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

The Queen Scallop Aequipecten opercularis is 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 . Modem shells were investigated to determine if carbonate is precipitated in isotopic equilibrium in this species, and to confirm that growth continu es through the year. Specimens of A. opercularis were cultured under monito red semi-natural conditions over an autumn/winter period, and the analyses of these specimens were supplemented with isotope data from preserved indig enous North Sea specimens. The results indicated that the cultured specimen s grew in relatively low temperatures, that oxygen isotopes of shell-exteri or carbonate were precipitated in isotopic equilibrium, and that a large pr oportion of the ambient temperature range was recorded in shell carbonate. The indigenous North Sea specimens exhibited a good correspondence to predi cted oxygen isotope values for summer; the isotopic record related to winte r was, however, somewhat truncated due to growth decelerations in the speci mens, Carbon,isotopes of shell-exterior carbonate were calculated as being derived primarily from external sources, and showed comparatively little in fluence from metabolically derived carbon; this suggests that carbon isotop es were at least close to equilibrium with ambient waters. The limited inco rporation of metabolic products suggests that A. opercularis would be a goo d monitor of changing carbon isotope ratios in seawater-dissolved inorganic carbon. The occurrence of equilibrium shell precipitation is explained usi ng a 'pteriomorph shell-calcification model', which proposes that transport of ions through the periostracum caused the composition of the extrapallia l fluid to be effectively the same as seawater. Carbonate secretion is, the refore, not influenced significantly by biological (vital) effects. (C) 199 9 Elsevier Science B.V. All rights reserved.