Combination Of Stable Isotopes Research Articles

Quantification of mixing processes in ore-forming hydrothermal systems by combination of stable isotope and fluid inclusion analyses

In the Schwarzwald area, southwest Germany, more than 400 hydrothermal veins hosting different gangue and ore mineral assemblages cross-cut the crystalline basement rocks. Many of the post-Variscan fluorite–barite–quartz veins are considered to have precipitated through mixing of a deep saline brine with meteoric, low salinity waters. This hypothesis was tested using carbon, sulfur, and oxygen isotope data of sulfides, sulfates and calcite, coupled with fluid inclusion studies. Primary hydrothermal calcites from the deposits show a positive correlation of their δ 13C (V-PDB) and δ 18O (V-SMOW) values, which range from −12 to −3‰ and from 12 to 18.5‰, respectively. Carbon and oxygen isotope compositions of paragenetically young, remobilized calcite types are shifted towards higher values and range from −12 to −1‰ and from 20 to 25‰, respectively. We developed an improved calculation procedure for modeling the covariation of carbon and oxygen isotopes in calcite resulting from mixing of two fluids with different isotopic compositions and total carbon concentrations. In our model, the carbon speciation in the two model fluid end-members and the fluid mixtures are calculated using a speciation and reaction path code. The carbon and oxygen isotope covariation of primary Schwarzwald calcites can effectively be modeled by a mixing trend of a deep saline brine and a meteoric, low salinity water. Sulfur isotope data of barites from 44 hydrothermal fluorite–barite–quartz veins vary from 9 to 18‰ (CDT), sulfide ore minerals show δ 34S values between −14.4 and 2.9‰. Calculated sulfide–sulfate equilibrium temperatures are in the range between 300 and 350 °C. These temperatures differ significantly from the formation temperatures of 150 to 200 °C of most of the deposits as estimated from fluid inclusions, and are interpreted as preserved paleotemperatures of the deep aquifer. This assumption has been carefully checked against possible contamination of an equilibrated sulfide–sulfate system from the deep aquifer with sulfate from surface-derived sources, considering also the kinetics of the sulfide-sulfate isotope exchange. A combination of the S isotopic results with microthermometric fluid inclusion data and constraints on the temperature of the meteoric water was used to calculate mixing ratios of the two fluid end-members. The results indicate that mass fractions of the deep saline brine in the mixed fluid were between 0.5 and 0.75. Considering all geologic, geochemical and isotopic information, we propose that the majority of the post-Variscan hydrothermal veins in the Schwarzwald area were precipitated by district-scale mixing of a homogeneous deep saline brine with meteoric waters.

Food partitioning within the species-rich benthic fish community of Lake Malawi, East Africa

Temporal diet patterns were studied for nine species of haplochromine cichlids in Lake Malawi, East Africa, using a combination of stable isotope and stomach content analyses. The results illustrate the complementarity of the two approaches in elucidating dietary differences. Evidence was found that benthic algal production contributed to the energy requirements of offshore fishes that occupy depths between 10 and 30 m. The importance of the lake fly Chaoborus edulis as a food source for demersal fishes was confirmed, supporting the recent hypothesis that deep demersal fish production is principally sustained through the pelagic food chain rather than from benthic detrital sources. Isotopic differences were observed among species with apparently similar diets, feeding behavior, and depth preferences, suggesting that important resource partitioning exists among Lake Malawi benthic haplochromine cichlids.

Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space

A combination of stable isotope and acetylene (0.01% v/v) inhibition techniques were used for the first time to determine N2O production during denitrification, autotrophic nitrification and heterotrophic nitrification in a fertilised (200 kg N ha−1) silt loam soil at contrasting (20–70%) water-filled pore space (WFPS). 15N-N2O emissions from 14NH415NO3 replicates were attributed to denitrification and 15N-N2O from 15NH415NO3 minus that from 14NH415NO3 replicates was attributed to nitrification and heterotrophic nitrification in the presence of acetylene, as there was no dissimilatory nitrate reduction to ammonium or immobilisation and remineralisation of 15N-NO3−. All of the N2O emitted at 70% WFPS (31.6 mg N2O-N m−2 over 24 days; 1.12 μg N2O-N g dry soil−1; 0.16% of N applied) was produced during denitrification, but at 35–60% WFPS nitrification was the main process producing N2O, accounting for 81% of 15N-N2O emitted at 60% WFPS, and 7.9 μg 15N-N2O m−2 (0.28 ng 15N-N2O g dry soil−1) was estimated to be emitted over 7 days during heterotrophic nitrification in the 50% WFPS treatment and accounted for 20% of 15N-N2O from this treatment. Denitrification was the predominant N2O-producing process at 20% WFPS (2.6 μg 15N-N2O m−2 over 7 days; 0.09 ng 15N-N2O g dry soil−1; 85% of 15N-N2O from this treatment) and may have been due to the occurrence of aerobic denitrification at this WFPS. Our results demonstrate the usefulness of a combined stable isotope and acetylene approach to quantify N2O emissions from different processes and to show that several processes may contribute to N2O emission from agricultural soils depending on soil WFPS.

Transpiration and groundwater uptake from farm forest plots of Casuarina glauca and Eucalyptus camaldulensis in saline areas of southeast Queensland, Australia

Plantings of salt-tolerant tree species are commonly used to manage shallow saline water tables in agricultural lands in Australia. Eucalyptus camaldulensis is often used for this purpose due to its salt tolerance and ability to use groundwater. Salt tolerance studies suggest that Casuarina glauca would also have groundwater management potential, however, little is known about its transpiration and groundwater uptake properties. A combination of stable isotope ( 2 H and 18 O ) techniques and sapflow measurements were used to determine groundwater discharge by both species at three saline sites. This included the use of a three-layer model for determining the proportions of water utilised from various soil depths. At one site, groundwater was the dominant water source for C. glauca in over 70% of sampling times. However, at a higher rainfall site, groundwater was the major water source for this species in only 40% of sampling times. E. camaldulensis relied less on groundwater than C. glauca, utilising water from mid-soil layers to a greater extent. Groundwater was the dominant water source for this species in 50% and 25% of sampling times at each of the two sites, with lower use of groundwater at the higher rainfall site. Low water use rates were observed in both the species; 1.5 to 3 mm day −1 in C. glauca and 1 to 3 mm day −1 in E. camaldulensis. Due to the low water use rates, groundwater discharge rates seldom exceeded 2 mm day −1, with the maximum discharge rate of 2.6 mm day −1 being recorded for C. glauca. As C. glauca relied more heavily on groundwater than did E. camaldulensis, and both species exhibited conservative water use rates, under the conditions of this study, C. glauca showed greater potential than E. camaldulensis to discharge saline groundwater.

Growth history and ecology of the Atlantic surf clam, Spisula solidissima (Dillwyn), as revealed by stable isotopes and annual shell increments

The shells of the Atlantic surf clam, Spisula solidissima (Dillwyn), contain a record of both life history and environmental changes. These shell records were investigated using oxygen and carbon stable isotopic analyses ( δ 18 O, δ 13 C) and shell growth increment analyses. δ 18 O variations across annual shell increments reflect the yearly cycle of sea-water temperatures off the New Jersey coast, further documenting the proposed annual periodicity of the major shell increments. The 11-yr shell record analyzed here confirms that shell growth is most rapid in spring-early summer, slow in late summer-fall, and extremely slow or non-existent in winter. Shell growth appears to occur in isotopic equilibrium with sea water and measured δ 18 O values are used to refine the aragonite-water temperature scale. Variations in the timing of annual growth increment formation are noted as well as ontogenetic effects upon the range of isotopic values recorded in shell carbonate. Both the δ 18 O and δ 13 C profiles are influenced by changes in the sea-water temperature regime over the 11-yr period studied (1965–1976) and record these effects in the shell. The combination of stable isotope and growth increment analyses provides a powerful tool for interpreting the shell records of both modern and fossil molluscs.