Oxygen isotopic composition of fruit carbonate in Lithospermeae and its potential for paleoclimate research in the Mediterranean

Calcareous pericarps of the tribe Lithospermeae (fam. Boraginaceae) are a common component of archaeobotanical macroremain assemblages in the Mediterranean region. In this study, the relationship between oxygen isotopic composition of fruit biogenic carbonate and climatic conditions was examined. δ18O and δ13C values of biogenic carbonate were measured in modern Lithospermeae fruits from seven Eurasian sites (Berlin, Kirchentellinsfurt, Göttingen, Athens, Ankara, Tbilisi, and Almaty) and in fossil fruits from three archaeological sites in the eastern Mediterranean (Troy, Kumtepe, and Hirbet ez-Zeraqon). Additionally, three 14C measurements were performed on ancient fruit carbonate from Hirbet ez-Zeraqon. The δ18O and δ13C values varied from − 9 to 5‰ PDB and between − 35 and − 7‰ PDB respectively. In modern fruits, δ18O of biogenic carbonate was correlated to local summer precipitation amounts (inversely proportional) and summer air temperatures (proportional). In fossil fruits, the δ18O values of carbonate from Troy and Kumtepe were significantly lower than that from Hirbet ez-Zeraqon (ca. − 5 vs. 2‰ PDB respectively). The vertical distribution of stable isotopic values and 14C dates in cultural layers of Hirbet ez-Zeraqon indicate that fruit biogenic carbonate can persist in sediment without appreciable diagenetic alteration. These findings suggest that biogenic carbonate of Lithospermeae fruits can be useful as a paleoclimate proxy at least in the Mediterranean.