Oxygen isotope fractionation in marine aragonite of coralline sponges

Oxygen isotope values of the extant Caribbean coralline sponge Ceratoporell a nicholsoni are compared with published temperatures and delta(18)O of wat er calculated from salinities. The measured values from aragonitic sponge s keletons have a mean offset of 1.0 +/- 0.1 parts per thousand from calculat ed calcite equilibrium values (alpha(aragonite-calcite) = 1.0010). This is in good agreement with published values from synthetic aragonite. They furt her agree with published near-equilibrium oxygen isotope values of temperat e and cold water molluscs and foraminifera extrapolated to the temperature range of the coralline sponges. These results and the mode of skeleton form ation of Ceratoporella nicholsoni suggest that these sponges precipitate ar agonite close to isotopic equilibrium. The temperature dependence of oxygen isotopic fractionation between the ara gonite of Ceratoporella nicholsoni and water is only roughly constrained by the available data, due to the narrow temperature range of the Caribbean r eef 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 spong e 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) - de lta(w)); for 3 degrees < T < 28 degrees. Copyright (C) 2000 Elsevier Scienc e Ltd.