Evidence of temporal changes in the marine ¹⁴C reservoir in the South Pacific and implications for improved dating resolution across a radiocarbon plateau (2600-2350 cal BP)

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.