STABLE ISOTOPE VARIATION BETWEEN GROWTH LINES ON THE BLASTOID PENTREMITES

Blastoids (class Blastoidea) are a group of extinct, stalked echinoderms that ranged from the Late Ordovician to the end Permian. They are composed of external stereom plates that developed in a regular, observable pattern. The plates are composed of high Mg calcite that undergoes rapid syntaxial cementation with low Mg calcite, filling in the porous stereom. Blastoids increase calyx volume by secreting calcite in between plate boundaries. During primary plate expansion, growth lines develop that parallel the plate boundaries, are continuous around the entire plate, and may represent pauses in the secretion of calcite. It is unknown how much time passes between sequential growth lines since there has previously been no way to measure the speed of blastoid growth. As the blastoid secretes calcite, the carbon and oxygen isotopes should equilibrate to the composition of the surrounding sea water as they are incorporated 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. This means that the relative isotopic values collected across the individual should still hold the ocean water signal, even if the original values have been overprinted.

Samples of Pentremites were collected from outcrops of Late Mississippian, Chesterian age rock. By using a Cameca IMS 7f GEO magnetic sector Secondary Ion Mass Spectrometer, stable isotope data can be collected at precise locations along the blastoid plate. Data can be collected between the growth lines, allowing measurements of stable isotopes throughout the entire ontogeny of the individual. This method represents a first attempt at ascertaining a possible temporal signal of isotopic change based on ocean temperature fluctuation, and we anticipate that it may prove useful in exploring the timing of blastoid growth line development. Preliminary data suggests that plate growth occurs intermittently, but requires further study to demonstrate statistically.