Offsets from isotopic equilibrium in biogenic carbonates have complicated paleoclimate reconstructions for decades. We use a new archive of climate, deep-sea corals, to evaluate the calcification processes, independent of photosynthesis, that contribute to these offsets. Carbon and oxygen stable isotope data from six modern deepsea corals show strong linear trends between δ 13 C and δ 18 O. Slopes of these trends between samples are similar and range between 2.1-2.6 for ∆δ 13 C/∆δ 18 O. Linear trends intersect isotopic equilibrium for δ 18 O and are slightly depleted for δ 13 C. Variations in the isotopic ratios are strongly correlated with the density banding structure. Isotopically depleted aragonite is associated with light, fast precipitating bands, while 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 δ 18 O and δ 13 C. This deviation, where δ 13 C is anomalously heavy for the δ 18 O, 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 biogenic carbonates that is based on a thermodynamic response to a biologically induced pH gradient in the calcifying region.

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