Towards Applications of B-isotope Geochemistry for Paleo-ocean Chemistry Reconstruction

Ocean acidification is predicted to pose a significant societal problem stemming from anthropogenic input of CO2. Marine organisms are adversely affected by decreased surface water pH leading to biodiversity losses that cascade through food webs. Recent compilations present ways to interpret ancient ocean acidification crises in the geologic record, but these inferences of ocean acidification in millions-of-years-old strata lack a quantitative measure of paleo-pH of seawater. Boron stable isotopes are widely used in foraminifera of Cenozoic successions to reconstruct paleo-pH and paleo-atmospheric CO2. However, the application of the boron isotope system to other marine invertebrates and to older fossil material is hampered by the ease of diagenetic alteration of B isotopes in carbonate minerals; species-specific vital effects during uptake of borate into the carbonate shell material; and lack of knowledge of secular variation of the isotope composition of total dissolved boron in seawater. While there has been substantial progress in developing the boron isotope proxy, the proxy requires a more thorough, quantitative analysis in order to substantiate its validity. Herein we present a new, rigorous approach to using B isotopes as a paleo-proxy for pH. We prepared and screened Carboniferous and Jurassic Terebratulid brachiopod shells for application of the boron isotope proxy. To constrain the secular variation in the isotope composition of total dissolved boron, we will measure boron isotopes in phyllosilicates in marine sedimentary strata and terrestrial lacustrine cherts. Boron isotopic data will be cross-referenced for covariation with carbon, oxygen, and strontium isotopic data obtained from the same sampled brachiopods. These methods will enable us to better understand the importance of ocean acidification for paleo-ecosystems and as a response/forcing of perturbations to global biogeochemical cycles