CD Budget Research Articles

Controls on the cadmium isotope composition of modern marine sediments

Continental margin sediments have been identified as the dominant sink in the marine budget of cadmium (Cd). The isotopic composition of this important output flux is, however, unknown. Here we present, with measurements on the Argentine continental margin, the first observational constraints on the isotopic composition of Cd in modern marine oxic and sub-oxic sediments. We identify two main removal mechanisms of Cd; in organic material, and by sulfide formation. Surface margin sediments (0–0.5 cm), with dissolved O2 below detection from ∼0.5 cm, are isotopically lighter than overlying oxygenated waters. A mass balance for these surface sediments indicates that Cd is present dominantly as organically-bound particulate Cd. In sub-surface sediments, Cd concentrations increase in the zone of nitrate reduction, and attain similar isotopic compositions as the water that overlies the sediment (i.e. ∼0.35‰ in deep waters). These observations are consistent with a downward diffusive flux of seawater Cd and redox-driven quantitative removal of that Cd during sulfide precipitation. In combination, these two routes of Cd removal lead to burial of isotopically light organic Cd in margin sub-oxic sediments that enables the global isotopic Cd budget to be balanced.

Limited impact of eolian and riverine sources on the biogeochemical cycling of Cd in the tropical Atlantic

We present coupled cadmium (Cd) concentrations and Cd isotopic compositions (expressed as δ114Cd) in seawater profiles along the northeast-southwest GEOTRACES GA11 section in the tropical Atlantic Ocean. The GA11 section encompasses three contrasting regions including the Amazon River plume, the North African dust plume, and the Guinea Dome oxygen deficient zone (ODZ). Given the long oceanic residence time of Cd (104 to 105 yr), local inputs such as atmospheric and riverine sources are generally considered to be of little importance for the open ocean Cd budget, and the limited Cd isotope dataset available thus far has prevented any unambiguous conclusions on the importance of these processes. The GA11 section is ideally located for assessing the influence of external, natural and anthropogenic riverine and eolian inputs, as well as internal processes, on the Cd mass balance in the tropical Atlantic Ocean. As in previous Cd isotope studies, this dataset documents that both surface consumption–regeneration of micronutrients within the water column and deep water mass mixing exert the prime control on the cycling of Cd. However, we do observe some near-surface samples with lower-than-expected δ114Cd signatures along the western section of the transect. Surface waters sampled at the margin of the Amazon freshwater plume show no resolvable difference in [Cd] and δ114Cd compared to waters outside of the plume, suggesting that the Amazon River is not an important source of Cd to the open ocean. Although the eastern GA11 transect is directly downwind of the Saharan dust plume, atmospheric Cd deposition accounts for <1% of the inferred upwelling flux, indicating that atmospheric inputs to the surface North Atlantic Ocean, even within the main Saharan dust plume, only have a relatively minor influence on the Cd budget. In the subsurface tropical Atlantic (100–200 m water depth), there is a clear deviation from a tight linear Cd-PO4 correlation toward lower Cd concentrations for a given PO4 concentration. Our new Cd data show that this Cd-PO4 decoupling is likely a feature of the entire tropical Atlantic and may possibly reflect PO4 enrichment via preferential re-mineralization of organically-bound phosphorus. Alternatively, the decoupling may arise from subsurface Cd depletion caused by precipitation of Cd sulphide within sinking organic particulate microenvironments, as has been suggested in recent studies.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Cadmium distribution in Pb-Zn slags from Upper Silesia, Poland: Implications for cadmium mobility from slag phases to the environment

Cadmium is a toxic element, generally highly mobile and easily transferred from Cd-bearing ore minerals to the environment as shown by numerous leaching experiments and studies of biota. However, the behaviour of Cd is less understood during smelting activities, when it is transferred from Cd-bearing ore to the metal as well as to the smelting by-product: slag. Slag material, with Cd concentrations over 500mg/kg, occurs at an abandoned slag pile after Pb-Zn metal smelting in Swiętochłowice (Upper Silesia). Such material, extremely enriched in Cd (15mg/kg is a legislative guideline for industrial soils), offers a possibility to study the distribution of Cd within slag fragments composed of silicates and oxides, gaining insight into Cd mobility. Electron microprobe analyses of primary slag show that Cd partitioning between different phases is not controlled by site preference and Cd does not substitute for elements with similar radius, such as Zn in Zn-spinel or Ca in Pb-hardystonite. Cadmium is strongly incompatible and enriched in late melts that, in the case of the studied slag, crystallize Pb-hardystonite rims, late Pb-silicates (probably alamosite) and rare phosphates. These phases contain most of the Cd budget, but in a small volume (5–10μm phases). As such, Cd behaviour and distribution in silicate slags may contrast to that of other potentially toxic elements, which are generally selectively partitioned into one or more major phases. The study of another slag fragment, composed of secondary Fe-(hydro)oxides, formed after sulphide weathering, shows low Cd contents suggesting that either Cd was mobilized from the sulphide during weathering or lost in the smelting process. The concentration of Cd within slag in parts easily accessible to fluids and weathering (enrichment at rims and in late phases occurring at grain boundaries) and not in secondary phases, suggests that Cd has a high potential mobility from the slag to the environment.

High- and low-affinity binding sites for Cd on the bacterial cell walls of Bacillus subtilis and Shewanella oneidensis

Bulk Cd adsorption isotherm experiments, thermodynamic equilibrium modeling, and Cd K edge EXAFS were used to constrain the mechanisms of proton and Cd adsorption to bacterial cells of the commonly occurring Gram-positive and Gram-negative bacteria, Bacillus subtilis and Shewanella oneidensis, respectively. Potentiometric titrations were used to characterize the functional group reactivity of the S. oneidensis cells, and we model the titration data using the same type of non-electrostatic surface complexation approach as was applied to titrations of B. subtilis suspensions by Fein et al. (2005). Similar to the results for B. subtilis, the S. oneidensis cells exhibit buffering behavior from approximately pH 3–9 that requires the presence of four distinct sites, with p K a values of 3.3 ± 0.2, 4.8 ± 0.2, 6.7 ± 0.4, and 9.4 ± 0.5, and site concentrations of 8.9(±2.6) × 10 −5, 1.3(±0.2) × 10 −4, 5.9(±3.3) × 10 −5, and 1.1(±0.6) × 10 −4 moles/g bacteria (wet mass), respectively. The bulk Cd isotherm adsorption data for both species, conducted at pH 5.9 as a function of Cd concentration at a fixed biomass concentration, were best modeled by reactions with a Cd:site stoichiometry of 1:1. EXAFS data were collected for both bacterial species as a function of Cd concentration at pH 5.9 and 10 g/L bacteria. The EXAFS results show that the same types of binding sites are responsible for Cd sorption to both bacterial species at all Cd loadings tested (1–200 ppm). Carboxyl sites are responsible for the binding at intermediate Cd loadings. Phosphoryl ligands are more important than carboxyl ligands for Cd binding at high Cd loadings. For the lowest Cd loadings studied here, a sulfhydryl site was found to dominate the bound Cd budgets for both species, in addition to the carboxyl and phosphoryl sites that dominate the higher loadings. The EXAFS results suggest that both Gram-positive and Gram-negative bacterial cell walls have a low concentration of very high-affinity sulfhydryl sites which become masked by the more abundant carboxyl and phosphoryl sites at higher metal:bacteria ratios. This study demonstrates that metal loading plays a vital role in determining the important metal-binding reactions that occur on bacterial cell walls, and that high affinity, low-density sites can be revealed by spectroscopy of biomass samples. Such sites may control the fate and transport of metals in realistic geologic settings, where metal concentrations are low.

Distribution of protons and Cd between bacterial surfaces and dissolved humic substances determined through chemical equilibrium modeling

Bacteria and dissolved humic substances are capable of binding significant concentrations of metals in natural environments. Recent advances in understanding bacteria-metal and humic-metal complexation have provided a framework for directly comparing the binding capacities of these components. In this study, we use chemical equilibrium modeling to construct an internally consistent set of thermodynamic equilibrium constants for proton and Cd binding onto dissolved humic substances, using a variety of published data sets. Our modeling approach allows for the direct comparison of humic substance binding constants and site densities to those previously published for proton and Cd binding onto natural consortia of bacteria. We then combine these constants into a unified model that accounts for the competition between bacterial surfaces and humic and fulvic acids in order to determine the relative importance of each component on the total Cd budget. The combined model is used to examine the relative contributions of bacteria and dissolved humic substances to Cd complexation in natural settings. Calculations are performed for three representative systems: (1) one with a maximum realistic concentration of bacteria and a minimum realistic concentration of humic substance, (2) one with a maximum realistic concentration of humic substance and a minimum concentration of bacteria, and (3) one with an intermediate concentration of both components. Our modeling results indicate that dissolved humic substances have 2 orders of magnitude more available binding sites than bacterial surfaces (per gram). Humic substances also have a greater affinity than bacterial surfaces for binding Cd over circumneutral pH ranges. The combined model results demonstrate that, depending upon their relative concentrations, both Cd-humic and Cd-bacteria complexes are capable of dominating Cd-speciation in specific natural environments. This modeling approach is useful in that it can easily be extended to include other metals and binding ligands; however, thermodynamic data must be gathered on additional components to facilitate the modeling of more realistic systems.

Influence of resist process on proximity bias

When linewidths in optical lithography are reduced, proximity bias, defined here as isolated to dense linewidth offsets, can consume large parts of the CD budget. This paper describes the resist thickness influence on proximity bias. If reflective substrates are used, proximity bias shows a cyclic behaviour. This can be seen as secondary swing caused by the spatial location of the remnant standing wave which results in photo active compound (PAC) concentration differences near the surface after diffusion. PAC acts as a dissolution barrier and influences the CD, resulting in different proximity biases for different resist thicknesses. When low-reflective substrates are used, as is the case with bottom anti-reflective coatings (BARCs), standing waves are less pronounced. On BARC substrates, a constant proximity bias was found, both in i-line and deep UV.

Time series for dissolved cadmium at a coastal station in the Western Baltic Sea

Abstract Concentrations of dissolved Cd, Cd(diss), were measured weekly from June 1991 to June 1994 at a coastal station in the western Baltic Sea. The mean concentration of 204 pmol/dm 3 is about 50% higher than in open Baltic Sea surface waters. A distinct seasonal cycle was observed with elevated concentrations in winter and spring (272 pmol/dm 3 ) and lower values in summer and autumn (131 pmol/dm 3 ). Relating the seasonal changes in Cd(diss) to the nutrient cycle revealed Δ Cd(diss) Δ NO 3 and Δ Cd(diss) Δ PO 4 ratios which are consistent with other measurements and seem to confirm the concept of a nutrient-like biogeochemistry of Cd. However, a time shift of two to three months exists between the depletion of nutrients in spring and the depletion of Cd(diss). Possibly, this indicates a decoupling of Cd(diss) from nutrients during the spring plankton bloom. However, no final conclusions can be drawn yet. Cd(diss) concentrations decreased significantly during the three year measurement period, whereas nitrate concentrations increased. A possible linkage between eutrophication and the Cd budget of the Baltic Sea is discussed.

Glacial enrichments of authigenic Cd And U in subantarctic sediments: A climatic control on the elements' oceanic budget?

We examine the possibility that glacial increase in the areal extent of reducing sediments might have changed the oceanic Cd inventory, thereby decoupling Cd from PO4. We suggest that the precipitation of Cd‐sulfide in suboxic sediments is the single largest sink in the oceanic Cd budget and that the accumulation of authigenic Cd and U is tightly coupled to the organic carbon flux into the seafloor. Sediments from the Subantarctic Ocean and the Cape Basin (South Atlantic), where oxic conditions currently prevail, show high accumulation rates of authigenic Cd and U during glacial intervals associated with increased accumulation of organic carbon. These elemental enrichments attest to more reducing conditions in glacial sediments in response to an increased flux of organic carbon. A third core, overlain by Circumpolar Deep Water (CPDW) as are the other two cores but located south of the Antarctic Polar Front, shows an approximately inverse pattern to the Subantarctic record. The contrasting patterns to the north and south of the Antarctic Polar Front suggest that higher accumulation rates of Cd and U in Subantarctic sediments were driven primarily by increased productivity. This proposal is consistent with the hypothesis of glacial stage northward migration of the Antarctic Polar Front and its associated belt of high siliceous productivity. However, the increase in authigenic Cd and U glacial accumulation rates is higher than expected simply from a northward shift of the polar fronts, suggesting greater partitioning of organic carbon into the sediments during glacial intervals. Lower oxygen content of CPDW and higher organic carbon to biogenic silica rain rate ratio during glacial stages are possible causes. Higher glacial productivity in the Cape Basin record very likely reflects enhanced coastal up welling in response to increased wind speeds. We suggest that higher productivity might have doubled the areal extent of suboxic sediments during the last glacial maximum. However, our calculations suggest low sensitivity of seawater Cd concentrations to glacial doubling of the extent of reducing sediments. The model suggests that during the last 250 kyr seawater Cd concentrations fluctuated only slightly, between high levels (about 0.66 nmol kg−1) on glacial initiations and reaching lowest values (about 0.57 nmol kg−1) during glacial maxima. The estimated 5% lower Cd content at the last glacial maximum relative to modern levels (0.60 nmol kg−1) cannot explain the discordance between Cd and δ13C, such as observed in the Southern Ocean. This low sensitivity is consistent with foraminiferal data, suggesting minimal change in the glacial Cd mean oceanic content.

Sensitivity of the phosphate‐cadmium‐carbon isotope relation in the ocean to cadmium removal by suboxic sediments

Reconstructions of past deep ocean nutrient concentrations rely largely on the carbon isotopic composition and cadmium content of shells of benthic foraminifera. When comparing records of these two paleonutrient tracers, a relation similar to the phosphate‐cadmium‐carbon isotope relation in the water column today is generally assumed. We present new water column, sediment, and pore water data from the California continental margin which demonstrate that Cd accumulation is enhanced in suboxic sediments. These results suggest that the oceanic Cd inventory is sensitive to changes in the areal extent and redox intensity of suboxic sediments and thus that the phosphate‐cadmium relation in the ocean could have been different in the past. The magnitude of this effect and its evolution through time is presently hard to quantify because the oceanic Cd budget is poorly constrained.

Precipitation and Post-Depositional Mobility of Cadmium in Reducing Sediments: Implications for the Cd Oceanic Budget

Reconstructions of past nutrient distribution and deep ocean circulation patterns are typically inferred from the 513C and Cd composition of benthic foraminifera from deep sea sediments; the modern oceanic distribution of 5 t3C and Cd reflect the biological cycling of nutrients (e.g. PO4) as modified by deep ocean circulation. This correlation between Cd and PO4 is the basis for using Cd, as recorded in foraminiferal shells, for reconstructing changes in deepwater circulation patterns (Boyle, 1988). Because of the relatively long residence time of Cd and PO4 (100,000-200,000 years) glacial-interglacial changes in Cd can be interpreted as reflecting variations in nutrients distribution. However, Cd and PO4 are controlled by different diagenetic processes. The strong tendency of Cd to form highly insoluble sulphide phases in the presence of HES explains its substantial enrichment in sulphide-bearing sediments as already was noted by Goldschmidt (1958). In recent studies we have investigated the possibility that temporal changes in the areal extent of reducing sediments may affect the Cd oceanic budget thus leading to de-coupling of Cd from the nutrient cycle (Rosenthal et al., 1993). The current work focuses on the precipitation of authigenic Cd in suboxic sediments and its mobility in response to post-depositional reoxidation of reducing sediments. The implications of these processes for the global Cd budget and for paleoceanographic interpretation are discussed below.