Inter- and intra-specimen variability masks reliable temperature control on shell Mg/Ca ratios in laboratory- and field-cultured Mytilus edulis and Pecten maximus (bivalvia)
Freitas, P. S.
Clarke, L. J.
Kennedy, H. A.
Richardson, C. A.
The Mg/Ca ratios of biogenic calcite is commonly seen as a valuable
palaeo-proxy for reconstructing past ocean temperatures. The temperature
dependence of Mg/Ca ratios in bivalve calcite has been the subject of
contradictory observations. The palaeoceanographic use of a geochemical
proxy is dependent on initial, rigorous calibration and validation of
relationships between the proxy and the ambient environmental variable to be
reconstructed. Shell Mg/Ca ratio data are reported for the calcite of two
bivalve species, <i>Mytilus edulis</i> (common mussel) and <i>Pecten maximus</i> (king scallop), which were grown in
laboratory culturing experiments at controlled and constant aquarium
seawater temperatures over a range from ~10 to ~20°C.
Furthermore, Mg/Ca ratio data of laboratory- and field-grown <i>M. edulis</i> specimens were
compared. Only a weak, albeit significant, shell Mg/Ca ratio–temperature
relationship was observed in the two bivalve species: <i>M. edulis</i> (<i>r</i><sup>2</sup>=0.37, <i>p<</i>0.001 for
laboratory-cultured specimens and <i>r</i><sup>2</sup>=0.50, <i>p<</i>0.001
for field-cultured specimens) and <i>P. maximus</i> (<i>r</i><sup>2</sup>=0.21, <i>p<</i>0.001 for
laboratory-cultured specimens only). In the two species, shell Mg/Ca ratios
were not found to be controlled by shell growth rate or salinity. The Mg/Ca
ratios in the shells exhibited a large degree of variability among and
within species and individuals. The results suggest that the use of bivalve
calcite Mg/Ca ratios as a temperature proxy is limited, at least in the
species studied to date. Such limitations are most likely due to the
presence of physiological effects on Mg incorporation in bivalve calcite.
The utilization is further limited by the great variability both within and
among shells of the same species that were precipitated under the same