Influence of Persistent Exchangeable Oxygen on Biogenic Silica δsup18/supO in Deep Sea Cores

The removal of exchangeable oxygen from biogenic opal prior to IRMS analysis is critical during sample preparation. Exchangeable oxygen is found in the form of hydroxyl and between defects within the amorphous silicate lattice structure. Typical analytical procedures utilize a variety of dehydroxylation methods to eliminate this exchangeable oxygen, including vacuum dehydroxylation and prefluorination. Such methods are generally considered sufficient for elimination of non-lattice bound oxygen that would obfuscate environmental oxygen isotopic signals contained within the silicate tetrahedra. δ ¹⁸ O data that are then empirically calibrated against modern hydrographic data, and applied down core in paleoceanographic applications.

We have conducted a suite of experiments on purified marine opal samples using the new microfluorination method (Menicucci et al., 2013). Our data demonstrate that the amount of exchangeable oxygen in biogenic opal decreases as sample age/depth in core increases. These changes are not accounted for by current researchers. Further, our experimental data indicate that vacuum dehydroxylation does not eliminate all exchangeable oxygen, even after hydroxyl is undetectable. We have conducted experiments to quantify the amount of time necessary to ensure vacuum dehydroxylation has eliminated exchangeable oxygen so that opal samples are stable prior to δ ¹⁸ O _diatom analysis. Our experiments suggest that previously generated opal δ ¹⁸ O data may contain a variable down-core offset due to the presence of exchangeable, non-lattice bound oxygen sources. Our experiments indicate that diatom silica requires dehydroxylation for ≥ 44 hours at 1060 ^o C to quantitatively remove all non-lattice bound oxygen. Further, this variable amount of exchangeable oxygen may be responsible for some of the disagreement between existing empirical calibrations based on core-top diatom frustule remains. Analysis of δ ¹⁸ O _diatom values after this long vacuum dehydroxylation time is necessary for quantitative comparisons of stable isotopic values across geologic time periods.