Characterising the decay of organic metal complexes in speleothem-forming cave waters

Abstract

Organic metal complexes (OMCs) transport trace metals (e.g., Co, Ni, Cu) from surface soils, via the unsaturated zone, to sites of cave carbonate (speleothem) formation. OMCs clearly imprint on speleothem trace element chemistry, but the role of kinetic factors in the signal transfer process has yet to be elucidated. We investigated whether OMCs may viably link metal concentrations in stalagmites and local hydrology (i.e., drip rate), via their time-sensitive dissociation (or ‘decay’) in the speleothem water thin-film. We performed competitive ligand exchange experiments using water and soil samples from eight geographically diverse Aotearoa New Zealand caves, providing a first comparative characterisation of speleothem-specific OMC kinetics. Critically, this approach corroborated that NOM ligands limit transition metal availability at the dripwater-speleothem interface, exhibiting stabilities on the order Cu ≈ Co > Ni, whereas organic complexation of the alkaline earth metals Mg and Sr was virtually absent. Systematic variations of OMC stability with natural organic matter characteristics were not observed amongst water samples, whilst enhanced complexation was clearly evident in the comparably organic-rich soil extracts. Our results imply that the supply of transition metals to speleothems is inversely related to drip rate, increasing with drip interval via the decay of OMCs. This process appears most sensitive on time-scales relevant to typical speleothem-forming settings (<0 to ca. 40 drips min−1, corresponding to ca. <5.6 mL min−1), and therefore provides a general, mechanistic link to a quantitative proxy of palaeoclimatic cave drip rates.