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Intra-shell boron isotope ratios in the symbiont-bearing benthic foraminiferan Amphistegina lobifera : Implications for d11B vital effects and paleo-pH reconstructions

C. Rollion-Bard, J. Erez

Geochimica et Cosmochimica Acta 74 (2010) 1530–1536

Abstract The boron isotope composition of marine carbonates is considered to be a seawater pH proxy. Nevertheless, the use of d11B has some limitations such as the knowledge of the fractionation factor (a4-3) between boric acid and the borate ion and the amplitude of “vital effects” on this proxy that are not well constrained. Using secondary ion mass spectrometry (SIMS) we have examined the internal variability of the boron isotope ratio in the shallow water, symbionts bearing foraminiferan Amphistegina lobifera. Specimens were cultured at constant temperature (24 ± 0.1 C) in seawater with pH ranging between 7.90 and 8.45. Intra-shell boron isotopes showed large variability with an upper limit value of 30&. Our results suggest that the fractionation factor a4-3 of 0.97352 (Klochko et al., 2006) is in better agreement with our experiments and with direct pH measurements in seawater vacuoles associated with the biomineralization process in these foraminifera. Despite the large variability of the skeletal pH values in each cultured specimen, it is possible to link the lowest calculated pH values to the experimental culture pH values while the upper pH limit is slightly below 9. This variability can be interpreted as follows : foraminifera variably increase the pH at the biomineralization site to about 9. This increase above ambient seawater pH leads to a range in d11B (D11B) for each seawater pH. This D11B is linearly correlated with the culture seawater pH with a slope of 13.1 per pH unit, and is independent of the fractionation factor a4-3, or the d11Bsw through time. It may also be independent of the pKB (the dissociation constant of boric acid) value. Therefore, D11B in foraminifera can potentially reconstruct paleo-pH of seawater.

Voir en ligne : http://dx.doi.org/doi:10.1016/j.gca...

publié vendredi 5 février 2010