Occurrence and distribution of biomarkers in loess-paleosol sequences of Tajikistan (Central Asia): Implication for archaeo-ecological studies

Abstract

Finding biomarkers preserved in loess-paleosol sequences is challenging and rarely undertaken in the archaeological studies that occur in these settings. Here, we investigate the preservation of n-alkanes and PAHs collected in association with “Loessic Paleolithic” sites (archaeological sites entrained in fine-grained eolian sediments dating to the Paleolithic era 700,000 to 70,000 years ago) in the Khovaling region of southern Tajikistan. This region has been subject to archaeological inquiry since Soviet research projects in the 1970s found evidence of hominin occupations from Lower and Middle Paleolithic stone tool assemblages in excavations. Questions about the timing and ecological circumstances of putative ancient migrations into the region have prompted new geoarchaeological research at the sites of Lakhuti-IV, Obi-Mazar, Kuldara, and Khonako-II to constrain landscape evolutionary histories better using vegetation (straight-chain [n-]alkanes) and fire (polycyclic aromatic hydrocarbon [PAH]) biomarkers. We collected sediment samples from on–, sparse on–, near-, and off-site contexts relative to known archaeological sites for biomarker extraction and identification. The homologous series of n-alkanes ranging from nC₁₅-nC₃₃ are identified and quantified, in which the long-chain length of n-alkanes (nC₂₇-nC₃₃) has shown odd-over-even predominance. We found the abundance of PAH compounds with two to five rings in the sedimentary organic matter, in which PAH compounds with four or more rings have shown the highest abundance. The short (≤ nC₂₁) and mid-chain (nC₂₂-nC₂₆) n-alkanes have shown a positive correlation with PAH abundance from study sites. This is likely the result of the combustion of terrestrial plant matter on the landscape at elevated temperatures (≥ 400 °C).