ENVIRONMENTAL STRESSES PRECEDING THE END-TRIASSIC EXTINCTION IN THE PANTHALASSIC OCEAN

The end-Triassic mass extinction has been closely linked to the initial emplacement of the Central Atlantic Magmatic Province (CAMP). However, several sections from the Panthalassic Ocean show evidence for changing environmental conditions beginning hundreds of thousands of years prior to the extinction. We present results from the Kurusu section, near Inuyama, Japan. The section is part of the Mino Terrane, an accretionary complex of late Paleozoic and Mesozoic sediments deposited at abyssal water depths in the open ocean, providing a unique window into the Triassic-Jurassic transition in a pelagic setting. The rhythmically bedded nature of these radiolarian cherts allowed construction of a floating astronomical age model, tied to the radiolarian-defined Triassic-Jurassic boundary.

We report the first identification of mercury deposition at the TJB from a pelagic setting, almost antipodal to the location of CAMP extrusive centers. While most mercury deposition in this setting is strongly correlated with sulfide deposition, the TJB excursion appears to be a unique event. Below this horizon, a series of discrete excursions in sulfide content appear in the 1.2 million years preceding the TJB, recording a precursor interval of environmental stress that correlates with changes in calculated primary productivity and the ecological composition of the planktonic community.

We propose that these environmental changes represent the development of water column stratification, and can be detected across a range of Panthalassic environments. We compare the record from the Kurusu section to other Panthalassic sections, to trace the development of water column stratification across the basin. We argue that the onset of environmental stress precedes evidence for extrusive CAMP volcanism, and that the end-Triassic extinction in Panthalassic environments coincides with the irruption of reducing waters into photic zone and shelf environments.