GLACIAL-INTERGLACIAL CHANGES IN MONSOON PRECIPITATION IN ARABIA-CONTROL BY GLACIAL BOUNDARY CONDITIONS

On timescales of 10-100 ky, records of continental wetness from areas affected by the Indian Ocean Monsoon are dominated by a 100ky cyclicity. Speleothems from both Oman and Yemen record periods of more rapid growth only during interglacial periods. In Hoti Cave in northern Oman, U/Th age determinations on 8 speleothems show rapid growth during discrete intervals: early to mid-Holocene, and times equivalent to marine isotope stages (MIS) 5a, 5e, 7a and 9. Similarly, preliminary dating of a stalagmite from southeastern Yemen indicates growth during MIS 5e, 7 and 9. Importantly, in both areas, individual stalagmites record growth during MIS 5, 7 and 9, suggesting that the intervening periods were continually dry. In all cases, the *18O values of speleothem carbonate are highly negative compared to values compatible with the modern arid climate, which is attributable to monsoon rainfall. Other records of continental wetness support our chronology. Analyses of pollen in marine sediments from both sides of equatorial Africa show peaks in tropical humid species during the early Holocene and in sediments deposited during MIS 5a and 5e (Van Campo et al., 1982; Prell and Van Campo, 1986; Dupont et al., 2000). These continental records of monsoon precipitation contrast greatly with records of Indian Ocean monsoon intensity derived from marine sediments in the Arabian Sea and Indian Ocean, which are based on indicators of wind strength or wind-driven upwelling. The majority of these records vary with a periodicity of ~ 20 ka, close to the precessional cycle. (e.g. Anderson and Prell, 1993; Rosteck et al., 1997; Clemens and Prell, 1990; Reichart et al., 1997). The differences between the continental and marine records of the monsoon suggest that monsoon rainfall is closely tied to glacial boundary conditions, while wind intensity is more directly related to solar forcing. Continental wetness appears to be more closely tied to control by glacial boundary conditions on the location of the ITCZ during Northern Hemisphere summers.