Decoupled carbonate chemistry controls on the incorporation of boron into Orbulina universa

In order to fully constrain paleo-carbonate systems, proxies for two out of seven parameters, plus temperature and salinity are required. The boron isotopic composition (δ11B) of planktonic foraminifera shells is a powerful tool to reconstruct changes in past surface ocean pH. As B(OH)4− is substituted into the biogenic calcite lattice in place of CO32−, it has been suggested that B/Ca ratios in biogenic calcite are a possible proxy for [CO32−]. However, differentiating between the effects of pH and [CO32−] is problematic, as they co-vary closely in natural systems, and so the major control on boron incorporation remains unclear. To deconvolve the effects of pH and [CO32−] on the B/Ca ratio and to test whether δ11B remains constant at constant pH, but under changing [CO32−] (pH 8.05 with 238, 285 and 532 µmol kg−1 CO32−) and vice versa, we decoupled pH and [CO32−] (276 ± 19.5 µmol kg−1 CO32− with pH 7.7, 7.9 and 8.05) and grew the planktonic foraminifer Orbulina universa in these manipulated culture media. Measurements of the isotope composition of boron and the B/Ca ratio were performed simultaneously using a femtosecond laser ablation system coupled to an MC ICP-MS. Results show that δ11B is controlled by pH and does not respond to changes in [CO32−]. On the other hand, the B/Ca ratio is driven by [HCO3−] independently of pH. This suggests that B/Ca ratios in foraminiferal calcite may be used as a second, independent, proxy for paleo-carbonate system reconstructions.

Howes E. L., Kaczmarek K., Raitzsch M., Mewes A., Bijma N., Horn I., Misra S., Gattuso J.-P. & Bijma J., 2016. Decoupled carbonate chemistry controls on the incorporation of boron into Orbulina universa. Biogeosciences Discussions 1-26. Article.

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