Quaternary climatic changes over Southern Arabia and the Thar Desert, India

The distribution of sand dunes over the southern half of Arabia conforms to the influence of two wind systems: the northern Shamal, which is a strong wind that blows to the SSE down the Persian (Arabian) Gulf and then swings to the SW across the hyperarid Rub al Khali towards North Yemen; and the strong winds of the SW Monsoon system, which were responsible for forming linear dunes that trend north-south in the Wahiba Sands of eastern Oman and SW-NE in the Thar Desert (NW India). In the Thar Desert, the SW Monsoon alternates with the weaker NE Monsoon. The dating of exposures of older dune systems by isotopic, radiometric and optically stimulated luminescence (OSL) analyses has shown that the Shamal was active throughout the latter part of the Quaternary period, and probably as long ago as Mid-Miocene time (c. 15 Ma). At times of glacial maxima, when global sea level was some 100-120 m or more lower than now, siliciclastic and carbonate grains were deflated from the exposed surface of the Persian Gulf and transported into the NE Rub al Khali within the United Arab Emirates. It is suspected that occasionally the Shamal also transported some quartz sands from the NW onto the exposed narrow continental shelf of SE Arabia, with silt-size particles being carried into the Arabian Sea. The SW Monsoon, on the other hand, was re-established over the coast of SE Arabia several millennia after the last glacial maximum and was fully established near the coast of SE Arabia during the early Holocene interglacial after the atmospheric high-pressure system associated with the glacial period had become weaker. Early during the Holocene interglacial periods when the SW Monsoon dominated, a combination of quartz and carbonate sands was deflated from the exposed continental shelf and transported to the north into the Wahiba Sands. Aeolian activity in the Thar Desert also peaked during this period of transition from full glacial to interglacial conditions. The dune systems of SE Arabia overlie the distal edges of older alluvial fans that in Oman date back at least 350 ka. The sediments of some of these fluvial sequences in Oman reached the Arabian Sea via Wadi Batha, only to be removed by along-shore currents driven by the SW Monsoon. In the Thar Desert, the supply of aeolian sediment is mostly from fluvial sources. Marine sediments from the Arabian Sea between Arabia and Thar record the contrasting effects of the Shamal and the SW Monsoon: the former mostly as a source of wind-blown dust from Arabia and the latter by causing upwelling of nutrient-rich waters leading to organic blooms