THE VENDIAN-CAMBRIAN DELTA-C-13 RECORD, NORTH IRAN - EVIDENCE FOR OVERTURNING OF THE OCEAN BEFORE THE CAMBRIAN EXPLOSION

Continuous fossiliferous successions across the Precambrian/Cambrian ( PC/C) boundary in the Elburz Mountains of Northern Iran show a remarka ble negative delta(13)C excursion just below the PC/C boundary. High c oncentrations of manganese, phosphorus, barium, and high abundances of fossil phytoplankton, and black shale coincide with the excursion. Wo rldwide stratigraphic correlation shows that the isotopic anomaly is a global event. The initial Metazoan diversification, coupled with C-13 enrichment, occurs stratigraphically just above the excursion. We pro pose the following scenario for oceanic environmental changes before t he Cambrian Faunal Explosion based on new data from Iran: A global war m climate following the last Precambrian glaciation resulted in a gene rally stagnant oceanic condition, so that surface water was oxic; deep water was dysoxic, depleted in C-13, and enriched in nutrients, Massi ve upwelling of deep water (vertical advection of nutrients and C-13-d epleted CO2) caused enhanced phytoplankton productivity and a sharp dr op in delta(13)C in shallow water carbonate and organic carbon. We con clude that latest Cryptozoic overturning of ocean stratification prece ded the Cambrian Explosion.