IR Laser Extraction Technique Applied to Oxygen Isotope Analysis of Small Biogenic Silica Samples

Abstract

An IR-laser fluorination technique is reported here for analyzing the oxygen isotope composition (delta18O) 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 delta18O 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/O2 delta18O analysis is lower than or equal to +/-0.26 per thousand (1 SD; n = 99) for phytoliths and +/-0.17 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 per thousand for phytoliths (1 SD; n = 99; CIE > 5) and +/-0.54 per thousand (1 SD; n = 47; CIE = 13) for diatoms. A minimum reproducibility of +/-0.5 per thousand leads to an estimated uncertainty on delta18Osilica close to +/-0.5 per thousand. Resulting uncertainties on reconstructed temperature and delta18Oforming water are, respectively, +/-2 degrees C and +/-0.5 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.

The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations

“…It has been shown that up to a temperature of 70 • C the extraction has no effect on the δ 18 O (Crespin et al, 2008). We verified that it did not have any effect on the 17 O-excess either, using our internal standard MSG extracted at 60 and 70 • C (Crespin et al, 2008). The obtained 17 O-excess values were similar (−211 and −243 per meg, respectively) given our reproducibility of ±34 per meg (see Sect.…”

“…Phytoliths from soils were extracted following Crespin et al (2008) using HCl, H 2 O 2 , C 6 H 5 Na 3 O 7 and Na 2 O 4 S3-H 2 O at 70 • C, and a ZnBr 2 heavy liquid separation. It has been shown that up to a temperature of 70 • C the extraction has no effect on the δ 18 O (Crespin et al, 2008).…”

“…Phytolith samples of 1.6 mg were dehydrated and dehydroxylated under a flow of N 2 (Chapligin et al, 2010) and oxygen extraction was performed using the IR Laser-Heating Fluorination Technique at CEREGE (Aix-en-Provence, France) (Alexandre et al, 2006;Crespin et al, 2008;Suavet et al, 2010). The purified oxygen gas (O 2 ) was passed through a −114 • C slush to refreeze gases interfering with the mass 33 (e.g., NF), potentially produced during the fluorination of residual organic N, before being sent to the dual-inlet mass spectrometer (ThermoQuest Finnigan Delta Plus).…”

Stable isotopes from diatom silica

“…To remove these and other contaminants from diatom samples, a number of chemical and physical clean-up methodologies have been used (e.g. JuilletLeclerc, 1986;Shemesh et al, 1988;Shemesh et al, 1995;Singer and Shemesh, 1995;Morley et al, 2004;Rings et al, 2004;Swann et al, 2006;Tyler et al, 2007;Crespin et al, 2008). The success of the purification process is controlled by factors such as: (a) the ability of chemical reagents to fully oxidize certain components (e.g.…”

“…Under CIE, oxygen in the -Si-OH layer of the diatom is exchanged with water containing a known δ 18 O ratio (Labeyrie and Juillet, 1982;Juillet-Leclerc and Labeyrie, 1987;Crespin et al, 2008).…”