Vital effects in foraminifera do not compromise the use of DB as a paleo-pH indicator: Evidence from modeling

[1] The stable boron isotope composition (dB) of fossil foraminiferal shells is used as a paleo-pH recorder and is therefore one of the most promising paleocarbonate chemistry proxies (‘‘paleoacidimetry’’). One crucial question regarding this proxy is whether foraminifera record the pH of the bulk seawater or the pH of the microenvironment (diffusive boundary layer, 500 mm), which is strongly influenced by life processes. Here we present a novel theoretical approach to address this question by using a diffusion-reaction model. Model results indicate that the dB in planktonic foraminifera is primarily controlled by the pH of the microenvironment. We therefore predict that the dB of different species (e.g., symbiont-bearing versus symbiont-barren) or of foraminifera grown in the dark and in the light should be offset from the dB of inorganic calcite. This theoretical prediction was experimentally confirmed while this paper was written [Hönisch et al., 2003]. Most importantly, the model predicts that this offset is constant over a wide pH range. Thus the use of dB as a paleopH indicator is not compromised through vital effects as modeled here.