Elemental and isotopic behaviour of Zn in Deccan basalt weathering profiles: Chemical weathering from bedrock to laterite and links to Zn deficiency in tropical soils
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NILS THORSTEN SUHR, Ronny Schoenberg, David Chew, Carolina Rosca, Mike Widdowson, Balz S. Kamber, 'Elemental and isotopic behaviour of Zn in Deccan basalt weathering profiles: Chemical weathering from bedrock to laterite and links to Zn deficiency in tropical soils', 2018, Science of the Total Environment;, 619-620;Download Item:
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Abstract:
Zinc (Zn) is a micronutrient for organisms and essential for plant growth, therefore knowledge of its elemental
cycling in the surface environment is important regarding wider aspects of human nutrition and health. To explore
the nature of Zn cycling, we compared its weathering behaviour in a sub-recent regolith versus an ancient
laterite profile of the Deccan Traps, India – an area of known soil Zn deficiency.We demonstrate that progressive
breakdown of primary minerals and the associated formation of phyllosilicates and iron oxides leads to a depletion
in Zn, ultimately resulting in a loss of 80% in lateritic residues. This residue is mainly composed of resistant
iron oxides and hydroxides ultimately delivering insufficient amounts of bio-available Zn. Moreover, (sub)-tropical
weathering in regions experiencing extended tectonic quiescence (e.g., cratons) further enhance the development
of old and deep soil profiles that become deficient in Zn. This situation is clearly revealed by the spatial
correlation of the global distribution of laterites, cratons (Africa, India, South America and Australia) and known
regions of Zn deficient soils that result in health problems for humans whose diet is derived from such land.
We also investigate whether this elemental depletion of Zn is accompanied by isotope fractionation. In the
saprolitic horizons of both weathering profiles, compositions of δ66ZnJMC-Lyon lie within the “crustal average” of
+0.27±0.07‰δ66ZnJMC-Lyon. By contrast, soil horizons enriched in secondary oxides show lighter isotope compositions.
The isotopic signature of Zn (Δ66Znsample-protolith up to ~−0.65‰) during the formation of the ferruginous-
lateritic weathering profile likely resulted from a combination of biotically- and kinetically-controlled
sorption reactions on Fe-oxyhydroxides. Our findings suggest that oxide rich soil types/horizons in (sub)-tropical regions likely exert a control on riverine Zn isotope compositions such that these becomeheavier than the crustal
average. This isotopic behaviour invites a broader study of global soils to test whether light isotope composition
alone could serve as an indicator for reduced bioavailability of Zn.
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Grant Number
Marie Curie
608069
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Author: SUHR, NILS THORSTEN; Schoenberg, Ronny; Chew, David; Rosca, Carolina; Widdowson, Mike; Kamber, Balz S.
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Science of the Total Environment;619-620;
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