Paper No. 40-40
Presentation Time: 9:00 AM-5:30 PM
CHALLANGING NORTHWARD SHIFTS OF THE INDIAN OCEAN BOREAL SUMMER ITCZ AND THE ROLE OF THE INDIAN SUMMER MONSOON IN INCREASING EARLY-MID HOLOCENE AND EARLIER INTERGLACIAL PRECIPITATION OVER ARABIA: REVISITING THE PALEOHYDROLOGICAL RECORDS
Holocene and earlier interglacials lacustrine deposits and speleothems from around the Arabian Sea have been used in paleohydrologic reconstruction and, in turn, in advocating large northward latitudinal shift of the boreal summer ITCZ from its supposedly current northernmost position along the coast of south Arabia, and the associated intensification of the Indian summer monsoon (ISM) impact on Arabia. For the early-middle Holocene, dramatic increases in Arabian rainfall have been proposed. Currently, the weak Indian Ocean ITCZ hardly migrates north of 5oN. Furthermore, the ISM forces total summer drought, not rains, in the Levant and its neighbouring Arabian desert due to large-scale air subsidence. In southernmost coastal Arabia, orography-lifted air produces rain. The drought is assisted by increased upwelling limiting the rainfall inland. These observations raise questions of how large the actual changes in paleohydrology were in the Arabian Peninsula, and what the real causes, if not the ISM, of these changes were. To address these questions we summarize paleohydrologic information from Arabia and specifically revisit the paleolake status of all lacustrine-like deposits and their basins in Arabia. Reinterpreting these and sedimentology and fauna data, we conclude that these basins were occupied by shallow marsh environments, not lakes. Consequently, the hydrologic changes required to support such restricted wetland (versus lakes) were much smaller. These conclusions are supported by the temporal and spatial distribution of indicators such as pollen and speleothems: (a) rainfall changes were very small in the heart of and northern Arabia and (b) these changes were only at the elevated edges of southern Arabian Peninsula. There it rains also at present, mainly due to orographic effects on precipitation in the presence of increased moisture supply. We propose that (a) latitudinal and slight inland impact expansion of the North African summer monsoon rains across the Red Sea, and (b) its uplifted air to southwestern Arabia highlands, rather than rains associated with intensification of ISM, increased rains in that region, producing the modest paleowetlands in downstream hyperarid basins. The ITCZ probably did not even reach Arabia and the ISM cannot penetrate inland.