Several of the world's large stratiform Mn deposits formed in shallow water environments at the Cenomanian‐Turonian boundary (CTB; ca. 92 mya). Earth history at this time is also characterized by increased tectonism, the highest Mesozoic‐Cenozoic sea level stand, and widespread reducing conditions in the oceans. Here we present evidence for a causal relationship between these global‐scale phenomena. A high resolution geochemical analysis of pelagic sediments from two sites on the Exmouth Plateau (off northwest Australia) indicates significant Mn depletion and reducing conditions coincident with deposit formation. We suggest the coupling between deep water Mn depletions and shallow water Mn enrichments involved the sequestering of hydrothermally derived Mn within expanded oxygen minimum zones via diagenetic remobilization and/or direct entrapment of hydrothermal effluents, and its subsequent deposition on shelf substrates at the top of a redoxcline.