Late <scp>H</scp>olocene coral reef environment recorded in <scp>T</scp>ridacnidae shells from archaeological sites in <scp>O</scp>kinawa‐jima, subtropical southwestern <scp>J</scp>apan

<jats:title>Abstract</jats:title><jats:p>Seasonally resolved records of carbon and oxygen isotopic composition (δ<jats:sup>18</jats:sup><jats:styled-content style="fixed-case">O</jats:styled-content>) in fossil shells of giant clams (<jats:styled-content style="fixed-case">T</jats:styled-content>ridacnidae) recovered from the <jats:styled-content style="fixed-case">K</jats:styled-content>ogachibaru <jats:styled-content style="fixed-case">S</jats:styled-content>hell <jats:styled-content style="fixed-case">M</jats:styled-content>ound (26°26′<jats:styled-content style="fixed-case">N</jats:styled-content>, 127°49′<jats:styled-content style="fixed-case">E</jats:styled-content>) and the <jats:styled-content style="fixed-case">S</jats:styled-content>econd <jats:styled-content style="fixed-case">A</jats:styled-content>ragusuku‐<jats:styled-content style="fixed-case">S</jats:styled-content>hichabaru <jats:styled-content style="fixed-case">R</jats:styled-content>uin (26°18′<jats:styled-content style="fixed-case">N</jats:styled-content>, 127°46′<jats:styled-content style="fixed-case">E</jats:styled-content>) in <jats:styled-content style="fixed-case">O</jats:styled-content>kinawa‐jima, southwestern <jats:styled-content style="fixed-case">J</jats:styled-content>apan, were investigated to reconstruct subtropical coral reef environments of the past. The samples, mainly composed of aragonite shells with limited amounts of diagenetically‐altered calcite cements, were selected for geochemical analyses. The radiocarbon dating of the samples for stable isotope analyses yields about 4000 yr cal. <jats:styled-content style="fixed-case">BP</jats:styled-content> for the <jats:styled-content style="fixed-case">K</jats:styled-content>ogachibaru <jats:styled-content style="fixed-case">S</jats:styled-content>hell <jats:styled-content style="fixed-case">M</jats:styled-content>ound and about 1900–1700 yr cal. <jats:styled-content style="fixed-case">BP</jats:styled-content> for the <jats:styled-content style="fixed-case">S</jats:styled-content>econd <jats:styled-content style="fixed-case">A</jats:styled-content>ragusuku‐<jats:styled-content style="fixed-case">S</jats:styled-content>hichabaru <jats:styled-content style="fixed-case">R</jats:styled-content>uin, respectively, that correspond to the early and middle <jats:styled-content style="fixed-case">S</jats:styled-content>hell <jats:styled-content style="fixed-case">M</jats:styled-content>ound periods in <jats:styled-content style="fixed-case">O</jats:styled-content>kinawa‐jima. These show good agreement with the respective ages inferred from excavation. The shell δ<jats:sup>18</jats:sup><jats:styled-content style="fixed-case">O</jats:styled-content> values roughly show seasonal variations, coincident with the occurrence of annual growth bands. Applying corrections for the effect of diagenetic alteration, the averages of annual, summer, and winter δ<jats:sup>18</jats:sup><jats:styled-content style="fixed-case">O</jats:styled-content> values of fossil shells were significantly lower (by 0.1–0.7‰) than those of aragonite theoretically precipitated in present‐day coral reef water of <jats:styled-content style="fixed-case">O</jats:styled-content>kinawa‐jima. These results demonstrate that annual mean seawater temperature and salinity at the sites where the <jats:styled-content style="fixed-case">T</jats:styled-content>ridacnida spp. lived were about 1–3°C higher and/or about 1–2 lower than at present. This variation cannot be fully explained by global climate change, sea level change, and topographic growth of coral reef. Therefore, it is likely that the giant clams lived in extremely small and/or closed coral‐reef lagoons with less water circulation where seawater is highly susceptible to insolation‐induced temperature increase and input of fresh water; the effect could be enhanced by the fisheries lifestyle that stonewalling would be constructed at shallow waters through the use of tidal variation during the early and middle <jats:styled-content style="fixed-case">S</jats:styled-content>hell <jats:styled-content style="fixed-case">M</jats:styled-content>ound period in <jats:styled-content style="fixed-case">O</jats:styled-content>kinawa‐jima.</jats:p>