A review of the Holocene hydroclimate in the arid West Asia

As a key component of the mid-latitude arid belt in Eurasia, arid West Asia is characterized by perennial aridity, low precipitation, intense evaporation, and fragile ecosystems, making it highly sensitive to coupled precipitation-temperature variations. Clarifying the climatic evolution patterns in West Asia is therefore crucial for predicting future climate trends and ensuring regional economic development and social stability. However, long-term trend analyses of precipitation in West Asia remain challenging due to the sparsity of paleoclimate records, which are often contradictory. This study synthesizes hydroclimatic records from the past three decades to summarize Holocene moisture patterns in West Asia and discuss their potential drivers. Existing pollen records suggest a long-term increase in effective moisture during the Holocene. In contrast, speleothem and lacustrine carbonate δ18O records demonstrate a progressive aridification trend during the Holocene. After reviewing existing hydroclimatic records in West Asia, we infer that the δ18O variations in secondary carbonates in West Asia are dominated by seawater δ18O dynamics in eastern Mediterranean, rather than directly recording local precipitation amount. Based on multi-proxy hydroclimatic records and reliability assessments of paleoclimate indicators in West Asia, this study infers that the millennial-scale climatic patterns in the arid regions of West Asia during the Holocene were closely linked to the intensity and latitudinal shifts of the westerlies, which were primarily modulated by summer insolation. During the early Holocene, increased summer insolation led to higher surface temperatures, driving a northward shift of the westerly circulation. It reduced moisture transport by the westerlies to West Asia, resulting in drier conditions. However, increased precipitation in the East African monsoon region enhanced freshwater discharge from the Nile and other rivers into the Mediterranean Sea, resulting in a negative δ18O excursion in Mediterranean seawater. Consequently, this modified moisture source signature led to depleted δ18O values in precipitation records across West Asia. The late Holocene exhibited opposite characteristics.