Shells of Mytilus edulis as Bioarchives of the Rare Earth Element and Yttrium Distribution in Seawater and the Potential Impact of pH and Temperature Variations

Abstract:

The importance of bivalve shells as proxy archives for changes in environmental conditions is increasingly recognized. However, data for trace metal concentrations, specifically rare earth elements and yttrium (REY) in bivalve shells, are quite scarce. The chemistry of these elements in terms of solution speciation and complexation (mostly by carbonate) makes them particularly useful in acting as geochemical proxies of oceanic change due to the impact of temperature and pH on the activity of CO₃^2- in seawater.

We approach this issue via an in situ culture experiment using Mytilus edulis from a site in the North Sea. The M.edulis used in our study were cultured offshore with no contact to the ocean floor. We compare the REY distribution in the shells and ambient seawater with the aim of investigating which REY species is incorporated into the shell. The shells show consistent shale-normalized (“SN”) REY patterns and display distinct REY features of the North Sea. The REY concentrations obtained from the shells were used to calculate apparent bulk REY partition coefficients between shells and ambient seawater (_appD_REY^{shell/seawater}) and acquired results were then used in the investigation of the potential effects of pH and temperature on REY partitioning. Further comparison of _appD_REY^{shell/seawater} patterns to the REY speciation in seawater suggests that the free REY³⁺ may be the most likely REY species that are actually incorporated from seawater into the M. edulis shell. We then modeled the REY_SN patterns of a hypothetical mussel shell at pH 8.2 and 7.6 and at temperatures of 25°C and 5°C assuming that only REY³⁺ are incorporated into the carbonate’s crystal lattice.

Our findings provide the basis to further understand how we can use shells of mussels like M. edulis as bioarchives that host geochemical proxies for paleoceanic environmental reconstructions. It appears that M. edulis shells are bioarchives of some REY features of seawater and results suggest that with lower pH, REY concentrations in shells increase, 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 only minor effects on REY concentrations.