Organic complexation in karst cave systems: implications for transition metal incorporation into speleothems
The accurate prediction of future climate scenarios heavily relies on the robust understanding of the magnitude and drivers of past climate variability. Amongst the various materials used to study historic climate beyond the instrumental record, secondary cave carbonate deposits, or speleothems, have proven to present particularly valuable archives of numerous environmental processes over a range of temporal and spatial scales. In this context, trace elements included in speleothems are increasingly used to bolster speleothem-based records of past climates and environments (often primarily based on stable isotope proxies), with Mg/Ca and Sr/Ca ratios presenting the most widely interpreted elemental signatures indicative of hydrological processes in the karst. More recently, however, a novel proxy system based on the concentrations of first-row transition metals in stalagmites has been proposed to potentially offer a first means to quantitatively reconstruct past cave drip rates, and hydroclimate by extension. The transport from the surface to the site of speleothem formation of transition metals, such as Co, Ni, or Cu, is understood to be largely governed by the formation of organic metal complexes (OMCs) with ligands present in natural organic matter (NOM). This organic association can evidently facilitate the deposition of organically-complexed metals in response to strong infiltration events, however, has also been suggested to enable a mechanistically distinct and largely undefined inclusion pathway for metals into calcite. Specifically, the availability of transition metals at speleothem surfaces for partitioning into the crystal phase may arguably be driven by the rate-dependant dissociation (or ‘decay’) of OMCs, which is in turn predicted to depend on their residence time at speleothem surfaces. OMC decay kinetics may thus present a potentially viable link between metal concentrations in stalagmites, and the residence time of OMCs at their surfaces. This thesis further investigates these interactions between selected first-row transition metals (Co, Ni, Cu) with NOM, aiming to advance the applicability of respective metal signatures in speleothems to palaeoclimatic reconstructions. In the first research chapter, the decay of OMCs is characterised in a first comparative study of in-cave OMC dissociation kinetic signatures by means of competitive ligand exchange experiments. Performed on water samples and soil extracts from eight Aotearoa New Zealand caves, the findings demonstrated that natural organic ligands decisively limit transition metal availability (by example of Co, Ni, Cu) at the dripwater-speleothem interface, whereas alkaline earth metals (here: Mg, Sr) are essentially unaffected by organic interactions in solution. OMC stability was found to occur in the hierarchy of Cu ≈ Co > Ni, with a variable fraction of all three metals bound very strongly to effectively inert complexes. OMC stability was overall enhanced in soil extracts, presumably due to higher organic content and aromaticity. The study further uses empirical estimates of OMC decay rate constants to assess the time-dependent release of metals at stalagmite surfaces in a simple forward model. This exercise predicted that the decay of transition metal complexes was most sensitive on time-scales relevant to typical cave drip points (up to ca. 40 drips min-1), and increasingly so towards lower flow rates. The second research chapter comprises experimental and field-based measurements of the inclusion rates of Co, Ni, Cu, as well as Mg, Sr. Firstly, a set of ten cave-analogue experiments were aimed to test for kinetic signatures in transition metal concentrations linked to the decay of OMCs during calcite precipitation. Performed at a range of drip rates and with variable concentrations of organic ligands in solution (Suwannee River Fulvic Acid (SRFA) and nitrilotriacetic acid (NTA)), however, the experimental calcite precipitates primarily depicted a direct inclusion of OMCs without prior dissociation. This was particularly pronounced for metal-SRFA complexes due to a considerable degree of co-precipitation of SRFA. Signatures attributable to OMC decay were in turn not discernible, presumably due to experimental conditions preventing their resolution. The study further yielded new estimates of inorganic partition coefficients for Co (≈1.8), Ni (≈0.4), Mg (≈0.04), Sr (≈0.09), and Cu (≈13 on average, but up to ca. 57), with the latter showing a pronounced positive dependency on drip rate. In the second part of this study, a wide range of new and previously published datasets on experimental and in-cave metal partitioning were compiled, which collectively allowed for the establishment of a conceptual framework around hypothesised system-specific conditions determining the dominant drivers of transition metal concentrations in dripwater and stalagmites. The final research chapter explores elemental systematics in two caves on the South Pacific island of Niue, aiming to utilise modern field observations to corroborate stalagmite-based palaeoclimate reconstructions of Holocene climate variability. Intermittent cave monitoring between September 2019 and November 2022 in principle supported the assumption that stalagmites from these caves record environmental conditions in their chemistry and binary lamination. Although reliable chronologies could not be established for the two stalagmite samples analyses in this study, their trace elemental and reconnaissance stable isotope measurements suggested that the most pronounced elemental signal of Mg/Ca ratios reflected a combined control of water-rock interactions, prior calcite precipitation (both indicative of rainfall amount), and marine aerosol inputs. The analysis further implied that Ni/Ca (and Cu, Co, and Zn to a lesser extent) also primarily reflected local hydrology, predominantly exhibiting behaviour consistent with that expected for a pervasive kinetic drip rate control. Based on preliminary interpretations, OMC decay presented the primary driver of transition metal concentrations in the deposits from Anapala Cave, while short-lived ‘soil-flushing’ peaks in various elemental concentrations in response to heavy rainfall events only sporadically defined the record. Collectively, albeit warranting further investigation, the results of this thesis provide new systematic insights into the role of organic complexation for transition metal incorporation into calcite speleothems, thereby presenting important precedent for the further development of a novel (semi-)quantitative class of hydrological proxies.
Carbonate, Climate, Geochemistry, Cave, South Pacific, Trace metal, Organic matter
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