The eruption of the Central Atlantic Magmatic Province (CAMP)—the largest igneous 12 province known—has been linked to the end-Triassic mass extinction event, however reconciling 13 the response of the biosphere (at local and nonlocal scales) to potential CAMP-induced 14 geochemical excursions has remained challenging. Here we present a combined sedimentary and 15 biological response to an ecosystem collapse in Triassic-Jurassic strata of the southwest United 16 Kingdom expressed as widely distributed carbonate microbialites and associated biogeochemical 17 facies. The microbialites (1) occur at the same stratigraphic level as the mass extinction (2), host a negative isotope excursion in δ13Corg found in other successions around the world, and (3) co- 19 occur with an acme of prasinophyte algae ‘disaster taxa’ also dominant in Triassic-Jurassic 20 boundary strata of other European sections. Although the duration of microbialite deposition is 21 uncertain, it is likely that they formed rapidly (perhaps fewer than ten thousand years), thus 22 providing a high-resolution glimpse into the initial carbon isotopic perturbation coincident with 23 the end-Triassic mass extinction. These findings indicate microbialites from the SW UK capture 2 24 a nonlocal biosedimentary response to the cascading effects of massive volcanism and add to the 25 current understanding of paleoecology in the aftermath of the end-Triassic extinction.