| Summary: | The Permian-Triassic boundary-interval (PTB) witnessed the most severe environmental crisis in Earth history, which dictated the course for evolution of life until today. Current lines of evidence on causation point towards massive flood-basalt volcanism from Siberian traps, involving a combination of global warming by ˜6◦C, substantial input of relatively light carbon to the atmosphere, sporadic to widespread anoxia or euxinia, and ocean acidification, however the trigger mechanism is yet to be fully understood. In order to reconstruct potential changes in seawater chemistry during this time interval, we examined the boron isotope composition (δ11B) of pristine brachiopod shells. Although to-date hardly applied in Paleozoic settings, the δ11B of marine biogenic carbonates is considered to be one of the most reliable paleo-pH proxy. Brachiopods present an advantageous and largely underutilised archive for Phanerozoic reconstructions considering their high abundance in the geological record and its origin dating back to the early Cambrian. Moreover, their low-magnesium calcite shell renders them more resilient to post-depositional diagenetic alteration of primary chemical signals. Using carefully chosen pristine specimens (class Rhynchonellata and Strophomenata), selected δ11B to pH relationships, and bulk seawater δ11B scenarios we present a high-resolution seawater pH record for the Tethys Ocean. This interval covers the negative carbon isotope excursion in excess of 4 and is associated with major climate and environmental changes that led to the mass extinction event. Our results show a significant decline in δ11B succeeding the δ13C excursion, suggesting a substantial drop in seawater pH at the onset of the extinction event in the Late Permian related to carbon cycle perturbations. Combining our pH record with paired δ13C data and a quantitative modelling approach, we delineate the unfolding carbon cycle dynamic and budget that may have been responsible for initiating the catastrophic extinction event.
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