Abstract: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 Southeast 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 cal BP. Here we present new research using a combination of stable isotope (δ 13 C and δ 18 O) and 14 C 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 cal. BP (1250-1130 BC). We recommend a similar isotopic evaluation for other sites in the Marianas that are dated by marine shell.

The colonisation of the Pacific is an important chapter in human dispersal for which chronological control is primarily provided by radiocarbon ( 14 C) dates. In this context, the ability to reliably date shellfish is important because alternative dating materials, such as charcoal and bone, are typically highly degraded. However, the interpretation of shell 14 C results is not always black and white because 14 C is not evenly distributed throughout the marine environment, with estuarine taxa more likely to incorporate terrestrial sources of carbon. Regions where water has percolated through limestone bedrock provide an additional problem since ancient carbon is introduced into the estuarine waters. This "hardwater" has been put forward to explain old 3500 cal. BP results from culturally significant shells recovered from the site of Unai Bapot (Bapot-1) on the island of Saipan (Petchey et al. 2017). While arguments for (Carson and Hung 2017) and against (Rieth and Athens 2017) early settlement dates remain polarised, little attention has been given to the idea of change in the marine 14 C reservoir over time, or to possible species-specific offsets in shell 14 C.In this paper, we further develop a tri-isotope approach using 14 C, δ 13 C, δ 18 O to identify carbon source. To investigate which shellfish are more prone to erroneous ages we have selected shell taxa that cover a range of nearshore environments commonly found in Pacific archaeological sites; including Anadara antiquata, Gafrarium pectinatum (both estuarine) and Tridacna (marine/reef). To test the possibility of change over time we extend the dating of the site beyond the earliest occupation layers to deposits considered to post-date the end of the mid-Holocene drawdown in sea-level.

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