A Laser-Extraction Technique for Oxygen Isotope Analysis of Diatom Frustules

Biogenic opal in the form of diatom frustules is abundant in lacustrine and marine environments throughout the Cenozoic. Oxygen isotope ratios preserved in the SiO2 of diatom frustules provide a valuable archive of paleoclimatic data, such as changes in temperature and precipitation/evaporation. However, oxygen isotope analysis of diatom silica is complicated by the presence of >15 wt. % H2 O that must be removed prior to analysis for accurate δ18Odiatom measurements to be obtained. Various techniques have been used to remove the hydrous silica including vacuum dehydration, isotope exchange, and stepwise fluorination. Here we present a laser-extraction technique for oxygen isotope analysis of biogenic opal that is rapid and requires far less material than other techniques. Batches of approximately twenty 1 to 2 mg aliquots of pure diatoms are loaded in a laser chamber and pre-fluorinated with F2 gas, similarly to previously published stepwise fluorination techniques to remove the hydrous portion of biogenic silica. The remaining SiO2 is then volatilized in the presence of BrF5 with a CO2 laser to produce O2 gas. The δ18Odiatom values produced through the laser-extraction technique increase with increasing pre-fluorination times as the hydrous portion of the silica is removed. A plateau with a constant, reproducible δ18O value is obtained once all of the hydrous silica is removed. The diatoms samples analyzed in this study are from sediments collected from the Valles Caldera in New Mexico, USA, and consist of a coarse (>50 micron) and fine (<50 micron) size samples from different locations in the caldera. Two different coarse grained samples were analyzed using the laser-extraction technique and had δ18Odiatom values that range from 31.2 to 32.4 ‰ and 31.6 to 32.5 ‰. In these samples values plateaued at 32.3 (±0.1) ‰ and 32.2 (±0.2) ‰. A finer grained sample, also from the Valles Caldera, had δ18Odiatom values that ranged from 28.5 to 29.0 ‰ and plateaued at 28.7 ‰ (±0.2). Additional diatom samples used in this study came from a variety of modern and ancient lacustrine and marine sources and were used to compare the laser extraction technique with other techniques. δ18Odiatom values obtained with laser extraction are 2 to 4 ‰ higher than δ18Odiatom values obtained using conventional vacuum dehydration of the same samples, suggesting that there is more complete removal of the hydrous material with the laser extraction technique or less exchange of the hydrous component during dehydration. Reproducibility of δ18Odiatom values obtained by the laser-extraction technique is ± 0.2 ‰. We propose that the laser-extraction technique for obtaining oxygen isotope values from diatom silica has advantages over other techniques in terms of sample size, removal of the hydrous material, and analytical procedure.