Reconstructing Paleoenvironmental Changes from Frame-Building Hypercalcified Sponges in the Late Permian Changhsingian Reefs of Eastern Sichuan Basin, China

Globally, Late Permian reefs were predominantly constructed by frame-building hypercalcified sponges, which are crucial proxies for reconstructing paleoenvironmental changes and sea-level fluctuations. The eastern Sichuan Basin, located in southwestern China, developed extensive Changhsingian (Late Permian) reefs dominated by hypercalcified sponges, but the relationships between frame-building hypercalcified sponge assemblages and their paleoenvironments remain poorly constrained. This study aims to fill this gap by investigating the Panlongdong and Yanggudong sections, representative outcrops of Changhsingian sponge reefs in the eastern Sichuan Basin, to clarify the characteristics of hypercalcified sponge assemblages and their responses to paleogeographic and paleoenvironmental changes, thereby enhancing understanding of Late Permian reef ecosystems and paleoenvironmental evolution. Results show that the Panlongdong reef, a platform-margin reef, forms a large domal structure with well-differentiated fore-reef and back-reef facies, while the Yanggudong reef, an intraplatform reef, is a small mound without distinct facies differentiation, both exhibiting three vertical reef-building cycles. Hypercalcified sponges are the primary frame-builders, accompanied by hydrozoans and bryozoans, with Archaeolithoporella as the main binder in Panlongdong and cyanobacteria in Yanggudong, and Tubiphytes as a secondary binder in both. Reef-dwelling organisms differ significantly: Panlongdong has a diverse stenohaline assemblage dominated by brachiopods, while Yanggudong has an euryhaline assemblage dominated by echinoderms and gastropods. The Panlongdong reef exhibits a succession of hypercalcified sponge assemblages: Amblysiphonella Assemblage (low diversity, subtidal), Amblysiphonella-Preperonidella Assemblage (high diversity, intertidal, peak reef development), and Preperonidella-Sollasia Assemblage (environmental deterioration). The Yanggudong reef has a distinct succession: Preperonidella-Intrasporeocoelia Assemblage (subtidal), Preperonidella-Sollasia Assemblage (intertidal, peak), and Sollasia-Uvanella Assemblage (decline). These successions reflect variations in hydrodynamic energy, seawater circulation, and paleosalinity driven by sea-level fluctuations and paleogeographic differences. Both reefs show ecological weakening prior to the end-Permian mass extinction, with the platform-margin reef affected by open-marine stress and the intraplatform reef by restricted conditions. This study demonstrates that frame-building hypercalcified sponge assemblages in the Changhsingian reefs effectively recorded paleoenvironmental changes, with their taxonomic composition, abundance, and succession closely linked to paleogeographic setting and sea-level fluctuations. The findings provide critical insights into the evolution of Late Permian reef ecosystems and their responses to regional and global environmental changes, laying a foundation for further research on Permian sponge biogeography and paleoceanography.