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Project Progress

Three protocols have been developed to measure δ¹³C of organic matter: 1) 6 μm beam spots with two faraday cup (FC) detectors, 2) 1 μm beam with a FC for ¹²C and an electron multiplier (EM) for ¹³C, and 3) 10 μm, high intensity beam with one FC and one EM for materials with very low carbon concentration (including microfossils in chert). Average reproducibility (2 SD) for these three protocols is 0.4, 0.8, and 2.1‰, respectively. A series of experiments was conducted to assess the impact of variable H/C and mineral matrix on the accuracy of in situ organic carbon isotope measurements for the first two analytical protocols. These experiments led to the development of a suite of 12 standard organic materials with a wide range of H/C. A subset of these standards is analyzed at the beginning of each session to calibrate the relationship of ¹³CH/¹³C to instrumental bias and allows correction factors to be applied based upon ¹³CH/¹³C measurements made on each spot during δ¹³C analysis of samples. A suite of carbonaceous cherts with a range of H/C is being evaluated to assess this effect for the high intensity beam conditions used to analyze microfossils, and this will allow a similar calibration if necessary. In terms of mineral matrix effects, we find that reproducibility is stable at ¹²C count rates down to ~10% that of the standard for the first two protocols, and ~25% of the standard for the microfossil protocol. Additionally, we have demonstrated that it is possible with a 6 μm beam to analyze small organic carbon domains together with up to 70% carbonate mineral matrix with the same reproducibility as analyses made in a silicate matrix, due to the extremely low ionization efficiency and lower© of carbonate relative to organic carbon.

1. In situ carbon isotope analysis of Archean kerogen

More than 100 in situ analyses of kerogen and pyrobitumen from samples of the SV1 (Tumbiana Formation, ~2.7 Ga), WRL1 (Wittenoom Formation, Marra Mamba Iron Formation, and Jeerinah Formation, ~2.6 Ga), RHDH2a (Carrawine Dolomite and Jeerinah Formation, ~2.6 Ga) and ABDP-9 (Mount McRae Shale, ~2.5 Ga) drill cores have revealed significant μm-scale heterogeneity in δ¹³C. Pyrobitumen nodules around a central, uraniferous mineral grain are typically ¹³C-enriched compared to kerogen without such mineral associations in the same sample. Overall, these results support previous evidence for depth gradients in oxidation driven by oxygenic photosynthesis and methane metabolisms in Neoarchean environments.

2. In situ carbon isotope analysis of Proterozoic microfossils

More than 50 in situ δ¹³C analyses were made of Proterozoic microfossils from the Gunflint (1880 Ma), Bitter Springs (850 Ma), Min’yar (740 Ma), and Chichkan (650 Ma) Formations. Average reproducibility based upon ~50 analyses of a carbonaceous chert from the Fig Tree Group (3250 Ma) was 2‰ (2 SD). Microfossils from individual localities showed up to 10‰ variability in δ¹³C, and average δ¹³C of microfossils was similar to average bulk δ¹³C_org. Taxonomic and anatomical trends in microfossil δ¹³C are apparent and likely result from metabolic and biosynthetic differences. This is the initial phase of a comprehensive study of Proterozoic microfossil preservation that will incorporate in situ carbon isotope analysis, laser Raman, confocal laser scanning microscopy, and other techniques.