Reappraisal of lagoon specific local marine reservoir effects in Tongatapu, the Kingdom of Tonga, over the last 3000 years

Lagoon-specific local marine reservoir effects (ΔR_lagoon) were defined as a new proxy to estimate the radiocarbon (¹⁴C) activity change in the lagoon freshwater proportion [1]. In Tongatapu island, previous research showed that ΔR_lagoon values increased linearly between ∼2.6 and 0.4 ka, which suggested that ΔR_lagoon value increase was caused by the near-closure of the Fanga ‘Uta Lagoon [1]. However, the calculated values contained large errors. Here, we refine those study results and present 26 new ¹⁴C values from G. tumidum combined with the previously published values and a ΔR_lagoon value recalculated using Marine20. Moreover, we investigated whether the ¹⁴C value of G. tumidum reflected that of the dissolved inorganic carbon in the lagoon water. Although the new ΔR_lagoon values were similar between 0.4 ka and 1.2 ka, the values increased from 2.6 ka to 1.2 ka. This timing was consistent with geophysical comparisons reported in our previous study and archaeological studies.