An experiment to assess the effects of diatom dissolution on oxygen isotope ratios

Rationale Current studies which use the oxygen isotope composition from diatom silica (δ 18 O diatom ) as a palaeoclimate proxy assume that the δ 18 O diatom value reflects the isotopic composition of the water in which the diatom formed. However, diatoms dissolve post mortem, preferentially losing less silicified structures in the water column and during/after burial into sediments. The impact of dissolution on δ 18 O diatom values and potential misinterpretation of the palaeoclimate record are evaluated. Methods Diatom frustules covering a range of ages (6 samples from the Miocene to the Holocene), environments and species were exposed to a weak alkaline solution for 48 days at two temperatures (20 °C and 4 °C), mimicking natural dissolution post mucilage removal. Following treatment, dissolution was assessed using scanning electron microscope images and a qualitative diatom dissolution index. The diatoms were subsequently analysed for their δ 18 O values using step‐wise fluorination and isotope ratio mass spectrometry. Results Variable levels of diatom dissolution were observed between the six samples; in all cases higher temperatures resulted in more frustule degradation. Dissolution was most evident in younger samples, probably as a result of the more porous nature of the silica. The degree of diatom dissolution does not directly equate to changes in the isotope ratios; the δ 18 O diatom value was, however, lower after dissolution, but in only half the samples was this reduction outside the analytical error (2σ analytical error = 0.46‰). Conclusions We have shown that dissolution can have a small negative impact on δ 18 O diatom values, causing reductions of up to 0.59‰ beyond analytical error (0.46‰) at natural environmental temperatures. These findings need to be considered in palaeoenvironmental reconstructions using δ 18 O diatom values, especially when interpreting variations in these values of <1‰. Copyright © 2015 John Wiley & Sons, Ltd.