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Cadmium Isotopic Composition Indicates Multiple Geological Sources in Cadmium Anomaly in Jamaican Soils.

INTRODUCTION

Large areas (>500 km2) of Jamaica are uniquely characterized by soils containing more than 50 mg kg-1 Cd, several orders of magnitude greater than what is typically expected for pedo-geochemical levels of the metal. However, the source of the Cd remains controversial but is presumably natural (geological) since there are no sources of contamination in the area large enough to account for the widespread high levels.

EXPERIMENTAL 

picture1_1.pngIn attempting to constrain the sources of Cd-rich soils developed on White Limestone Supergroup in Jamaica, we determine the Cd isotopic composition (ε114/110Cd) of old, highly weathered bauxitic soils, dust deposits, and samples of a cadmiferous phosphorite soil concretion (guano; Fig. 1), karst bauxite (DRB) and volcanic ash.

Fig. 1: Map of simplified bedrock geology of Jamaica superimposed by map of geospatial distribution of Cd in soils.


RESULTS AND DISCUSSION

The elemental (Cd) concentration and isotopic composition of the samples varied considerably (Fig. 2), suggesting that they do not solely reflect the underlying karstic geology.

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Fig. 2                                                                                                                  Fig. 3                                                                                                                                                                                                                                                 

Fig. 2: Lognormal distribution plot of Cd in study samples. Inset illustrates variations in the elemental concentration and isotopic composition of Cd. ε114/110Cd = [(114Cd/ 110Cd)sample/(114Cd/ 110Cd)JMC Cd Münster – 1] x 104.

There is also evidence to suggest that immature pedo-geochemical matrices dust deposits contain lower concentrations of Cd (< 1 mg kg -1; Fig. 3).

Fig. 3: Cd concentration of surface soils, dust deposits and other matrices as a function of Fe/Na ratio. Cumulative probability plot of the Fe/Na ratio (inset) provides a relative measure of maturity.

Further, we utilize a suite of elements Cr, Th, Sc as well as the REE La (the lightest REE) to Lu (the heaviest REE; measured in this and previous studies) that are considered, based on high ionic potential, to be immobile in low-temperature, near surface environments as provenance indicators, and the results presented in Figures 4 to 6

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Fig. 4. Cr-Th-Nd and Sc-Th-La ternary diagrams. Compositional overlaps indicate different candidate parent materials and has been confirmed (Fig. 6).

 

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Fig. 5. CI-chondrite normalized REE concentrations of different sample types (this study). Concentrations vary widely and are typically higher in soils. Samples generally exhibit negative Eu/Eu* and are enriched in LREE relative to HREE.  

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Fig. 6. Plots of (A) (Sm/Yb)N vs. (La/Yb)N and (B) Eu/Eu* vs. (La/Yb)N.


CONCLUSIONS

Given the local topography and purity (98%) of the underlying carbonate host, Jamaican bauxitic soils have long been considered to have developed from Miocene volcanic ash falls. Here we combine stable isotope and allied geochemical data to advance the argument that anomalous Cd concentrations in Jamaican soils do not solely reflect the underlying parent material, but rather a complex and not fully understood mixture of African dust carried west across the Atlantic Ocean and volcanogenic inputs from Central America and the Lesser Antilles Arc.