Stable isotopes in deep-sea corals and a new mechanism for "vital effects"

Offsets from isotopic equilibrium in biogenic carbonates have complicated paleoclimate reconstructions for decades. A new archive of climate, deep-sea corals, is used to evaluate the calcification processes, independent of photosynthesis, that contribute to these offsets. Carbon and oxygen stable isotope data from six modem deep-sea corals show strong linear trends between delta(13)C and delta(18)O. Slopes of these trends between samples are similar and range between 1.9 to 2.6 for Deltadelta(13)C/Deltadelta(18)O. Linear trends intersect isotopic equilibrium for delta(18)O and are slightly depleted for delta(13)C. Variations in the isotopic ratios are strongly correlated with the density banding structure. Isotopically depleted aragonite is associated with light, quickly precipitating bands, whereas isotopically enriched points correspond to slowly accumulating, less dense aragonite. The densest white band at the trabecular center is furthest from isotopic equilibrium for both carbon and oxygen. Data from this region fall off the linear trend between delta(18)O and delta(13)C. This deviation, where delta(13)C remains constant while the delta(18)O continues to decrease, does not support "vital effect" mechanisms that call upon kinetic fractionation to explain offsets from isotopic equilibrium. We propose a new mechanism for vital effects in these deep-sea corals that is based on a thermodynamic response to a biologically induced pH gradient in the calcifying region. Copyright (C) 2003 Elsevier Science Ltd.