Boron isotopes in Southern Ocean deep sea corals

Today the Southern Ocean is experiencing changes in carbonate chemistry with potential impacts on calcifying fauna. Similarly there were likely to have been large changes in pH in the past, associated with transient shifts in upwelling. Studying past changes in Southern Ocean carbonate chemistry, and its effect on calcifying organisms may help to better assess their fate in the light of anthropogenic ocean acidification.

Carbonate formed by marine organisms incorporates boron, and the speciation of boron and fractionation of its isotopes are sensitive to the pH of seawater. In order to test a new archive for past seawater pH we have analysed the boron chemistry of modern stylasterid corals collected from 130m to 2000m in the Drake Passage where there is a pH gradient with depth. Stylasterid corals have a global distribution and concentrically banded skeletons, but little is known about their growth rates or biomineralisation.

Boron concentrations range from ~60-160ppm and vary between species. The $11B values also show a large range, which again appear to be species dependent. Single species exhibit lighter values with decreasing pH (increasing water depth) similar to empirical calibrations of benthic foraminifera and theoretical predictions. The high boron concentrations provide the possibility of using these corals for ultra-highresolution reconstruction of deep and high latitude pH in the past, but will require analyses of additional modern samples to confirm the pH-boron calibration.

Robinson, L. F., Nisch, B. H., Auro, M. E., 2010. Boron isotopes in Southern Ocean deep sea corals. Geochimica et Cosmochimica Acta, Goldschmidt 2010: Earth, Energy, and the Environment 74(12; Supplement 1): A873. Abstract (subscription required).

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