A remarkable sea-level drop and relevant biotic responses across the Guadalupian-Lopingian (Permian) boundary in low-latitude mid-Panthalassa: Irreversible changes recorded in accreted paleo-atoll limestones in Akasaka and Ishiyama, Japan

摘要

The Capitanian (Upper Guadalupian) to Wuchiapingian (Lower Lopingian) shallow-marine limestones at Akasaka and Ishiyama in central Japan record unique aspects of the extinction-related Guadalupian-Lopingian boundary (G-LB) interval. The ca. 140 m-thick Akasaka Limestone consists of the Capitanian black limestone (Unit B; 112 m) and the Wuchiapingian light gray dolomitic limestone (Unit W; 21 m), with a black/white striped limestone (Unit S; 9 m) between them. The G-LB horizon is assigned at the base of Unit W, on the basis of the first occurrence of the Wuchiapingian fusulines. The Capitanian Unit B and the Wuchiapingian Unit W were deposited mostly in the subtidal zone of a lagoon, whereas the intervened Unit S and the lowermost Unit W were in the intertidal zone. A hiatus with a remarkable ero-sional feature was newly identified at the top of Unit S. These records indicate that the sea-level has dropped significantly around the G-LB to have exposed the top of the atoll complex above the sea-level. The Ishiyama Limestone, located ca. 10 km to the north of the Akasaka limestone, retains almost the same depositional records. The extinction of large-tested fusuline (Yabeina) and large bivalves (Alatoconchidae) occurred in the upper part of Unit B, and the overlying 20 m-thick limestone (the uppermost Unit B and Unit S) below the hiatus represents a unique barren interval. The upper half of the barren interval is more depleted in fossils than the lower half, and this likely represents a duration of the severest environmental stress(es) for the shallow-marine protists/animals on the mid-oceanic paleo-atoll complex. Small-tested fusulines re-appeared at the base of Unit W above the hiatus. These facts prove that the elimination of shallow-marine biota occurred during the Capitanian shallowing of Akasaka paleo-atoll before the sub-aerial exposure/erosion across the G-LB. The overall shallowing and the development of such a clear hiatus at the top of a mid-oceanic seamount, in accordance with the contemporary sea-level curve based on continental shelf records, indicates that a remarkable sea-level drop has occurred globally during the latest Capitanian. This further suggests that a cool climate likely has appeared even in the low-latitude domains in Panthalassa to cause the decline of the Middle Permian shallow-water protists/animals that adapted to warmer seawater. The Wuchiapingian biota first appeared immediately after this erosional episode, i.e., during the onset of warming after the G-LB.