The shell of the Queen Scallop Aequipecten opercularis (L.) as a promising tool for palaeoenvironmental reconstruction: evidence and reasons for equilibrium stable-isotope incorporation

The Queen Scallop Aequipecten opercularis has been an important part of the marine fauna of the northeast Atlantic region since the Miocene, and is a suitable candidate for palaeoenvironmental studies using stable isotopes. Modern shells were investigated to determine if carbonate is precipitated in isotopic equilibrium in this species, and to confirm that growth continues through the year. Specimens of A. opercularis were cultured under monitored semi-natural conditions over an autumn/winter period, and the analyses of these specimens were supplemented with isotope data from preserved indigenous North Sea specimens. The results indicated that the cultured specimens grew in relatively low temperatures, that oxygen isotopes of shell-exterior carbonate were precipitated in isotopic equilibrium, and that a large proportion of the ambient temperature range was recorded in shell carbonate. The indigenous North Sea specimens exhibited a good correspondence to predicted oxygen isotope values for summer; the isotopic record related to winter was, however, somewhat truncated due to growth decelerations in the specimens. Carbon isotopes of shell-exterior carbonate were calculated as being derived primarily from external sources, and showed comparatively little influence from metabolically derived carbon; this suggests that carbon isotopes were at least close to equilibrium with ambient waters. The limited incorporation of metabolic products suggests that A. opercularis would be a good monitor of changing carbon isotope ratios in seawater-dissolved inorganic carbon. The occurrence of equilibrium shell precipitation is explained using a `pteriomorph shell-calcification modelʹ, which proposes that transport of ions through the periostracum caused the composition of the extrapallial fluid to be effectively the same as seawater. Carbonate secretion is, therefore, not influenced significantly by biological (vital) effects.