DO STABLE ISOTOPES TRACK ENVIRONMENTAL CHANGE IN ECHINOID PLATES? A TEST USING MODERN ECHINOIDS

Stable oxygen and carbon isotope ratios were collected for transects across the plates of modern echinoids in order to test their responses to changes in sea water temperature and available nutrient content. The ultimate goals of this study are to 1) determine whether it would be possible to collect stable isotope signatures from fossil to subfossil echinoderm material to track paleoenvironmental changes in sea water, and 2) estimate plate growth rates in echinoderms. Three modern echinoid tests were examined; two sea urchins (an echinometrid and a strongylocentrotid) and one heart urchin (Clypeaster rosaceus). Echinoderms secrete high-magnesium calcite along the edges of plate boundaries. Round and/or square-shaped plates should have equivalent rates of growth along each boundary and in each axial direction. Although there would be some dissolution and reprecipitation of plate material through the life of the individual, in general the edge of the plate should represent a time-line, i.e. should have grown at the same time and in the same marine ambient water conditions assuming that the calcite is in equilibrium with sea water. Concentric regions of the test, moving inward toward the center, should also represent simultaneous growth on any radius and should have grown under identical ambient conditions. This would suggest that isotopic signatures collected near the edge of the plate should be equivalent along any axis, and signatures moving inward equal distances toward the center of the plate should be equivalent. We used a stabilized dental drill with scope to collect a transect of 2 mm-spaced samples along the axis of growth. A second linear transect was taken across the plates perpendicular to the first transect for two of the specimens. δ¹⁸O VPDB values ranged from -6.22 to -0.34, and δ¹³C VPDB values ranged from -6.52 to 3.21 for all three specimens. One echinoid with elongate plating showed no variation in either oxygen or carbon ratios along the plate. This implies either fast growth or an extremely stable environment. Two of the echinoids with ovoid plates displayed a symmetric pattern in both isotopes along the perpendicular transects. This study suggests that echinoderm plates can be used to track sea water chemistry changes including variations in seasonal temperature.