analyte. Much attention has focused on the use of this approach for lead isotope analysis using a thallium dopant and the technique has also been applied to copper–zinc isotope analysis. The most successful applications of the doping approach have established empirical mass bias relationships between dopant and analyte isotope ratios but this often has to be done for single analytical sessions. Insufficient intra-session variation in mass discrimination often leads to poor constraints on these relationships. Moreover, with the Tl–Pb system there is some doubt over whether samples and standards exhibit the same relationship. Here we show that for the Cu–Zn system, two improvements on previous approaches lead to precise and accurate isotope ratios for unknowns. Firstly, addition of Sr to mixed Cu–Zn standard solutions generates extreme variation in mass bias so that empirical mass bias relationships between analyte and dopant are much better constrained. Secondly, we show that inadequate chemistry, specifically the inefficient removal of matrix Fe and Ti, seriously compromises the Cu–Zn isotope analysis of samples but that with clean chemistry, samples with complex matrices demonstrably yield similar mass discrimination relationships between Cu and Zn isotopes to standards. We also document previously unreported aspects of Cu–Zn isotope analysis: (1) that Cu–Zn mass bias relationships depend critically on the Cu/Zn ratio of the solution; (2) that for an introduction system with a desolvating membrane, the behaviour of standards is highly variable, perhaps due to variations in the oxidation state of Cu in the solution, and that this can be overcome by the passage of standards through the ion exchange procedure used to purify samples. Finally, we document chemical separation and mass spectrometric techniques that permit the isotopic analysis of order of magnitude smaller samples than previously achieved and report values for BCR-1 basalt standard of δ66Zn = 0.20 ± 0.09‰ (n = 12) and δ65Cu = 0.07 ± 0.08‰ (n = 6) at the 95% confidence level.

Coupled Fe and S isotope evidence for Archean microbial Fe(III) and sulfate reduction

Direct fossil evidence for early microbial life on Earth is rare. Microbiological data indicate that sulfate and iron reduction are both among the earliest forms of microbial respiration, and direct evidence for the early origin of sulfate reduction comes from sulfur isotopic anomalies in ancient sediments. Fe isotope geochemistry potentially provides a new way of identifying microbial iron reduction early in Earth9s history. We present Fe isotopic data for sedimentary pyrite from the 2.7 Ga Belingwe sedimentary basin in Zimbabwe. Isotopically light Fe and a remarkable covariation between Fe and S isotopes provide strong evidence for coexisting Fe and S reduction. Our results are consistent with an early origin for sulfate reduction and provide direct geochemical evidence for the antiquity of bacterial Fe reduction. The covariation of Fe and S isotopes may provide a useful new tracer of microbial evolution early in Earth history.

Isotopic constraints on the source of Argentinian loess – with implications for atmospheric circulation and the provenance of Antarctic dust during recent glacial maxima

We present rare-earth element (REE) and Sr–Nd isotopic data for Argentinian loess (28–38°S) with two aims: (1) to examine the source regions of Argentinian loess and the constraints that these put on palaeo-wind directions; (2) to further investigate the source of Antarctic ice-core dust and to test the hypothesis that some of it could be derived from a region to the north of Patagonia – into which the dry, dusty, westerly dominated Patagonian climate expanded during Quaternary glacial maxima. Sr–Nd isotopic data for Argentinian loess from north of 37°S are distinct from Patagonian loess compositions in that they have more radiogenic Sr (87Sr/86Sr=0.7059–0.7123) and less radiogenic Nd (ϵNd=−0.8 to −6.4). REE patterns and Sr–Nd isotopic values are relatively homogeneous for multiple samples taken from single loess sections but show significant differences between sections. In general, there is a northward change from Patagonia-like REE patterns and isotopic values away from volcanogenic signatures and towards those that are more like the continental crust. The latitudinal Nd isotopic pattern is remarkably similar to that for Andean volcanic rocks and suggests derivation of loess from the Andes by more-or-less direct westerly transport. For loess sections in the north, the data imply a contribution from Palaeozoic gneisses to the northwest in the Chilean Altiplano. Sr–Nd data for extra-Patagonian Argentinian loess north of 37°S do not support a significant role for this source region in supplying dust to Antarctica at the last glacial maximum. This conclusion contrasts with previous studies that suggest a significant northward shift in the climatic belts – and in particular the westerlies and the Antarctic Polar Front – during Quaternary glacial maxima. Very systematic relations between the Sr–Nd isotopic composition of loess and their Andean source highlights shifts in the Sr isotopic composition of loess to more radiogenic values and strongly suggests that the slight offset between Patagonian loess and ice-core dust identified by previous workers is due to grain-size differentiation effects.

Resolution of inter-laboratory discrepancies in Mo isotope data: an intercalibration

The molybdenum (Mo) stable isotope system has been applied to a variety of geochemical and environmental problems. In the absence of a universally accepted zero-delta reference material, different groups report their data relative to their adopted in-house standards. Rigorous comparison of results generated in different laboratories using different analytical approaches is only possible if the in-house standards are of identical Mo isotope composition. To determine potential isotopic differences among various standards, the δ98Mo (98Mo/95Mo) values of ten Mo standard solutions were measured as part of this study. For six of these solutions, four laboratories carried out an intercalibration. In contrast to previous results, δ98Mo of various in-house standards were found to differ by up to 0.37‰. Renormalisation of our new and published Mo-isotope data available for seawater taken from various sites and the USGS rock reference material SDO-1 relative to NIST-SRM-3134, provides a much better agreement among reported δ98Mo values for these samples. Relative to NIST-SRM-3134, the δ98Mo of SDO-1 is 0.80 ± 0.14‰ (2s), while oxic, open-ocean seawater is characterised by an average δ98Mo of 2.09 ± 0.10‰ (2s). This intercalibration provides a solid platform for comparing and amending existing δ98Mo values. In addition, we recommend that future Mo isotope studies adopt NIST-SRM-3134 as a universal zero-delta reference material.

A biomarker based on the stable isotopes of nickel

The new stable isotope systems of transition metals are increasingly used to understand and quantify the impact of primitive microbial metabolisms on the modern and ancient Earth. To date, little effort has been expended on nickel (Ni) isotopes but there are good reasons to believe that this system may be more straightforward, and useful in this respect, than some others. Here, we present Ni stable isotope data for abiotic terrestrial samples and pure cultures of methanogens. The dataset for rocks reveals little isotopic variability and provides a lithologic baseline for terrestrial Ni isotope studies. In contrast, methanogens assimilate the light isotopes, yielding residual media with a complementary heavy isotopic enrichment. Methanogenesis may have evolved during or before the Archean, when methane could have been key to Earths early systems. Our data suggest significant potential in Ni stable isotopes for identifying and quantifying methanogenesis on the early planet. Additionally, Ni stable isotope fractionation may well prove to be the fundamental unambiguous trace metal biomarker for methanogens.

The oceanic budgets of nickel and zinc isotopes: the importance of sulfidic environments as illustrated by the Black Sea

Isotopic data collected to date as part of the GEOTRACES and other programmes show that the oceanic dissolved pool is isotopically heavy relative to the inputs for zinc (Zn) and nickel (Ni). All Zn sinks measured until recently, and the only output yet measured for Ni, are isotopically heavier than the dissolved pool. This would require either a non-steady-state ocean or other unidentified sinks. Recently, isotopically light Zn has been measured in organic carbon-rich sediments from productive upwelling margins, providing a potential resolution of this issue, at least for Zn. However, the origin of the isotopically light sedimentary Zn signal is uncertain. Cellular uptake of isotopically light Zn followed by transfer to sediment does not appear to be a quantitatively important process. Here, we present Zn and Ni isotope data for the water column and sediments of the Black Sea. These data demonstrate that isotopically light Zn and Ni are extracted from the water column, probably through an equilibrium fractionation between different dissolved species followed by sequestration of light Zn and Ni in sulfide species to particulates and the sediment. We suggest that a similar, non-quantitative, process, operating in porewaters, explains the Zn data from organic carbon-rich sediments. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Inter-calibration of a proposed new primary reference standard AA-ETH Zn for zinc isotopic analysis

We have prepared a large volume of pure, concentrated and homogenous zinc standard solution. This new standard solution is intended to be used as a primary reference standard for the zinc isotope community, and to serve as a replacement for the nearly exhausted current reference standard, the so-called JMC-Lyon Zn. The isotopic composition of this new zinc standard (AA-ETH Zn) has been determined through an inter-laboratory calibration exercise, calibrated against the existing JMC-Lyon standard, as well as the certified Zn reference standard IRMM-3702. The data show that the new standard is isotopically indistinguishable from the IRMM-3702 zinc standard, with a weighted δ66/64Zn value of 0.28 ± 0.02‰ relative to JMC-Lyon. We suggest that this new standard be assigned a δ66/64Zn value of +0.28‰ for reporting of future Zn isotope data, with the rationale that all existing published Zn isotope data are presented relative to the JMC-Lyon standard. Therefore our proposed presentation allows for a direct comparison with all previously published data, and that are directly traceable to a certified reference standard, IRMM-3702 Zn. This standard will be made freely available to all interested labs through contact with the corresponding author.

The oceanic cycles of the transition metals and their isotopes

The stable isotope systems of the transition metals potentially provide constraints on the current and past operation of the biological pump, and on the state of ocean redox in Earth history. Here we focus on two exemplar metals, nickel (Ni) and zinc (Zn). The oceanic dissolved pool of both elements is isotopically heavier than the known inputs, implying an output with light isotope compositions. The modern oceanic cycle of both these elements is dominated by biological uptake into photosynthesised organic matter and output to sediment. It is increasingly clear, however, that such uptake is associated with only very minor isotope fractionation. We suggest that the isotopic balance is instead closed by the sequestration of light isotopes to sulphide in anoxic and organic-rich sediments, so that it is ocean chemistry that controls these isotope systems, and suggesting a different but equally interesting array of questions in Earth history that can be addressed with these systems.

Nd Isotopes and Geochemistry of Phanerozoic Clastic Sedimentary Rocks from the Yangtze Block and Their Tectonic Implications

Abstract This article presents Sm-Nd and geochemical data on fine-grained sediments of the northern margin from the Yangtze block, China, to understand the variations of Nd isotopic compositions and crustal evolution history in this area. The results are as follows: (1) Nd isotopic compositions for clastic sedimentary rocks of the Middle-Late Proterozoic have relatively positive Nd(t) values (+ 2.72 to + 0.69), with Nd model ages from 1.38 Ga to 1.55 Ga, corresponding to the contemporaneous volcanic rocks from the Xixiang ( ) Group. This indicates that the arc-related materials from Middle-Late Proterozoic dominate the provenances of the Middle-Late Proterozoic periods. (2) The gradual decrease in ɛNd(t) during the Cambrian-Carboniferous periods is likely to reflect the progressively increasing proportion of erosion materials from the Foping ( ) and Qinling ( ) complexes, corresponding to a gradually decreasing trend in the La/Th ratios. (3) A prominent increase in the ɛNd(t) value of the Late Permian strata probably reflects the significant incorporation of the mantle-derived materials. The trace element data are compared with data of the Emeishan ( ) flood basalts. These data indicate that the volcanic dust has been added to the Late Permian strata during the Late Permian, represented by periods of extremely high Emeishan flood basalt activity in the south-eastern margin of the Yangtze block.

Erratum to: The oceanic cycles of the transition metals and their isotopes

The article ‘‘The oceanic cycles of the transition metals and their isotopes’’ written by Derek Vance, Corey Archer, Susan H. Little, Michael Kobberich, Gregory F. de Souza was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 3 May 2017 without open access. With the author(s)’ decision to opt for Open Choice, the copyright of the article changed on [8 August 2017] to The Author(s) 2017 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licen ses/by/4.0/) which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made. The original article was corrected.