Assessment of Cadmium Isotopes as a Paleoclimate Proxy

Lead Research Organisation: Imperial College London
Department Name: Earth Science and Engineering

Abstract

It has long been known that the biological activity of the oceans is regulated by the availability of the major nutrient ions phosphate, nitrate, and silicate. More recently, however, it has been recognized that micronutrient trace elements also play an important role in limiting marine productivity. As biological activity draws down CO2 from the atmosphere, micronutrients play a significant role in regulating the Earth's carbon cycle and climate. This study focuses on the micronutrient element cadmium (Cd). The geochemistry of Cd in seawater has attracted significant interest for more than 30 years because its marine distribution mimics the distribution of the macronutrient phosphate. This correlation forms the basis for the application of foraminiferal Cd/Ca ratios as a paleoclimate proxy. The application of this proxy is hindered, however, by our limited understanding of the role and cycling of bioactive Cd in the oceans. A recent pilot investigation of the PI indicates that analyses of Cd stable isotope compositions are able to address such limitations. In particular, the pilot study was the first investigation to identify large Cd isotope variations in seawater, which primarily reflect isotope fractionation from biological uptake of dissolved seawater Cd. The strikingly systematic nature of the fractionations, provide new insights into the marine cycling of Cd and demonstrate that Cd isotopes may be a useful paleoclimate proxy. The present study will build on and verify the results of the pilot investigation. To this end, we will first acquire a significantly larger Cd isotope dataset for seawater. We will then carefully evaluate this new dataset to re-examine the conclusion, that combined analyses of seawater Cd contents and isotope compositions uniquely inform on climate-relevant processes, such as variations in marine productivity. Whilst the interpretation of the new analytical data is expected to be straightforward, we will also address this goal by expanding a currently available global ocean model of Cd cycling to Cd isotopes. This approach will allow us to assess whether the model can reproduce reasonable Cd concentrations and isotope distributions for the oceans, based on known processes and fractionation factors. Any discrepancies between the model results and data will thus help to identify deficiencies in our understanding of the processes that regulate marine Cd contents and isotope compositions. A confirmation of the hypothesis that combined Cd concentration and isotope measurements provide unique constraints on the cycling of Cd and other nutrients in the oceans would be exciting. Such a result provides a basis for the application of Cd isotopes as a paleonutrient proxy in climate research. For example, combined Cd/Ca and Cd isotope data for foraminifera from sediment cores could be used to investigate temporal changes in marine nutrient utilization and the upwelling of nutrient-rich water masses. Such studies are important because they allow an assessment of past changes in the marine carbon cycle and their effect on climate.

Publications

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Description It has long been known that the biological activity of the oceans is regulated by the availability of the major nutrients phosphate, nitrate, and silicate. More recently, however, it has been recognized that micronutrient trace elements also play an important role in limiting marine productivity. As biological activity draws down CO2 from the atmosphere, micronutrients thus play a significant role in regulating the Earth's carbon cycle and climate.



This study focused on the element cadmium (Cd). The geochemistry of Cd in seawater has attracted significant interest for more than 30 years because its marine distribution mimics the distribution of the macronutrient phosphate, presumably because Cd belongs to the group of marine micronutrients. The investigation followed on from a recent pilot study of the PI, which first identified large Cd isotope variations in seawater that were thought to primarily reflect isotope fractionation from biological uptake of dissolved seawater Cd. The observed fractionations were strikingly systematic and thus promised to provide new insights into the marine cycling of Cd and hinted that Cd isotopes may have a potential as a paleoclimate proxy. A primary goal of the present study was to verify these preliminary conclusions.



To this end, we developed, validated and employed new analytical techniques to carry out combined Cd isotope and concentration analyses for more than 100 seawater samples from the Atlantic, Pacific, Southern and Arctic Oceans. These measurements significantly increased the available Cd isotope database for seawater, which encompassed only about 50 samples prior to this study.



Our analytical results confirm the earlier observation that larger Cd isotope fractionations are common in the surface ocean, whilst deeper water masses typically have nearly constant Cd isotope compositions. However, the new, larger dataset for seawater also indicates that samples from (i) "normal" open ocean settings, (ii) so called HNLC locations (that feature High Nutrient but Low Chlorophyll contents) and (iii) zones where nutrient-rich deep water masses rise to the surface, feature distinct Cd isotope systematics. This distinct behaviour is particularly striking in diagrams of Cd isotope compositions versus Cd concentrations, where seawater samples from the different settings define distinct trends.



This is an important a result. Firstly, the different trends are indicative of and can be used to better understand different modes of marine nutrient cycling and utilization. Secondly, our observations strengthen the case for the application of Cd isotopes as a paleonutrient proxy in climate research, which may be employed to better constrain past changes in marine nutrient utilization and their effect on the oceanic carbon cycle and climate.



In addition, the data obtained for samples from the Siberian Shelf confirm that Cd typically behaves as a conservative element during estuarine mixing of river and seawater, whilst additional release of Cd from riverine particles is observed in a few instances. In one particular case, the isotope data indicate that anthropogenic Cd is being released into the shelf sea. This is an extremely important result because it suggests that large quantities of toxic Cd of anthropogenic origin may be transported in rivers in relatively inert, particulate form for hundreds of kilometres but can later be transformed into dissolved, and hence readily bioavailable, Cd in estuaries. Importantly, such a release of anthropogenic Cd can be distinguished from the release of natural Cd from particles using our novel isotopic techniques. This suggests that it may be useful to carry out more Cd isotope analyses of shelf water samples to investigate potential release of anthropogenic Cd in such environments.
Exploitation Route Using methods that were developed based on our novel Cd isotope techniques, my research group is now exploring how isotopic tracing can be applied for the detection of engineered ZnO and Ag nanomaterials in natural and experimental systems against high background concentrations. This work is being carried out in our laboratory as part of PROSPECT, a private-public partnership that provides the UK contribution to the global nanoparticle study of the OECD Working Party on Manufactured Nanomaterials. The PROSPECT research involves close collaboration between academic and industrial project partners and our research thus supports, for example, the development of new devices for the in situ detection of engineered nanomaterials (by Naneum Ltd, Canterbury). The novel isotopic techniques that were developed and applied in our laboratory as part of this (and further) projects, are finding direct use in other investigations and are creating impact in fields that extend far beyond the present research. Three prominent examples of this are:

(1) The improved Cd isotope methods that were developed here, are currently also being used in our laboratory with great benefit to study early solar system processes and the accretion of the Earth.

(2) The methods and expertise that were developed in this study have also helped to advance presently active investigations, which apply novel isotopic techniques for medical and life science research and, most recently, in the rapidly growing field of nanoparticle toxicology. Such research is currently being conducted by our group in collaboration with academics from a wide range of fields (biologists, toxicologists, medical researchers) as well as industrial partners and is of relevance to both industry and policymakers.

(3) PI Rehkamper also applies his extensive isotopic expertise to address security related issues in nuclear forensics, which is relevant to policymakers and government.
Sectors Education,Environment

 
Description The findings provide an improved understanding of the biogeochemical cycling of cadmium in marine environments. This is important as Cd is a highly toxic element and may also have an impact on marine productivity and hence the carbon cycle.
First Year Of Impact 2010
Sector Education,Environment
Impact Types Societal

 
Description Ocean Micronutrient Cycles: UK GEOTRACES
Amount £294,000 (GBP)
Funding ID NE/H005390/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 09/2010 
End 09/2014