39 results found
Packman H, Little SH, Baker AR, et al., 2022, Tracing natural and anthropogenic sources of aerosols to the Atlantic Ocean using Zn and Cu isotopes, Chemical Geology, Vol: 610, Pages: 1-14, ISSN: 0009-2541
Anthropogenic activities have significantly enhanced atmospheric metal inputs to the ocean, which has potentially important consequences for marine ecosystems. This study assesses the potential of Zn and Cu isotope compositions to distinguish between natural and anthropogenic atmospheric inputs of these metals to the surface ocean. To this end, the isotopic compositions of Zn and Cu in aerosols collected from the eastern tropical Atlantic Ocean on the GEOTRACES GA06 cruise are examined. Enrichment factors and fractional solubility measurements indicate the presence of a significant anthropogenic component in the aerosols collected furthest from the North African dust plume for both Zn and Cu. The mean δ65CuNIST SRM 976 for the fully digested aerosols is +0.07 ± 0.39 ‰ (n = 9, 2 SD), which is indistinguishable from the lithogenic value, and implies that Cu isotopes are not an effective tracer of aerosol sources in this region. The mean δ66ZnJMC-Lyon value for the aerosols that underwent a total digestion is +0.17 ± 0.22 ‰ (n = 11, 2 SD). The aerosols leached with ammonium acetate have similar Zn isotope compositions, with a mean of +0.15 ± 0.16 ‰ (n = 7, 2 SD). The aerosols were collected in a region with prevalent mineral dust but, despite this, exhibit isotopically lighter Zn than lithogenic Zn with δ66Zn ≈ +0.3 ‰. When coupled with the previously published Pb isotope data, the aerosols exhibit coupled Zn-Pb isotope systematics that are indicative of mixing between mineral dust (δ66Zn = +0.28 ‰ and 206Pb/207Pb = 1.205) and anthropogenic emissions (δ66Zn = −0.22 ‰ and 206Pb/207Pb = 1.129). This demonstrates the potential of Zn isotopes to trace atmospheric Zn inputs from anthropogenic sources to the surface ocean.
de Souza GF, Vance D, Sieber M, et al., 2022, Re-assessing the influence of particle-hosted sulphide precipitation on the marine cadmium cycle, Geochimica et Cosmochimica Acta, Vol: 322, Pages: 274-296, ISSN: 0016-7037
It has been inferred that the marine distributions of the micronutrient cadmium (Cd) and its stable isotope composition (expressed as δ114Cd) bear widespread and unambiguous evidence for loss of Cd from the shallow water column through the formation of particle-associated cadmium sulphide (CdS) in oxygen minimum zones (OMZs). In this review, we bring together elemental and isotopic datasets from the dissolved and particulate Cd pools in order to unravel the multiple, overlapping controls on the distribution of Cd and δ114Cd, and demonstrate that the global dataset challenges this view. By far the most important control on the marine Cd distribution is the extreme plasticity in the cadmium:phosphorus (Cd:P) stoichiometry of biological uptake and, in consequence, particulate export. We show that δ114Cd systematics in low-latitude OMZs that have been taken to reflect Cd loss in fact come about mainly through the interaction between the physical circulation and the variable stoichiometry of biological Cd uptake at high and low latitudes; water-column evidence for Cd loss is thus much less widespread than has previously been inferred. Subtle but consistent signals in particulate elemental and dissolved isotopic data from the open tropical Atlantic and Pacific Oceans allow us to identify the signal of a Cd loss associated with the oxycline of the shallow tropical subsurface, as has previously been suggested. However, this Cd loss appears to be ubiquitous throughout the tropics, rather than confined to oxygen-poor waters, speaking against CdS formation as the driving mechanism. Although its true identity remains unknown, this tropical Cd loss may be related to biological activity. Finally, we show how the processes we consider – the remineralisation of biogenic particles with variable Cd:P stoichiometry, and ubiquitous tropical oxycline Cd loss – bear upon the role of particle-hosted CdS formation in the marine mass balance of Cd, which is lik
Anand P, Appiah F, Lawrence A, et al., 2022, Assessing diversity and inclusion within the UK&#8217;s geochemistry academic workforce
<jats:p>&lt;p&gt;Geochemistry is applied across Earth, environmental and planetary geoscience research. Yet, the first specific workforce diversity data for geochemistry is only now being collected (e.g. EAG-GS led 2022 Global Geochemistry Community Survey). Additionally, national effort is underway to scrutinise detailed and intersectional diversity data (e.g., race, ethnicity, gender identity, sexual orientation, disability, socioeconmic background, career path) for UK geochemists via &amp;#8216;Evaluating Diversity and Inclusion within the (geochemistry) Academic Ladder (E-DIAL)&amp;#8217;, a project funded by the UK&amp;#8217;s Natural Environmental Research Council. This project will also collect data to evidence workplace structures and policies specific to the UK&amp;#8217;s geochemists.&lt;/p&gt;&lt;p&gt;We will present key findings linked to the geochemistry workforce, including laboratory support staff, from among our survey results to provide a snapshot of the demographics and intersectional representation among the UK geochemistry community within Higher Education Institutes (HEIs). An important facet of this work is our focus on how the allocation, accessibility, and support of geochemistry laboratories are distributed and experienced by all members of the UK community. Furthermore, the study will report on salient aspects of COVID-19 generated impacts and inequities within the geochemistry community. We also present specific findings for experiences that capture evidence of some persisting barriers to individuals and/or geochemistry groups. These exclusionary hurdles include cultural and other obstacles for which we suggest remedies that will advance the representation and further the success of people of minoritised identities within and across the academic ladder.&amp;#160;&lt;/p&gt;&lt;p&gt;We aim to use our project findings
Maters E, Appiah F, Lawrence A, et al., 2022, Are diverse geochemists retained and thriving on the UK academic ladder?
<jats:p>&lt;p&gt;Evaluating Diversity and Inclusion within the (geochemistry) Academic Ladder (E-DIAL), funded by the UK&amp;#8217;s Natural Environment Research Council, provides a snapshot of diversity and identifies barriers resulting in underrepresentation among intersectional identity groups across the UK geochemistry community. The project emphasis is on the academic ladder within UK higher education institutions (HEIs). As a multi-faceted discipline, geochemistry is central to Earth, environmental and planetary sciences, yet the first specific geochemistry workforce data is only now being collected (e.g., through the 2022 Global Geochemistry Community Survey launched by the European Association of Geochemistry and Geochemical Society). In complement to this timely society-led effort serving the international geochemistry community, our project collects original data and evidence through a UK-wide survey that captures coupled diversity and academic progression and retention data. These data are vital to catalyse policies that actively improve geochemistry career prospects for diverse talents, given that a community&amp;#8217;s scientific potential can only be reached by including everyone. Specifically, the UK-wide survey data examines information on past and recent rates of appointment, progression and retention of both majority and under-represented groups across all levels of seniority, including research students, within UK HEIs.&lt;/p&gt;&lt;p&gt;In profiling geochemists&amp;#8217; career pathways and evidencing lived experiences (e.g., among postdocs), exclusionary obstacles are identified with resultant understanding driving revision of prevailing policies, attitudes and practices while assessing implementation effectiveness at HEIs with, for example, differing diversity certifications (e.g., Athena Swan or Race Equality Charters). Findings will test the hypothesis that &a
Pourret O, Anand P, Bots P, et al., 2022, Gender balance and geographical diversity in editorial boards of Geochimica et Cosmochimica Acta and Chemical Geology, European Science Editing, Vol: 2022, ISSN: 0258-3127
Background: Members of editorial boards of academic journals are often considered gatekeepers of knowledge and role models for the academic community. Editorial boards should be sufficiently diverse in the background of their members to facilitate publishing manuscripts representing a wide range of research paradigms, methods, and cultural perspectives. Objectives: To critically evaluate changes in the representation of binary gender and geographic diversity over time on the editorial boards of Chemical Geology and Geochimica et Cosmochimica Acta, flagship geochemistry journals, respectively, from the European Association of Geochemistry and the Geochemical Society – Meteoritical Society partnership. Methods: The composition of editorial boards was ascertained as given in the first issue of each year, over 1965–2021 for Chemical Geology and 1950–2021 for Geochimica et Cosmochimica Acta, and members of the editorial boards were coded for their country of affiliation (the country of origin may have been different) and for their binary gender. Results: Gender parity, limited to men and women, and the number of countries of affiliation increased steadily between the late 1980s and 2021. However, the geographic distribution remained dominated by affiliations from North America and Western Europe. The editor-in-chief or board of editors had a significant impact on the diversity of the editorial boards, and both geographic and gender diversity may evolve with nearly every newly appointed editor. However, the persistently substantial under-representation on editorial boards of affiliations outside North America and Europe is of concern and needs to be the focus of active recruitment and ongoing monitoring. This approach will ensure that traditionally low geographic diversity is increased and maintained in the future. Conclusion: Improving diversity and inclusion of editorial boards of academic journals and strengthening journal and disciplinary reputations
Horner TJ, Little SH, Conway TM, et al., 2021, Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES-Era Data, GLOBAL BIOGEOCHEMICAL CYCLES, Vol: 35, ISSN: 0886-6236
- Author Web Link
- Citations: 12
Little SH, Wilson DJ, Rehkämper M, et al., 2021, Cold-water corals as archives of seawater Zn and Cu isotopes, Chemical Geology, Vol: 578, Pages: 1-20, ISSN: 0009-2541
Traditional carbonate sedimentary archives have proven challenging to exploit for Zn and Cu isotopes, due to the high concentrations of trace metals in potential contaminants (e.g., Fe-Mn coatings) and their low concentrations in carbonate. Here, we present the first dataset of δ66ZnJMC-Lyon and δ65CuSRM 976 values for cold-water corals and address their potential as a seawater archive. Extensive cleaning experiments carried out on two corals with well-developed Fe-Mn rich coatings demonstrate that thorough physical and chemical cleaning can effectively remove detrital and authigenic contaminants. Next, we present metal/Ca ratios and δ66Zn and δ65Cu values for a geographically diverse sample set of Holocene age cold-water corals. Comparing cold-water coral δ66Zn values to estimated ambient seawater δ66Zn values (where Δ66Zncoral-sw = δ66Zncoral – δ66Znseawater), we find Δ66Zncoral-sw = +0.03 ± 0.17‰ (1SD, n = 20). Hence, to a first order, cold-water corals record seawater Zn isotope compositions without fractionation. The average Holocene coral Cu isotope composition is +0.59 ± 0.23‰ (1SD, n = 15), similar to the mean of published deep seawater δ65Cu values at +0.66 ± 0.09‰, but with considerable variability. Finally, δ66Zn and δ65Cu data are presented for a small subset of four glacial-age corals. These values overlap with the respective Holocene coral datasets, hinting at limited glacial-interglacial changes in oceanic Zn and Cu cycling.
Horner T, Little S, Conway T, et al., 2021, Bioactive trace metals and their isotopes as paleoproductivity proxies: An assessment using GEOTRACES-era data
Chen L, Little SH, Kreissig K, et al., 2021, Isotopically light Cd in sediments underlying oxygen deficient zones, Frontiers in Earth Science, Vol: 9, Pages: 1-17, ISSN: 2296-6463
Cadmium is a trace metal of interest in the ocean partly because its concentration mimics that of phosphate. However, deviations from the global mean dissolved Cd/PO4 relationship are present in oxygen deficient zones, where Cd is depleted relative to phosphate. This decoupling has been suggested to result from cadmium sulphide (CdS) precipitation in reducing microenvironments within sinking organic matter. We present Cd concentrations and Cd isotope compositions in organic-rich sediments deposited at several upwelling sites along the northeast Pacific continental margin. These sediments all have enriched Cd concentrations relative to crustal material. We calculate a net accumulation rate of Cd in margin settings of between 2.6 to 12.0 × 107 mol/yr, higher than previous estimates, but at the low end of a recently published estimate for the magnitude of the marine sink due to water column CdS precipitation. Cadmium in organic-rich sediments is isotopically light (δ114/110CdNIST-3108 = +0.02 ± 0.14‰, n = 26; 2 SD) compared to deep seawater (+0.3 ± 0.1‰). However, isotope fractionation during diagenesis in continental margin settings appears to be small. Therefore, the light Cd isotope composition of organic-rich sediments is likely to reflect an isotopically light source of Cd. Non-quantitative biological uptake of light Cd by phytoplankton is one possible means of supplying light Cd to the sediment, which would imply that Cd isotopes could be used as a tracer of past ocean productivity. However, water column CdS precipitation is also predicted to preferentially sequester light Cd isotopes from the water column, which could obfuscate Cd as a tracer. We also observe notably light Cd isotope compositions associated with elevated solid phase Fe concentrations, suggesting that scavenging of Cd by Fe oxide phases may contribute to the light Cd isotope composition of sediments. These multiple possible sources of isotopically light Cd
Little SH, Archer C, Chatterjee A, et al., 2021, Towards balancing the oceanic Ni budget (vol 547, 116461, 2020), EARTH AND PLANETARY SCIENCE LETTERS, Vol: 557, ISSN: 0012-821X
Sherman DM, Little SH, 2020, Isotopic disequilibrium of Cu in marine ferromanganese crusts: Evidence from ab initio predictions of Cu isotope fractionation on sorption to birnessite, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 549, ISSN: 0012-821X
- Author Web Link
- Citations: 11
Little SH, Archer C, McManus J, et al., 2020, Towards balancing the oceanic Ni budget, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 547, ISSN: 0012-821X
- Author Web Link
- Citations: 22
Horner T, Little S, Conway T, et al., 2020, Bioactive trace metals and their isotopes as paleoproductivity proxies: An assessment using GEOTRACES-era data
Wang Q, Zhou L, Little SH, et al., 2020, The geochemical behavior of Cu and its isotopes in the Yangtze River, Science of The Total Environment, Vol: 728, Pages: 1-12, ISSN: 0048-9697
Copper (Cu) isotopes can be a useful tool to constrain the interaction of water and the environment, but they have not been widely applied to riverine research in the preceding decades. Isotopically heavy Cu in rivers (global average: about +0.7‰) compared to rocks (at about 0‰) has been attributed to: a) the mobilization of heavy Cu during oxidative weathering, and b) partitioning between an isotopically heavy, organically complexed dissolved pool, and an isotopically light pool adsorbed to particulates. Here, we report Cu concentrations and isotope ratios of the main stream of the Yangtze River and its several tributaries. We find that the Yangtze River exhibits anomalously heavy Cu isotope compositions compared to other rivers: δ65CuNIST 976 of dissolved Cu for the main stream, from Chongqing to Nanjing, ranges from +0.59 to +1.65‰, while the tributaries vary from +0.48 to +1.20‰. A negative correlation is observed between Cu concentrations and Cu isotope compositions.We attribute the anomalous Cu isotope geochemistry of the Yangtze River to two key features of the basin: first, the influence of the Three Gorges Dam (TGD), and second, the presence of extensive Cu sulphide deposits close to the lower reaches of the river. In the upper reaches, downstream towards the TGD, δ65Cu values increase as Cu concentrations decrease, reflecting the preferential adsorption of light Cu by sedimenting particulate phases. δ65Cu values continue to increase to a maximum of +1.65‰ in the middle reaches, at Guangxingzhou. The lower reaches, from Jiujiang to Tongling, are characterized by less positive values of δ65Cu (at about +0.60‰), due to the oxidative weathering of Cu sulphide deposits. The overall Cu-δ65Cu trend in the river reflects mixing of these waters from the lower reaches, influenced by Cu sulphides, with waters from upstream, which have lower Cu concentrations and elevated δ65Cu values.
Griffiths A, Packman H, Leung YL, et al., 2020, Evaluation of optimized procedures for high-precision Pb isotope analyses of seawater by MC-ICP-MS, Analytical Chemistry, Vol: 92, Pages: 11232-11241, ISSN: 0003-2700
The application of Pb isotopes to marine geochemistry is currently hindered by challenges associated with the analysis of Pb isotopes in seawater. The current study evaluates the performance of MC-ICP-MS measurements of seawater Pb isotope compositions following Pb separation by either solid-phase extraction with Nobias Chelate PA-1 resin or co-precipitation with Mg(OH)2, and using either a Pb double-spike or external normalization to Tl for mass bias correction. The four analytical combinations achieve results of similar quality when measuring 1–7 ng of seawater Pb, with reproducibilities (2SD) of 100–1200 ppm for 206Pb/207Pb, 208Pb/207Pb and 300–1700 ppm for ratios involving the minor 204Pb isotope. All four procedures enable significantly improved sample throughout compared to an established TIMS double-spike method and produce unbiased seawater Pb isotope compositions with similar or improved precision. Nobias extraction is preferable to co-precipitation due to its greater analytical throughput and suitability for analyses of large seawater samples with high Si(OH)4 contents. The most accurate Pb isotope data are produced following Nobias extraction and double-spike correction as such analyses are least susceptible to matrix effects. However, Nobias extraction with Tl-normalization constitutes an attractive alternative as, unlike the double-spike procedure, only a single mass spectrometric measurement is required, which improves analytical throughput and optimizes Pb consumption for analysis. Despite the advantages of solid-phase extraction, co-precipitation represents a useful Pb separation technique for samples with low to moderate Si contents as it is inexpensive, simple to implement and the data are only marginally less accurate, especially when combined with a Pb double-spike for mass bias correction.
Schleicher NJ, Dong S, Packman H, et al., 2020, A global assessment of copper, zinc, and lead isotopes in mineral dust sources and aerosols, Frontiers in Earth Science, Vol: 8, Pages: 1-20, ISSN: 2296-6463
The stable isotope compositions of Cu and Zn in major geochemical reservoirs are increasingly studied with the aim to develop these isotope systems as tools to investigate the global biogeochemical cycles of these trace metals. The objectives of the present study were (i) to expand the range of Cu, Zn, and Pb isotope compositions of mineral dust by analyzing samples from major mineral dust sources in Asia and Africa (Chinese Loess Plateau, Chinese deserts, Thar desert, Sahel region) and (ii) to assess the potential impact of human activities on the isotope composition of aerosols by synthesizing published Cu and Zn isotope compositions in aerosols and natural and anthropogenic sources. For the newly analyzed mineral dust areas in Asia and Africa, δ65CuNIST−976 values range from −0.54 to +0.52‰, δ66ZnJMC−Lyon values from −0.07 to +0.57‰, and 206Pb/204Pb values from 18.522 to 19.696. We find a significant geographic control with samples from the Thar Desert having the heaviest isotopic compositions (δ65CuNIST−976 = +0.48 ± 0.06‰, δ66ZnJMC−Lyon = +0.49 ± 0.11‰) and samples from the Sahel and the Badain Jaran desert having the lightest Zn isotope composition (δ66ZnJMC−Lyon = +0.19 ± 0.15‰ and +0.07 ± 0.07‰, respectively). We find important variations in the isotope signatures between particle size fractions with heavier isotopic compositions in the smallest and largest particle size fractions and lighter isotopic compositions in the mid particle size fractions. Associations with the mineralogical composition are less clear. Newly analyzed aerosol samples for Beijing and Xi'an show δ65CuNIST−976 values of +0.29 ± 0.19‰ and +0.16 ± 0.04‰, δ66ZnJMC−Lyon values of −0.36 ± 0.04‰ and +0.02 ± 0.06‰, and 206Pb/204Pb values of 18.129 ± 0.003 and 18
Little S, van De Flierdt T, Wilson D, et al., 2020, Zn isotopes in deep sea corals: a useful palaeoceanographic archive?
<jats:p> &lt;p&gt;Zinc (Zn) is an important bioessential trace element. Its distribution in the modern oceans reflects a combination of biological uptake, remineralization and the physical ocean circulation. Furthermore, the partitioning behaviour of Zn (D&lt;sub&gt;Zn&lt;/sub&gt;) and its isotopes (&amp;#948;&lt;sup&gt;66&lt;/sup&gt;Zn) in carbonates has been linked to ambient seawater carbonate chemistry [1-3].&lt;/p&gt;&lt;p&gt;Development of Zn isotopes in carbonates as a palaeoceanographic tool has been hampered by the high concentrations of Zn in contaminating material, such as lithogenic or authigenic (e.g. Fe-Mn oxide) phases. However, deep-sea corals are large enough to be subjected to aggressive physical and chemical cleaning, enabling effective removal of contaminating phases. They also have several other advantages over traditional palaeoclimate archives, including the ability to assign precise absolute ages to individual specimens based on uranium-series dating .&lt;/p&gt;&lt;p&gt;Here we present Zn/Ca and &amp;#948;&lt;sup&gt;66&lt;/sup&gt;Zn data for a suite of modern and recent (&lt;1000 yr) deep sea corals from six ocean regions spanning the far North Atlantic to the Tasman Sea. We observe what appears to be species-specific Zn partitioning behaviour, but no clear links between D&lt;sub&gt;Zn&lt;/sub&gt; or coral &amp;#948;&lt;sup&gt;66&lt;/sup&gt;Zn and ambient seawater carbonate chemistry. Overall, there is good agreement between measured or best-guess modern seawater &amp;#948;&lt;sup&gt;66&lt;/sup&gt;Zn and coral aragonite &amp;#948;&lt;sup&gt;66&lt;/sup&gt;Zn values, suggesting that
Little S, 2020, Message in a fossil? Lessons from the last plants on Antarctica, Weather, Vol: 75, Pages: 30-31, ISSN: 0043-1656
Little SH, Munson S, Prytulak J, et al., 2019, Cu and Zn isotope fractionation during extreme chemical weathering, GEOCHIMICA ET COSMOCHIMICA ACTA, Vol: 263, Pages: 85-107, ISSN: 0016-7037
- Author Web Link
- Citations: 32
Pratt N, Chen T, Li T, et al., 2019, Temporal distribution and diversity of cold-water corals in the southwest Indian Ocean over the past 25,000 years, Deep Sea Research Part I: Oceanographic Research Papers, Vol: 149, ISSN: 0967-0637
Fossil cold-water corals can be used to reconstruct physical, chemical, and biological changes in the ocean because their skeleton often preserves ambient seawater signatures. Furthermore, patterns in the geographic and temporal extent of cold-water corals have changed through time in response to environmental conditions. Here we present taxonomic and dating results from a new collection of subfossil cold-water corals recovered from seamounts of the Southwest Indian Ocean Ridge. The area is a dynamic hydrographic region characterised by eastward flow of the Agulhas Return Current and the northernmost fronts of the Antarctic Circumpolar Current. In total, 122 solitary scleractinian corals and 27 samples of colonial scleractinian material were collected from water depths between 172 and 1395 m, corresponding to subtropical waters, Antarctic Intermediate Water (AAIW), and Upper Circumpolar Deep Water (UCDW). Fifteen species were identified, including eight species new to the region. The assemblage reflects the position of the seamounts in a transition zone between Indo-Pacific and Subantarctic biogeographic zones. Morphological variation in caryophyllids and the restriction of dendrophylliids to the southern seamounts could result from genetic isolation or reflect environmental conditions. Uranium-series dating using both rapid laser ablation and precise isotope dilution methods reveals their temporal distribution from the Last Glacial Maximum to the present day. Only one specimen of glacial age was found, while peaks in abundance occur around Heinrich Stadial 1 and the Younger Dryas, times at which ocean chemistry and food supply were likely to have presented optimal conditions for cold-water corals. A widespread regional preference of cold-water corals for UCDW over AAIW depths during the deglacial, the reverse of the modern situation, could be explained by higher dissolved oxygen concentrations and a temperature inversion that persisted into the early Holocene.
Vance D, Little SH, 2019, The History, Relevance, and Applications of the Periodic System in Geochemistry, PERIODIC TABLE I: HISTORICAL DEVELOPMENT AND ESSENTIAL FEATURES, Editors: Mingos, Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 111-156, ISBN: 978-3-030-40024-8
Little SH, Archer C, Milne A, et al., 2018, Paired dissolved and particulate phase Cu isotope distributions in the South Atlantic, Chemical Geology, Vol: 502, Pages: 29-43, ISSN: 0009-2541
Copper (Cu) is both an essential micronutrient and toxic to photosynthesizing microorganisms at low concentrations. Its dissolved vertical distribution in the oceans is unusual, being neither a nutrient-type nor scavenged-type element. This distribution is attributed to biological uptake in the surface ocean with remineralisation at depth, combined with strong organic complexation by dissolved ligands, scavenging onto particles, and benthic sedimentary input. We present coupled dissolved and particulate phase Cu isotope data along the UK-GEOTRACES South Atlantic section, alongside higher resolution dissolved and particulate phase Cu concentration measurements. Our dissolved phase isotope data contribute to an emerging picture of homogeneous deep ocean δ65Cu, at about +0.65‰ (relative to NIST SRM 976). We identify two pools of Cu in the particulate phase: a refractory, lithogenic pool, at about 0‰, and a labile pool accessed via a weak acidic leach, at about +0.4‰. These two pools are comparable to those previously observed in sediments. We observe deviations towards lighter δ65Cu values in the dissolved phase associated with local enrichments in particulate Cu concentrations along the continental slopes, and in the surface ocean. Copper isotopes are thus a sensitive indicator of localised particle-associated benthic or estuarine Cu inputs. The measurement of Cu isotopes in seawater is analytically challenging, and we call for an intercalibration exercise to better evaluate the potential impacts of UV-irradiation, storage time, and different analytical procedures.
Howarth S, Prytulak J, Little SH, et al., 2018, Thallium concentration and thallium isotope composition of lateritic terrains, Geochimica et Cosmochimica Acta, Vol: 239, Pages: 446-462, ISSN: 0016-7037
Continental weathering plays a key role in modifying the geochemical budget of terrestrial reservoirs. Laterites are the products of extreme sub-aerial continental weathering. This study presents the first investigation of thallium (Tl) abundances and stable isotopic compositions of lateritic terrains. Two laterite profiles from India of differing protolith and age are studied. Thallium concentrations range between 7 – 244 ng/g for a basalt-based lateritic profile and 37 – 652 ng/g within a greywacke lateritic profile. The average Tl stable isotope composition of the two profiles is similar to many typical igneous materials, however, the intense tropical weathering causes a small but resolvable fractionation of Tl stable isotopes towards heavy values in the residual soils. The profiles are dominated by significant positive isotope excursions (reported as ε205Tl relative to standard NBS997) of + 3.5 ± 0.5 2sd and + 6.2 ± 0.5 2sd at the inferred palaeowater tables within both laterite profiles. These signatures likely reflect combined changes in redox state and mineralogy. Extensive mineral dissolution under through-flowing fluids alongside the formation of new phases such as phyllosilicates and Mn- and Fe- oxides and hydroxides likely account for some of the Tl mobilisation, sorption and coprecipitation. In the case of laterites, the formation of the new phases and role of surface sorption likely contribute to stable Tl isotope fractionation. The identification of strong isotope excursions at inferred palaeowater tables encourages future research to determine specific mineral phases that may drive significant fractionation of Tl stable isotopes. This study showcases the magnitude of natural variation possible in terrestrial soils. Such information is key to the nascent applications of Tl isotope compositions as tracers of anthropogenic pollution.
Hayes CT, Anderson RF, Cheng H, et al., 2018, Replacement Times of a Spectrum of Elements in the North Atlantic Based on Thorium Supply, GLOBAL BIOGEOCHEMICAL CYCLES, Vol: 32, Pages: 1294-1311, ISSN: 0886-6236
Schlitzer R, Anderson RF, Dodas EM, et al., 2018, The GEOTRACES Intermediate Data Product 2017, Chemical Geology, Vol: 493, Pages: 210-223, ISSN: 0009-2541
The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the previous IDP2014. For the first time, the IDP2017 contains data for a large suite of biogeochemical parameters as well as aerosol and rain data characterising atmospheric trace element and isotope (TEI) sources. The TEI data in the IDP2017 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at crossover stations. The IDP2017 consists of two parts: (1) a compilation of digital data for more than 450 TEIs as well as standard hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing an on-line atlas that includes more than 590 section plots and 130 animated 3D scenes. The digital data are provided in several formats, including ASCII, Excel spreadsheet, netCDF, and Ocean Data View collection. Users can download the full data packages or make their own custom selections with a new on-line data extraction service. In addition to the actual data values, the IDP2017 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering and for statistical analysis. Metadata about data originators, analytical methods and original publications related to the data are linked in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2017 as section plots and rotating 3D scenes. The basin-wide 3D scenes combine data from many cruises and provide quick overviews of large-scale tracer distributions. These 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of tracer plumes near ocean margins or alo
de Souza GF, Khatiwala SP, Hain MP, et al., 2018, On the origin of the marine zinc–silicon correlation, Earth and Planetary Science Letters, Vol: 492, Pages: 22-34, ISSN: 0012-821X
The close linear correlation between the distributions of dissolved zinc (Zn) and silicon (Si) in seawater has puzzled chemical oceanographers since its discovery almost forty years ago, due to the apparent lack of a mechanism for coupling these two nutrient elements. Recent research has shown that such a correlation can be produced in an ocean model without any explicit coupling between Zn and Si, via the export of Zn-rich biogenic particles in the Southern Ocean, consistent with the observation of elevated Zn quotas in Southern Ocean diatoms. Here, we investigate the physical and biological mechanisms by which Southern Ocean uptake and export control the large-scale marine Zn distribution, using suites of sensitivity simulations in an ocean general circulation model (OGCM) and a box-model ensemble. These simulations focus on the sensitivity of the Zn distribution to the stoichiometry of Zn uptake relative to phosphate (PO4), drawing directly on observations in culture. Our analysis reveals that OGCM model variants that produce a well-defined step between relatively constant, high Zn:PO4 uptake ratios in the Southern Ocean and low Zn:PO4 ratios at lower latitudes fare best in reproducing the marine Zn–Si correlation at both the global and the regional Southern Ocean scale, suggesting the presence of distinct Zn-biogeochemical regimes in the high- and low-latitude oceans that may relate to differences in physiology, ecology or (micro-)nutrient status. Furthermore, a study of the systematics of both the box model and the OGCM reveals that regional Southern Ocean Zn uptake exerts control over the global Zn distribution via its modulation of the biogeochemical characteristics of the surface Southern Ocean. Specifically, model variants with elevated Southern Ocean Zn:PO4 uptake ratios produce near-complete Zn depletion in the Si-poor surface Subantarctic Zone, where upper-ocean water masses with key roles in the global oceanic circulation are formed. By setting th
Vance D, Archer C, Little SH, et al., 2018, The oceanic cycles of the transition metals and their isotopes (vol 36, pg 359, 2017), ACTA GEOCHIMICA, Vol: 37, Pages: 151-151, ISSN: 2096-0956
Vance D, Archer C, Little SH, et al., 2017, The oceanic cycles of the transition metals and their isotopes, Acta Geochimica, Vol: 36, Pages: 359-362, ISSN: 2096-0956
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.
Little SH, Vance D, McManus J, et al., 2017, Copper isotope signatures in modern marine sediments, Geochimica et Cosmochimica Acta, Vol: 212, Pages: 253-273, ISSN: 1872-9533
The development of metal stable isotopes as tools in paleoceanography requires a thorough understanding of their modern marine cycling. To date, no Cu isotope data has been published for modern sediments deposited under low oxygen conditions. We present data encompassing a broad spectrum of hydrographic and redox regimes, including continental margin and euxinic (sulphide-containing) settings. Taken together with previously published data from oxic settings, these data indicate that the modern oceanic sink for Cu has a surprisingly homogeneous isotopic composition of about +0.3‰ (δ65Cu, relative to NIST SRM976). We suggest that this signature reflects one of two specific water-column processes: (1) an equilibrium isotope fractionation between soluble, isotopically heavy, Cu complexed to strong organic ligands and an isotopically light pool sorbed to particles that deliver Cu to the sediment, or (2) an equilibrium isotope fractionation between the same isotopically heavy ligand-bound pool and the particle reactive free Cu2+ species, with the latter being scavenged by particulates and thereby delivered to the sediment. An output flux of about +0.3‰ into sediments is isotopically light relative to the known inputs to the ocean (at around +0.6‰) and the seawater value of +0.6 to +0.9‰, suggesting the presence of an as yet unidentified isotopically light source of Cu to the oceans. We hypothesize that this source may be hydrothermal, or may result from the partial dissolution of continentally derived particles.
Vance D, Little SH, De Souza GF, et al., 2017, Silicon and zinc biogeochemical cycles coupled through the Southern Ocean, Nature Geoscience, Vol: 10, Pages: 202-206, ISSN: 1752-0908
Zinc is vital for the physiology of oceanic phytoplankton. The striking similarity of the depth profiles of zinc to those of silicate suggests that the uptake of both elements into the opaline frustules of diatoms, and their regeneration from these frustules, should be coupled. However, the zinc content of diatom opal is negligible, and zinc is taken up into and regenerated from the organic parts of diatom cells. Thus, since opaline frustules dissolve deep in the water column while organic material is regenerated in the shallow subsurface ocean, there is little reason to expect the observed close similarity between zinc and silicate, and the dissimilarity between zinc and phosphate. Here we combine observations with simulations using a three-dimensional model of ocean circulation and biogeochemistry to show that the coupled distribution of zinc and silicate, as well as the decoupling of zinc and phosphate, can arise in the absence of mechanistic links between the uptake of zinc and silicate, and despite contrasting regeneration length scales. Our simulations indicate that the oceanic zinc distribution is, in fact, a natural result of the interaction between ocean biogeochemistry and the physical circulation through the Southern Ocean hub. Our analysis demonstrates the importance of uptake stoichiometry in controlling ocean biogeochemistry, and the utility of global-scale elemental covariation in the ocean in understanding these controls.
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