GLOBAL CHANGE AND MANGANESE DEPOSITION AT THE CENOMANIAN-TURONIAN BOUNDARY

Several of the world's large stratiform Mn deposits formed in shallow water environments at the Cenomanian-Turonian boundary (CTB; ca. 92 my a). Earth history at this time is also characterized by increased tect onism, the highest Mesozoic Cenozoic sea level stand, and widespread r educing conditions in the oceans. Here we present evidence for a causa l relationship between these global scale phenomena. A high resolution geochemical analysis of pelagic sediments from two sites on the Exmou th Plateau (off northwest Australia) indicates significant Mn depletio n and reducing conditions coincident with deposit formation. We sugges t the coupling between deep water Mn depletions and shallow water Mn e nrichments involved the sequestering of hydrothermally derived Mn with in expanded oxygen minimum zones via diagenetic remobilization and/or direct entrapment of hydrothermal effluents, and its subsequent deposi tion on shelf substrates at the top of a redoxcline.