The settlement of the Pacific is an important chapter in human dispersal for which radiocarbon (14C) dates provide chronological control. At some point during the last 3000 years, Polynesian culture developed within Tonga and Samoa out of earlier ancestral populations. Unfortunately, between ~2650 and 2350 cal BP, the terrestrial 14C calibration curve used to convert 14C ages into calendar dates is relatively flat, resulting in low precision date interpretations. At this time, the South Pacific marine 14C reservoir appears to have diverged from the terrestrial reservoir for reasons that are not yet understood. This divergence may be key to refining the chronology of coastal sites dating to this broad 14C plateau. We investigate this at the site of Talasiu in Tongatapu – a site with evidence of both late Lapita and early Polynesian ceramics. Moreover, we show how careful evaluation of δ13C and δ18O, in combination with shell taxa diet and habitat, enables shell 14C dates impacted by hard water to contribute and refine chronologies blurred by this 2650-2350 cal BP plateau. Our finding has significant global implications for understanding the relationship between the marine and terrestrial 14C reservoirs and is of interest to those interested in refining dates from this time.