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

CLIMATE records from lake sediments in tropical Africa, Central Americ a and west Asia show several century-scale arid intervals during the H olocene(1-10). These may have been caused by temporary weakening of th e monsoonal circulation associated with reduced northward heat transpo rt by the oceans' or by feedback processes stimulated by changes in tr opical land-surface conditions(10). Here we use a lake-sediment record from the montane Mediterranean zone of Morocco to address the questio n of whether these events were also felt in temperate continental regi ons. We find evidence of arid intervals of similar duration, periodici ty and possibly timing to those in the tropics. But our pollen data sh ow that the forest vegetation was not substantially affected by these events, indicating that precipitation remained adequate during the sum mer growing season. Thus, the depletion of the groundwater aquifer tha t imprinted the dry events in the lake record must have resulted from reduced winter precipitation. We suggest that the occurrence of arid e vents during the summer in the tropics but during the winter at temper ate latitudes can be rationalized if they are both associated with coo ler sea surface temperatures in the North Atlantic.