Boron isotopes in foraminifera as proxy for seawater pH: Application and challenges

Abstract

One of the main goals for climate scientists is to reconstruct past pCO2 levels. Combined with accurate temperature reconstructions, these insights will help to predict climate change as a function of ongoing emissions of greenhouse gasses. Since seawater pH and atmospheric CO2 concentrations are directly related, several pH proxies have been developed and applied. The boron isotope composition of foraminiferal calcite (δ11B) is a potentially accurate estimate for past seawater pH and thereby, atmospheric pCO2. Here, I outline the underlying principals of this proxy and discuss challenges associated with its paleo-application. Offsets from the theoretical pH-boron isotope fractionation curve are of particular importance since this hints to a biological control on the incorporation of boron in foraminiferal calcite and needs to be understood and quantified for reliable paleo-pH reconstructions. Secondly, the concentration of boron over calcium in seawater (B/Casw) also affects foraminiferal δ11B and needs to be corrected for when using fossil boron isotopes as a paleo-pH proxy.