The middle Holocene climatic records from Arabia: Reassessing lacustrine environments, shift of ITCZ in Arabian Sea, and impacts of the southwest Indian and African monsoons

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

Anomalous atmospheric circulation and large winter floods in six subregions of the southwestern United States

EXTRACT (SEE PDF FOR FULL ABSTRACT): We examined atmospheric circulation conditions conducive to occurrence of winter floods that exceed the 10-year peak discharge on rivers in six hydroclimatic subregions in Arizona, southern Utah, Nevada, and California. ... This relationship between flooding and broad-scale atmospheric patterns in the modern record will aid in paleoclimatic interpretations of paleoflood records over the last few thousand years

Both differential and equatorial heating contributed to African monsoon variations during the mid-Holocene

The Sahara was significantly greener 11-5 kya and during multiple earlier interglacial periods. But the mechanisms related to the greening of the Sahara remain uncertain as most climate models severely underestimate past wet conditions over north Africa. The variations in the African monsoon related to the greening of the Sahara are thought to be associated with the variations in the inter-hemispheric differential heating of Earth, caused by orbital variations. However, how orbital variations affect regional climate is not well understood. Using recent theory that relates the position of the tropical rain belt to the atmospheric energy budget, we study the effect of orbital forcing during the mid-Holocene on the African monsoon in simulations provided by the third phase of the Paleo Model Intercomparison Project (PMIP3). We find that energy fluxes in the African sector are related to orbital forcing in a complex manner. Contrary to generally accepted theory, orbital modulation of seasonal differential heating alone is shown to be a weak driver of African monsoon variations. Instead, net atmospheric heating near the equator, which modulates the intensity and extent of seasonal migrations of the tropical rain belt, is an important but overlooked driver of African monsoon variations. A conceptual framework that relates African monsoon variations to both equatorial and inter-hemispheric differential solar heating is presented

Dead Sea drawdown and monsoonal impacts in the Levant during the last interglacial

Sediment cores recovered by the Dead Sea Deep Drilling Project (DSDDP) from the deepest basin of the hypersaline, terminal Dead Sea (lake floor at ∼725 m below mean sea level) reveal the detailed climate history of the lake's watershed during the last interglacial period (Marine Isotope Stage 5; MIS5). The results document both a more intense aridity during MIS5 than during the Holocene, and the moderating impacts derived from the intense MIS5e African Monsoon. Early MIS5e (∼133–128 ka) was dominated by hyperarid conditions in the Eastern Mediterranean–Levant, indicated by thick halite deposition triggered by a lake-level drop. Halite deposition was interrupted however, during the MIS5e peak (∼128–122 ka) by sequences of flood deposits, which are coeval with the timing of the intense precession-forced African monsoon that generated Mediterranean sapropel S5. A subsequent weakening of this humidity source triggered extreme aridity in the Dead Sea watershed and resulting in the biggest known lake level drawdown in its history, reflected by the deposition of thick salt layers, and a capping pebble layer corresponding to a hiatus at ∼116–110 ka. The DSDDP core provides the first evidence for a direct association of the African monsoon with mid subtropical latitude climate systems effecting the Dead Sea watershed. Combined with coeval deposition of Arabia and southern Negev speleothems, Arava travertines, and calcification of Red Sea corals, the evidence points to a climatically wet corridor that could have facilitated homo sapiens migration “out of Africa” during the MIS5e peak. The hyperaridity documented during MIS5e may provide an important analogue for future warming of arid regions of the Eastern Mediterranean–Levant

Tropical and subtropical moisture and southerly displaced North Pacific storm track: factors in the growth of late Quaternary lakes in the Mojave Desert

Historical flood events produced lakes in the Mojave River watershed in southeastern California and represent climatic conditions similar to those in the late Quaternary when perennial lakes formed in the Mojave Desert. Historical lakes are related to tropical and subtropical sources of moisture and an extreme southward shift of storm tracks. It is suggested that this atmospheric pattern occurred frequently during earlier periods with perennial lakes in the Mojave River drainage basin

Synoptic-scale to mesoscale atmospheric circulation connects fluvial and coastal gravel conveyors and directional deposition of coastal landforms in the Dead Sea basin

Streams convey coarse-clastic sediments towards coasts, where interactions with deltaic and coastal processes determine their resultant sedimentology and geomorphology. Extracting hydroclimatic signals from such environments is a desired goal, and therefore studies commonly rely on interpreting available paleoclimatic proxy data, but the direct linking of depositional and geomorphic processes with the hydroclimate remains obscure. This is a consequence of the challenge of linking processes that are often studied separately and span across large spatial and temporal scales, including synoptic-scale hydroclimatic forcing, streamflows, water body hydrodynamics, fluvial and coastal sediment transport, and sedimentation. Here, we explore this chain of connected processes in the unique setting of the Dead Sea basin, where present-day hydroclimatology is closely tied with geomorphic evolution and sediment transport of streams and coasts that rapidly respond to lake-level fall. We use a 5-year (2018-2022) rich dataset of (i) high-resolution synoptic-scale circulation patterns, (ii) continuous wind-wave and rain-flood records, and (iii) storm-scale fluvial and coastal sediment transport of "smart" and marked boulders. We show the significance of Mediterranean cyclones in the concurrent activation of fluvial (floods) and coastal (wind waves) sediment conveyors. These synoptic-scale patterns drive the westerlies necessary for (i) delivering the moisture across the Judean desert, which is transformed into floods, and at the same time, (ii) the coeval, topographically funneled winds that turn into surface southerlies (>10 m s(-1)) along the Dead Sea rift valley. During winter, these mesoscale southerlies generate 10-30 high-amplitude, northward-propagating storm waves, with 5 times more frequent than flash floods, coarse-clastic beach berms and fan deltas are deposited preferentially north of the delivering channel mouths. This asymmetric depositional architecture, controlled by the regional hydroclimatology, is identified for both the modern and late Pleistocene coast and delta environments, implying that the dominance of present-day Mediterranean cyclones also persisted in the region during the late Pleistocene when Lake Lisan occupied the basin.ISSN:2196-632XISSN:2196-631

Lake levels and sequence stratigraphy of Lake Lisan, the late Pleistocene precursor of the Dead Sea

Lake Lisan, the late Pleistocene precursor of the Dead Sea, existed from ∼70,000 to 15,000 yr B.P. It evolved through frequent water-level fluctuations, which reflected the regional hydrological and climatic conditions. We determined the water level of the lake for the time interval ∼55,000-15,000 cal yr B.P. by mapping offshore, nearshore, and fan-delta sediments; by application of sequence stratigraphy methods; and by dating with radiocarbon and U-series methods. During the studied time interval the lake-level fluctuated between ∼340 and 160 m below mean sea level (msl). Between 55,000 and 30,000 cal yr B.P. the lake evolved through short-term fluctuations around 280-290 m below msl, punctuated (at 48,000-43,000 cal yr B.P.) by a drop event to at least 340 m below msl. At ∼27,000 cal yr B.P. the lake began to rise sharply, reaching its maximum elevation of about 164 m below msl between 26,000 and 23,000 cal yr B.P., then it began dropping and reached 300 m below msl at ∼15,000 cal yr B.P. During the Holocene the lake, corresponding to the present Dead Sea, stabilized at ca. 400 m below msl with minor fluctuations. The hypsometric curve of the basin indicates that large changes in lake area are expected at above 403 and 385 m below msl. At these elevations the lake level is buffered. Lake Lisan was always higher than 380 m below msl, indicating a significantly large water contribution to the basin. The long and repetitious periods of stabilization at 280-290 m below msl during Lake Lisan time indicate hydrological control combined with the existence of a physical sill at this elevation. Crossing this sill could not have been achieved without a dramatic increase in the total water input to the lake, as occurred during the fast and intense lake rise from ∼280 to 160 m below msl at ∼27,000 cal yr B.P. C 2002 University of Washington

Quaternary evolution of a hyperarid drainage under climatic fluctuations and rift-margin base-level fall, NE Negev, Israel

International audienc

Magnitude and frequency of Holocene palaeofloods in the southwestern United States: A review and discussion of implications

Data about the magnitude and time of occurrence of palaeofloods from the lower Colorado River basin enable us to test two long-standing hypotheses which have affected many studies and applications in the field of flood hydrology. The two hypotheses are (a) the existence of an upper boundary to flood magnitudes and whether there is a possibility of determining it from the existing data, and (b) the random occurrence versus clustering of the large floods through time. Earlier observations on regional flood envelope curves indicated the existence of an upper limit for flood magnitudes, but these studies limited their conclusions because of the short length of the systematic gauged data. This limitation is overcome here because palaeoflood data cover a much longer period of observation. Palaeoflood studies provide information about the largest individual floods experienced in many rivers in a specific region occurring over the last millennia. In the southwestern US, this information demonstrates that, even when the length of observational data increases to centuries and millennia, there is no change in the stabilized, regional envelope curves constructed from gauged and historical flood records. This pattern supports the hypothesis of an upper limit to flood magnitudes and points to a method for testing this hypothesis in other regions. There are surprising similarities between the envelope curve of the palaeoflood data and the envelope curve for the gauged and historical data in the lower Colorado River basin. These similarities indicate that in regions of the world where flood data is sparse envelop curves based on palaeoflood studies can provide basic data for engineering design purposes and other hydrological applications. The random occurrence of large floods in time is tested by constructing chronologies for the largest palaeofloods in several basins in the lower Colorado River basin. These chronologies indicate a clustering of the large floods in specific time periods. The similarity between the various time periods characterized by high- and low-flooding and other palaeoclimatic indicators from the southwestern United States seems best explained by a climatic control on flood frequency over the last 5000 years