Evaluating Isotopic Variability in Cave Faunal Assemblages: Preliminary Ecological Insights from Biak-na-Bato, Bulacan, Philippines

Eleanor Marie S. Lim1,2*, Katherine L. Pernia3, Angelo A. Guirgio4, Armand Salvador B. Mijares5, Edilberto Larin Jr.6, Roland V. Rallos7, and Michelle S. Eusebio5

The application of stable isotope analysis on archaeological faunal assemblages has given essential data for paleoenvironmental reconstruction in Southeast Asia. However, with the dearth of local isotopic baselines, characterization of the archipelago’s microhabitats remains largely constrained. This study explores the isotopic variability within faunal assemblages from two archaeological cave sites in Biak-na-Bato, Bulacan, Philippines – Tanggapan and Munsayac, which appear to indicate notably diverse ecological windows. Tanggapan Cave provides a basal date of 8319-8244 cal BP, which serves as a benchmark for the region’s early environmental conditions; whereas Munsayac Cave’s sequence situates its occupation during the Protohistoric Period, from 964-559 cal BP (c. 10th-14th centuries AD). Our preliminary results suggest a notable divergence in isotopic niche width between these contexts. The fauna from Tanggapan shows a tightly clustered ∂13C signature (average at -22.18‰). These isotopic values relatively fall within a “closed-canopy” forest environment, which yields depleted carbon values. Conversely, the Munsayac assemblage illustrates a broader degree of carbon enrichment, where four samples show ∂13C values as high as -6.30‰. While comparative macroscopic analysis identifies that these samples belong to the Cervidae family, their isotopic profiles deviate significantly from the expected C3 forest baseline mostly associated with cervids in Southeast Asia. This isotopic breadth speculates a likely partitioned environment, suggesting that past inhabitants of Bulacan were acquiring fauna from varied ecological niches. This research establishes an initial isotopic framework for Bulacan and the broader Central Luzon Island, as well as a starting point for discussing human exploitation of tropical landscapes.