A universal carbonate ion effect on stable oxygen isotope ratios in unicellular planktonic calcifying organisms

[摘要] The oxygen isotopic composition (δ¹⁸O) of calcium carbonate ofplanktonic calcifying organisms is a key tool for reconstructing both pastseawater temperature and salinity. The calibration of paloeceanographicproxies relies in general on empirical relationships derived from fieldexperiments on extant species. Laboratory experiments have more often thannot revealed that variables other than the target parameter influence theproxy signal, which makes proxy calibration a challenging task.Understanding these secondary or "vital" effects is crucial for increasingproxy accuracy. We present data from laboratory experiments showing thatoxygen isotope fractionation during calcification in the coccolithophoreCalcidiscus leptoporus and the calcareous dinoflagellate Thoracosphaera heimii is dependent on carbonate chemistry ofseawater in addition to its dependence on temperature. A similar result haspreviously been reported for planktonic foraminifera, supporting the ideathat the [CO₃²⁻] effect on δ¹⁸O is universal forunicellular calcifying planktonic organisms. The slopes of the δ¹⁸O/[CO₃²⁻] relationships range between –0.0243‰ (μmol kg⁻¹)⁻¹ (calcareousdinoflagellate T. heimii) and the previously published –0.0022‰ (μmol kg⁻¹)⁻¹ (non-symbioticplanktonic foramifera Orbulina universa), while C. leptoporus has a slope of –0.0048 ‰ (μmol kg⁻¹)⁻¹. We present a simpleconceptual model, based on the contribution of δ¹⁸O-enrichedHCO₃⁻ to the CO₃²⁻ pool in the calcifying vesicle, whichcan explain the [CO₃²⁻] effect on δ¹⁸O for thedifferent unicellular calcifiers. This approach provides a new insight intobiological fractionation in calcifying organisms. The large range in δ¹⁸O/[CO₃²⁻] slopes should possibly be explored as a meansfor paleoreconstruction of surface [CO₃²⁻], particularly throughcomparison of the response in ecologically similar planktonic organisms.

[学科分类] 地球化学与岩石