Precise determination of cadmium isotope fractionation in seawater by double spike MC-ICPMS (original) (raw)

Cadmium isotope fractionation in seawater — A signature of biological activity

Earth and Planetary Science Letters, 2007

Investigations of cadmium isotope variations in the oceans may provide new insights into the factors that control the marine distribution and cycling of this element. Here we present the results of Cd isotope and concentration analyses for 22 seawater samples from the Atlantic, Southern, Pacific, and Arctic Oceans. The results reveal, for the first time, large and well resolved Cd isotope fractionations in the marine environment. The majority of the seawater samples display an inverse relationship between dissolved Cd contents and isotope compositions, which range from ε 114/110 Cd≈ +3 ± 0.5 for Cd-rich waters (0.8-1.0 nmol/kg) to ε 114/110 Cd ≈ 38 ± 6 for surface water with a Cd concentration of only 0.003 nmol/kg (all ε 114/110 Cd data are reported relative to the JMC Cd Münster standard). This suggests that the Cd isotope variations reflect kinetic isotope effects that are generated during closed system uptake of dissolved seawater Cd by phytoplankton. A few samples do not follow this trend, as they exhibit extremely low Cd contents (b 0.008 nmol/kg) and nearly un-fractionated Cd isotope compositions. Such complexities, which are not revealed by concentration data alone, require that the Cd distribution at the respective sites was affected by additional processes, such as water mass mixing, atmospheric inputs of Cd and/or adsorption. Uniform isotope compositions of ε 114/110 Cd= +3.3 ± 0.5 (1 S.D.) were determined for seawater from ≥900 m depth, despite of Cd concentrations that display the expected increase along the global deep-water pathway from the Atlantic (∼0.3 nmol/kg) to the Pacific Ocean (∼0.9 nmol/kg). This indicates that the biomass, which is remineralized in the deeper ocean, is also characterized by a very constant Cd isotope composition. This observation is in accord with the interpretation that the Cd distribution in surface waters is primarily governed by Rayleigh fractionation during near-quantitative uptake of dissolved seawater Cd.

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.

A new methodology for precise cadmium isotope analyses of seawater

Analytical and Bioanalytical Chemistry, 2012

Previous studies have revealed considerable Cd isotope fractionations in seawater, which can be used to study the marine cycling of this micronutrient element. The low Cd concentrations that are commonly encountered in nutrient-depleted surface seawater, however, pose a particular challenge for precise Cd stable isotope analyses. In this study, we have developed a new procedure for Cd isotope analyses of seawater, which is suitable for samples as large as 20 L and Cd concentrations as low as 1 pmol/L. The procedure involves the use of a 111 Cd-113 Cd double spike, co-precipitation of Cd from seawater using Al(OH) 3 , and subsequent Cd purification by column chromatography. To save time, seawater samples with higher Cd contents can be processed without co-precipitation. The Cd isotope analyses are carried out by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The performance of this technique was verified by analyzing multiple aliquots of a large seawater sample that was collected from the English Channel, the SAFe D1 seawater reference material, and several samples from the GEOTRACES Atlantic intercalibration exercise. The overall Cd yield of the procedure is consistently better than 85% and the methodology can routinely provide ε 114/110 Cd data with a precision of about ±0.5 ε (2sd, standard deviation) when at least 20-30 ng of natural Cd is available for analysis. However, even seawater samples with Cd contents of only 1-3 ng can be analyzed with a reproducibility of about ±3 to ±5 ε. A number of experiments were furthermore conducted to verify that the isotopic results are accurate to within the quoted uncertainty.

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.

Cadmium in northeast Pacific waters

Limnology and Oceanography, 1978

Northeast Pacific water was collected by five different methods and the Cd in it was preconcentrated by both chelex-ion exchange and chelation-organic extraction techniques. All sampling and preconcentnation methods yielded essentially the same data. Cadmium was very significantly correlated with phosphate and nitrate at all depths and it appears that the resulting equations, ng Cd* liter-' = -3.6 + 34.9 (Mmol PO,. liter-') and ng Cd. liter-l = 5.1 + 2.45 (pmol NOR-liter-'),

The Marine Biogeochemisty of Cadmium: Studies of Cadmium Isotopic Variations in the Southern Ocean

2011

Cadmium (Cd) in the oceans closely mimics the behavior of the macronutrient phosphate ) and can be used in the enzyme carbonic anhydrase (CA), suggesting a biological uptake of Cd. This relationship between Cd and PO 4 3has been used extensively as a paleoproxy for historic nutrient cycling. However, the validity of this proxy is questionable due to the complexity of the Cd /PO 4 3relationship. To this end, Cd isotopic studies can provide critical insight into the mechanism controlling Cd uptake and may, in itself, be a First I would like to thank my supervisors, Claudine Stirling, Russell Frew, and Keith Hunter. Thank you all for the many intense hours of thesis reading over the last weeks. This would not have been possible without your comments, advice, and support. Thank you Claudine for being our FILTER boss, all of your advice both in the lab and in life over the past four years was invaluable. Thank you for being enthusiastic throughout the entire process, without you, I may have given up long ago. Thank you Russell for the many impromptu discussions and brainstorming about the oceans, and I always appreciate our talks about farming and lambs. Thank you Keith for building such an amazing research group and accepting me to be a part of it. Waterworld"s knowledge of the oceans is incredible and has provided such a unique environment for the studies of marine biogeochemistry. Thank you for your enthusiasm about science and the numerous hours that you made available for me. Thank you to the three examiners, Michael Ellwood, Ken Bruland and Sylvia Sander for the time and energy spent reading this thesis and making corrections. All of your comments are discussion are very much appreciated. Thank you to Frank Wombacher, Joel Baker, and Mark Rehkamper for providing cadmium standards and double spike advice, aiding in the positive outcomes of this research. I would like to thank all of my past and present colleagues from Waterworld and the chemistry department for all of their help, brainstorming, motivation and enthusiasm for the work we are doing, including: Especially to Malcolm Reid for all of your help on the mass spec and all aspects of the lab and science. As our FILTER solution, you were always there to help me solve my problems. To Evelyn Armstrong, Eike Breitbarth and Linn Hoffmann for all of your help with my phytoplankton culturing when my lack of a green thumb shone through. And to Robert Strzepek for all of the help and advice and for making this culture work possible. Thank you to Kim Curry for organizing every sampling trip and helping through the hours of seasickness and frustrations of compressors, pumps, and tubing that seemed to be inevitable no matter how organized we were. And especially to Bill Dixon and Phil Heseltine, the captain and crewman of the R/V Polaris II, for being patient, and more than helpful in every aspect of the sampling process. Thank you to Hugh Doyle for being seasick with/for me, more than anyone else. Thank you to Katherine Baer and Sylvia Sander for their friendship and helping keep me healthy, or at least trying to, on all of our lunchtime runs. Thank you especially to my FILTER girls, Martine Poffet, Mihoko Numata, and Angela Kaltenbach. I could not have asked for better friends to share a lab with. The hours of lunches, coffee breaks, weekends, bbq"s, and camping trips will never be forgotten. To my parents, Carol Gault and Gordon Ringold and the rest of my family, thank you for supporting me throughout my life and helping me get to this point. I could not have done this without your love and encouragement. To my friends from all over the world, thank you for not giving up on me, despite my thesis brain and my lack of communication over the last year.