Stable isotope variation along the direction of growth in echinoderm plates

The plates of echinoderms are composed of calcite that undergoes rapid syntaxial cementation, filling in the porous stereom. During primary plate expansion, many echinoderms display growth lines that develop parallel to the plate boundaries. These lines circumscribe the plate and may represent pauses in the secretion of calcite. In extinct echinoderms such as the blastoids, the rate of development is unknown, and how much time passes between sequential growth lines remains ambiguous. In an attempt to address this question, a Cameca IMS 7f GEO magnetic sector Secondary Ion Mass Spectrometer was used to collect stable isotope data on plates of Paleozoic blastoids. As the echinoderm secretes calcite, the carbon and oxygen isotopes should equilibrate to the composition of the surrounding sea water and incorporate into the plates. Changes in oceanic isotopic values associated with annual temperature changes should be detectable in these blastoid plate growth lines. Although post-burial diagenetic fluids will almost certainly imprint isotopic values different from the ocean waters onto the plates, the buffering of burial fluids with surrounding rock and the rapid speed of syntaxial cementation on echinoderm plates should keep diagenetic isotope values equivalent through time. Therefore, relative isotopic values should still hold the ocean water signal, even if the original values have been overprinted. Oxygen and carbon relative isotope values were measured along the growth direction Pentremites plates, collected from outcrops of Late Mississippian, Chesterian age rock. A cyclic pattern can be demonstrated on the plates that may represent temperature changes in the water through the growth of the individual. The width of these cycle periods can be calibrated to the number of growth lines to calculate an age and rate of development on the individual.