Growth environment of the Pacific oyster (Crassostrea gigas) reefs recorded by shells and sediment since mid-Holocene on the coastal plain of the Western Bohai Sea, China

High-resolution paleoenvironmental reconstruction is crucial for understanding past climate dynamics and predicting future climate changes. Holocene oyster reefs (Crassostrea gigas) on the coastal plain of the western Bohai sea serve as valuable archives for deciphering the paleoclimate fluctuations. Micro-scale sclerochronology study of individual oyster shells and macro-scale analysis on reef development can track the different time-resolution paleoenvironmental changes. This study investigated buried Holocene oyster reefs from three sites on the western coast of Bohai Bay: Dawuzhuang (DW), Lingtou (LT), and Huanggang (HG). Multi-proxy approaches, including radiocarbon dating, stable and radiogenic isotopes (C, O, Sr) and morphological analysis, were applied to the oyster shells. In addition, argillaceous sediments in the oyster reefs were analyzed for organic carbon, total nitrogen, total sulfur and their isotopes. Results indicated that the DW reef, characterized by relatively lower δ13C, δ18O, and δ13Corg values, was situated in a proximal estuarine setting with significant freshwater influence. In contrast, the HG reef exhibited higher isotopic values, suggesting minimal freshwater input and higher salinity. The LT reef showed the highest TOC/TN ratio, indicating substantial contributions from freshwater aquatic plants. The DW reef oysters, benefited from high plankton productivity due to terrestrial input, developed broader, thicker, and larger shells. Oysters at the HG reef, lied in a shallow marine area away from the estuary with the high salinity and hydrodynamics, displayed the largest shell height and growth rate but the shortest lifespan. At the LT reef, the low-energy habitat of a lagoon or semi-enclosed bay, fine sediment and reduced sediment stress supported the longest oyster lifespans and slowest shell height and weight growth rate. This integrated analysis of shell and sediment proxies successfully reconstructs distinct paleoenvironmental conditions and ecological adaptations across the Bohai Bay oyster reefs since middle Holocene.