Kinetic metals: reconstructing past cave drip rates using the “decay” of organic metal complexes (OMCs)

Speleothems (secondary cave carbonates) are exceptional archives for the study of past climate and environments over a range of temporal and spatial scales. Conventional speleothem proxies, such as stable oxygen isotopes (δ¹⁸O), are increasingly bolstered by trace element to Ca ratios, providing increased certainty regarding local/regional hydroclimatic dynamics.

Most studies utilising trace element records limit analyses to the alkaline earth metals (primarily Mg, Sr, Ba), which are most commonly interpreted to reflect drying and wetting within the karst system. This interpretation is based on the susceptibility of these elements to prior carbonate precipitation (PCP) and their relatively predictable partitioning between infiltrating water and the carbonate crystal phase. However, this approach rarely allows for quantitative hydroclimate reconstructions, and in many cases may unperceivably be compromised by similar chemical signals generated by other processes in the karst (e.g., incongruent calcite dissolution). While numerous other trace elements are incorporated into speleothems, their systematics and controls are far less constrained, and typically require statistical models to derive any potential links with environmental processes.

Here we aim to develop a more mechanistic understanding of the partitioning of selected transition metals (Ni, Co, Cu) with view to establishing a novel quantitative hydrological proxy. The transport of these elements from the surface to the cave is governed by binding to organic matter present in percolating waters. The rate of dissociation, or “decay”, of such organic metal complexes (OMCs) at the dripwater-stalagmite interface is suggested to determine the availability of these elements for inclusion into the carbonate (Hartland & Zitoun, 2018). By extension, this link between OMC dissociation and metal availability for capture by stalagmites offers an opportunity to quantify past drip rates because the resulting carbonate metal concentrations are time-dependent.

We present results from a series of Competitive Ligand Exchange (CLE) experiments aimed to assess OMC dissociation kinetics in water samples collected from various New Zealand caves. Our study demonstrates how organic ligands constrain transition metal partitioning from dripwaters to speleothems, and provides first quantitative estimates of the time-dependent release of metals for the inclusion in the latter. We argue that in absence of detrital contamination, this kinetic control presents the overriding mechanism for metal availability at stalagmite surfaces, and thus effectively dictates M/Ca ratios in stalagmites.

References:

Hartland, A., Zitoun, R. (2018) Transition metal availability to speleothems controlled by organic binding ligands. Geochem. Persp. Let. 8, 22–25.