UCLA

Element-to-calcium (X/Ca) ratios within biogenic calcium carbonate minerals are used as proxies to reconstruct past seawater temperature and composition. This study focuses on examining Li/Ca, B/Ca, and Mg/Ca ratios in sea urchins cultured at different temperatures and pCO₂ levels.

In situ secondary ion mass spectrometry (SIMS) analyses were conducted on two species of sea urchins cultured under controlled conditions and several species of paleo-echinoderms. A temperate species, Arbacia punctulata, was cultured at various pCO₂ levels (400, 600, 900, 2850 ppmv) with temperature held constant at 25°C. A tropical species, Echinometra viridis, was cultured in variable pCO₂ (400 and 1000 ppmv) and temperature (20°C and 30°C) conditions.

In the tropical species, a positive correlation was found between temperature and the analyzed (Li, B, Mg) element-to-calcium ratios of the spines, while variable responses to pCO₂ were observed. In the temperate species, a variable response to increasing pCO₂ was observed, with B/Ca having a similar trend to the calcification rate found in a previous study by Ries et al. (2009). Paleo-echinoderm samples varied in age from 518 to 53 Ma, and have a global distribution. The X/Ca ratios from paleo-echinoderms were used, along with the full range of observed partition coefficients (D^cX/Ca=X/Ca_carbonate/X/Ca_fluid, with X=Li, B, or Mg in this study) from the cultured sea urchins, to model element-to-calcium ratios of paleo-seawater.