The Late Triassic–Early Jurassic was a time of major global change; however, the fundamental processes driving these changes are less than clear. We have determined the Re and Os abundances, and Os isotope compositions, of marine mudrock samples that span the Triassic-Jurassic boundary in the UK and have established that major isotopic and geochemical shifts in the composition of seawater took place in the latest Triassic. We argue that these shifts were caused by the sudden initiation of widespread igneous activity within the Central Atlantic magmatic province, associated with rifting of the supercontinent Pangea. The Os isotope composition of seawater responded rapidly to these events, demonstrating that the seawater Os isotope system has great potential in identifying the nature and precise timing of major environmental change.

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