286-12 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

See more from this Division: Topical Sessions
See more from this Session: Sclerochronological Archives from Rivers to the Sea: Documentation, Interpretation, and Utility

Wednesday, 8 October 2008: 11:00 AM
George R. Brown Convention Center, 330A

Bernd R. Schoene1, Zengjie Zhang1, David P. Gillikin2, Dorrit E. Jacob1, Thomas Tütken3, Dieter Garbe-Schönberg4, Ted McConnaughey5 and Analia Soldati1, (1)Geosciences, University of Mainz, Mainz, Germany
(2)Department of Earth Science and Geography, Vassar College, Poughkeepsie, NY
(3)Steinmann Institute of Geology, Mineralogy and Palaeontology, University of Bonn, Bonn
(4)Geosciences, University of Kiel, Kiel, Germany
(5)Boise, ID
Abstract:
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.

See more from this Division: Topical Sessions
See more from this Session: Sclerochronological Archives from Rivers to the Sea: Documentation, Interpretation, and Utility