干旱区生态环境知识资源中心

Pollen diagrams based on marine cores from the Eastern Mediterranean and Arabian Seas and land cores from Greece, Turkey, the Levant and western Iran during the last deglaciation reveal a similar succession of conspicuous pollen abundance maxima: an early phase with Chenopodiaceae signaling the highest aridity, a late phase with Pistacia and Gramineae in the Mediterranean domain, and tropical humid taxa in the Arabian sea domain, indicating conditions of mildest winter and wettest summer. Between these two phases, the record for deciduous oak in the Mediterranean and Near East shows a continuous increase that points to rising moisture.

The chronology of this succession and its exact relationship with the marine isotopic record of global climate are established using marine cores. The marine records dated by the AMS C-14 method show that the Chenopodiaceae phase correlates with the Younger Dryas chronozone, globally dated 11,000 to 10,000 BP, during which the climate reversed to quasi-glacial conditions at least in the northern hemisphere. They also show that the Pistacia phase, during which the Mediterranean climate reached its seasonal optimum, occurred from 9000 to 6000 C-14 years BP (Boreal and early Atlantic chronozones defined in NW Europe) in the first half of the Holocene. The oak pollen abundance increases rapidly from 10,000 to 9000 BP (Preboreal chronozone).

Thus, in terms of climate, the Younger Dryas chronozone is regionally expressed by widespread desert conditions, as a cold and arid period. During the Pistacia phase, the climate was most favourable both for plants that require frost-free winters, such as the Mediterranean Pistacia, and those that need spring/summer moisture, such as Gramineae at low annual moisture level(seasonally similar to 300 mm), and deciduous oak at higher levels (similar to 600 mm all year, without summer drought). During the brief intermediary phase, the climate changed drastically through an increase of winter temperature and of spring/summer moisture.

The chronologies of the land cores, however, based on few C-14 dates, show significant discrepancies with the marine-based time-frame. These discrepancies imply a lack of coherence in the climate history of this mediterranean-irano-turanian region. We propose using the chronology derived from the marine records for the entire region. The local C-14 dates generally appear too old, perhaps because of geological contamination of the material by older carbon.