Intra-shell boron isotope ratios in the symbiont-bearing benthic foraminiferan Amphistegina lobifera: Implications for δ11B vital effects and paleo-pH reconstructions

The boron isotope composition of marine carbonates is considered to be a seawater pH proxy. Nevertheless, the use of delta B-11 has some limitations such as the knowledge of the fractionation factor (alpha(4-3)) 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 degrees 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 approximate to 30 parts per thousand. Our results suggest that the fractionation factor alpha(4-3) 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 delta B-11 (Delta B-11) for each seawater pH. This Delta B-11 is linearly correlated with the culture seawater pH with a slope of -13.1 per pH unit, and is independent of the fractionation factor alpha(4-3), or the delta B-11(SW) through time. It may also be independent of the pK(B) (the dissociation constant of boric acid) value. Therefore, All B in foraminifera can potentially reconstruct paleo-pH of seawater. (c) 2009 Elsevier Ltd. All rights reserved.