The modern and LGM hydrological cycles of the Eastern Mediterranean and the Levant from a water isotope perspective

Long-term averages of the isotopic composition of precipitation from 15 sites in Israel and reanalyzed vapor data from the Mediterranean are used to assess the full modern hydrological cycle of sea surface water–vapor–distillation–precipitation–evaporation in the E. Mediterranean and the Levant. The results show that once the source effect is accounted for, the long-term isotopic composition of precipitation in Israel is governed primarily by distillation and to a lesser degree by temperature. These processes govern the amount, altitude and the distance from the ocean effects.

Based on our understanding of the modern processes, we modeled the distillation as a function of relative humidity during the LGM using δ¹⁸O stalagmites and Mediterranean foraminifera. The model defines three scenarios of LGM precipitation: a) distillation and relative humidity were higher then today; b) both were similar to today; c) both were lower then today. Based on the Lisan high we assume an increase in precipitation occurred over Israel, strengthening options a and b, thus promoting the following scenarios: 1) an increase occurred in the amount of precipitation of each event which would demand Mediterranean relative humidity values of ~85% (as apposed to 65% today) and a d-excess of 10⁰/₀₀ (as apposed to 23⁰/₀₀ today), or 2) an increase occured in the frequency of rain events, which would require a rain event every 1.5 days during the rainy months (as apposed to an event every 3.3 days at present), in this case, the relative humidity and d-excess would be similar to present day. By emphasizing the importance of the distillation effect and possible changes in relative humidity over the Mediterranean, our model reconciles the conundrum of the Lake Lisan high-stand during the LGM and the similar offset between the δ¹⁸O values from Soreq Cave stalagmites and Mediterranean foraminifera in modern and LGM times.