Precise carbon isotopic ratio analyses of micro amounts of carbonate and non-carbonate in basalt using continuous-flow isotope ratio mass spectrometry.

RATIONALE: Continuous-flow isotope ratio mass spectrometry (CF-IRMS) is a specialized technique used to quickly analyze very small amounts of sample. We have used CF-IRMS to assess the influences of sample weight and relative carbon content on the accuracy and precision of the δ13 C values of micro amounts of carbonate and non-carbonate in silicate rocks. METHODS: The analytical work was performed on a Gasbench II (GB) sample preparation device and on an Elemental Analyzer (EA), which were both interfaced to CF-IRMS instruments. Potential silicate matrix effects on the carbon isotopic analyses were investigated by measuring mixtures of calibrated carbon reference materials and quartz powder. The calibration lines, established by the measured raw values and the known values of three reference materials mixed with quartz powder, were used to calibrate the δ13 C values of basalt samples from eastern China. RESULTS: The δ13 C values measured by GB-CF-IRMS of one national carbonate reference material, GBW04416, deviate slightly from the known value for approximately 20-70 µg of carbonate contained in 4.5-mL vials; the smaller the sample size, the lower the measured δ13 C values. External precision better than 0.1‰ (1σ, n = 26) is achieved at a signal intensity for mass 44 of between 868 and 1614 mV, corresponding to a sample weight of 30.8-50.2 µg, whereas it is reduced to 0.27‰ (1σ, n = 34) at a signal intensity between 519 and 1614 mV, corresponding to a sample weight of 21.1-50.2 µg. In the EA-CF-IRMS experiments for non-carbonate carbon, at high carbon concentration (greater than 800 ppm) and at optimum sample weights, the accuracy and precision are both better than 0.2‰. For carbon concentrations less than 500 ppm, the measured δ13 C values deviate from the average by up to -1.2‰ and the precision is 0.74‰. CONCLUSIONS: The measured δ13 C values decrease substantially at lower carbon concentration and higher sample weights, and poorer precision is obtained. Suggestions are made to measure repeatedly the same carbon concentration of sample and reference materials in order to obtain not only reproducible, but also accurate carbon isotope ratios.

An online method combining a Gasbench II with continuous flow isotope ratio mass spectrometry to determine the content and isotopic compositions of minor amounts of carbonate in silicate rocks.

An online method using continuous flow isotope ratio mass spectrometry (CF-IRMS) interfaced with a Gasbench II device was established to analyze carbon and oxygen isotopic compositions and to estimate the content of minor amounts of carbonate in silicate rocks. The mixtures of standard materials and high-purity quartz are firstly used to calibrate different quantities of carbonate in silicates. The results suggest that the accuracy and precision of the online analysis are both better than those obtained using an offline method. There is a positive correlation between the carbonate weight and the Mass44 ion beam intensity (or peak area). When the weight of carbonate in the mixtures is greater than 70 microg (equal to approximately 1800 mV Mass44 ion beam intensity), the delta(13)C and delta(18)O values of samples usually have accuracy and precision of +/-0.1 per thousand and +/-0.2 per thousand (1sigma), respectively. If the weight is less than 70 microg, some limitations (e.g., not perfectly linear) are encountered that significantly reduce the accuracy and precision. The measured delta(18)O values are systematically lower than the true values by -0.3 to -0.7 per thousand; the lower the carbonate content, the lower the measured delta(18)O value. For samples with lower carbonate content, the required phosphoric acid doses are higher and more oxygen isotope exchanges with the water in the phosphoric acid. To guarantee accurate results with high precision, multiple analyses of in-house standards and an artificial MERCK sample with delta(13)C values from -35.58 to 1.61 per thousand and delta(18)O from 6.04 to 18.96 per thousand were analyzed simultaneously with the unknown sample. This enables correction of the measured raw data for the natural sample based on multiple-point normalization. The results indicate that the method can be successfully applied to a range of natural rocks.

Enhanced weathering as a trigger for the rise of atmospheric O2 level from the late Ediacaran to the early Cambrian.

A shift toward a higher oxygen level in both ocean and atmosphere systems during the late Ediacaran to the early Cambrian has been suggested from multiple indirect proxies. However, the mechanism and magnitude of this oxidation remain unclear. To solve this issue, we measured carbon isotopes in both carbonate and organic matter as well as their trace element compositions for an Ediacaran-Cambrian sequence in the Lower Yangtze basin, South China. The δ13Corg and δ13Ccarb excursions of this sequence are coupled and can be compared with contemporaneous global carbon isotope curves. A 2‰ rise in Δ13Ccarb-org occurred from the late Ediacaran to the early Cambrian, suggesting a substantial increase in atmospheric oxygen level from 16% to 30% of the present atmospheric level (PAL). Furthermore, the distribution pattern of rare earth elements and the concentrations of water-insoluble elements in the carbonates indicate a sudden enhancement in chemical weathering of the continental crust during the early Cambrian, which may be a trigger for the rise of atmospheric O2 level. Both the supply of a large amount of nutrients due to the enhanced continental weathering and the contemporary increase of atmospheric oxygen concentrations may have promoted the appearance of large metazoans in the early Cambrian.