STRONTIUM ISOTOPIC COMPOSITION OF MIDCRETACEOUS SEAWATER

The Sr-87/Sr-86 ratio in fish teeth separated from mid-Cretaceous marl and black shale from the northeastern Apennines and Venetian Alps (Italy) define three periods of low Sr-87/Sr-86 ratio at 121 to 124 million years ago (Ma), 110 to 115 Ma, and 89 to 91 Ma. The Sr-87/Sr-86 excursions correspond to oceanic anoxic events represented by the Livello Selli, Livello 113, Livello Urbino, and Livello Bonarelli black shale marker beds and probably reflect an increase in the low-Sr-87/Sr-86 hydrothermal strontium flux associated with the emplacement of the Ontong-Java and Kerguelen plateaus (120 to 110 Ma) and the Caribbean Plateau (89 to 91 Ma). The modeled flux is consistent with the volumes and eruption rates of the oceanic plateaus but is far smaller than expected from the proposed Cretaceous crustal production rates of 50 to 100 percent greater than modern.