Yoshitaka Kakuwa

The Vendian-Cambrian δ13C record, North Iran: evidence for overturning of the ocean before the Cambrian Explosion

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.

Permian-Triassic mass extinction event recorded in bedded chert sequence in southwest Japan

Rock types in southwest Japan change across the Permian-Triassic boundary. Late Permian bedded chert grades upward into latest Permian siliceous claystone, carbonaceous mudstone of unknown age, early Triassic siliceous claystone with carbonaceous mudstone and chert interbeds, and then into middle Triassic to Jurassic bedded chert. These stratigraphic lithologic variation also accompany the decline and recovery of radiolarians in the Changxingian stage and Smithian stage, respectively. The Permian-Triassic boundary is not precisely defined because of poor stratigraphic control, but it is thought to occur at the boundary between the carbonaceous mudstone and the Changxingian siliceous claystone, or somewhere in the carbonaceous mudstone. During the Griesbachian stage there was deposition of carbonaceous mudstone in deep pelagic seas. Bioturbation structures and the massive nature of carbonaceous mudstone indicate dysaerobic, but not anoxic condition in a strict sense. The dramatic decrease in productivity of radiolarians preceded the sedimentation of carbonaceous mudstone, which suggests that oxygen-deficient conditions were not the primary cause of mass extinction. Red tide-type conditions inferred from the increased primary productivity in the pelagic seas and the oxygen deficiency in the epicontinental seas are the last hit against the biosphere in the chain of events that started from late Permian and caused the greatest mass extinction of the Phanerozoic. Correlation between lithologic variations of bedded cherts and δ13C excursions of carbonate rocks clarifies the processes of simultaneous massive decline and recovery of biomass as represented in both group of rocks.