Centennial- to decadal-scale monsoon precipitation variations in the upper Hanjiang River region, China over the past 6650 years

Highlights

• Monsoon precipitation reconstruction in the upper Hanjiang River over the past 6650 years.

• Dramatically dry climate existed during the 5.0 ka and 2.8 ka events.

• Wet 4.2 ka event and the Little Ice Age were observed.

• Interplay of Westerly jet and Asian summer monsoon controlled regional precipitation changes.

Abstract

The upper Hanjiang River region is the recharge area of the middle route of South-to-North Water Transfer Project. The region is under construction of the Hanjiang-Weihe River Water Transfer Project in China. Monsoon precipitation variations in this region are critical to water resource and security of China. In this study, high-resolution monsoon precipitation variations were reconstructed in the upper Hanjiang River region over the past 6650 years from ${\delta }^{18}$O and ${\delta }^{13}$C records of four stalagmites in Xianglong cave. The long term increasing trend of stalagmite ${\delta }^{18}$O record since the middle Holocene is consistent with other speleothem records from monsoonal China. This trend follows the gradually decreasing Northern Hemisphere summer insolation, which indicates that solar insolation may control the orbital-scale East Asian summer monsoon (EASM) variations. Despite the declined EASM intensity since the middle Holocene, local precipitation may not have decreased remarkably, as revealed by the ${\delta }^{13}$C records. A series of centennial- to decadal-scale cyclicity was observed, with quasi-millennium-, quasi-century-, 57-, 36- and 22-year cycles by removing the long-term trend of stalagmite ${\delta }^{18}$O record. Increased monsoon precipitation during periods of 4390–3800 a BP, 3590–2960 a BP, 2050–1670 a BP and 1110–790 a BP had caused four super-floods in the upper reach of Hanjiang River. Dramatically dry climate existed in this region during the 5.0 ka and 2.8 ka events, coinciding with notable droughts in other regions of monsoonal China. Remarkably intensified and southward Westerly jet, together with weakened summer monsoon, may delay the onset of rainy seasons, resulting in synchronous decreasing of monsoon precipitation in China during the two events. During the 4.2 ka event and the Little Ice Age, the upper Hanjiang River region was wet, which was similar to the climate conditions in central and southern China, but was the opposite of drought observed in northern China. We propose that weakened summer monsoon and less strengthened or normal Westerly jet may cause rain belt stay longer in the southward region, which reduced rainfall in northern China but enhanced it in central and southern China.

Introduction

Hanjiang River is the longest branch of Yangtze River. The upper Hanjiang River region is the recharge area of the middle route of South-to-North Water Transfer Project in China. It will also supply water for a recently under construction project named Hanjiang-Weihe River Transfer Project in Shaanxi province, China. Precipitation variations in the upper Hanjiang River region not only affect regional environment and economy but also have an important influence on the water security of China. Understanding the precipitation variability and mechanisms in this region on different timescales is critical to make strategic plans for sustainable use of water resources of Hanjiang River.

Modern instrumental records in China are too short (often began from the 1950s) to reveal the precipitation changes on millennial- to centennial- and decadal-timescales. Therefore, it is crucial to reconstruct past precipitation variations by using various geological, biological, and historical archives to extend the instrumental records. Paulsen et al. (2003) reconstructed the climate variations in central China over the past 1270 years by using a stalagmite from Buddha cave. The stalagmite ${\delta }^{18}$O and ${\delta }^{13}$C records revealed generally dry “Medieval Warm Period” and “Modern Warm Period”, but a wet “Little Ice Age” (LIA) (Paulsen et al., 2003). Tan et al., 2009, Tan et al., 2015a further reconstructed high-resolution monsoon precipitation changes in the upper Hanjiang River region during the last 750 years based on stalagmite and historical records from Dayu Cave in southern Shaanxi province, China. They also observed a wet LIA in this region (Tan et al., 2009). This observation is the opposite of a dry LIA in north central China (Tan et al., 2011a). By combining stalagmite and historical records, they suggested droughts and even modest events interrupting wet intervals in historical times had caused serious social crises in this region (Tan et al., 2015a). The observed regional differences between the upper Hanjiang River region and north central China during LIA is consistent with a recent review by Chen et al. (2015b). They synthesized moisture/precipitation records during the past 1000 years in China, and suggested a dry LIA in the northern part of monsoonal China and a wet LIA in the southern part, with the boundary at about 34°N (Chen et al., 2015b), north of Dayu Cave.

Despite the existing studies, it remains unclear about the characteristics and mechanisms of centennial- to decadal-scale precipitation variability in the upper Hanjiang River region during the Holocene. How did precipitation in this region respond to Holocene abrupt events, such as 5.0 ka, 4.2 ka and 2.8 ka events (Mayewski et al., 2004)? Were there any differences between precipitation changes in the late and middle Holocene under different solar insolation conditions? What's the relationship between extreme flood and climate change? These are open questions.

Stalagmite is an ideal natural archive for Quaternary climate reconstruction, with the advantages of accurate dating, continuous growth and high resolution (Fairchild et al., 2006). Multiple proxies in stalagmite, such as stable oxygen and carbon isotopes (Fairchild et al., 2006), trace elements (Fairchild and Treble, 2009), lipid biomarker (Blyth et al., 2007), fossil water and noble gases in fluid inclusions (Krüger et al., 2011), magnetism (Zhu et al., 2017), growth rate (Tan et al., 2013), and fluorescent intensity (Baker and Genty, 1999), can be used to reconstruct paleoenvironmental and climatic changes. Here we reconstruct monsoon precipitation changes in the upper Hanjiang River region over the past 6650 yr by using four stalagmites ${\delta }^{18}$O records from Xianglong cave in southern Shaanxi province, China, with an average 9-yr resolution. The variations, periodicities and mechanisms are further discussed.