Crystal fabrics and element impurities (Sr/Ca, Mg/Ca, and Ba/Ca) in shells of Arctica islandica-Implications for paleoclimate reconstructions

Sr/Ca, Mg/Ca and Ba/Ca values are heterogeneously distributed in shells of Arctica islandica. These patterns are largely associated with crystal fabrics (size, habit and orientation of crystals) or the processes controlling them. The outer sublayer of the outer shell layer (oOSL; homogenous and irregular simple prismatic crystal fabrics,) contained element/Ca values up to 62% higher than the inner sublayer (iOSL; crossed-acicular, crossed-lamellar, fine crossed-lamellar and irregular simple prismatic crystal fabrics). A gradual decrease in Sr/Ca and Mg/Ca values was observed from the outer portions of the oOSL toward the oOSL/iOSL transition zone. This chemical shift was accompanied by a gradual transition from homogenous crystal fabrics into crossed-lamellar/acicular crystal fabrics. Near annual growth lines (irregular simple prisms), i.e., during periods of slow growth, Sr and Mg seemed to be deposited in equilibrium with the ambient environment because the Sr/Ca and Mg/Ca values approached values typical for seawater. During the remainder of the growing season, Sr/Ca and Mg/Ca remained far below values expected for thermodynamic equilibrium. Ba/Ca peaks, however, occurred erratically at different times of the year without any noticeable changes in crystal fabrics. Likely, the environmental information contained in these peaks was less severely filtered by vital effects than in Sr and Mg. The findings of the present study can help to develop new techniques with which extract environmental signals from the metal-to-calcium ratios of bivalve shells.