Carbon and oxygen isotopic composition of mollusk shells from marine and fresh-water environments

Abstract

Results are given of a systematic survey of differences in the isotopic composition of carbon and oxygen of modern mollusk shells from marine and continental environments. Marine shells analysed show a range of δC 13 (relative to Chicago PDB standard) from +4.2 to −1.7 %., whereas the fresh-water mollusk shells have relatively C 13-deficient carbon, in the range δC 13 = −0.6 to −15.2. There is a similar difference in O 18 content. Within the marine group, environmental sub-groups differ mainly in O 18 content and the differences are consistent with the temperature dependence previously studied by other investigators. Within the fresh-water group, the most striking difference between sub-groups is in the C 13 content of pelecypod shells from large lakes (δC 13 = −2.4 to 6.0%.), and from rivers (δC 13 = − 8.3 to −15.2 %.), a difference which is shown to be environment-controlled rather than speciescontrolled. The soft parts of pelecypods also show characteristic differences of carbon isotopic composition from marine to lacustrine to fluvial specimens. The ligament of pelecypods is found to consist partly of aragonite fibers which are isotopically different from shell carbonate. It is concluded that the carbon isotope ratio in mollusk shells is considerably influenced by the proportional amount of land-plant derived carbon included in the food of the mollusks or contributed by humus decay to dissolved bicarbonate in the water. It appears likely that the observed isotopic differences can be applied to the environmental study of fossils and sedimentary carbonate rocks, and to the source identification of shell artifacts of archaeological interest.