Influence of exchangeable oxygen on biogenic silica oxygen isotope data

Application of biogenic opal oxygen isotope ratios from fossil and subfossil diatoms to paleoceanographic problems has been hampered by analytical and calibration issues in concert with a lack of experimental data to test fundamental assumptions about sample processing techniques. Here we present experiments where we react purified sediment trap and sediment core diatom samples to waters of different oxygen isotopic composition to quantify oxygen isotope exchange between laboratory processing solutions and δ18Odiatom values. We generate δ18Odiatom data using a microfluorination technique, and present FTIR data for samples both before and after vacuum dehydroxylation in order to investigate the mineralogic behavior of biogenic silica after dehydroxylation occurs. Our data demonstrate that exposure of diatoms to different δ18Oequil. water solutions during sample preparation alters final δ18Odiatom values and this alteration occurs during sample dehydroxylation prior to IRMS analysis. In addition, we present data that show structural diatom hydroxyl is persistent in sediment core samples, but the degree of equilibration with surrounding water decreases with sample age and/or core depth increase. Based on FTIR data, we propose that OH- loss from biogenic silica occurs post depositionally and may occur after heating in the laboratory, producing molecular scale reorganization of the silica tetrahedra within the biogenic opaline structure. Finally, we provide preliminary estimates of the time necessary for complete dehydroxylation of diatom silica, which would result in fully mature biogenic opal. These data suggest that high latitude marine sediments are slow to mature, and that the rate of exchangeable oxygen reduction varies exponentially at the locations we examined. Maturation time estimates require millions of years, and likely vary depending on core location.