Trace element and kinetic data from competitive ligand exchange experiments on water and soils from eight Aotearoa-New Zealand caves

This dataset provides all data discussed and presented in the manuscript "Characterising the decay of organic metal complexes in speleothem-forming cave waters" by Sebastian N. Höpker, Sebastian F. M. Breitenbach, Megan Grainger, Claudine H. Stirling, and Adam Hartland.

The dataset is the basis for the kinetic characterisation of organic metal complex (OMC) dissociation rates in water samples (drips and cave streams) and soil extracts (1:10 weight:volume CaCl2 extraction) from eight cave systems in Aotearoa-New Zealand. The study aimed to explore the magnitude and variability of OMC stability in various karst systems to assess the role of OMC decay in determining speleothem (cave carbonate) trace metal concentrations. Kinetic experiments were conducted using competitive ligand exchange (CLE) with Chelex-100 resin as competing ligand. Cave water samples were spiked with 5 ppb of Co, Ni, Cu prior to analysis. Experiments were carried out at solution pH 8.5 and ca. 22 °C. Upon addition of Chelex-100, 11 aliquots were taken over the course of 2 hours via a syringe, and immediately filtered using 0.45 micron filters. Quantification of metal concentrations as reported here was carried out by inductively-coupled plasma mass spectrometry (ICP-MS). Most experiments clearly depict an exponential decrease in metal concentrations in the test solutions over time, reflecting the dissociation of OMCs present in the solution. The data were used to fit kinetic models and derive dissociation rate constants discussed in the manuscript. The dataset further includes all manuscript tables and supplementary tables, as well as data used in a simple forward model presented in the manuscript. Data are provided in individual tabs. A brief description of each tab and variable is provided in the tab "README".

Our experiments 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 NOM 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.