Inter-species, Intra-species and Intra-shell Variations in the Stable Isotope Composition of Late Ordovician Brachiopods From the Ellis Bay Formation, Anticosti Island, Quebec

Oxygen and carbon isotope results have been obtained for Late Ordovician (Hirnantian) brachiopod shells from the Ellis Bay Formation of Anticosti Island, Quebec. This location contains one of the world's most complete Ordovician-Silurian boundary successions and is renown for its abundance of well-preserved brachiopods. The site therefore offers an opportunity to explore the extent of preservation of primary oxygen and carbon isotope compositions in its ancient fossil materials. Three to four samples of each of eight species that lived contemporaneously in very similar environments were analyzed. Several analyses were performed along distinct growth lines of each shell; matrix carbonate and secondary calcite formed during diagenesis were also measured. Mean oxygen isotope values among the different species ranged from -4.1 to -3.1 per mil (VPDB) and mean carbon isotope values ranged from 0.4 to 1.8 per mil. The maximum variation among shells of the same species was 1.0 per mil for oxygen and 0.8 per mil for carbon (both for Parastrophinella). Intra-shell variation along growth lines for all brachiopod species was characterized by systematically higher carbon isotope values (up to 0.8 per mil for Vellamo) with distance from the hinge. Such variations in oxygen isotope composition were not observed. Such behaviour can be explained as a function of brachiopod metabolism. A brachiopod's shell grows radially from the central hinge structure. Young brachiopods experience faster growth rates than older brachiopods, which should produce lower carbon isotope compositions relative to those expected from equilibrium fractionation. The observed pattern in carbon isotope values suggests that original shell compositions have remained intact at this locality. The small but significant differences in shell isotopic compositions within and among shell species may also indicate preservation of local variations in environmental conditions and species-dependent metabolic behaviours. The significantly different isotopic values of secondary calcite from the interior of some shells also suggest that primary compositions were not reset during diagenesis. These data suggest that brachiopod shells from this locality can provide good proxies for the oxygen isotope composition of seawater and climatic conditions at this point in geological time.