Continuous fossilliferous successions across the Precambrian/Cambrian (PC/C) boundary in the Elburz Mountains of Northern Iran show a remarkable negative δ13C excursion just below the PC/C boundary. High concentrations of manganese, phosphorus, barium, and high abundances of fossil phytoplankton, and black shale coincide with the excursion. Worldwide stratigraphic correlation shows that the isotopic anomaly is a global event. The initial Metazoan diversification, coupled with13C enrichment, occurs stratigraphically just above the excursion. We propose the following scenario for oceanic environmental changes before the Cambrian Faunal Explosion based on new data from Iran: A global warm climate following the last Precambrian glaciation resulted in a generally stagnant oceanic condition, so that surface water was oxic; deep water was dysoxic, depleted in13C, and enriched in nutrients. Massive upwelling of deep water (vertical advection of nutrients and13C-depleted CO2) caused enhanced phytoplankton productivity and a sharp drop in δ13C in shallow water carbonate and organic carbon. We conclude that latest Cryptozoic overturning of ocean stratification preceded the Cambrian Explosion.
