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

Smith, Andrew C.; Leng, Melanie J.; Swann, George E.A.; Barker, Philip A.; Mackay, Anson W.; Ryves, David B.; Sloane, Hilary J.; Chenery, Simon R.N.; Hems, Mike

Abstract

Rationale: Current studies which use the oxygen isotope composition from diatom silica (δ18Odiatom) as a palaeoclimate proxy assume that the δ18Odiatom 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 δ18Odiatom 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 δ18O 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 δ18Odiatom 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 δ18Odiatom 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 δ18Odiatom values, especially when interpreting variations in these values of <1‰.

Citation

Smith, A. C., Leng, M. J., Swann, G. E., Barker, P. A., Mackay, A. W., Ryves, D. B., Sloane, H. J., Chenery, S. R., & Hems, M. (2016). An experiment to assess the effects of diatom dissolution on oxygen isotope ratios. Rapid Communications in Mass Spectrometry, 30(2), 293-300. https://doi.org/10.1002/rcm.7446

• Journal Article Type: Article
• Acceptance Date: Oct 28, 2015
• Online Publication Date: Dec 14, 2015
• Publication Date: Jan 1, 2016
• Deposit Date: Jan 4, 2016
• Publicly Available Date: Jan 4, 2016
• Journal: Rapid Communications in Mass Spectrometry
• Print ISSN: 0951-4198
• Electronic ISSN: 1097-0231
• Publisher: Wiley
• Peer Reviewed: Not Peer Reviewed
• Volume: 30
• Issue: 2
• Pages: 293-300
• DOI: https://doi.org/10.1002/rcm.7446
• Keywords: Palaeoclimate, biogenic silica, oxygen isotopes, dissolution, sedimentation
• Public URL: https://nottingham-repository.worktribe.com/output/768077
• Publisher URL: http://onlinelibrary.wiley.com/doi/10.1002/rcm.7446/abstract
• Additional Information: This is the pre-peer reviewed version of the following article: Smith, A. C., Leng, M. J., Swann, G. E. A., Barker, P. A., Mackay, A. W., Ryves, D. B., Sloane, H. J., Chenery, S. R. N., and Hems, M. (2016) An experiment to assess the effects of diatom dissolution on oxygen isotope ratios. Rapid Commun. Mass Spectrom., 30: 293–300, which has been published in final form at doi:10.1002/rcm.7446. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.
• Contract Date: Jan 4, 2016