LA-ICP-MS-derived U-concentrations and microstructural domains within biogenic aragonite of Arctica islandica shell.

Abstract

Understanding of the uranium uptake processes (both in vivo and post-mortem) into the skeletal structures of marine calcifiers is a subject of multi-disciplinary interest. U-concentration changes within the molluscan shell may serve as a paleoceanographic proxy of the pH history. A proxy of this type is needed to track the effects of fossil fuel emissions to ocean acidification. Moreover, attaining reliable U-series dates using shell materials would be a geochronological breakthrough. Picturing the high-resolution changes of U-concentrations in shell profiles is now possible by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Here, we analyzed in situ U-concentration variations in sub-fossilized shells of ocean quahog (Arctica islandica), a commonly studied bivalve species in Quaternary geoscience, using LA-ICP-MS. Microstructural details of the shell profiles were achieved by the scanning electron microscopy (SEM). Comparison of the shell aragonite microstructure with the changes in U-concentration revealed that uranium of possibly secondary origin is concentrated into the porous granular layers of the shell. Our results reinforce the hypothesis that U-concentration variations can be linked with microstructural differences within the shell. A combination of LA-ICP-MS and SEM analyses is recommended as an interesting approach for understanding the U-concentration variations in similar materials.