Articles | Volume 13, issue 3
Biogeosciences, 13, 751–760, 2016
Biogeosciences, 13, 751–760, 2016

Research article 11 Feb 2016

Research article | 11 Feb 2016

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

A. Ponnurangam1, M. Bau1, M. Brenner2,3, and A. Koschinsky1 A. Ponnurangam et al.
  • 1Jacobs University Bremen, Department of Physics and Earth Sciences, Campus Ring 1, 28759 Bremen, Germany
  • 2Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
  • 3University of Rostock, Department of Aquaculture and Sea Ranching, Justus von Liebig Weg 6, 18059 Rostock, Germany

Abstract. Mussel shells are potential bioarchives of proxies for changes in the physicochemical conditions in the bivalve's habitat. One such proxy is the distribution of 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 ultratrace concentrations of REY in bulk bivalve shells (comprised of calcite and aragonite) that includes sample treatment with NaOCl followed by REY separation and preconcentration. The data obtained were used to calculate REY partition coefficients between bulk bimineralic shells of Mytilus edulis (calcite aragonite mix) and ambient seawater, and the results acquired were then used to investigate the potential effects of pH and temperature on REY partitioning.

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 and decrease from the middle REY to the heavy REY. Despite being different from 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 REY partition coefficients between shells and seawater (appDTot.REYshell/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, modDFreeREY3+shell/seawater values are higher and comparatively 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, such as 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 modeled 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 free REY3+ are incorporated into the carbonate's crystal lattice and that vital effects do not obliterate the REY signal of the shells. The results suggest that with lower pH, REY concentrations in 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. Hence, after additional calibration studies, the REY systematics in mussel shells may become a valuable proxy for paleo-pH and ocean acidification.

Short summary
Our study demonstrates that rare earth elements and yttrium (REY) accumulating in mussel shells emerge as potential proxies for environmental changes. Focusing on pH and temperature variation effects on the distribution of REY in seawater, we show that shells incorporate the free REY3+ species and that decreasing pH leads to increased REY concentrations, while rising temperatures impact the REY distribution pattern with minor effects on the absolute REY concentrations in shells.
Final-revised paper