Cadmium Isotope Analytical Method And Its Paleo-ocean Environmental Significance

With the improvement of analytical techniques and the development of multiple-collector inductively coupled plasma mass spectrometry(MC-ICP-MS) since 1990s, the non-traditional stable isotopes are wide applied in the earth science, and provides a new means for explore the early eatth evolution, the origin of life, environmental change, and the deep geological process etc, and thus promoted a series of fundamental research from low-temperature biological effect to high-temperature magmatic process.Recently, the research result of the non-traditional stable isotopie, shows that the evaporation/condensation, biological processes and adsorption process of ferromanganese nodules could lead significant cadmium isotope fractionation. However, the geochemical research of the cadmium isotope is just in its infancy, conducting an investigation into the cadmium isotopic analytical techniques, the cadmium isotope fractionation mechanism, the distribution characteristics in natural reservoirs and a unified cadmium isotope standard calibration system could promote the application of the cadmium isotope in geosciences and environmental sciences.In this paper, based on the related literature, and modern instrucment on the State Key Laboratory of Biogeology and Environmental Geology, we discussed to establish a method to accurate determination of trace cadmium in various geological samples using closed acid digestion with ETAAS determine, the chemical pre-treatment and the measurement method of the cadmium isotopic ratio in geological samples using the Neptune Plus MC-ICP-MS, and then, used for 12 geochemical reference materials to obtain their cadmium isotopic compositions and characteristics. The proposed method were applied to determine the cadmium isotopic ratio in the samples collected from the strata of Permian and Triassic in Meishan, Zhejiang Province, and Majiashan, Chaohu, Anhui Province. Along with their trace element and cadmium isotope features, we carried out systemic study on the paleo-ocean environment indicative significance of cadmium isotope, the evolution process of paleo-ocean environment and primary productivity during Permian and Triassic.The major experimental works and conclusions of the application research are follows:(1) Establishing a method to accurate determination of trace cadmium in various geological samples.A method of accurate determination of trace Cd in various geological standard reference materials (SRMs) were established using closed acid digestion with ETAAS.0.05% Pd(NO3)2 solution was assayed as matrix modifiers, and the optimal pyrolysis and atomization temperatures for the real geological sample (SRM AGV-2) were 500 ℃ and 1600 ℃, which obtained the best sensitivity and lower background signal. The limits of quantitation (LOQ) and characteristic mass was 0.75 ng g-1 and 0.8 ± 0.1 pg, respectively. The proposed method was applied to determination of cadmium in 11 USGS geological related SRMs and 40 Chinese geological SRMs (soil, sediment and rock etc.). Most of the results (N=47) were found to be in good agreement with the certified values, or published literature values determined by isotope dilution mass spectrometry (ID-ICP-MS or ID-TIMS). Cadmium concentrations in dolerite DNC-la, granodiorite GSP-2, and carbonates COQ-1 SRMs are determined to be 0.079 ± 0.007 μg·g-1, 0.106 ± 0.008 μg·g-1, and 0.556 ± 0.042 μg·g-1, respectively. There are no previously reported Cd levels for these samples available. Ten separate aliquots of these SRMs were digested and analyzed over a period of three months and consistent results were obtained, indicating that our determined values are reasonable.(2) Establishing the pre-treatment method of separation and enrichment for cadmium in geological samples.By comparing the leaching effect of different amounts of resin and eluent, improved the pre-treatment method of separation and enrichment for cadmium in geological samples, cadmium was effective separated from the matrix by elution with AG-MP-1M anion exchange resin, The matrix elements (K, Na, Ca, Al, Fe, and Mg etc.), polyatomic interfered elements (Ge, Ga, Zr, Nb, Ru, and Mo), and isobaric interfered elements (In, Pd and most of Sn) were eluted using HCl with gradient descent concentrations (2,0.3,0.06,0.012 and 0.0012 mol·L-1). Against the problem of Sn incomplete separation in the conventional pre-treatment processes, the experiment increase the amount of resin and twice leaching. The results showed that the relative content of Sn was reduced effectively, the range of Sn/Cd content ratio from 0.15～0.18 to 0.004～0.018 in the leaching solution for receiving cadmium, and kept the recovery of Cd greater than 90%. The total procedural blank was 230pg and the ratio of δ114/110Cd reference solution was-0.03±0.08(N=3,2sd). The effects of procedural blank and column chemistry fractionation were effectively eliminated in the cadmium isotope analysis.(3) Establishing a high presision measurement method of the cadmium isotopic compositons using MC-ICP-MS.The cone combination of J+X was optimize to the cadmium isotope analysis in Neptune Plus multi-collector inductively coupled plasma mass spectrometry. A precision of 0.001～0.002% (RSD), an relative error of 0.06～0.14 (δ114/110Cd,2σ), and a long-term reproducibility of 0.12 (δ114/110Cd,2σ) were obtained. This experiment investigated the concentration effect and polyatomic ion interference effect on the determination of cadmium isotopes. The results showed that the Cd isotopic compositions are statistically indistinguishable from the reference value when sample/reference ratios are between 0.80 and 1.20; and the elements of Zr and Nb could cause uncorrectable interference to cadmium isotope analysis when the Zr/Cd, Nb/Cd content ratio greater than 0.01. Owing to pre-treament, the content of Zr and Nb was less than 10pg in the eluent of receving Cd which can ignore their effect on the cadmium isotope results.(4) Determinating the cadmium isotopic compositions in geochemical reference materials (reported as δ114/110Cd relative to NIST 3108 standard).Based on the established measurement method for geological samples of the cadmium isotopic compositions, the cadmium isotopic compositons were determined in different geochemical reference materials including of basalt BCR-2, BHVO-2, stream sediments GSD-3a, GSD-5a, GSD-7a, GSD-6, GSD-9, GSD-10, GSD-11, GSD-12, GSD-23 and soil GSS-1 (reported as δ114/110Cd relative to NIST 3108 standard). The isotopic composition significantly in GSS-1(δ114/110 value of 0.20) heavier than the NIST 3108 and GSD-12. The cadmium isotopic composition of GSD-11(δ144/110Cd=-0.19) was lighter than the NIST 3108. The cadmium isotopic composition were found to be in good agreement with the published literature values. The δ114/110Cd value in the basalt BCR-2 and BHVO-2 were 1.01±0.23 and 0.94±0.52, respectively. The cadmium isotopic compositions of stream sediments were in the range of -0.25～+0.26, obviously different from the one of basalt BCR-2, BHVO-2.(5) Determinating the trace elements and cadmium isotope composition of Permian-Triassic transitional sections at Meishan and Chaohu.There were several mass extinction events in the earth’s history, and the Permian-Triassic event is one of the most extensive, which destroyed 90% marine species,70% land vertebrate genera and most land vegetation. This thesis studied the trace elements and cadmium isotope composition of Permian-Triassic transitional sections at Meishan (Changxing, Zhejiang Province) and Chaohu (Anhui Province), South China. The Cd isotope index was applicated to trace the evolution process of paleo-ocean environment and primary productivity during Permian and Triassic. Using modern cadmium-isotopic cycling as our interpretative framework, we suggest that the cadmium-isotopic composition of Upper Permian rocks reflects the substable ocean condition of high primary productivity-vigorous upwelling-incomplete consumption, and Lower Triassic rocks reflects the stagnation ocean condition of low primary productivity-incomplete consumption-anoxia. The primary productivity declined rapidly and the ocean environment deteriorated sharply during Permian-Triassic transition.