The eastern Mediterranean climate at times of sapropel formation: a review

Abstract

Sapropel formation in the eastern Mediterranean coincided closely with minima in the precession index. Such minima occur approximately every 21000 years. At such times perihelion falls within Northern Hemisphere summer. Minima in the precession index are characterized by intensified Indian Ocean (summer) SW monsoonal circulation, which enhanced discharge of the river Nile into the eastern Mediterranean. However, by compiling paleoclimatological data from the literature, the influence of the monsoon is shown to have reached only as far as the southern Sinai Desert. Therefore, it does not account for contemporary humid phases in the northern borderlands of the eastern Mediterranean, which seem to have been characterized mainly by increased summer precipitation. We argue that increased (summer) precipitation along the northern borderlands of the eastern Mediterranean, at times of sapropel formation, was probably due to increased activity of Mediterranean (summer) depressions. Forming predominantly in the western Mediterranean and tracking eastwards, such depressions tend to lower the excess of evaporation from the eastern Mediterranean relative to that from the western basin. Picking up additional moisture along their eastward path, such depressions also redistribute freshwater within the complex eastern Mediterranean water balance. The increase in runoff and the related flux of nutrients and continental organic matter that resulted from the increased precipitation on the northern borderlands of the eastern Mediterranean, at times of sapropel formation, presumably provided a substantial addition to that which entered the eastern Mediterranean via the Nile.