Phosphorus and arsenic distributions in a seasonally-stratified, iron- and manganese-rich lake: microbiological and geochemical controls

Seasonal stratification in temperate lakes greater than a few metres deep provides conditions amenable to pronounced vertical zonation of redox chemistry. Such changes are particularly evident in eutrophic systems where high phytoplankton biomass often leads to seasonally-established anoxic hypolimnia and profound changes in geochemical conditions. In this study, we investigated the behaviour of trace elements in the water column of a seasonally-stratified, eutrophic lake. Two consecutive years of data from Lake Ngapouri, North Island, New Zealand, demonstrate the occurrence of highly correlated profiles of phosphorus (P), arsenic (As), iron (Fe) and manganese (Mn), all of which increased in concentration by 1-2 orders of magnitude within the anoxic hypolimnion. Stoichiometric and mass-balance considerations demonstrate that increases in alkalinity in hypolimnetic waters were consistent with observed changes in sulfate, Fe and Mn concentrations with depth, corresponding to dissimilatory reduction of sulfate, Fe(III) and Mn(IV) hydroxides. Thermodynamic constraints on Fe, Mn and Al solubility indicate that amorphous Fe(III), Mn(IV) hydroxides most probably controlled Fe and Mn in the surface mixed layer (~0 to 8 m) while Al(III) hydroxides were supersaturated throughout the entire system. Surface complexation modelling indicated that iron hydroxides (HFO) potentially dominated As speciation in the lake. It is likely that other colloidal phases such as allophanic clays also limited HPO42- activity, reducing competition for HAsO42- adsorption to iron hydroxides. This research highlights the coupling of P, As, Fe and Mn in Lake Ngapouri, and the apparent role of multiple colloidal phases in affecting P and As activity within overarching microbiological and geochemical processes

Transition metal availability to speleothems controlled by organic binding ligands

Speleothems are important archives of Quaternary palaeoclimate. However, climate proxies based on trace elements in speleothems are currently limited to the metals which exhibit simple partitioning (Mg²⁺, Sr²⁺, Ba²⁺). This study aims to expand understanding of the processes controlling the divalent first row transition metals (M²⁺) in these systems. Adsorptive cathodic stripping voltammetry was used to determine Cu speciation in speleothem dripwater, stream and pool samples from five limestone caves located in diverse climatic settings. Our results demonstrate that Cu binding and stabilisation by organic ligands (L) is a universal property of cave waters, which decreases the available Cu concentration ([Cu’]) by ~5 orders of magnitude relative to total Cu concentration ([CuT]). Furthermore, [Cu’] does not change meaningfully with increases in either [CuT] or [L], meaning that Cu²⁺ substitution in Ca²⁺ valence sites in precipitating CaCO₃ speleothems is likely to be inhibited by organic complexation. We suggest that the residence time of speleothem thin water films (1/drip rate), the dissociation rates of labile metal-organic complexes, and the stability of adsorbed (ternary) metal-organic complexes will determine M²⁺ incorporation in speleothems

Use of cadmium isotopes to distinguish sources of cadmium in New Zealand agricultural soil: Preliminary results

In New Zealand’s agricultural soils, phosphate fertiliser applications are the main source of cadmium (Cd). In 1997, the NZ fertiliser industry discontinued sourcing rock phosphate from Nauru (about 450 mg Cd/ Kg P) and began producing superphosphate from other rock phosphate sources (such as Morocco), which have generally lower concentrations of Cd. Research on the concentration of Cd in soils from the long-term irrigation trials at the Winchmore research farm (Canterbury) indicates that Cd accumulation rates have started to slow in the period since 1997 (Fig. 1) (McDowell 2012)

Tracing sources of cadmium in agricultural soils using cadmium stable isotopes

The application of phosphate fertilizers has, on a global basis, resulted in long-term accumulation of cadmium (Cd) in agricultural soils [1]. While this accumulation has led to concern over potential environmental consequences, we currently lack a viable tool to track fertilizer- derived Cd in terrestrial environments. In 1997, the main source of phosphate fertilizers in New Zealand (NZ) was changed from Nauru to a mixed product sourced from other phosphorites with lower concentrations of Cd. Around the same time, Cd accumulation in a 66-year-long field trial (Winchmore Farm, South Island, NZ) showed an apparent plateau [2]. In this study, Cd isotope ratios (ɛ114/110Cd) were used to trace Cd sources in Winchmore soil and determine the cause of this plateau. The ɛ114/110Cd was measured in archived phosphate fertilizer, phosphorite and topsoil (0-7.5 cm) samples from Winchmore. The ɛ114/110Cd of fertilized topsoils and fertilizers was distinct from control (unfertilized) subsoils by around +0.6‰. Bayesian isotope modelling using pre- and post-2000 fertilizers and control soil as the endmembers, confirmed the dominant contribution of Cd is from pre-2000 fertilizers (ɛ114/110Cd=2.48±0.37) with signature comparable to source rocks (ɛ114/110Cd=2.19± 0.39) but distinct from control subsoil (ɛ114/110Cd=-3.33 ±0.41). The decline in Cd concentration after 2000 followed the reduction in fertilizer Cd concentration. The ɛ114/110Cd of soil remained quite constant following the source change, confirming that soil Cd represents the historical burden of Cd (originating from Nauru phosphorites) and concurrent applications of fertilizer are not resulting in further accumulation of Cd

Speleothem growth intervals reflect New Zealand montane vegetation response to temperature change over the last glacial cycle

Flowstone speleothem growth beneath Mount Arthur, New Zealand shows a clear relationship to vegetation density and soil development on the surface above. Flowstone does not currently form beneath sub-alpine Nothofagus forest above ca. 1000–1100 m altitude but U-Th dating shows it has formed there during past intervals of warmer-than-present conditions including an early–mid Holocene optimum and the last interglacial from ca. 131–119 ka. Some flowstones growing beneath ca. 600 m surface altitude, currently mantled with dense broadleaf-podocarp forest, grew during full glacial conditions, indicating that local tree line was never below this altitude. This implies that Last Glacial Maximum annual temperature was no more than ca. 4 °C cooler than today. Flowstone growth appears to be a robust indicator of dense surface vegetation and well-developed soil cover in this setting, and indicates that past interglacial climates of MIS 7e, 5e, the early–mid Holocene and possibly MIS 5a were more conducive to growth of trees than was the late Holocene, reflecting regional temperature changes similar in timing to Antarctic temperature changes. Here, flowstone speleothem growth is a sensitive indicator of vegetation density at high altitude, but may respond to other factors at lower altitudes

Condensation corrosion alters the oxygen and carbon isotope ratios of speleothem and limestone surfaces

Condensation corrosion is a natural process which enhances the chemical weathering of limestone cave chambers and speleothems. We evaluated the use of carbonate tablets for detecting condensation corrosion in Glowworm Cave, New Zealand, using local limestone and speleothem as experimental substrates (herein tablets). Evidence for condensation corrosion was assessed via three methods: gravimetric (mass wasting), microscopic (surface pitting, recrystallization) and isotopic (δ¹³C and δ¹⁸O changes). Our results show little evidence of tablet mass loss throughout a 6-month deployment period. However, SEM imaging and isotope analysis (δ¹³C and δ¹⁸O) of the upper ∼50 μm layer of the tablets, suggest that condensation corrosion operates in the cave, especially in sectors affected by large diurnal microclimate variations. Most notably, condensation water altered the tablet surface δ¹³C and δ¹⁸O values. Small, positive shifts in surface δ¹³C and δ¹⁸O values are considered to reflect pure dissolution (where dissolution favours the removal of lighter isotopologues). In contrast, tablets that exhibited large positive shifts in δ¹³C in tandem with large negative shifts in δ¹⁸O values, are interpreted as showing calcite recrystallization and the inheritance of higher DIC δ¹³C values (¹³C fractionation by CO₂ degassing), lighter water δ¹⁸O values and/or kinetic fractionation of δ¹⁸O. This study therefore demonstrates that stable isotopes could be applied to detect paleoclimatic episodes of condensation corrosion in speleothems

Cadmium accumulation in three contrasting New Zealand soils with the same phosphate fertilizer history

Cadmium (Cd) concentration in New Zealand (NZ) agricultural soils has increased due to phosphate fertilizer application, but it is not clear whether soils with different properties accumulate Cd at similar rates for given P loadings. Here, the distribution of Cd was measured in three soils: the well-drained Horotiu series (Orthic Allophanic Soil in NZ soil classification, Typic Hapludand in US soil taxonomy), poorly-drained Te Kowhai series (Orthic Gley Soil in NZ classification, Typic Humaquept in US soil taxonomy) and an intergrade between them, Bruntwood series (Impeded Allophanic Soil in NZ soil classification, Aquic Hapludand in US soil taxonomy). All three soils often occur in the same paddock with the same fertilizer history, but have differing drainage and mineralogical characteristics, permitting an assessment of the potential for varying accumulation/translocation of Cd in contrasting soil conditions. Thirty soil profiles from ten paddocks on a dairy farm near Hamilton, NZ, with a uniform fertilizer history were sampled to depth of 60 cm. The Cd concentration in topsoil (0–7.5 cm) samples (mean of 0.79 mg kg−1 ) was about 7–8 times greater than in deeper horizons (P b 0.001). No significant differences in Cd concentration or fractionation among the soil series were detected. Cluster analysis showed that Cd, phosphorus (P) and uranium (U) were highly correlated, consistent with a common source, most likely phosphate fertilizer. The absence of a difference in the Cd depth profiles in the three soils indicates that Cd was preferentially adsorbed to the topsoil and was not significantly mobilized by drainage in the soils. The lack of difference in Cd distribution between contrasting soil series supports the use of one Cd management system tool for all of these soils

Preservation of NOM-metal complexes in a modern hyperalkaline stalagmite: Implications for speleothem trace element geochemistry

AbstractWe 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

Sampling rate-corrected analysis of irregularly sampled time series

The analysis of irregularly sampled time series remains a challenging task requiring methods that account for continuous and abrupt changes of sampling resolution without introducing additional biases. The edit distance is an effective metric to quantitatively compare time series segments of unequal length by computing the cost of transforming one segment into the other. We show that transformation costs generally exhibit a nontrivial relationship with local sampling rate. If the sampling resolution undergoes strong variations, this effect impedes unbiased comparison between different time episodes. We study the impact of this effect on recurrence quantification analysis, a framework that is well suited for identifying regime shifts in nonlinear time series. A constrained randomization approach is put forward to correct for the biased recurrence quantification measures. This strategy involves the generation of a type of time series and time axis surrogates which we call sampling-rate-constrained (SRC) surrogates. We demonstrate the effectiveness of the proposed approach with a synthetic example and an irregularly sampled speleothem proxy record from Niue island in the central tropical Pacific. Application of the proposed correction scheme identifies a spurious transition that is solely imposed by an abrupt shift in sampling rate and uncovers periods of reduced seasonal rainfall predictability associated with enhanced El Niño-Southern Oscillation and tropical cyclone activity