Cadmium isotope fractionation in seawater — A signature of biological activity (original) (raw)

Cadmium isotopic composition in the ocean

Geochimica Et Cosmochimica Acta, 2006

The oceanic cycle of cadmium is still poorly understood, despite its importance for phytoplankton growth and paleoceanographic applications. As for other elements that are biologically recycled, variations in isotopic composition may bring unique insights. This article presents (i) a protocol for the measurement of cadmium isotopic composition (Cd IC) in seawater and in phytoplankton cells; (ii) the first Cd IC data in seawater, from two full depth stations, in the northwest Pacific and the northwest Mediterranean Sea; (iii) the first Cd IC data in phytoplankton cells, cultured in vitro. The Cd IC variation range in seawater found at these stations is not greater than 1.5 e Cd/amu units, only slightly larger than the mean uncertainty of measurement (0.8 e Cd/amu ). Nevertheless, systematic variations of the Cd IC and concentration in the upper 300 m of the northwest Pacific suggest the occurrence of Cd isotopic fractionation by phytoplankton uptake, with a fractionation factor of 1.6 ± 1.4 e Cd/amu units. This result is supported by the culture experiment data suggesting that freshwater phytoplankton (Chlamydomonas reinhardtii and Chlorella sp.) preferentially take up light Cd isotopes, with a fractionation factor of 3.4 ± 1.4 e Cd/amu units. Systematic variations of the Cd IC and hydrographic data between 300 and 700 m in the northwest Pacific have been tentatively attributed to the mixing of the mesothermal (temperature maximum) water (e Cd/amu = À0.9 ± 0.8) with the North Pacific Intermediate Water (e Cd/amu = 0.5 ± 0.8). In contrast, no significant Cd IC variation is found in the northwest Mediterranean Sea. This observation was attributed to the small surface Cd depletion by phytoplankton uptake and the similar Cd IC of the different water masses found at this site. Overall, these data suggest that (i) phytoplankton uptake fractionates Cd isotopic composition to a measurable degree (fractionation factors of 1.6 and 3.4 e Cd/amu units, for the in situ and culture experiment data, respectively), (ii) an open ocean profile of Cd IC shows upper water column variations consistent with preferential uptake and regeneration of light Cd isotopes, and (iii) different water masses may have different Cd IC. This isotopic system could therefore provide information on phytoplankton Cd uptake and on water mass trajectories and mixing in some areas of the ocean. However, the very small Cd IC variations found in this study indicate that applications of Cd isotopic composition to reveal aspects of the present or past Cd oceanic cycle will be very challenging and may require further analytical improvements. Better precision could possibly be obtained with larger seawater samples, a better chemical separation of tin and a more accurate mass bias correction through the use of the double spiking technique.

Nonspecific uptake and homeostasis drive the oceanic cadmium cycle

Proceedings of the National Academy of Sciences, 2013

The global marine distributions of Cd and phosphate are closely correlated, which has led to Cd being considered as a marine micronutrient, despite its toxicity to life. The explanation for this nutrient-like behavior is unknown because there is only one identified biochemical function for Cd, an unusual Cd/Zn carbonic anhydrase. Recent developments in Cd isotope mass spectrometry have revealed that Cd uptake by phytoplankton causes isotopic fractionation in the open ocean and in culture. Here we investigate the physiochemical pathways that fractionate Cd isotopes by performing subcellular Cd isotope analysis on genetically modified microorganisms. We find that expression of the Cd/Zn carbonic anhydrase makes no difference to the Cd isotope composition of whole cells. Instead, a large proportion of the Cd is partitioned into cell membranes with a similar direction and magnitude of Cd isotopic fractionation to that seen in surface seawater. This observation is well explained if Cd is mistakenly imported with other divalent metals and subsequently managed by binding within the cell to avoid toxicity. This process may apply to other divalent metals, whereby nonspecific uptake and subsequent homeostasis may contribute to elemental and isotopic distributions in seawater, even for elements commonly considered as micronutrients.

The isotopic composition of Cadmium in the water column of the South China Sea

Geochimica et Cosmochimica Acta, 2012

We determined the Cd isotopic composition of seawater and sinking particles collected in the deep basin of the northern South China Sea (SCS) to investigate the controlling mechanisms on the Cd isotopic composition in the water column. The isotopic composition in the water column decreased with depth, with e 114/110 Cd values (e 114/110 Cd = [( 114 Cd/ 110 Cd) sample / ( 114 Cd/ 110 Cd) JMC Cd Mü nster À 1] Â 10 4 ) ranging from +8.7 to +9.9 in the top 80 m, from +4.6 to +5.5 between 100 and 150 m, decreasing from +5.5 to +3.6 at depths from 150 to 1000 m, and remaining at +3.4 ± 0.5 from 1000 to 3500 m. The isotopic composition and concentrations of Cd observed in the deep waters of the SCS are similar to the values that were previously reported in the North Pacific Ocean. In the thermocline, the variations in the Cd isotopic composition and concentrations were consistent with the relative volumetric percentages of the subsurface water, the intermediate water, and the deep water in the water column, indicating that water mixing is the dominant process determining the isotopic composition in the thermocline. Comparable to the isotopic composition value in the seawater of the mixed layer, the e 114/110 Cd in the sinking particles collected at 30 m was +9.3 ± 0.9. Because our previous studies demonstrated that the particulate Cd was predominantly biogenic organic matter, the comparable isotopic composition between the surface seawater and the sinking particles indicates that net biological isotopic fractionation on Cd in the surface water was insignificant. The result indicates that phytoplankton do not necessarily take up relatively light Cd in the oceanic surface waters. It is necessary to directly and systematically investigate how marine phytoplankton fractionate Cd isotopes.

Natural and Anthropogenic Cd Isotope Variations

2012

Cadmium is a transition metal with eight naturally occurring isotopes that have atomic mass numbers of between 106 and 116. The large Cd isotope anomalies of meteorites have been subject to investigation since the 1970s, but improvements in instrumentation and techniques have more recently enabled routine studies of the smaller stable Cd isotope fractionations that characterize various natural and anthropogenic terrestrial materials. Whilst the current database is still comparatively small, pilot studies have identified two predominant mechanisms that routinely generate Cd isotope effects -partial evaporation/condensation and biological utilization. Processes that involve evaporation and condensation appear to be largely responsible for the Cd isotope fractionations of up to 1‰ (for 114 Cd/ 110 Cd) that have been determined for industrial Cd emissions, for example from ore refineries. Cadmium isotope measurements hence hold significant promise for tracing anthropogenic sources of this highly toxic metal in the environment. The even larger Cd isotope fractionations that have been identified in the oceans (up to 4‰ for 114 Cd/ 110 Cd) are due to biological uptake and utilization of dissolved seawater Cd. This finding confirms previous work, which identified Cd as an essential marine micronutrient that exhibits a phosphate-like distribution in the oceans. The marine Cd isotope fractionations are of particular interest, as they can be used to study micronutrient cycling and its impact on ocean productivity. In addition, they may also inform on past changes in marine nutrient utilization and how these are linked to global climate, if suitable archives of seawater Cd isotope compositions can be identified.

Isotopically Light Cd in Sediments Underlying Oxygen Deficient Zones

Frontiers in Earth Science

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 ...

Precise determination of cadmium isotope fractionation in seawater by double spike MC-ICPMS

Geochimica et Cosmochimica Acta, 2007

A new technique has been developed for the accurate and precise determination of the stable Cd isotope composition of seawater. The method utilizes a 110 Cd-111 Cd double spike, and it involves separation of Cd from seawater by column chromatography and isotopic analyses by multiple collector inductively coupled plasma mass spectrometry. As a by-product, it also generates precise Cd concentration data. Repeated analyses of three pure Cd reference materials and three seawater samples yielded reproducibilities of about ±1.0 to ±1.6 e 114/110 Cd (2 SD), based on measurements that each consumed about $8 ng of natural Cd (e 114/110 Cd is the deviation of the 114 Cd/ 110 Cd isotope ratio of a sample from the standard in parts per 10,000). This demonstrates that the new double spike technique is superior to published methods of Cd isotope analyses, with regard to the acquisition of precise data for samples of limited size. Additional experiments showed that as little as 1-5 ng of seawater Cd could be analyzed with a precision of about ±2 to ±6 e 114/110 Cd (2 SD). The accuracy of the seawater isotope data was ascertained by experiments in which a Cd-free seawater matrix was doped with small quantities of isotopically well-characterized Cd. Repeated mass scans that were carried out on purified Cd fractions of several samples furthermore demonstrated the absence of significant spectral interferences. The isotope data that were acquired for the three seawater samples reveal, for the first time, small but resolvable Cd isotope fractionations in the marine environment. Cadmium-rich intermediate water from the North Pacific was found to have an isotope composition of e 114/110 Cd = 3.2 ± 1.0. In contrast, Cd-depleted seawater from the upper water column of the Atlantic and Arctic Oceans displayed isotope compositions of e 114/110 Cd = 6.4 ± 1.1 and 6.6 ± 1.6, respectively. These observations are in accord with the interpretation that the isotope effects are due to the biological fractionation that occurs during the uptake of dissolved seawater Cd by phytoplankton.