Abstract
We report the first quantitative study of the capture of colloidal natural organic matter (NOM) and NOM-complexed trace metals (V, Co, Cu, Ni) in speleothems. This study combines published NOM–metal dripwater speciation measurements with high-resolution laser ablation ICPMS (LA-ICPMS) and sub-annual stable isotope ratio (δ18O and δ13C), fluorescence and total organic carbon (TOC) analyses of a fast-growing hyperalkaline stalagmite (pH ∼11) from Poole’s Cavern, Derbyshire UK, which formed between 1997 and 2008 AD. We suggest that the findings reported here elucidate trace element variations arising from colloidal transport and calcite precipitation rate changes observed in multiple, natural speleothems deposited at ca. pH 7–8. We find that NOM–metal(aq) complexes on the boundary between colloidal and dissolved (∼1nm diameter) show an annual cyclicity which is inversely correlated with the alkaline earth metals and is explained by calcite precipitation rate changes (as recorded by kinetically-fractionated stable isotopes). This relates to the strength of the NOM–metal complexation reaction, resulting in very strongly bound metals (Co in this system) essentially recording NOM co-precipitation (ternary complexation). More specifically, empirical partition coefficient (Kd) values between surface-reactive metals (V, Co, Cu, Ni) [expressed as ratio of trace element to Ca ratios in calcite and in solution] arise from variations in the ‘free’ fraction of total metal in aqueous solution (fm). Hence, differences in the preservation of each metal in calcite can be explained quantitatively by their complexation behaviour with aqueous NOM. Differences between inorganic Kd values and field measurements for metal partitioning into calcite occur where [free metal]≪[total metal] due to complexation reactions between metals and organic ligands (and potentially inorganic colloids). It follows that where fm≈0, apparent inorganic Kd app values are also ≈0, but the true partition coefficient (Kd actual) is significantly higher. Importantly, the Kd of NOM–metal complexes [organic carbon–metal ratio) approaches 1 for the most stable aqueous complexes, as is shown here for Co, but has values of 24–150 for V, Ni and Cu. This implies that ternary surface complexation (metal–ligand co-adsorption) can occur (as for NOM–Co), but is the exception rather than the rule. We also demonstrate the potential for trace metals to record information on NOM composition as expressed through changing NOM–metal complexation patterns in dripwaters. Therefore, a suite of trace metals in stalagmites show variations clearly attributable to changes in organic ligand concentration and composition, and which potentially reflect the state of overlying surface ecosystems.
Highlights
IntroductionCave systems in humid and temperate climates are typically characterised by a distinct seasonality in both external temperature and infiltrating precipitation (affecting ventilation and hydrogeochemical processes), and this seasonality is reflected in the chemical, isotopic and optical compositions of speleothems (Fairchild et al, 2001; Frisia et al, 2011; Fairchild and Baker, 2012;)
Cave systems in humid and temperate climates are typically characterised by a distinct seasonality in both external temperature and infiltrating precipitation, and this seasonality is reflected in the chemical, isotopic and optical compositions of speleothems (Fairchild et al, 2001; Frisia et al, 2011; Fairchild and Baker, 2012;)
We investigate the degree to which speleothems capture natural organic matter (NOM)-metal complexes from solution by focusing on a hyperalkaline system where the aqueous NOM–metal species have been previously characterised (Hartland et al, 2011)
Summary
Cave systems in humid and temperate climates are typically characterised by a distinct seasonality in both external temperature and infiltrating precipitation (affecting ventilation and hydrogeochemical processes), and this seasonality is reflected in the chemical, isotopic and optical compositions of speleothems (Fairchild et al, 2001; Frisia et al, 2011; Fairchild and Baker, 2012;). Include the increased stability of natural organic matter (NOM) in colloidal (1–1000 nm) and particulate (>1000 nm) size ranges arising from electrostatic effects (Hartland et al, 2010a,b); and stronger binding of metals by NOM at increasing pH demonstrated by higher Mz+/ NOM ratios and slower dissociation kinetics than observed at circum-neutral pH values (Fairchild and Hartland, 2010; Hartland et al, 2011); This study capitalises on the stronger binding between metals (most notably Co) and NOM in hyperalkaline dripwater PE1 (Hartland et al, 2011) to investigate the partitioning of NOM–metal complexes into the conjugate, fast-growing PC-08-1 stalagmite. We review the data from experiments and studies of normal (ca. pH 7–8) cave waters and conjugate speleothems before discussing the most relevant information from recent studies at Poole’s Cavern
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
