A dramatic increase in regional summer rainfall amount has been proposed for the Arabian Peninsula during the middle Holocene (ca. 9-5. ka BP) based on lacustrine sediments, inferred lake levels, speleothems, and pollen. This rainfall increase is considered primarily the result of an intensified Indian summer monsoon as part of the insolation-driven, northward shift of the boreal summer position of the Inter-Tropical Convergence Zone (ITCZ) to over the deserts of North Africa, Arabia, and northwest India. We examine the basis for the proposed drastic climate change in Arabia and the shifts in the summer monsoon rains, by reviewing paleohydrologic lacustrine records from Arabia. We evaluate and reinterpret individual lake-basin status regarding their lacustrine-like deposits, physiography, shorelines, fauna and flora, and conclude that these basins were not occupied by lakes, but by shallow marsh environments. Rainfall increase required to support such restricted wetlands is much smaller than needed to form and maintain highly evaporating lakes and we suggest that rainfall changes occurred primarily at the elevated edges of southwestern, southern, and southeastern Arabian Peninsula. These relatively small changes in rainfall amounts and local are also supported by pollen and speleothems from the region. The changes do not require a northward shift of the Northern Hemisphere summer ITCZ and intensification of the Indian monsoon rainfall. We propose that (a) latitudinal and slight inland expansion of the North African summer monsoon rains across the Red Sea, and (b) uplifted moist air of this monsoon to southwestern Arabia highlands, rather than rains associated with intensification of Indian summer monsoon, as proposed before, increased rains in that region; these African monsoon rains produced the modest paleo-wetlands in downstream hyperarid basins. Furthermore, we postulate that as in present-day, the ITCZ in the Indian Ocean remained at or near the equator all year round, and the Indian summer monsoon, through dynamically induced air subsidence, can reduce rather than enhance summer rainfall in the Levant and neighboring deserts, including Arabia. Our summary suggests a widening to the north of the latitudinal range of the rainfall associated with the North African summer monsoon moisture crossing the Red Sea to the east. We discuss other mechanisms that could have potentially contributed to the formation and maintaining of the modest paleo-wetlands.
Bibliographical noteFunding Information:
Dr. Ron Kahana (Met Office Hadley Centre, Exeter) provided an excellent critical colleague review. We thank H. Gildor, J. Pigati, and J. Rech for fruitful discussions over the years on the latitudinal changes in monsoons and ITCZ and on the meaning of “lacustrine environments”. We also thank the GCP editors for inviting this review and the editor (T. Cronin) and an anonymous reviewer for their efforts in editing this long manuscript. We acknowledge financial support from Lamont–Doherty Earth Observatory (Columbia University) to YE and The Lady Davis Foundation and The Fredy and Nadine Herrmann Institute of Earth Sciences (The Hebrew University of Jerusalem) to YK and JQ that enabled mutual visits to focus on this work. YK was also supported by the U.S. National Oceanic and Atmospheric Administration (NOAA) grant NA10OAR4320137 . YE research is funded by the Israel Science Foundation—The Dead Sea Deep Core, Center of Excellence (Grant No. 1436/14 ). This article carries the designated LDEO publication # 7890.
© 2015 Elsevier B.V.
- African monsoon
- Holocene paleoclimate
- Indian monsoon
- Somali jet