Ocean acidification and mass extinction

Ocean acidification is implicated in many of the most profound mass extinction events in the geological record (Hönisch et al. 2012). However until recently it has not been possible to reconstruct past pH change, so the role of acidification in mass extinction remains largely untested (Clarkson et al. 2015).

This project aims to transform our understanding of pH change through geological time by application of the boron isotope pH proxy in the rock record. The use of the boron isotope proxy has grown rapidly in recent years due to new MC-ICPMS methods and improved understanding of the proxy's systematics (e.g. Rae et al. 2011; Foster & Rae 2016). However to date the application of boron isotopes beyond the reach of deep sea sediment cores has been limited.

The initial phase of this project will develop new methods of extracting primary carbonate signatures from geological samples. By pairing a variety of elemental and isotopic measurements (including strontium and sulphur isotopes), we will screen for diagenetic influences and obtain complementary geochemical reconstructions. Preliminary data have yielded coherent signals from biogenic carbonates taken from ~200 Myr micrite, illustrating the vast potential of this technique.

We will then generate boron isotope records over major carbon cycle perturbations in the geological record. A particular focus will be times of massive volcanic eruptions from large igneous provinces (LIPs). LIPS are commonly associated with mass extinction, but some events leave the biosphere relatively unscathed. Our data will allow us to constrain the relative importance of buffering in the ocean-atmosphere carbonate system versus resilience in marine communities.

To explore these data in a quantitative framework we will perform a series of experiments with the GENIE Earth System model. We will test the influence of different eruption rates (constrained with the latest geochronology data) on the carbon cycle over a range of boundary conditions. For instance did the evolution of a buffering blanket of seafloor carbonate during the Mid-Mesozoic dampen the ability of LIPS to drive extensive acidification? Or is the major ion chemistry of the ocean a more crucial control?