IR laser extraction technique applied to oxygen isotope analysis of small biogenic silica samples

An IR-laser fluorination technique is reported here for analyzing the oxygen isotope composition (delta O-18) of microscopic biogenic silica grains (phytoliths and diatoms). Performed after a controlled isotopic exchanged (CIE) procedure, the laser fluorination technique that allows one to visually check the success of the fluorination reaction is faster than the conventional fluorination technique and allows analyzing delta O-18 of small to minute samples (1.6-0.3 mg) as required for high-resolution paleoenvironmental reconstructions. The long-term reproducibility achieved with the IR laser-heating fluorination/O-2 delta O-18 analysis is lower than or equal to +/- 0.26 parts per thousand (1 SD; n = 99) for phytoliths and +/- 0.17 parts per thousand (1 SD; n = 47) for diatoms. When several CIE are taken into account in the SD calculation, the resulting reproducibility is lower than or equal to +/- 0.51 parts per thousand for phytoliths (1 SD; n = 99; CIE > 5) and +/- 0.54 parts per thousand (1 SD; n = 47; CIE = 13) for diatoms. A minimum reproducibility of +/- 0.5 parts per thousand leads to an estimated uncertainty on delta O-18(silica) close to +/- 0.5 parts per thousand. Resulting uncertainties on reconstructed temperature and delta O-18(forming) (water) are, respectively, +/- 2 degrees C and +/- 0.5 parts per thousand and fit in the precisions required for intertropical paleoenvironmental reconstructions. Several methodological points such as optimal extraction protocols and the necessity or not of performing two CIE prior to oxygen extraction are assessed.