Shell-gathering from mangroves and the seasonality of the Southeast Asian Monsoon using high-resolution stable isotopic analysis of the tropical estuarine bivalve (Geloina erosa) from the Great Cave of Niah, Sarawak: methods and reconnaissance of molluscs

This paper describes a reconnaissance, high-resolution, study of stable isotopes (δ18O and δ13C) from modern shells of the estuarine bivalve Geloina erosa, and those dated to the early Holocene that were harvested by people from mangroves near the Great Cave of Niah on the north coast of Borneo. This reconnaissance study provides high-resolution palaeoclimatic-palaeohydrological information concerning early human activity in the region and the past character of the Southeast Asian Monsoon. Laser ablation continuous flow isotope ratio mass spectrometry (LA-CF-IRMS) on modern shells of Geloina erosa revealed ‘saw-tooth’ stable isotopic profiles that bear a close resemblance to peaks and troughs of trends in recent local rainfall, including the 1998 El Niño drought, highlighting the potential of Geloina erosa for reconstructing seasonality of the Southeast Asian Palaeomonsoon. LA-CF-IRMS analysis of prehistoric shells of Geloina erosa held in the Harrisson Archives of the Sarawak Museum revealed cyclical shifts in δ18O with similar amplitudes of variation as found in modern shells of Geloina erosa. As at the present day, this probably reflects the changing seasonality of the monsoon rains with shifts to negative δ18O values during periods of high runoff. Lighter mean δ18O values of the prehistoric shells, however, may indicate a greater annual surplus of rainfall and possibly consistent with the early Holocene strengthening of the summer monsoon at that time. The similarity of the last growth increment δ18O of the prehistoric shells to their mean δ18O profile values suggests that gathering took place during times of moderate runoff.