Mussel shells of Mytilus edulis as bioarchives of the distribution of rare earth elementsand yttrium in seawater and the potential impact of pH and temperature ontheir partitioning behavior

[摘要] Mussel shells are potential bioarchives of proxies for changes in thephysicochemical conditions in the bivalve's habitat. One such proxy is thedistribution of rare earths and yttrium (REY) in seawater, as REYspeciation in seawater is sensitive to pH and temperature variations, due tothe impact of these parameters on the activity of CO₃²⁻ inseawater. We present a new protocol for sample preparation and determinationof ultratrace concentrations of REY in bulk bivalve shells (comprised ofcalcite and aragonite) that includes sample treatment with NaOCl followedby REY separation and preconcentration. The data obtained were used tocalculate REY partition coefficients between bulk bimineralic shellsof Mytilus edulis (calcite aragonite mix) and ambient seawater, and the results acquired werethen used to investigate the potential effects of pH and temperature on REYpartitioning.

Shells of Mytilus edulis mussels from the North Sea show consistent shale-normalized(SN) REY patterns that increase from the light REY to the middle REY anddecrease from the middle REY to the heavy REY. Despite being different from the general seawater REY_SN pattern, the shells still display distinctREY features of seawater, such as a negative Ce_SN anomaly and smallpositive Y_SN and Gd_SN anomalies. Apparent REY partitioncoefficients between shells and seawater (_appD_Tot.REY^{shell/seawater}) are low and decrease stronglyfrom the light REY (4.04 for La) to the heavy REY (0.34 for Lu). However,assuming that only the free REY³⁺ are incorporated into the shell,_modD_FreeREY3+^{shell/seawater} values are higher andcomparatively similar for all REY (102.46 for La; 113.44 for Lu) but show a slightmaximum at Tb (199.18). Although the impact of vital effects, such as REYspeciation in a mussel's extrapallial fluid from which the carbonateminerals precipitate, cannot be quantified yet, it appears that M. edulis shells arebioarchives of some REY features of seawater.

We modeled the REY_SN patterns of a hypothetical mussel shell at pH 8.2and 7.6 and at temperatures of 25 and 5 °C, assumingthat only free REY³⁺ are incorporated into the carbonate's crystallattice and that vital effects do not obliterate the REY signal of theshells. The results suggest that with lower pH, REY concentrations in shellsincrease, but with little effect on the shape of the REY_SN patterns,while a temperature change has an impact on the REY_SN pattern but onlyminor effects on REY concentrations. Hence, after additional calibrationstudies, the REY systematics in mussel shells may become a valuable proxyfor paleo-pH and ocean acidification.

[学科分类] 地球化学与岩石