RELATION BETWEEN CENTURY-SCALE HOLOCENE ARID INTERVALS IN TROPICAL AND TEMPERATE ZONES

CLIMATE records from lake sediments in tropical Africa, Central America and west Asia show several century-scale arid intervals during the Holocene(1-10). These may have been caused by temporary weakening of the monsoonal circulation associated with reduced northward heat transport by the oceans' or by feedback processes stimulated by changes in tropical land-surface conditions(10). Here we use a lake-sediment record from the montane Mediterranean zone of Morocco to address the question of whether these events were also felt in temperate continental regions. We find evidence of arid intervals of similar duration, periodicity and possibly timing to those in the tropics. But our pollen data show that the forest vegetation was not substantially affected by these events, indicating that precipitation remained adequate during the summer growing season. Thus, the depletion of the groundwater aquifer that imprinted the dry events in the lake record must have resulted from reduced winter precipitation. We suggest that the occurrence of arid events during the summer in the tropics but during the winter at temperate latitudes can be rationalized if they are both associated with cooler sea surface temperatures in the North Atlantic.