Effect of Organic Matrices on the Determination of Minor Elements (Mg, Sr) and Mg:Ca and Sr:Ca Ratios of Aragonitic Bivalve Shells (Arctica islandica) – Comparison of ICP-OES and LA-ICP-MS Data

The element chemistry of biogenic carbonates can provide important data on past environments. However, the Sr/Ca and Mg/Ca ratios of biological carbonates, especially aragonitic bivalves often depart from apparent thermodynamic equilibrium. When measured by means of LA-ICP-MS, the Mg concentration is often substantially enriched (> 200%) near the organic-rich, annual growth lines. To test the hypothesis that organic components exert a major influence on the skeletal metal content, the element chemistry of different shell components (insoluble organic matrix, dissolved CaCO₃ and soluble organics) of Arctica islandica was measured by means of ICP-OES and LA-ICP-MS. ICP-OES data indicate that the IOM is strongly enriched in Mg (150ppm) and depleted in Sr and Ca (11ppm and 0.25%, respectively) when compared to the whole biomineral (Mg: 78ppm, Sr: 1,218ppm, Ca: 41.25%). Although the average relative abundance of the IOM barely exceeds 0.46wt%, its chemical composition can significantly increase estimates of bulk Mg content of the shell if measured by LA-ICP-MS. This overestimation is related to internal standardization problems in the case of the LA-ICP-MS technique and to the heterogeneous distribution of the IOM across the shells. For example, the Mg concentration of shell portions with higher than average IOM content is overestimated by LA-ICP-MS. Removal of the IOM prior to the chemical analysis by LA-ICP-MS or mathematical correction for the IOM-derived magnesium concentrations is therefore strongly advised. For determination of the Mg concentration and Mg/Ca ratios in biominerals with moderate IOM content, wet chemical methods such as ICP-OES can be applied without special sample pretreatment procedures. The precision error induced by the IOM is less than 1%. Strontium levels and Sr/Ca ratios of bivalve shells, however, can be determined with high fidelity by ICP-OES and LA-ICP-MS techniques. The results of this study can significantly improve our understanding of shell-based climate and environmental proxies.