The ancient giant clam shells from Balabok Rockshelter, Philippines as potential recorders of environmental changes: a diagenesis assessment and sclerochronology study

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The study explores the preservation state of ancient giant clams excavated from the Balobok Rockshelter Archaeological Site, Philippines. The Hippopus hippopus and Tridacna maxima are from the cultural layer of Early Occupational Phase (8760 +/130 BP) and Later Occupational Phase (5140 +/100 BP), respectively. The research presents the mineral composition, microstructure characteristics and microgrowth patterns of the Tridacnidae shells. Both species were subjected to Fourier Transform Infrared Spectrometry (FTIR) and Scanning Electron Microscopy (SEM). These two methods aim to recognize its mineralogy and microstructure of the studies material. Only the H. hippopus was analyzed through sclerochronological method to understand its growth patterns. The results of the analyses imparted a record on the diagenesis alteration of the Tridacnidae shells. Both shells are still aragonite; however, altered prismatic and crossed-lamellar microstructure can be observed, which is not favorable for geochemical study. On the other hand, sclerochronology study on H. hippopus determines the age and internal growth changes of the shell, which is approximately one year and started to development in the last quarter. This concludes that sclerochronology is feasible because of the shell’s preserved inner growth increments. However, larger (older) shells should be selected for study to obtain palaeoenvironmental information.