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RESEARCH PRODUCT

Effects of organic matter complexation on partitioning of transition metals into calcite: Cave-analogue crystal growth experiments

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We highlight the potential for first row transition metals to carry paleohydrological signals in speleothems, and argue that these metals constitute valuable proxies for climate reconstructions. Metal availability to speleothems is hypothesised to be restricted by organic complexation, which strongly limits the free ion activity of transition metals (Co2+, Ni2+ and Cu2+) in dripwater, thereby creating a kinetic overprint on metal concentrations and isotope ratios in speleothem calcite. This study presents the results of the first cave-analogue experiments of transition metal partitioning into calcite in the absence and presence of organic ligands. The Geological Microclimate (GeoMic) system establishes homeostatic control of air temperature and atmospheric pCO2 (set to 20 ± 0.1 °C and 1000 ± 10 ppm, respectively), and regulates relative humidity to cave-analogue levels (>90%). Calcite was precipitated from flowing aqueous films on inclined glass plates to assess the effect of prior calcite precipitation (PCP) on metal partitioning. We report speleothem-specific inorganic Kd values for Co, Ni and Cu of ∼4, 1, and 44, respectively. Pronounced PCP effects were observed in Co and Cu in the inorganic experiments, leading to strong reduction in Cu/Ca and Co/Ca, whereas Ni/Ca molar ratios remained constant. Introduction of the organic chelating ligand nitrilotriacetic acid (NTA) overturned the behaviour of all three metals, leading to stable solution Cu/Ca ratios and progressive enrichment of Co/Ca and Ni/Ca ratios as PCP progressed and apparent Kd values <1 for all three metals. This result is confirmed in a Cu-fulvic acid experiment. We show that in the presence of organic ligands with stability constants close to those observed in natural systems, the partitioning of transition metals into calcite is dependent on the fluid residence time and the stability of the metal–ligand complexes. Therefore, drip rate should be the first-order control on transition metal concentrations in stalagmites: opening the way to a novel and potentially quantitative paleohydrological proxy.