Boron isotopes in carbonates as proxy for seawater paleo-pH: back to basics – CARBORIC

A major scientific challenge is to determine the CO2 content of the past atmosphere or associated ocean acidity. The boron isotopic composition of marine carbonates was first proposed as a proxy of ancient seawater pH 20 years ago. Since that time, a large body of literature has been published using boron isotopes in carbonates to reconstruct paleo seawater pH and paleo atmospheric CO2 content. Several empirical calibration curves between the boron isotopic composition of carbonates and seawater pH have been published. These calibration curves exhibit different patterns depending on the type of carbonate and species of the calcifying organisms. As a consequence, the use of boron isotopes as a paleo environmental proxy is limited by ambiguities and large uncertainties resulting from our poor knowledge of the fundamental mechanisms controlling its co-precipitation with carbonates and subsequent isotopic fractionation.
The aim of this project is to revisit the different hypotheses that have been made for using boron isotopes in carbonates as a pH indicator. For this purpose, five complementary teams with a long standing expertise in boron isotopes, boron speciation and carbonate geochemistry have joined their efforts to create this multidisciplinary project, involving experimental and theoretical geochemists, isotope geochemists, mineralogists and NMR experts. This project is comprised of a set of 5 interrelated tasks. Task 1 uses two novel experimental methods to determine the fractionation factor between boric acid and borate ion, a fundamental parameter in pH determination. Task 2 is to measure directly boron isotopic fractionation and speciation during its co-precipitation with carbonate minerals and to built a model of boron incorporation. Task 3 addresses the natural heterogeneity of carbonate minerals for boron isotopes and aims at defining an analytical protocol reconciling in situ and bulk isotopic analyses in carbonates. Task 4 addresses the spatial and temporal variability of seawater boron isotope composition. Task 5 proposes two application studies to natural carbonates in the context of ocean acidification resulting from anthropogenic CO2 emissions. Combined techniques and expertise will allow us to address the important issue of isotopic “upscaling” in natural carbonates, and to search for fine spatial variations of the boron isotopic composition of seawater, susceptible to affect the use of boron isotopes as a paleo pH proxy. This project is not a paleo-oceanography project and does not considerer that all previous and on-going studies using boron isotopes to reconstruct ocean pH are not correct. It is a “back to basics” project aiming at a refining isotopic partition coefficients of boron, at a better understanding of boron coprecipitation in carbonates, at solving the upscaling issue between in situ and bulk measurements and addressing seawater variability. It is a timely and necessary project. The result will benefit to paleo-oceanography and B isotope geochemistry communities but also, more generally it will help understanding some of the fundamental mechanisms that fractionate stable isotopes in nature. The techniques used in this project associate “state to the art” isotope and spectroscopic measurements and require a very high level and unique of expertise that has been achieved by the proposing teams over the last decade.