In-situ Mg isotopes measurements of biogenic carbonates using Laser Ablation-Multi-Collector Inductively Coupled Plasma Mass Spectrometry: a new tool to understand biomineralisation.

Rationale: Magnesium is one of the most abundant elements in the earth's crust and in seawater. Fractionation of its stable isotopes has been shown to be a useful indicator of many geological, chemical, and biological processes. For example, biogenic carbonates display an similar to 5 parts per thousand range of delta Mg-26 values, which is attributed to variable degrees of biological control on Mg ions during biomineralisation. Understanding this biological control is essential for developing proxies based on biogenic carbonates. Methods: In this work, we present a new approach of measuring Mg isotopes in biogenic carbonates using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS). Results: Our results show that this microanalytical approach provides relatively fast, high spatial resolution (<0.2 mu m) measurements with high precision and accuracy down to 0.2 parts per thousand (2SE). To achieve high levels of precision and accuracy, baseline interferences need to be monitored and a carbonate standard with a relatively low trace metal composition similar to biogenic carbonates should be used. We also demonstrate that the matrix effect on Mg isotopes in carbonates with low Fe and Mn is limited to less than 0.2 parts per thousand fractionation under different laser parameters and low oxide condition (<0.3% ThO/Th). Conclusions: Our newly developed LA-MC-ICPMS method and its applications to biogenic carbonates show significant advantages provided by the microanalytical approach in understanding complex processes of biomineralisation in marine calcifiers.