IR Laser Extraction Technique Applied to Oxygen Isotope Analysis of Small Biogenic Silica Samples Julien Crespin, Anne Alexandre,* Florence Sylvestre, Corinne Sonzogni, Christine Paille ` s, and Vincent Garreta CEREGE, CNRS, Universite ´ Paul Ce ´ zanne Aix-Marseille III and IRD, Europo ˆ le de l’Arbois, BP 80, 13545, Aix en Provence Cedex 04, France An IR-laser fluorination technique is reported here for analyzing the oxygen isotope composition (δ 18 O) of mi- croscopic 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 δ 18 O of small to minute samples (1.6- 0.3 mg) as required for high-resolution paleoenvironmen- tal reconstructions. The long-term reproducibility achieved with the IR laser-heating fluorination/O 2 δ 18 O analysis is lower than or equal to (0.26‰ (1 SD; n ) 99) for phytoliths and (0.17‰ (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‰ for phytoliths (1 SD; n ) 99; CIE > 5) and (0.54‰ (1 SD; n ) 47; CIE ) 13) for diatoms. A minimum reproducibility of (0.5‰ leads to an estimated uncertainty on δ 18 O silica close to (0.5‰. Resulting un- certainties on reconstructed temperature and δ 18 O forming water are, respectively, (2°C and (0.5‰ 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 as- sessed. Investigating the oxygen isotope composition of biogenic silica (δ 18 O silica ) such as phytoliths and diatoms is of particular interest for continental paleoclimatic reconstructions as the oxygen isotopic composition of a mineral reflects both temperature and isotopic composition of the solution from which it precipitates (δ 18 O forming water ). Diatoms are micrometric unicellular algae (5- 400 μm) that precipitate in isotopic equilibrium. 1-5 δ 18 O silica values of diatoms from lacustrine sediments are commonly investigated for reconstructing past temperature and trends in lake moisture balance (e.g., refs 6-10). Phytoliths are micrometric particles (< 60-100 μm) that form in plants tissues, inside or between the cells. 11 Shahack-Gross et al. 12 and Webb and Longstaffe 13-16 demonstrated that when phytoliths precipitate in nontranspiring tissues, their δ 18 O silica value is related to the δ 18 O tissue water values and to the growing season temperature. Thus, δ 18 O silica value of phytoliths formed in nontranspiring tissue, such as wood, 17 should reflect atmospheric temperature and δ 18 O values of soil water, in turn controlled by (1) δ 18 O values of meteoric water, (2) potential input of groundwater, and (3) isotopic enrichment due to evapora- tion. 18 Although the potential of the δ 18 O value of continental biogenic silica as a proxy of atmospheric temperature and part of the water cycle is very promising, it faces analytical problems. Up to now, the conventional fluorination technique 19 is commonly used for extracting oxygen from the biogenic silica grains. 6,20,4,21,13 This method is time-consuming and the success of the fluorination * Corresponding author. E-mail: Alexandre@cerege.fr. Fax: +33 4 42 97 15 40. (1) Labeyrie, L. Nature 1974, 248, 40-42. (2) Juillet-Leclerc, A.; Labeyrie, L. Earth Planet. Sci. Lett. 1987, 84, 69-74. (3) Matheney, R. K.; Knauth, L. P. Geochim. Cosmochim. Acta 1989, 53, 3207- 3214. (4) Brandriss, M. E.; O’Neil, J. R.; Edlund, M. B.; Stoermer, E. F. Geochim. Cosmochim. Acta 1998, 62, 1119-1125. (5) Moschen, R.; Lu ¨cke, A.; Schleser, G.H. Geophys. Res. Lett. 2005, 32, L07708; doi:10.1029/2004GL022167. (6) Shemesh, A.; Charles, C. D.; Fairbanks, R. G. Science 1992, 256, 1434- 1436. (7) Rietti-Shati, M.; Shemesh, A.; Karlen, W. Science 1998, 281, 980-982. (8) Barker, P. A.; Street-Perrot, F. A.; Leng, M. J.; Greenwood, P. B.; Swain, D. L.; Perrott, R. A.; Telford, R. J.; Ficken, K. J. Science 2001, 292, 2307- 2310. (9) Polissar, P. J.; Abbott, M. B.; Shemesh, A.; Wolfe, A. P.; Bradley, R. S. Earth Planet. Sci. Lett. 2006, 242, 375-389. (10) Barker, P. A.; Leng, M. J.; Gasse, F.; Huang, Y. Earth Planet. Sci. Lett., in press. (11) Piperno, D. P. Phytolith Analysis-An Archaeological and Geological Perspective; Academic Press: New York, 1988. (12) Shahack-Gross, R.; Shemesh, A.; Yakir, D.; Weiner, S. Geochim. Cosmochim. Acta 1996, 60, 3949-3953. (13) Webb, E. A.; Longstaffe, F. J. Geochim. Cosmochim. Acta 2000, 64, 767- 780. (14) Webb, E. A.; Longstaffe, F. J. Geochim. Cosmochim. Acta 2002, 66, 1891- 1904. (15) Webb, E. A.; Longstaffe, F. J. Geochim. Cosmochim. Acta 2003, 67, 1437- 1449. (16) Webb, E. A.; Longstaffe, F. J. Geochim. Cosmochim. Acta 2006, 64, 767- 780. (17) Crespin, J.; Alexandre, A.; Sylvestre, F.; Sonzogni, C.; Hilbert, D. Abstracts of the 6th International Meeting on Phytolith Research; Barcelona, Spain, September 12-15, 2006. (18) Hsieh, J. C. C.; Chadwick, O. A.; Kelly, E. F.; Savin, S. M. Geoderma 1998, 82, 269-293. (19) Clayton, R. N.; Mayeda, T. K. Geochim. Cosmochim. Acta 1963, 27, 43-52. (20) Schmidt, M.; Botz, R.; Stoffers, P.; Anders, T.; Bohrmann, G. Geochim. Cosmochim. Acta 1997, 61, 2275-2280. (21) Leng, M.; Barker, P.; Greenwood, P.; Roberts, N.; Reed, J. J. Paleolim. 2001, 25, 343-349. Anal. Chem. 2008, 80, 2372-2378 2372 Analytical Chemistry, Vol. 80, No. 7, April 1, 2008 10.1021/ac071475c CCC: $40.75 © 2008 American Chemical Society Published on Web 02/28/2008