BIO- AND CHEMOSTRATIGRAPHY (D13C, 87SR/86SR) OF THE GUADALUPIAN-LOPINGIAN BOUNDARY INTERVAL OF THE MID-PANTHALASSAN PALEO-ATOLL LIMESTONE AT AKASAKA, JAPAN

In order to clarify the mid-oceanic environmental change across the Guadalupian-Lopingian boundary (G-LB), litho-, bio-, and chemostratigraphy (⁸⁷Sr/⁸⁶Sr, δ¹³C_carb) was analyzed in the Permian Akasaka Limestone (Japan) primarily deposited in low-latitude mid-Panthalassa.

Between the LAD of the Capitanian large fusulines (Yabeina) and the FAD of the Wuchiapingian fauna, a 20 m-thick G-LB extinction interval recorded two extinction episodes; i.e. the first extinction of the large fusulines, large bivalves (Alatoconchidae) and Dasycladacean algae at the base of the interval and the second one of smaller foraminifer and gastropod at the middle of the interval.

The ⁸⁷Sr/⁸⁶Sr ratios recorded extremely low values around 0.7068 throughout the Capitanian and started to rise rapidly to 0.7072 at the top of the G-LB interval. The extremely low ⁸⁷Sr/⁸⁶Sr ratios correspond to the “Guadalupian minimium”, the unique marker of the lowest seawater ⁸⁷Sr/⁸⁶Sr values during the Late Paleozoic. The present study confirmed that the duration of the minimum lasted for at least 5 million years.

The δ¹³C_carb values recorded very high over +5 ‰ in the uppermost Wordian to the Capitanian, followed by a decrease to +2 ‰ at the top of the G-LB interval. The drop of δ¹³C_carb and the rise of ⁸⁷Sr/⁸⁶Sr occurred almost at the same time around the G-LB. The high primary productivity and the sudden collapse at the top of the G-LB interval in mid-Panthalassa are concordant with the Kamura event that likely started in the Wordian.

As the large fusulines and large bivalves are regarded to have lived with algal symbiosis, the first extinction might reflect the collapse of shallow marine photosymbiotic community. The two extinctions occurred earlier than the δ¹³C_carb change, indicating that the primary productivity was never affected by marine diversity change. The Lopingian fauna appeared almost at the same time as the 87Sr/86Sr started to rise. This may indicate that the appearance of Lopingian fauna was possibly triggered by the increased nutrient supply. The rise of ⁸⁷Sr/⁸⁶Sr usually suggests the rise of the continental weathering rate; however, the ⁸⁷Sr/⁸⁶Sr rise cannot be explained by normal continental weathering under the Early Lopingian transgression. Instead, deglaciation and plume-related tectonics can be possible explanations.