Oxygen isotope fractionation in marine aragonite of coralline sponges

Oxygen isotope values of the extant Caribbean coralline sponge Ceratoporella nicholsoni are compared with published temperatures and delta(18)O of water calculated from salinities. The measured values from aragonitic sponge skeletons have a mean offset of 1.0 +/- 0.1 parts per thousand from calculated calcite equilibrium values (alpha(aragonite-calcite) = 1.0010). This is in good agreement with published values from synthetic aragonite. They further agree with published near-equilibrium oxygen isotope values of temperate and cold water molluscs and foraminifera extrapolated to the temperature range of the coralline sponges. These results and the mode of skeleton formation of Ceratoporella nicholsoni suggest that these sponges precipitate aragonite close to isotopic equilibrium. The temperature dependence of oxygen isotopic fractionation between the aragonite of Ceratoporella nicholsoni and water is only roughly constrained by the available data, due to the narrow temperature range of the Caribbean reef sites. However, as the data suggest oxygen isotopic equilibrium, we can calculate a well constrained temperature equation combining temperate and cold water equilibrium values from molluscs and foraminifera with our sponge data: 10(3)ln alpha(aragonite-water) = (18.45 +/- 0.4) * 10(3)/T (K) - (32.54 +/- 1.5) and T (degrees C) = (20.0 +/- 0.2) - (4.42 +/- 0.10) * (delta(a) - delta(w)); for 3 degrees < T < 28 degrees. Copyright (C) 2000 Elsevier Science Ltd.