Calcification depth of pteropods in the Coral Sea using stable oxygen isotopes

Pteropods are marine holoplanktonic gastropods inhabiting epipelagic and mesopelagic waters with a wide distribution from tropical to polar regions. However, their ontogeny and calcification depth/habitat depth are not well understood. To this end, we analyzed the stable oxygen isotopic composition of five pteropod species (Diacavolinia angulata, Diacavolinia longirostris, Heliconoides inflatus, Limacina bulimoides and Telodiacria quadridentata) from two multicores recovered offshore north-eastern Australia between 15°S and 26°S in the Coral Sea. We applied the Limacina dissolution index (LDX) to evaluate the preservation status of the pteropods, which revealed very good to moderate aragonite preservation at both locations. Comparison of the pteropod shell oxygen isotopic composition with predicted aragonite equilibrium oxygen isotope values implies calcification depths of 50 ± 20 m for D. longirostris and 75 ± 30 m for D. angulata, suggesting that they predominantly calcify in the mixed layer during austral summer. The apparent calcification depths of T. quadridentata and H. inflatus at 90 ± 30 m and 120 ± 30 m at both sites indicate a year-round, lower mixed layer and upper thermocline habitat depth, respectively, while T. quadridentata appear to calcify at temperatures above 22 °C. The calcification depth of L. bulimoides is deeper (100 ± 15 m) at the northern station and shallower (75 ± 30 m) at the southern station that might also hint to a temperature control. The relative narrow calcification depth ranges of the investigated pteropod species in the Coral Sea bolster their potential for reconstructing past ocean conditions at the mixed layer and thermocline.