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Removal of organic magnesium in coccolithophore calcite

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Blanco-Ameijeiras, S and Lebrato, M and Stoll, HM and Iglesias-Rodriguez, MD and Mendez-Vicente, A and Sett, S and Muller, M and Oschlies, A and Schulz, KG (2012) Removal of organic magnesium in coccolithophore calcite. Geochimica et Cosmochimica Acta, 89. pp. 226-239. ISSN 0016-7037

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Abstract

Coccolithophore calcite refers to the plates of calcium carbonate (CaCO3) produced by the calcifying phytoplankton, coccolithophores. The empirical study of the elemental composition has a great potential in the development of paleoproxies. However, the difficulties to separate coccolithophore carbonates from organic phases hamper the investigation of coccoliths magnesium to calcium ratios (Mg/Ca) in biogeochemical studies. Magnesium (Mg) is found in organic molecules in the cells at concentrations up to 400 times higher than in inorganically precipitated calcite in present-day seawater. The aim of this study was to optimize a reliable procedure for organic Mg removal from coccolithophore samples to ensure reproducibility in measurements of inorganic Mg in calcite. Two baseline methods comprising organic matter oxidations with (1) bleach and (2) hydrogen peroxide (H2O2) were tested on synthetic pellets, prepared by mixing reagent grade CaCO3 with organic matter from the non-calcifying marine algae Chlorella autotrophica and measured with an ICP-AES (inductively coupled plasmaatomic emission spectrometer). Our results show that treatments with a reductive solution [using hydroxylamine-hydrochloride (NH2OHHCl + NH4OH)] followed by three consecutive oxidations (using H2O2) yielded the best cleaning efficiencies, removing >99% of organic Mg in 24 h. P/Ca and Fe/Ca were used as indicators for organic contamination in the treated material. The optimized protocol was tested in dried coccolithophore pellets from batch cultures of Emiliania huxleyi, Calcidiscus leptoporus and Gephyrocapsa oceanica. Mg/Ca of treated coccolithophores were 0.151 ± 0.018, 0.220 ± 0.040, and 0.064 ± 0.023 mmol/mol, respectively. Comparison with Mg/Ca literature coccolith values, suggests a tight dependence on modern seawater Mg/Ca, which changes as a consequence of different seawater origins (<10%). The reliable determination of Mg/Ca and Sr/Ca, and the low levels of organic contamination (Fe/Ca and P/Ca) make this protocol applicable to field and laboratory studies of trace elemental composition in coccolithophore calcite.

Item Type: Article
Journal or Publication Title: Geochimica et Cosmochimica Acta
Page Range: pp. 226-239
ISSN: 0016-7037
Identification Number - DOI: 10.1016/j.gca.2012.04.043
Additional Information: Copyright 2012 Elsevier Ltd. All rights reserved.
Date Deposited: 01 Aug 2012 07:07
Last Modified: 18 Nov 2014 04:39
URI: http://eprints.utas.edu.au/id/eprint/14635
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