The tropical echinoid Echinometra viridis was reared in controlled laboratory experiments at temperatures of approximately 20°C and 30°C to mimic winter and summer temperatures and at carbon dioxide (CO2) partial pressures of approximately 487 ppm-v and 805 ppm-v to simulate current and predicted-end-of-century levels. Spine material produced during the experimental period and dissolved inorganic carbon (DIC) of the corresponding culture solutions were then analyzed for stable oxygen (δ18Oe, δ18ODIC) and carbon (δ13Ce, δ13CDIC) isotopic composition. Fractionation of oxygen stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (Δ18O = δ18Oe – δ18ODIC) was significantly inversely correlated with seawater temperature but not significantly correlated with atmospheric pCO2. Fractionation of carbon stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (Δ13C = δ13Ce – δ13CDIC) was significantly positively correlated with pCO2 and significantly inversely correlated with temperature, with pCO2 functioning as the primary factor and temperature moderating the pCO2-Δ13C relationship. Echinoid calcification rate was significantly inversely correlated with both Δ18O and Δ13C, both within treatments (i.e., pCO2 and temperature fixed) and across treatments (i.e., with effects of pCO2 and temperature controlled for through ANOVA). Therefore, calcification rate and potentially the rate of co-occurring dissolution appear to be important drivers of the kinetic isotope effects observed in the echinoid spines. Study results suggest that echinoid Δ18O monitors seawater temperature, but not atmospheric pCO2, and that echinoid Δ13C monitors atmospheric pCO2, with temperature moderating this relationship. These findings, coupled with echinoids’ long and generally high-quality fossil record, supports prior assertions that fossil echinoid Δ18O is a viable archive of paleo-seawater temperature throughout Phanerozoic time, and that Δ13C merits further investigation as a potential proxy of paleo-atmospheric pCO2. However, the apparent impact of calcification rate on echinoid Δ18O and Δ13C suggests that paleoceanographic reconstructions derived from these proxies in fossil echinoids could be improved by incorporating the effects of growth rate.
Courtney T. & Ries J. B., in press. Impact of atmospheric pCO2, seawater temperature, and calcification rate on the δ18O and δ13C composition of echinoid calcite (Echinometra viridis). Chemical Geology. Article (subscription required).
