Mussel shells of Mytilus edulis as bioarchives of the rare earth elements and yttrium distribution in seawater and the potential impact of pH and temperature on the partitioning behaviour

Mussel shells are potential bioarchives of proxies for changes of the physico-chemical conditions in the bivalve’s habitat. One such proxy is the distribution of the Rare Earths and Yttrium (REY) in seawater, as REY speciation in seawater is sensitive to pH and temperature variations, due to the impact of these parameters on the activity of CO32− in seawater. We present a new protocol for sample preparation and determination of REY concentrations in bivalve shells, that includes sample treatment with NaOCl followed by REY separation and preconcentration. The data obtained was further used to calculate REY partition coefficients between shells of M. edulis and ambient seawater, and acquired results were then used in the investigation of the potential effects of pH and temperature on REY partitioning.

Shells of M. edulis mussels from the North Sea show consistent shale-normalized (“SN”) REY patterns that increase from the light REY to the middle REY and decrease from the middle REY to the heavy REY. Despite being different to the general seawater REYSN pattern, the shells still display distinct REY features of seawater such as a negative CeSN anomaly and small positive YSN and GdSN anomalies. Apparent partition coefficients for the REY between the shell and seawater (appDREYshell/seawater) are low and decrease strongly from 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, appDREY3+shell/seawater values are higher and rather similar for all REY (102.46 for La; 113.44 for Lu), but show a slight maximum at Tb (199.18). Although the impact of vital effects i.e. REY speciation in a mussel’s extrapallial fluid from which the carbonate minerals precipitate, cannot be quantified yet, it appears that M. edulis shells are bioarchives of some REY features of seawater.

We modelled the REYSN patterns of a hypothetical mussel shell at pH 8.2 and 7.6 and at temperatures of 25 and 5 °C assuming that only REY3+ are incorporated into the carbonate’s crystal lattice. The results suggest that with lower pH, REY concentrations in a shells increase, but with little effect on the shape of the REYSN patterns, while a temperature change has an impact on the REYSN pattern, but only minor effects on REY concentrations.

Ponnurangam A., Bau M., Brenner M. & Koschinsky A., 2015. Mussel shells of Mytilus edulis as bioarchives of the rare earth elements and yttrium distribution in seawater and the potential impact of pH and temperature on the partitioning behaviour. Biogeosciences Discussions 12: 14911-14939. Article.

 


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