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, Michelle S. Eusebio5**

The application of stable isotope analysis on archaeological faunal assemblages has given essential data for palaeoenvironmental 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 to 8244 cal. BP, which serves as a benchmark for the region’s early environmental conditions. On the other hand, Munsayac Cave’s sequence situates its occupation during the Protohistoric Period, ranging from 964-559 cal. BP (c. 10th to 14th centuries AD). Our preliminary results suggest a notable divergence in isotopic niche width between these contexts. The fauna from Tanggapan Cave shows a tightly clustered δ13C signature (average at -22.18‰). These isotopic values relatively fall within a “closed-canopy” forest environment, which yields depleted carbon values. By comparison, the Munsayac assemblage illustrates a broader degree of carbon enrichment, where four samples show δ13C values as high as -6.30‰. While comparative macroscopic analysis identifies that these four 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 in future studies.