Integrating Microfossil and Geochemical Evidence to Refine our understanding of the Paleoenvironmental Evolution of the Szekou Formation

The Late Pleistocene Szekou Formation in southern Taiwan preserves an exceptional fossil assemblage that includes marine vertebrates, mollusks, and microfossils, which offers a unique peek into the paleo-lagoon environment during a period of rapid tectonic uplift and sea-level fluctuations. Previous studies utilizing sedimentological and lithofacies analyses, fossil assemblages, and stable isotope analyses, suggests a semi-open to open lagoonal system surrounded by barrier islands that provide partial isolation from the open ocean. However, the stratigraphic complexities, inconsistent sampling, and limited dating methods have led to contradictory interpretations and coarse age estimates, with the stratigraphic age roughly constrained between 30,000 and 260,000 years.

Recently, the National Museum of Natural Science's discovery of numerous in-situ preserved cetacean fossils, combined with new core data from the Geological Survey and Mining Management Agency, has provided new chronological evidence to further clarify the paleoenvironmental conditions. We integrated GNSS-based sampling location and elevation data to ensure accuracy. Carbon isotope ratio (δ13C) range from -22‰ to -25‰, while organic carbon-to-total nitrogen ratios (C/N) range between 7 and 29, suggesting C3 terrestrial plants or marine dissolved organic carbon as primary organic matter source(s). Additionally, foraminiferal assemblages indicate that the lagoon's water depth (60 meter) exceeded earlier estimates of 20 meters.

By integrating stratigraphically continuous core materials, stable isotope ratios and foraminifera data, this study reconciles prior discrepancies in paleoenvironmental interpretations of the Szekou formation. It reconstructs the sequential evolution of the Szekou lagoon, from its initial formation to later stages of development, emphasizing the influence of rapid uplift and sea level fluctuations on lagoon-ocean connectivity. Ultimately, this research seeks to contribute to broader understanding of evolution of coastal lagoons during the Late Pleistocene in southern Taiwan.