Controlling factors on a paleo-lake oxygen isotope record (Yammoûneh, Lebanon) since the Last Glacial Maximum

Late Quaternary paleoclimatic changes in the Levant are difficult to extract from carbonate oxygen isotope records partly because the factors controlling the terrestrial δ18O signals are not fully understood in this region characterized by sharp climatic gradients. Here, we address this purpose by presenting the first 14C-dated isotope record from the northern Levant. The record is based on oxygen isotopes from ostracod shells from lacustrine-palustrine deposits accumulated in a small karstic, hydrologically open basin (Yammoûneh), located on the eastern flank of Mount Lebanon. We have first constructed a composite isotopic record obtained from three different ostracod taxa. This yields an oxygen isotope record of ostracods (δost) related to the most widespread species (Ilyocypris inermis), and converted to δ18O values coeval with calcite precipitated in equilibrium (δC) with the lake water (δL). As with other records from the Mediterranean region, the Yammoûneh profile shows maximum and minimum δC values during the LGM and the early Holocene, respectively, and a slight late Holocene increase. In order to discuss the potential causes of the observed δ18O values fluctuations (e.g., changes in the isotopic composition of the moisture source, temperature, precipitation minus evaporation (P-E) balance, or atmospheric circulation), we tentatively reconstruct the lake water isotope composition by correcting the δC values for lake water temperature using regional paleotemperature estimates. Inferred δL values were then corrected for the isotopic composition of the Eastern Mediterranean sea surface water (the moisture source) derived from planktonic foraminifera δ18O values corrected for alkenone-based sea surface temperature. Our study suggests that Holocene δL fluctuations are primarily linked to changes in the sea surface water composition (source effect) amplified by enhanced inland rainfall during the early Holocene. During the LGM, low δL values at Yammoûneh are likely due to the ground-temperature effect on the rainfall isotope composition, possibly associated with a steeper altitudinal thermal gradient inland, and with changes in air mass trajectories over the Mediterranean Sea.