40Ar/39Ar dating and phytolith analysis of the Early Pleistocene sequence of Kvemo-Orozmani (Republic of Georgia): Chronological and palaeoecological implications for the hominin site of Dmanisi

摘要

The Kvemo-Orozmani locality contains a 15 m thick Early Pleistocene fluvio-lacustrine sequence bracketed by two lava flows. Located 13 km up the Masavera River Valley (west) from the Dmanisi Site, Kvemo-Orozmani is therefore of prime interest to improve the paleoecological framework as well as dating Dmanisi. ~(40)Ar/~(39)Ar ages obtained on the two lava flows indicate that Kvemo-Orozmani sequence brackets the Dmanisi sediments between 1.825 ± 0.021 Ma and 1.765 ± 0.021 Ma. Phytolith assemblages found at the bottom of the sequence indicate a warm and humid climate while the assemblages found at the top suggest dryer-colder climatic conditions. The shift in phytolith assemblages observed at Kvemo-Orozmani is similar to the one already described in Dmanisi and occurs near the end of the Oluduvai subchron. The new palaeoecological and chronological data confirm the hypothesis made in Dmanisi about the palaeoenvironmental context during the hominin occupation that suggests a temperate climate becoming drier. The age we obtained on the overlying lava flow establishes a minimum age for the climatic transition at both Kvemo-Orozmani and the Dmanisi Site of 1.765 ± 0.021 Ma.