Geochronological Constraints in Humid Tropical Environments: Methodological Insights from the Early Pleistocene Bumiayu Site, Java, Indonesia

Yves Belgiaswara Susilo1,2*, Yan Rizal1, Mika Rizki Puspaningrum1, Moh. Mu’alliful Ilmi2, Harman Dwi Rachmadan1,2, E. Olafianto Drespriputra Wisnuwardhana1,2, Muhammad Wildan Fadlillah2,3, Sofwan Noerwidi2, and Thomas Sutikna2

Establishing a precise chronological framework for the Quaternary period is fundamental to unravelling the complexities of hominin evolution and dispersal within the Southeast Asian archipelago. This study reconstructs the geochronology of the Bumiayu archaeo-paleontological site in Central Java, a critical Pleistocene Epoch locality characterized by its rich vertebrate fossil assemblages and lithic artifacts. The primary geochronological constraint in this region arises from the humid tropical environment. Persistent high precipitation and fluctuating groundwater tables facilitate intense chemical weathering and diagenetic alteration of fossil remains. These processes induce unstable uranium uptake—specifically leaching or secondary enrichment—and trigger the recrystallization of the mineral lattice in dental enamel, posing significant geochemical constraints for conventional absolute dating methods. To mitigate these environmental "noise" factors and ensure chronostratigraphic precision, this research implements a multi-proxy dating strategy. Combined U-series/Electron Spin Resonance (ESR) dating is applied directly to in situ fossil teeth recovered from the Kaliglagah, Mengger, and Gintung Formations. Crucially, to provide independent stratigraphic anchors, 40Ar/39Ar dating is conducted on volcanic minerals (hornblende, K-feldspar, etc,) within intercalated pyroclastic and volcanoclastic sequences. By synthesizing systematic geological mapping and high-resolution stratigraphic measurements with these cross-validated laboratory protocols, the study seeks to resolve long-standing taphonomic uncertainties. The expected outcome is a robust geochronology of the Bumiayu sequences, offering vital insights into the timing of Javanese tectonic uplift, faunal succession, and the broader paleoenvironmental context of early human occupation and behavioural patterns across the Pleistocene landscape.