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Preliminary results from the on-going inter-laboratory comparison of the oxygen isotope composition of biogenic silica

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Chapligin, B. , Alexandre, A. , Dodd, J. , Ijiri, A. , Leng, M. , Lücke, A. , Shemesh, A. , Webb, E. , Abelmann, A. , Longstaffe, F. , Meyer, H. , Moschen, R. , Okazaki, Y. , Sharp, Z. , Sloane, H. J. , Sonzogni, C. , Swann, G. , Sylvestre, F. and Tyler, J. (2010): Preliminary results from the on-going inter-laboratory comparison of the oxygen isotope composition of biogenic silica , Isotopes in Biogenic Silica (IBiS) Meeting 2010, Swansea, Wales (UK), February 8-9 .
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Abstract:

In 2009 the Alfred-Wegener-Institute organised an inter-laboratory comparison for analysing the oxygen isotope compositions of biogenic silica. The relationship of the oxygen isotope compositions of carbonate to climate-relevant parameters is widely utilized. However, challenges still exist in the use of biogenic silica. Problems arise during the sample preparation stage and during the isotopic analysis itself. The problems are commonly related to the removal of loosely bound exchangeable oxygen from the silica. Amorphous silica contains OH groups within the SiO2 skeleton as well as chemically combined water. The OH groups and chemically combined water must be removed prior to analysis as their oxygen is easily exchanged following silica formation and thus does not reflect the original isotopic composition of the water. Various methods have been established in the past 20 years for the dehydration and dehydroxylation of amorphous silica, including Controlled Isotopic Exchange (CIE) followed by fluorination, Stepwise Fluorination (SWF) and inductive High-Temperature carbon reduction (iHTR). A new method under consideration is Helium Flow Dehydration (HFD) followed by fluorination. These methods have never been compared in a comprehensive interlaboratory comparison.This inter-laboratory comparison should function as a method performance study as well as a material certification study. The studys goals are:1. To evaluate the agreement of δ18O results in terms of accuracy and reproducibility among different methods in different laboratories, and2. To provide the δ18OSi community with well-calibrated biogenic standards covering a large range of δ18O values.The second goal is related to the current existence of only one quartz standard (NBS-28) made available by IAEA as a reference material for analysis of oxygen isotopes from SiO2. There is a need for natural biogenic reference materials with oxygen isotopic compositions in the range of naturally occurring lacustrine and marine sediments. Six different samples so far only used as internal standards in the participating laboratories have been analysed by eight different laboratories using their typical analytical methods. The samples cover a wide-range of δ18O values (23 to 43 ) and originate from lacustrine and marine sediment deposits as well as from chemically precipitated amorphous silica. The samples consist of diatoms, phytoliths or synthetical nano-spheres. Each laboratory has analysed each sample at least ten times on various days to evaluate reproducibility. X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses have also been performed to verify the purity and the structure of the proposed standards. The XRF analysis of the six standards indicates SiO2 contents ranging from 94 to 99%. The XRD analyses showed that five of the standards have a broad, pure amorphous silica (opal-A) peak whereas one (Kieselguhr) is crystalline, with crystobalite bands resulting from pre-calcination performed by the supplying company.At the time of this abstract submission the laboratories were completing their analyses. Preliminary results will be presented at the meeting. Based on these results, we hope to recommend up to six biogenic silica standards for δ18OSi values with a reproducibility of better than ±0.3 . We will also evaluate the various methods in terms of the analytical uncertainties accompanying each approach.

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