We are analyzing oxygen (δ18O) and silicon (δ30Si) stable isotopes of biogenic opal (e.g. diatoms and radiolarians) (Fig. 1) to reconstruct past changes in hydrographic structures, including meltwater events, and nutrient dynamics particularly in polar regions like the polar North Pacific and the Southern Ocean (Link toprojects), which are characterized by rather opal-rich sediments.
Biogenic opal samples are generated from bulk sediment samples following an extensive purification process, which includes the separation and enrichment of diatoms radiolarians and sponge spicules (Maier et al. (2013), Abelmann et al. (2015).About 2 mg of the purified opal samples are then dehydrated at 1100°C to remove the hydroxyl groups within the opal structure (Figs. 1C, 2). Dehydrated samples are heated with a CO2 laser under BrF5 atmosphere to generate O2 and SiF4 (laser fluorination) (Fig. 2). The liberated O2 is analysed using a PDZ Europa 2020 mass spectrometer allowing for δ17O and δ18O measurements. The liberated SiF4 is transferred to pyrex glass tubes and then measured (as SiF3+) using a Finnigan MAT252 mass spectrometer configured for δ29Si and δ30Si measurements. For details on the analytics see Chapligin et al. (2010), Maier et al. (2013) and Abelmann et al. (2015).
This method allows for a combined analyses of both δ18O and δ30Si from the same sample aliquot and is performed in close collaboration with the isotope laboratory at AWI Potsdam, where at present sample dehydration, laser fluorination and oxygen isotope analysis is performed. Our opal isotope laboratory at AWI Bremerhaven, currently performing sample preparation and silicon isotope analysis, is at present under construction to allow for performing the complete analytics.
Figure 1. Scanning electron microscopic pictures of (A) two diatom species and (B) two radiolarian species. (C) Schematic illustration of the opal structure (SiO2*nH2O) showing an inner layer of silica tetrahedrons (Si-O-Si bonds) and an outer, hydrous layer (Si-OH bonds) (opal structure modified after Perry, 1983). Since the hydrous „outer layer“ contains exchangeable oxygen it needs to be removed before isotope analysis.
Abelmann, A. , Gersonde, R. , Knorr, G. , Zhang, X. , Chapligin, B. , Maier, E. , Esper, O. , Friedrichsen, H. , Lohmann, G. , Meyer, H. and Tiedemann, R. (2015) The seasonal sea-ice zone in the glacial Southern Ocean as a carbon sink , Nature Communication, 6 (8136), pp. 1-12 . doi:10.1038/ncomms9136 ,
Chapligin, B., Meyer, H., Friedrichsen, H., Marent, A., Sohns, E., Hubberten, H.-W. (2010). A high-performance, safer and semi-automated approach for the d18O analysis of diatom silica and new methods for removing exchangeable oxygen. Rapid Communications in Mass Spectrometry 24(17), 2655-2664, doi:10.1002/rcm.4689.
Maier, E., Chapligin, B., Abelmann, A., Gersonde, R., Esper, O., Ren, J., Friedrichsen, H., Meyer, H., Tiedemann, R. (2013), Combined oxygen and silicon isotope analysis of diatom silica from a deglacial subarctic Pacific record. Journal of Quaternary Science 28(6), 571-581, doi:10.1002/jqs.2649.
Perry, C.C. (1989), Chemical Studies of Biogenic Silica, in: Biomineralization: Chemical and Biochemical Perspectives, edited by Mann, S., Webb, J. Williams, R.J.P., VCH Verlagsgesellschaft, Weinheim, pp. 223-256.