Precise and fast determination of inorganic magnesium in coccolithophore calcite

Coccolithophores are calcifying marine phytoplankton playing a pivotal role in the production of calcium carbonate deposits. During the formation of calcium carbonate phase, calcium ions may be replaced by other divalent cations (e.g. Sr²⁺ and Mg²⁺) which results in a unique elemental signature showing environmental and, in case of biogenic precipitates, physiological conditions of its formation. It was shown that the ratio between magnesium and calcium is strongly correlated with surface sea temperature variation and can be used in paleoceanographic studies to reconstruct past environmental conditions and to understand biogeochemical cycles. The determination of the coccolithophore calcite Mg/Ca ratio, however, requires the efficient removal or quantification of organically bound magnesium, which is up to 400 times more abundant than the magnesium incorporated within the inorganic calcite. Classical methods, using oxidation to remove organic matter, are time consuming and require a considerable amount of sample material. So, the aim of our investigation is the development of a new reliable method for determination of inorganic magnesium and the Mg/Ca ratio in coccolithophore calcite. For this purpose, labile organic bound magnesium is replaced by the incubation with added Cu²⁺ for 10 min, and the released Mg²⁺ is determined via high performance chelation ion chromatography (HPCIC). This method enabled the determination of the coccolithophore calcite Mg/Ca ratio within 1 h using < 5 mg dry coccolithophore sample material. The method has been successfully tested with laboratory cultured calcifying and non-calcifying coccolithophores samples. The portable HPCIC instrumentation can be easily mounted on-board scientific research vessels and thus potentially represent a new tool for in-situ calcite analysis of coccolithophore surface bloom situations. However, a further work is required to make this method suitable for analysis of pelagic sediments and sediment traps, which may contain residues of Mg-rich clays.