Preservation of NOM-metal complexes in a modern hyperalkaline stalagmite: Implications for speleothem trace element geochemistry
Preservation of NOM-metal complexes in a modern hyperalkaline stalagmite: Implications for speleothem trace element geochemistry
复制标题
DOI：
10.1016/j.gca.2013.12.005
复制
发表时间：
2014-03
影响因子：
5
通讯作者：
A. Hartland;I. Fairchild;W. Müller;D. Domínguez-Villar
中科院分区：
文献类型：
--
作者：
A. Hartland;I. Fairchild;W. Müller;D. Domínguez-Villar
我们报道了首次对胶体天然有机质（NOM）和无机络合微量金属（V, Co, Cu, Ni）在洞穴中捕获的定量研究。这项研究结合了已发表的无机金属滴水形态测量、高分辨率激光烧蚀ICPMS （LA-ICPMS）和亚年稳定同位素比（δ18O和δ13C）、荧光和总有机碳（TOC）分析，这些分析来自英国德比郡Poole’s cave，形成于公元1997年至2008年之间。我们认为，本文报道的结果阐明了在pH 7-8的多个自然洞穴中观察到的胶体运输和方解石沉淀率变化引起的微量元素变化。我们发现，在胶体和溶解（~ 1 nm直径）之间的无机金属（aq）配合物显示出与碱土金属负相关的年周期性，并通过方解石沉淀率变化（由动力学分选的稳定同位素记录）来解释。这与NOM -金属络合反应的强度有关，导致非常强的结合金属（该体系中的Co）本质上记录了NOM共沉淀（三元络合）。更具体地说，表面活性金属（V, Co, Cu, Ni）之间的经验分配系数（Kd）值[以方解石和溶液中微量元素与Ca的比值表示]源于水溶液中总金属的“自由”部分（fm）的变化。因此，每种金属在方解石中保存的差异可以通过它们与水性NOM的络合行为来定量解释。在[游离金属]≪[总金属]的地方，由于金属与有机配体（以及潜在的无机胶体）之间的络合反应，无机物ickd值与金属在方解石中分配的现场测量值之间存在差异。可见，当efm≈0时，无机盐表观分配系数也≈0，但真正分配系数（实际分配系数Kd）明显更高。重要的是，对于最稳定的水相配合物，如图中所示，无机金属配合物的kdof（有机碳金属比）接近1，但对于V、Ni和Cu， kdof的值为24-150。这意味着三元表面络合（金属-配体共吸附）可以发生（如no - co），但这是例外，而不是规则。我们还证明了痕量金属通过改变水滴中NOM -金属络合模式来记录NOM组成信息的潜力。因此，石笋中痕量金属的变化明显可归因于有机配体浓度和组成的变化，并可能反映上覆地表生态系统的状态。
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 (∼1 nm 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 inorganicKdvalues 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 wherefm≈ 0, apparent inorganicKd appvalues are also ≈0, but the true partition coefficient (Kd actual) is significantly higher. Importantly, theKdof 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.