CAPITANIAN AND CHANGHSINGIAN (PERMIAN) ANOMALOUS CALCITE PRECIPITATES IN NORTHERN SICHUAN, CHINA: PRE-EXTINCTION UNUSUAL SEAWATER CONDITIONS IN THE ‘ARAGONITE SEA' ?

The abundant occurrence of anomalous carbonate precipitates was newly identified in the Capitanian and Changhsingian black calcareous mudstone/chert of deep slope facies at Chaotian in northern Sichuan. The several cm-thick carbonate precipitate beds are composed mostly of ca. 300 µm-long ‘bladed’ monocrystalline calcite crystals elongated along c-axis. Some crystals reach up to 2 cm in length, aligning parallel and perpendicularly to the bedding. Their mode of occurrence, together with the relatively low Sr content, suggests that the carbonate crystals precipitated primarily as calcite in sediment/water interface. The carbonate precipitates recorded the appearance of some unusual seawater conditions twice in the Capitanian and Changhsingian in South China. This unusual conditions might be caused by episodic upwelling of anoxic deep-ocean water enriched in bicarbonate ions from deeper basins to shallower slopes, even during the reputed Permian ‘aragonite sea’. It is also noteworthy that the same unusual condition has developed twice, as each of the events occurred slightly earlier than the two independent Permian extinctions immediately before the Guadalupian-Lopingian and P-T boundaries.