LATE DEVONIAN PELAGIC CHERTS AND EXTINCTION EVENT

The Devonian was a warming period with the most extensive reef development except a decline in the Late Devonian. The Late Devonian (the late Frasnian) extinction in reef habitats was regarded as a result of global cooling on the basis of the studies on shallow-water sediments. In order to reconstruct the global environmental change at that time, the author investigated on deep-sea cherts in the New England Fold Belt, northern New South Wales and southern Queensland, eastern Australia. The cherts represent ancient deep-sea pelagic sediments primarily deposited in the Paleo-Pacific. The cherts at 13 sites yield age-diagnostic radiolarians. The Middle Devonian and Early Carboniferous radiolarians occurred from red bedded cherts, while Late Devonian ones from gray to green cherts. The red cherts during the Middle Devonian and Early Carboniferous indicate well-ventilated and oxygenated bottom water. The Late Devonian gray to green cherts in Australia, and coeval black cherts and shallow-water black shales rich in organic carbon in North America suggest the extensive disoxic water, indicating a sluggish ocean circulation. This sluggish ocean circulation may have reflected a warm climate in the Late Devonian. The warm-water radiolarians, Nassellaria and Albaillellaria, proliferated in the Late Devonian and declined at the end-Devonian synchronous with the onset of glaciation. This also may suggest that the late Frasnian extinction occurred during the warming climate.