The colonisation of the Mariana Islands in Western Micronesia is likely to represent a long-distance ocean dispersal of more than 2000 km, and establishing the date of human arrival in the archipelago is important for modelling Neolithic expansion in Island South-East Asia and the Pacific. In 2010, Clark et al. published a paper discussing a number of radiocarbon dates from the Bapot-1 site on Saipan Island, but a disparity between charcoal and marine shell (Anadara sp.) results prevented the calculation of a definitive age for the site and left open the possibility that Bapot-1 was first settled as early as 3500 calBP. Here, we present new research using a combination of stable isotope (δ13C and δ18O) and 14C information to demonstrate that A. antiquata from the lowest layers of Bapot-1 is affected by hardwaters. These new results indicate human arrival at Bapot-1 occurred around 3200–3080 calBP (1250–1130 BC). We recommend a similar isotopic evaluation for other sites in the Marianas that are dated by marine shell.