Removal of organic magnesium in coccolithophore calcite
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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Official URL: http://dx.doi.org/10.1016/j.gca.2012.04.043
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.
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|Deposited By:||Ms MA Hazelwood|
|Deposited On:||01 Aug 2012 17:07|
|Last Modified:||01 Aug 2012 17:07|
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