Cd In Seawater Research Articles

Automated preconcentration of Fe, Zn, Cu, Ni, Cd, Pb, Co, and Mn in seawater with analysis using high-resolution sector field inductively-coupled plasma mass spectrometry

A rapid, automated, high-throughput analytical method capable of simultaneous analysis of multiple elements at trace and ultratrace levels is required to investigate the biogeochemical cycle of trace metals in the ocean. Here we present an analytical approach which uses a commercially available automated preconcentration device (SeaFAST) with accurate volume loading and in-line pH buffering of the sample prior to loading onto a chelating resin (WAKO) and subsequent simultaneous analysis of iron (Fe), zinc (Zn), copper (Cu), nickel (Ni), cadmium (Cd), lead (Pb), cobalt (Co) and manganese (Mn) by high-resolution inductively-coupled plasma mass spectrometry (HR-ICP-MS). Quantification of sample concentration was undertaken using isotope dilution for Fe, Zn, Cu, Ni, Cd and Pb, and standard addition for Co and Mn. The chelating resin is shown to have a high affinity for all analyzed elements, with recoveries between 83 and 100% for all elements, except Mn (60%) and Ni (48%), and showed higher recoveries for Ni, Cd, Pb, Co and Mn in direct comparison to an alternative resin (NOBIAS Chelate-PA1). The reduced recoveries for Ni and Mn using the WAKO resin did not affect the quantification accuracy. A relatively constant retention efficiency on the resin over a broad pH range (pH 5–8) was observed for the trace metals, except for Mn. Mn quantification using standard addition required accurate sample pH adjustment with optimal recoveries at pH 7.5 ± 0.3. UV digestion was necessary to increase recovery of Co and Cu in seawater by 15.6% and 11.4%, respectively, and achieved full break-down of spiked Co-containing vitamin B12 complexes. Low blank levels and detection limits could be achieved (e.g., 0.029 nmol L−1 for Fe and 0.028 nmol L−1 for Zn) with the use of high purity reagents. Precision and accuracy were assessed using SAFe S, D1, and D2 reference seawaters, and results were in good agreement with available consensus values. The presented method is ideal for high throughput simultaneous analysis of trace elements in coastal and oceanic seawaters. We present a successful application of the analytical method to samples collected in June 2014 in the Northeast Atlantic Ocean.

A graphene oxide based poly(2,6-diaminopyridine) composite for solid-phase extraction of Cd(II) prior to its determination by FAAS

The authors describe the preparation of a graphene oxide based poly(2,6-diaminopyridine) composite by in-situ polymerization. The material was characterized by Fourier transform infrared spectrometry, powder X-ray diffraction and scanning electron microscopy. The composite was placed in a column and used for solid phase extraction (SPE) of Cd(II) ions from water samples. The adsorbed Cd(II) was then eluted with HCl and injected into a flame atomic absorption spectrometer. The effects of pH values, volume and flow rate of sample, and the kind, volume and concentration of eluent were optimized. The calibration plot for the assay is linear in the 2 to 100 μg.L−1 Cd(II) concentration range. Other figures of merit include (a) a 0.47 μg.L−1 limit of detection, (b) a relative standard deviation of 1.6% (for n = 7; at 20 μg.L−1 of Cd(II)), and (c) a preconcentration factor of 50. The method was validated by analyzing the certified reference material NIST SRM 1643f and spiked real samples. The method was successfully employed to the determination of Cd(II) in tap, river, seawater and wastewater samples.

Distribution and stoichiometry of Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in seawater around the Juan de Fuca Ridge

A central theme of the ongoing GEOTRACES program is to improve the understanding of processes occurring at ocean interfaces with continents, sediments, and ocean crust. In this context, we studied the distributions of Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb around the Juan de Fuca Ridge (JdFR) in total dissolvable (td), dissolved (d), and labile particulate (lp) fractions, which represent a fraction in unfiltered samples, filtered samples through an AcroPak capsule filter, and the difference between td and d, respectively. Al and Fe were dominated by lp-species, while Ni, Zn, and Cd were dominated by d-species with undetectable amounts of lp-species. Major findings in this study are as follows: (1) The continental margin (CM) provided large sources of Al, Mn, Fe, and Co from the surface to ~2000 m in depth. The supply from CM caused high surface concentrations of dMn and dCo, a subsurface (100–300 m depth) maximum of dCo, and intermediate (500–2000 m depth) maxima of lpAl and lpFe. The supply of dFe from CM was ~10 times that from the high-temperature hydrothermal activity at station BD21, which is located at ~3 km from the Middle Valley venting site and ~ 200 km from Vancouver Island. (2) DPb was maximum at the top layer of North Pacific Intermediate Water, probably owing to isopycnal transport of anthropogenic Pb via advection of subducted surface waters. Although dCo and dPb had different sources in the upper water, they showed a strong linearity below 300 m (r 2 = 0.95, n = 38), indicating concurrent scavenging. (3) A high-temperature hydrothermal plume occurred at a depth of 2300 m at BD21, accounting for maxima of dAl, dMn, dFe, lpCu, and lpPb and a minimum of dCu. (4) Strong bottom maxima of lpAl, lpMn, lpFe, lpCo, and lpPb occurred above the abyssal plain at the western foot of the JdFR, indicating resuspension of sediments. However, bottom maxima of d-species were apparent only for dAl and dCu.

Exploration of the phycoremediation potential of Laminaria digitata towards diflubenzuron, lindane, copper and cadmium in a multitrophic pilot-scale experiment

The presence of contaminants in aquatic ecosystems can cause serious problems to the environment and marine organisms. This study aims to evaluate the phycoremediation capacity of macroalgae Laminaria digitata for pesticides (diflubenzuron and lindane) and toxic elements (cadmium and copper) in seawater in the presence or absence of mussels. The photosynthetic activity was monitored in the macroalgae to assess its “physiological status”. The results showed that the presence of algae decreased diflubenzuron concentration in mussels by 70% after 120 h of exposure. Additionally, this macroalgae was efficient to reduce lindane, Cu and Cd in seawater; even though it not was able to reduce these contaminants in mussels. The studied pollutants did not affect the physiological status of macroalgae. This study reveals that the application of phycoremediation with macroalgae can be an useful and effective mitigation strategy to remove/decrease contaminant levels from the aquatic environment.

Determination of Traces of Pb, Cu and Cd in Seawater around Thane Creek by Anodic Stripping Voltammetry Method.

Trace metals (Pb, Cu and Cd) in seawater samples were pre-concentrated for the simultaneous quantitative determination using solvent extraction procedure in the presence of ammonium pyrrolidine dithiocarbamate (APDC) as complexing agent and methyl isobutyl ketone (MIBK) as organic phase and acid exchange back - extraction followed by its determination by Differential Pulse Anodic Stripping Voltammetry (DPASV). Concentrations of Pb, Cu and Cd vary from 3.0 to 9.44ng/mL, 12.7 to 28.6ng/mL and 0.31 to 1.21ng/mL respectively in seawater samples collected from different locations across Thane Creek area, Mumbai, India. Pb, Cu, Cd concentration were observed to be higher in the eastern side of the Thane Creek as it is covered by various industries.

Direct Determination of Cadmium in Seawater by Standard Addition ICP-QMS/QMS with an ORC.

A method for direct and precise determination of Cd in seawater by ICP-QMS/QMS was developed, where a high matrix introducing unit permitted the direct introduction of seawater without clogging the cones while the matrix effect of seawater was canceled by standard addition. NH3 gas was selected as the reaction gas for removing the MoO interferences with the measurement of Cd isotopes, effectively improving the signal to background ratio. The lower limit of detection obtained at 114Cd was 0.0002 ng mL-1, which is enough for the determination of Cd in natural seawater. The validity of the present method was confirmed by the analysis of two certified reference materials, i.e. NASS-5 and NASS-6, whose observed values of Cd concentration agreed with the certified values. The analytical figures of merit of the present method were comparable to or better than those reported for direct analysis of seawater.

Transcriptomic change as evidence for cadmium-induced endocrine disruption in marine fish model of medaka, Oryzias javanicus

We evaluated cadmium (Cd)-induced acute toxicity in Oryzias javanicus (marine medaka or Javanese ricefish) and gathered transcriptomic evidence for the Cd-induced endocrine-disrupting effect. The median lethal concentrations for the fish were determined to be 44.25 and 27.80 mg/L after exposure to Cd in seawater for 24 and 48 h, respectively, and 2.84, 1.61, and 1.20 mg/L after exposure in freshwater for 24, 48, and 72 h, respectively. The differences in the bioavailability and activity of free Cd2+ caused by the salt concentration in seawater could explain these dramatic differences in the toxicity of Cd between marine and fresh water system. The genes differentially expressed in O. javanicus liver tissue after exposure to 280 μg/L CdCl2 for 48 h were profiled with a customized marine medaka cDNA microarray (HazChem Fish Array). We identified 204 differentially expressed genes; the expression of 66 genes was upregulated and that of 138 genes was downregulated (P<0.05). The total 31 genes were commonly expressed in fish exposed to Cd and two references of environmental disruptor (bisphenol A, or 17β-estradiol). These genes were used to predict the changes that occur in metabolic pathways and processes in response to Cd exposure. The database for annotation, visualization and integrated discovery (DAVID) was used for functional analysis for the differentially expressed genes. Significant changes were predicted in the steroid hormone and estrogen stimulus response, vitellogenin expression, sterol and cholesterol metabolic processes, lipid transport activity, defense response, innate immune response, and metal ion binding activity. These results extend our knowledge of the toxicity of Cd at the molecular level and indicate that Cd exposure causes endocrine disruption in aquatic organisms.

Novel Imprinted Polymer for the Preconcentration of Cadmium with Determination by Inductively Coupled Plasma Mass Spectrometry

ABSTRACTA novel Cd(II)-imprinted polymer was prepared with chemical immobilization using N-methacryloyl-L-Histidine as a vinylated chelating agent for online solid-phase extraction of Cd(II) for determination by inductively coupled plasma mass spectrometry. The Cd(II)–monomer complex was synthesized and copolymerized through bulk polymerization method in the presence of ethyleneglycoldimethacrylate cross-linker. The resulting polymer was leached with 1.0 mol L−1 HNO3 to generate the cavities in the polymer for Cd(II) ions. The experimental conditions, including load pH, solution flow rate, and eluent concentration for effective sorption of Cd(II), were optimized using a minicolumn of the imprinted polymer. A volume of 5.0 mL sample 5 µg L−1 Cd(II) solution at pH 6.5 was loaded on the column at 2.0 mL min−1 using a sequential injection system followed by elution with 1.0 mL of 0.75 mol L−1 HNO3. The relative selectivity coefficients of the imprinted polymer for Cd(II) were 38.5, 3.5, 3.0, 2.5, and 6.0 in the presence of Cu(II), Ni(II), Zn(II), Co(II), and Pb(II), respectively. Computational calculations revealed that the selectivity of the imprinted polymer was mediated by the stability of Cd(II)–N-methacryloyl-L-Histidine complex which was more stable than commonly used monomers including 4-vinyl pyridine, methacrylic acid, and vinylimidazole. The detection limit and relative standard deviation were 0.004 µg L−1 and 3.2%, respectively. The method was validated by the analysis of seawater certified reference material (CASS-4) and successfully used for the determination of Cd(II) in coastal seawater and estuarine water.

ICP-MSを用いる海水中ng L&lt;sup&gt;－1&lt;/sup&gt;レベルのCd定量分析：キレート樹脂濃縮分離法におけるMoの選択的除去

誘導結合プラズマ質量分析法（ICP-MS）で海水中ng L－1レベルのCdを定量する際に問題となるMoOの干渉を低減するために，キレート樹脂濃縮分離法におけるMoの選択的除去法を確立した．イミノ二酢酸を官能基に有するキレート樹脂固相カラムを使用し，海水試料に容積比で過酸化水素を2％ 添加することで，濃縮液中のMo濃度を0.1 μg L－1以下に低減することができた．その結果，ICP-MSによるCdの分析においてMoOによる多原子イオン干渉を完全に除去することが可能となった．本法を用いてpH 5.0において海水試料の25倍濃縮を実施した場合の添加回収率は98.6±1.0％（n＝4），分析検出限界は0.06 ng L－1であり，非汚染海域のCdの分析に十分適用可能なレベルであった．また，Fe, Ni, Cu, Zn, Pb, 希土類元素等の同時濃縮も可能であることが分かった．本法の有効性は，National Research Council Canada（NRCC）より頒布されている海水認証標準物質の分析により確認した．また，実際試料としてサンゴ礁海域の海水試料の分析に適用し，数ng L－1レベルのCdを精度よく定量することができた．

Cadmium regeneration within the North Atlantic

AbstractCadmium (Cd) incorporated into the benthic microfossils has been widely used in reconstructing the past water circulation in the North Atlantic. This requires a major control by conservative mixing over regeneration process on the seawater cadmium distribution in the North Atlantic. Through coupling the recently reported Cd data at depths below 300 m with quantitative water mass analysis along the GA03 transect, we tested two models for cadmium cycling within the North Atlantic: conservative mixing alone and conservative mixing plus regeneration which is termed as regenerative mixing. The results show that the regenerative mixing model reproduces the observations (slope = 0.99 and R2 = 0.97) much better than the conservative mixing model (slope = 0.99 and R2 = 0.88). The regenerative mixing model was applied to estimate the amount of dissolved cadmium regenerated “within” the North Atlantic. This regionally regenerated cadmium contributes to ~10% and &gt;50% of the total cadmium in the North Atlantic at depths &gt;1000 m and 300–1000 m, respectively, indicating that the microfossil‐reconstructed seawater Cd can be attributed to the mixing from the South Atlantic to the North Atlantic with high accuracy for depths below 1000 m, while within 300–1000 m cadmium is more controlled by the regeneration process than the mixing process, complicating the attribution of the microfossil‐reconstructed seawater Cd to the changes in water mass geometry within this 300–1000 m depth range. A regionally and vertically constant Cd/PO43− regeneration ratio ~262 pM/µmol/kg was derived from the regenerative mixing model which is comparable with ratios estimated in the recent studies for the North Atlantic.

Genetic Algorithm-Based Fuzzy Comprehensive Evaluation of Water Quality in Dongzhaigang

The concentrations of dissolved inorganic nitrogen (DIN; NO2−–N, NH3–N, and NO3−–N), PO43−–P, dissolved oxygen (DO), chemical oxygen demand (COD), five-day biological oxygen demand (BOD5), oil, Si, and seven heavy metals (Hg, Cr, Cu, As, Zn, Pb, and Cd) in seawater from the Dongzhaigang National Mangrove Nature Reserve of China in 2013 were determined. Except for the concentrations of the COD, BOD5, Cr, Hg, Cu, As, and Cd, each index in seawater were found to be over the limits of I-Class seawater standards. The index of organic pollution showed that the pollution level in this study area reached level 6; eutrophication levels indicated that the nutritional level reached level 4. According to the water quality index model, the sea area was slightly polluted by heavy metals. In a genetic algorithm-based fuzzy comprehensive evaluation, the results for organic pollutants, nutrients, and heavy metal pollution can be combined to evaluate the water quality as a whole. Results showed that the sea area in Dongzhaigang did not have a healthy water environment, but was seriously polluted by organic pollutants and nutrients.

Toxic Effect of Cadmium Assay in Contaminated Soil Earthworm Cell Using Modified Sensor.

A voltammetric toxic metal of cadmium detection was studied using a fluorine doped graphite pencil electrode (FPE) in a seawater electrolyte. In this study, square wave (SW) stripping and chronoamerometry were used for determination of Cd(II) in seawater. Affordable pencils and an auxiliary electrode were used as reference. All experiments in this study could be performed at reasonable cost by using graphite pencil. The application was performed on the tissue of contaminated soil earthworm. The results show that the method can be applicable for vegetables and in vivo fluid or medicinal diagnosis.

An off-line automated preconcentration system with ethylenediaminetriacetate chelating resin for the determination of trace metals in seawater by high-resolution inductively coupled plasma mass spectrometry

A novel automated off-line preconcentration system for trace metals (Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) in seawater was developed by improving a commercially available solid-phase extraction system SPE-100 (Hiranuma Sangyo). The utilized chelating resin was NOBIAS Chelate-PA1 (Hitachi High-Technologies) with ethylenediaminetriacetic acid and iminodiacetic acid functional groups. Parts of the 8-way valve made of alumina and zirconia in the original SPE-100 system were replaced with parts made of polychlorotrifluoroethylene in order to reduce contamination of trace metals. The eluent pass was altered for the back flush elution of trace metals. We optimized the cleaning procedures for the chelating resin column and flow lines of the preconcentration system, and developed a preconcentration procedure, which required less labor and led to a superior performance compared to manual preconcentration (Sohrin et al. [5]). The nine trace metals were simultaneously and quantitatively preconcentrated from ∼120g of seawater, eluted with ∼15g of 1M HNO3, and determined by HR-ICP-MS using the calibration curve method. The single-step preconcentration removed more than 99.998% of Na, K, Mg, Ca, and Sr from seawater. The procedural blanks and detection limits were lower than the lowest concentrations in seawater for Mn, Ni, Cu, and Pb, while they were as low as the lowest concentrations in seawater for Al, Fe, Co, Zn, and Cd. The accuracy and precision of this method were confirmed by the analysis of reference seawater samples (CASS-5, NASS-5, GEOTRACES GS, and GD) and seawater samples for vertical distribution in the western North Pacific Ocean.

Trace elements distribution and ecological risk assessment of seawater and sediments from Dingzi Bay, Shandong Peninsula, North China

Selected trace elements (Hg, As, Pb, Cu, Cd, Cr and Zn) in seawater and sediments from Dingzi Bay, a semi-enclosed bay suffering from severe degradation located in the Shandong Peninsula, were investigated to evaluate the spatial distribution and potential ecological risk. Results indicated that higher concentrations occurred in the inner bay. Calculation of pollution load index (PLI) showed overall low values while the concentration factor (CF) indicated that Hg, As and Cd were at moderate risk levels in the region. Based on the effects-range classification, As was likely to pose environment risk. Principal component analysis (PCA) revealed that in addition to background contributions, the trace elements contamination could also be affected by anthropogenic pollution sources. The results of present study provide useful background information for further marine investigation and management in the region.

An argon–nitrogen–hydrogen mixed-gas plasma as a robust ionization source for inductively coupled plasma mass spectrometry

Multivariate optimization of an argon–nitrogen–hydrogen mixed-gas plasma for minimum matrix effects, while maintaining analyte sensitivity as much as possible, was carried out in inductively coupled plasma mass spectrometry. In the presence of 0.1M Na, the 33.9±3.9% (n=13 elements) analyte signal suppression on average observed in an all-argon plasma was alleviated with the optimized mixed-gas plasma, the average being −4.0±8.8%, with enhancement in several cases. An addition of 2.3% v/v N2 in the outer plasma gas, and 0.50% v/v H2 to the central channel, as a sheath around the nebulizer gas flow, was sufficient for this drastic increase in robustness. It also reduced the background from ArO+ and Ar2+ as well as oxide levels by over an order of magnitude. On the other hand, the background from NO+ and ArN+ increased by up to an order of magnitude while the levels of doubly-charged ions increased to 7% (versus 2.7% in an argon plasma optimized for sensitivity). Furthermore, detection limits were generally degraded by 5 to 15 fold when using the mixed-gas plasma versus the argon plasma for matrix-free solution (although they were better for several elements in 0.1M Na). Nonetheless, the drastically increased robustness allowed the direct quantitative multielement analysis of certified ore reference materials, as well as the determination of Mo and Cd in seawater, without using any matrix-matching or internal standardization.

Benthic macroalgae as biological indicators of heavy metal pollution in the marine environments: A biomonitoring approach for pollution assessment

Metal pollution in the marine coastline environment is an important topical issue in the context of ecological disturbance and climate change. Heavy metal contaminations (Cd, Cr, Cu, Mn, Ni, Pb and Zn) in seawater and surficial sediments, as well as macroalgal diversity, were determined in six different locations along the coast of the Gulf of Kutch in India. The marine coastline environment was found to be enriched with Cd and Zn in comparison to other metals. Significant (p≤0.05) inter-elemental positive-correlations were observed between Fe–Mn, Fe–Cu, Fe–Cr, Fe–Zn, Cr–Cu, Cu–Mn, and Cd–Zn, as well as negative-correlations between Cd–Pb, Ni–Pb, and Zn–Pb. Though genus specific macroalgal responses to heavy metal accumulation were significant, species specific response was insignificant (p≤0.05). The relative abundance of metals in macroalgae followed the order of Fe>Zn>Mn>Cu>Cd>Cr>Ni>Pb. The high uptake of metals in green algae (Ulva lactuca and Enteromorpha intestinalis) and brown algae (Padina gymnospora and Dictyota bartayresiana) suggested that these algae may be used as potential biomonitors for heavy metal pollution. Three pollution indicators, Contamination Factor (CF), Enrichment Factor (EF) and Geochemical Index (Igeo) were calculated to determine the degree of metal pollution in the marine coastline and the contribution of anthropogenic influence.

Multielement Solid Phase Preconcentration Using a Chelating Resin of Styrene Divinylbenzene Copolymer and Application for the Analysis of Seawater and Fish Otoliths by Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

A new chelating resin was synthesized by immobilizing 4–(2–thiazolylazo) resorcinol (TAR) on to a styrene divinylbenzene copolymer. This material was examined for on-line solid phase extraction/preconcentration of Cd, Co, Cu, Ni, Pb, and Zn in seawater and fish otoliths for determination by inductively plasma mass spectrometry (ICP-MS). A volume of 5.0 mL sample solution was loaded on to a mini-column of the resin at 2.0 mL min−1 via a sequential injection system. The optimum pH for multielement preconcentration was around pH 5.5. Recoveries were better than 96% in artificial seawater. Elution was achieved with 1.0 mL of 0.75 mol L−1 HNO3. The resin possesses a large sorption capacity ranging from 82.0 µmol g−1 for Pb to 319 µmol g−1 for Cu. The detection limits (3σ) varied between 0.0016 µg L−1 (Cd) and 0.015 µg L−1 (Zn) for preconcentration of 5.0 mL blank solutions (pH 5.5). Relative standard deviation for three replicate runs was between 0.3% (Cd) and 6% (Zn) at the 1.0 µg L−1 level. The procedure was validated by analysis of a nearshore seawater and a fish otoliths certified reference material, and employed for coastal seawater and estuarine water samples.

Comment on “The isotopic composition of cadmium in the water column of the South China Sea”

Yang et al. (2012) found that marine biogenic particles and corresponding surface seawater from the South China Sea were characterized by essentially identical Cd isotope compositions, and inferred that biological Cd isotope fractionation is insignificant at this location. Based on results obtained for a box model that represents Cd cycling in the surface layer, and using published data to constrain the Cd source and sink fluxes, we show that this conclusion is likely to be incorrect. The modeling results indicate that the heavy Cd isotope signature of ε114/110Cd≈+9 observed by Yang et al. (2012) for the surface South China Sea is most likely a consequence of substantial isotope fractionation (of about 4–6 ε) during uptake of dissolved seawater Cd by phytoplankton.

Past 140-year environmental record in the northern South China Sea: Evidence from coral skeletal trace metal variations

About 140-year changes in the trace metals in Porites coral samples from two locations in the northern South China Sea were investigated. Results of PCA analyses suggest that near the coast, terrestrial input impacted behavior of trace metals by 28.4%, impact of Sea Surface Temperature (SST) was 19.0%, contribution of war and infrastructure were 14.4% and 15.6% respectively. But for a location in the open sea, contribution of War and SST reached 33.2% and 16.5%, while activities of infrastructure and guano exploration reached 13.2% and 14.7%. While the spatiotemporal change model of Cu, Cd and Pb in seawater of the north area of South China Sea during 1986–1997 were reconstructed. It was found that in the sea area Cu and Cd contaminations were distributed near the coast while areas around Sanya, Hainan had high Pb levels because of the well-developed tourism related activities.

Cadmium isotope variations in the Southern Ocean

Cadmium concentrations and isotope compositions were determined for 47 seawater samples from the high nutrient low chlorophyll (HNLC) zone of the Atlantic sector of the Southern Ocean. The samples include 13 surface waters from a transect of the Weddell Gyre and 3 depth profiles from the Weddell Sea and Drake Passage. The Southern Ocean mixed layer samples from this study and Abouchami et al. (2011) define a clear but broad ‘HNLC trend’ in a plot of εCd114/110 versus [Cd], which is primarily a consequence of isotopic fractionation associated with biological uptake (εCd114/110 is the deviation of the 114Cd/110Cd ratio of a sample from NIST SRM 3108 Cd in parts per 10,000). The trend is especially apparent in comparison to the large range of values shown by a global set of seawater Cd data for shallow depths. The Southern Ocean samples are also distinguished by their relatively high Cd concentrations (typically 0.2 to 0.6 nmol/kg) and moderately fractionated εCd114/110 (generally between +4 and +8) that reflect the limited biological productivity of this region. Detailed assessment reveals fine structure within the ‘HNLC trend’, which may record differences in the biological fractionation factor, different scenarios of closed and open system isotope fractionation, and/or distinct source water compositions.Southern Ocean seawater from depths ⩾1000 m has an average εCd114/110 of +2.5±0.2 (2se, n=16), and together with previous results this establishes a relatively constant εCd114/110 value of +3.0±0.3 (2se, n=27) for global deep waters. Significant isotopic variability was observed at intermediate depths in the Southern Ocean. Seawater from 200 m to 400 m in Weddell Sea has high Cd concentrations and εCd114/110 as low as +1, presumably due to remineralization of Cd from biomass that records incomplete nutrient utilization. Antarctic Intermediate Water, which was sampled at 150 to 750 m depth in the Drake Passage, features a distinct Cd isotope signature of εCd114/110≈+4, which reflects biological isotope fractionation at the surface and subsequent mixing into the ocean interior. Taken together, our results demonstrate that coupled Cd isotope and concentration data provide valuable insights into the distribution and biological cycling of Cd in the water column. The highly systematic nature of Cd isotope signatures may furthermore prove to be of utility for future research in marine geochemistry and paleoceanography.