Fractionation of the stable isotopes of carbon and oxygen in calcareous marine invertebrates - the Asteroidea, Ophiuroidea and Crinoidea
Quantitatively, the Eehinodermata constitute an important phylum of the carbonatesecreting marine invertebrates, comprising up to 90 per cent of the total biomass in some areas of the ocean. Locally, they may be important contributors to sediment as a result of their large numbers, the high proportion of skeletal carbonate in a given animal, the relatively short life span, and the rapidity with which disarticulated skeletal ossicles, ranging in size from the large spine of the slate-pencil urchin to microscopic pedicellariae, are distributed by submarine geological processes after death of the organism. The echinoderms differ from most carbonatesecreting marine organisms in the isotopic composition of their skeletal calcite. A pronounced biological effect with genetic control is evident in the fractionation of the stable carbon and oxygen isotopes by echinoderms. Five hundred and seventy-three specimens were selected to represent the population of living asteroids. All important taxonomic groups are included. The analyzed asteroids were taken from world-wide localities, from polar to tropical seas, and from littoral to abyssal depths. The major findings are: (1) Relatively large and systematic variations in δC13 and δO18 occur for different skeletal elements within a single animal(2) δC13 and δO18 values of these different skeletal elements are positively correlated (3) asteroid calcite is enriched in C 12 and O 16 with respect to "inorganic" carbonate (4) significant differences in isotopic composition exist at the species and higher levels. All of these observations are accounted for by an isotope exchange reaction between respiratory carbon dioxide and dissolved bicarbonate at or near the site of skeletal deposition, a consequence of the primitive respiratory system of the Asteroidea. The distribution of the stable carbon and oxygen isotopes in the skeletal calcite of ophiuroids (brittle stars), an important component of the carbonate-secreting invertebrate fauna over much of the sea floor, has been determined for 166 specimens referred to eleven families. The analyzed samples, taken from environments ranging from the cold waters of Alaska, Greenland and Siberia to the warm tropical seas of the central Pacific, and from depths ranging from littoral to 2086 m, are considered to adequately represent the known living ophiuroid population. Within a single animal, systematic differences in δC13 and δO18 to the extent of 1-2 permil are observed for different skeletal elements, and the δC13 and δO18 values are positively but poorly correlated. With the exception of the families Asteronychidae and Gorgonocephalidae, the mean skeletal C 13 and O 18 contents of the various ophiuroids are similar but are quite distinct from those of all other analyzed echinoderms (Echinoidea, Asteroidea, Crinoidea) except the irregular or "sand-dollar" echinoids. All of these observations are consistent with the mechanism of carbon and oxygen isotope fractionation in calcification of ephinoderms proposed to account for the unusual C 13:C 18 and O 18:O 16 found in echinoid and asteroid calcites. The skeletal calcite of living crinoids is appreciably enriched in C 12 and moderately enriched in O 16 with respect to calcium carbonate which might be precipitated inorganically from seawater at the same temperature. Crinoidal calcites are isotopically lighter than those of most other echinoderms, and the stable isotope fractionation model proposed for echinoids, ophiuroids and asteroids applies equally well to crinoids.