Dipole Pattern of Holocene Hydroclimate Variations Across the Asian Drylands: Critical Evidence From West Asia

Exploring the spatiotemporal differences of hydroclimate variations is crucial for managing future climate change. In the Asian drylands, West Asia (WA) and arid central Asia (ACA) are both climatically dominated by the westerlies and have shown a dipole pattern in precipitation variation during the past several decades. However, it is unclear whether such a difference exists during the Holocene, mainly because the dispute between the delta 18O-based early Holocene hydroclimate optimum and the pollen-based mid-Holocene optimum in WA. Here we present a precisely dated record based on lipid biomarkers from Almalou Peatland in the western Iranian Plateau. The chain length of n-alkanoic acids was interpreted as a hydroclimate indicator based on a proxy validation study. Our record reveals a wetting trend during 9-7.5 cal ka BP, a mid-Holocene hydroclimate optimum (7.5-3 cal ka BP), and a rapidly drying trend during 3-0 cal ka BP. The hydroclimate variation was supported by a water-level reconstruction from the same core. The most negative delta 18O values during the early Holocene could be partly attributed to the impacts of water vapor sources. Comparing our reconstruction from WA to those from ACA, we found a dipole pattern of hydroclimate variations on the millennial timescale during the Holocene. The simulation results revealed that, unlike the persistent wetting trend in ACA, the pivotal shift of spring insolation led to a transition from wetting to drying conditions in WA, ultimately leading to the generation of dipole pattern. This demonstrates that despite the consistent control by westerlies over the Asian drylands, there are distinct spatial differences in hydroclimate responses to natural forcings. Asian drylands are particularly vulnerable to the effects of climate change due to scarce water availability. Although West Asia (WA) and arid central Asia (ACA) within the Asian drylands are both climatically dominated by the westerlies, the precipitation variation in ACA (WA) showed a generally increasing (decreasing) trend during the anthropogenic warming period. To understand the long-term hydroclimate response to natural forcings, we investigated the variations in the hydroclimate during the Holocene. While the long-term wetting trend in ACA during the Holocene has been investigated, the contradictory hydroclimate reconstructions in WA hinder our ability to address this question. The alpine peatland serves as a sensitive monitor of regional hydroclimate variation. By analyzing biomarker and pollen in the peat sediments from WA, we could understand how the climate changed over time. Our study showed that climate became wetter from 9000 to 7500 years ago, had a period of optimal hydroclimate from 7500 to 3000 years ago, and then became rapidly drier in the last 3000 years. We compared our findings to other studies in ACA and found that the dipole pattern of hydroclimate variation existed on the millennial timescale within Asian drylands, representing a primary climatic characteristic in these vulnerable regions. Holocene hydroclimate is reconstructed in West Asia based on multiple proxies including lipid biomarkers and pollen spectra A mid-Holocene hydroclimate maximum is revealed, facilitating forest expansion in the large area of West Asia Despite the consistent control by westerlies in Asian drylands, there are spatial differences in hydroclimate responses to natural forcings