Description: Highlights • Pb concentrations and isotope ratios presented for GEOTRACES section GA06. • Northern and southern hemisphere water masses have distinct Pb isotope ratios. • Pb isotope ratios consistent with ventilation timescales of northern water masses. • Mixing complicates interpretation of Pb distributions in southern water masses. Abstract Anthropogenic emissions have dominated marine Pb sources during the past century. Here we present Pb concentrations and isotope compositions for ocean depth profiles collected in the eastern Tropical Atlantic Ocean (GEOTRACES section GA06), to trace the transfer of anthropogenic Pb into the ocean interior. Variations in Pb concentration and isotope composition were associated with changes in hydrography. Water masses ventilated in the southern hemisphere generally featured lower 206Pb/207Pb and 208Pb/207Pb ratios than those ventilated in the northern hemisphere, in accordance with Pb isotope data of historic anthropogenic Pb emissions. The distributions of Pb concentrations and isotope compositions in northern sourced waters were consistent with differences in their ventilation timescales. For example, a Pb concentration maximum at intermediate depth (600–900 m, 35 pmol kg−1) in waters sourced from the Irminger/Labrador Seas, is associated with Pb isotope compositions (206Pb/207Pb = 1.1818–1.1824, 208Pb/207Pb = 2.4472–2.4483) indicative of northern hemispheric emissions during the 1950s and 1960s close to peak leaded petrol usage, and a transit time of ∼50–60 years. In contrast, North Atlantic Deep Water (2000–4000 m water depth) featured lower Pb concentrations and isotope compositions (206Pb/207Pb = 1.1762–1.184, 208Pb/207Pb = 2.4482–2.4545) indicative of northern hemispheric emissions during the 1910s and 1930s and a transit time of ∼80–100 years. This supports the notion that transient anthropogenic Pb inputs are predominantly transferred into the ocean interior by water mass transport. However, the interpretation of Pb concentration and isotope composition distributions in terms of ventilation timescales and pathways is complicated by (1) the chemical reactivity of Pb in the ocean, and (2) mixing of waters ventilated during different time periods. The complex effects of water mass mixing on Pb distributions is particularly apparent in seawater in the Tropical Atlantic Ocean which is ventilated from the southern hemisphere. In particular, South Atlantic Central Water and Antarctic Intermediate Water were dominated by anthropogenic Pb emitted during the last 50–100 years, despite estimates of much older average ventilation ages in this region.

Description: 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 along ridges. The IDP2017 is the result of a truly international effort involving 326 researchers from 22 countries. This publication provides the critical reference for unpublished data, as well as for studies that make use of a large cross-section of data from the IDP2017.

Description: Anthropogenic emissions completely overwhelmed natural marine lead (Pb) sources during the past century, predominantly due to leaded petrol usage. Here, based on Pb isotope measurements, we reassess the importance of natural and anthropogenic Pb sources to the tropical North Atlantic following the nearly complete global cessation of leaded petrol use. Significant proportions of up to 30–50% of natural Pb, derived from mineral dust, are observed in Atlantic surface waters, reflecting the success of the global effort to reduce anthropogenic Pb emissions. The observation of mineral dust derived Pb in surface waters is governed by the elevated atmospheric mineral dust concentration of the North African dust plume and the dominance of dry deposition for the atmospheric aerosol flux to surface waters. Given these specific regional conditions, emissions from anthropogenic activities will remain the dominant global marine Pb source, even in the absence of leaded petrol combustion.

Description: Highlights: • GEOTRACES releases its first integrated and quality controlled Intermediate Data Product 2014 (IDP2014). • The IDP2014 digital data are available at http://www.bodc.ac.uk/geotraces/data/idp2014/ in 4 different formats. • The eGEOTRACES Electronic Atlas at http://egeotraces.org/ provides 329 section plots and 90 animated 3D tracer scenes. • The new 3D scenes provide geographical and bathymetric context crucial for tracer assessment and interpretation. Abstract: The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes.

Description: Mass fractionation laws relate the fractionation factor αA for one isotope ratio to the fractionation factor αB for a second isotope ratio of the same element, with a fractionation exponent β such that αA = αBβ. The exponent β defines the mass-dependence of the mass fractionation law and thus determines the slope of a mass fractionation line in linearized three isotope space. The generalized power law (GPL) defines β as a function of a variable exponent n. The laws that aim to describe equilibrium and kinetic isotope fractionations are special cases of the GPL with n = −1 and n → 0, respectively. Large isotope fractionations (up to 10% for 106Cd/114Cd) were found to accompany the evaporation of molten Cd into vacuum at about 180°C. The slopes of the fractionation lines (β-values) were obtained by analyzing the Cd isotope compositions of the evaporation residues relative to the starting material with two different multiple collector-ICPMS instruments. For the most fractionated sample, the difference between the theoretical β-values, that describe kinetic and equilibrium isotope fractionation, is 10 to 20 times larger than the measurement uncertainty. A mass-dependence with n = −0.35 was determined for this sample. This result differs significantly from the value that would be expected for simple kinetic evaporation (n → 0), which is governed by the diffusion of monatomic Cd from the melt into vacuum. The observed “non-kinetic” mass-dependence probably results from partial recondensation (back reaction) of Cd vapor into the melt phase. This interpretation requires that equilibrium evaporation of Cd at about 180°C is associated with significant isotope fractionation. The present study demonstrates that the mechanism of isotope fractionation can be investigated by studying the associated mass-dependence, which can be determined by measuring the isotope ratios of a fractionated product relative to the starting material. The quantification of mass fractionation line slopes with the GPL should aid the interpretation of mass-dependent and small mass-independent isotope effects.

Description: Stable isotope compositions can potentially be used to trace atmospheric Cd inputs to the surface ocean and anthropogenic Cd emissions to the atmosphere. Both of these applications may provide valuable insights into the effects of anthropogenic activities on the cycling of Cd in the environment. However, a lack of constraints for the Cd isotope compositions of atmospheric aerosols is currently hindering such studies. Here, we present stable Cd isotope data for aerosols collected over the Tropical Atlantic Ocean. The samples feature variable proportions of mineral dust-derived and anthropogenic Cd, yet exhibit similar isotope compositions, thus negating the distinction of these Cd sources using isotopic signatures in this region. Isotopic variability between these two atmospheric Cd sources may be identified in other areas, and thus warrants further investigation. Regardless, these data provide important initial constraints on the isotope composition of atmospheric Cd inputs to the ocean.

Description: Over the past decades, observations have confirmed decreasing oxygen levels and shoaling of oxygen minimum zones (OMZs) in the tropical oceans. Such changes impact the biogeochemical cycling of micronutrients such as Cd, but the potential consequences are only poorly constrained. Here, we present seawater Cd concentrations and isotope compositions for 12 depth profiles at coastal, nearshore and offshore stations from 4◦S to 14◦S in the eastern tropical South Pacific, where one of the world’s strongest OMZs prevails. The depth profiles of Cd isotopes display high δ114/110Cd at the surface and decreasing δ114/110Cd with increasing water depth, consistent with preferential utilization of lighter Cd isotopes during biological uptake in the euphotic zone and subsequent remineralization of the sinking biomass. In the surface and subsurface ocean, seawater displays similar δ114/110Cd signatures of 0.47 ± 0.23‰ to 0.82 ± 0.05‰ across the entire eastern tropical South Pacific despite highly variable Cd concentrations between 0.01 and 0.84 nmol/kg. This observation, best explained by an open system steady-state fractionation model, contrasts with previous studies of the South Atlantic and South Pacific Oceans, where only Cd-deficient waters have a relatively constant Cd isotope signature. For the subsurface to about 500 m depth, the variability of seawater Cd isotope compositions can be modeled by mixing of remineralized Cd with subsurface water from the base of the mixed layer. In the intermediate and deep eastern tropical South Pacific (〉500 m), seawater [Cd] and δ114/110Cd appear to follow the distribution and mixing of major water masses. We identified modified AAIW of the ETSP to be more enriched in [Cd] than AAIW from the source region, whilst both water masses have similar δ114/110Cd. A mass balance estimate thus constrains a δ114/110Cd of between 0.38‰ and 0.56‰ for the accumulated remineralized Cd in the ETSP. Nearly all samples show a tight coupling of Cd and PO4 concentrations, whereby surface and deeper waters define two distinct linear trends. However, seawater at a coastal station located within a pronounced plume of H2S, is depleted in [Cd] and features significantly higher δ114/110Cd. This signature is attributed to the formation of authigenic CdS with preferential incorporation of lighter Cd isotopes. The process follows a Rayleigh fractionation model with a fractionation factor of α114/110Cdseawater-CdS = 1.00029. Further deviations from the deep Cd–PO4 trend were observed for samples with O2 〈 10 μmol/kg and are best explained by in situ CdS precipitation within the decaying organic matter even though dissolved H2S was not detectable in ambient seawater.

Description: This contribution aims to report the reflections we had with the scientific community during two international workshops on reference materials for stable isotopes in Davos (2002) and Nice (2003). After evaluating the isotopic homogeneity of some existing reference materials, based on either certificates, literature data or specific inter-laboratory rounds, we confirm these as primary reference materials or propose new ones relative to which stable isotope compositions should be reported. We propose DSM-3 for Mg, NIST SRM 915a for Ca, L-SVEC for Li and NBS28 for Si. Cadmium does not yet have a well identified delta zero material, although three commercial mono-elemental Cd solutions have yielded the same isotopic composition relative to one another. In order to scale the linearity of any mass spectrometer, some secondary reference materials are also proposed: Cambridge-1 solution for Mg, the “Münster-Cd” and JEPPIM Cd solutions for Cd and the “Big Batch” silicate for Si. The team from Nancy propose to prepare a mixed spike solution for Li isotopes. Well-characterised natural samples such as ocean or continental waters, diatoms, sponges, rocks and minerals are needed to validate the entire analytical procedure, particularly to take into account the effect of sample mineralisation and of chemical manipulations for elemental separation prior to analysis.

Description: Neodymium isotopes have been used to show that the large instrumental mass discrimination of plasma source mass spectrometers cannot be accurately corrected with the common mass fractionation laws. The present study investigates the application of the generalised power law (GPL) for mass bias correction. To this end, the Nd isotope ratio data acquired with two Nu Plasma multiple collector ICP-MS instruments were compared with published Nd reference values determined by thermal ionisation mass spectrometry. The GPL applies a variable, n, such that it can be used to describe different mass-dependencies. In particular, the GPL is equivalent to the power law for n = 1 and approaches the exponential law for n → 0. The exponential and power law corrected data displayed significant (〉100 ppm) deviations from the reference values for some Nd isotope ratios, whereas the instrumental mass discrimination was accurately described by the GPL. For three datasets acquired under optimal operating conditions this was achieved with n ≈ −0.23, but during routine operation of the instrument the n-values were observed to vary between −0.2 and −0.4. This shows that the GPL is a valuable tool for data evaluation in isotope ratio mass spectrometry and for the characterisation of instrumental mass-dependencies. The observed mass-dependencies are in accord with isotope fractionation by kinetic transport processes, if some of the mass discrimination is governed by reduced masses.

Description: Notations for Cd stable isotope compositions and the use of reference materials are discussed. It is proposed that Cd stable isotope data should be reported as variations of the 114Cd/110Cd ratio/ using either the δ114/110Cd or ε114/110Cd notations. Future publications should report results for BAM-1012 Cd, the only currently available international Cd isotope reference material. It is also recommended that “Münster Cd” and a range of specified geological reference materials are used as additional reference materials. The final choice of a primary “zero-delta” reference standard remains the most important outstanding question.