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 (Delta R-lagoon) were defined as a new proxy to estimate the radiocarbon (C-14) activity change in the lagoon freshwater proportion [1]. In Tongatapu island, previous research showed that Delta R-lagoon values increased linearly between similar to 2.6 and 0.4 ka, which suggested that Delta R-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 C-14 values from G. tumidum combined with the previously published values and a Delta R-lagoon value recalculated using Marine20. Moreover, we investigated whether the C-14 value of G. tumidum reflected that of the dissolved inorganic carbon in the lagoon water. Although the new Delta R-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.

Automated summary

Delta R-lagoon is a new proxy used to estimate the change in radiocarbon activity in fresh water in a lagoon. By measuring G. tumidum samples and recalculating Delta R-lagoon values, we found that the values remained stable between 0.4 ka and 1.2 ka, but increased from 2.6 ka to 1.2 ka.