The 7.2 Ka Climate Event: Evidence from High-Resolution Stable Isotopes
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HOL0010.1177/0959683619875809The HoloceneFeng et al. 875809research-article2019 Research Paper The Holocene 1 –10 The 7.2 ka climate event: Evidence © The Author(s) 2019 Article reuse guidelines: sagepub.com/journals-permissions from high-resolution stable isotopes DOI:https://doi.org/10.1177/0959683619875809 10.1177/0959683619875809 and trace element records of stalagmite journals.sagepub.com/home/hol in Shuiming Cave, Chongqing, China Xiangxiang Feng,1,2 Yan Yang,1,2 Hai Cheng,3,4 Jingyao Zhao,3 Xinggong Kong,5 Ping Zhang,1,2 Zhili He,1,2 Xiao Shi1,2 and R Lawrence Edwards4 Abstract Speleothem records have radically changed our understanding of the paleo Asian monsoon (AM), but explanations for Asian summer monsoon (ASM) variations in the Holocene period are usually based on single proxy records, such as oxygen isotope (δ18O). The application of multi-proxy records to the study of the forcing mechanism and internal structure of climate events is more comprehensive and accurate in the analysis of paleoclimates than using single proxy records. Therefore, a multi-proxy speleothem record of weak ASM events in 8.0–7.0 BP from Chongqing, which showed a dry southwestern China during weak ASM episodes, was reported. The analysis of multi-proxy records of SMY2 stalagmite indicated that the beginning of the ‘7.2 ka event’ was at 7.29 ± 0.03 ka BP, and its culmination was at 7.1 ± 0.04 ka BP. Thus, this event lasted nearly 200 a, and the main drought period lasted 50 a. Power spectrum analysis showed a significant 61 a cycle signal during the study period in the δ18O record of SMY2, which coincided with the Atlantic multidecadal oscillation activity (AMOC) cycle. This study demonstrates that the ASM during 8.0–7.0 ka BP is strongly correlated with the changes in solar activity and also sensitive to the southward shift of Intertropical Convergence Zone (ITCZ) and AMOC cycle. Keywords 7.2 ka event, Chongqing, drought, Holocene, Shuiming Cave, stalagmite, trace elements Received 8 April 2019; revised manuscript accepted 20 July 2019 Introduction received extensive attention, and the forcing mechanism of this climate event lacks in-depth research (Liu et al., 2015). During the Holocene, variations in the Asian monsoon (AM) Shi et al. (1994) proposed that the Holocene Megathermal have been well-documented by accurately dated cave sediments mainly appeared during 8.5–3.0 ka BP, and several strong cli- (e.g. Band et al., 2018; Berkelhammer et al., 2012; Fleitmann matic fluctuations and cooling events were noted during this et al., 2003; Wang et al., 2005). The variation of Asian summer period. An unstable temperature fluctuation phase occurred monsoon (ASM) during the Holocene is closely related to sum- during 8.5–7.2 ka BP, followed by a stable warm and wet phase mer insolation at low latitudes. The ASM strengthened shortly from 7.2 to 6.0 ka BP (Megathermal Maximum). Duplessy after the beginning of the Holocene, the strongest in the early et al. (2001) found a warming trend from 10.0 to 7.8 ka BP, and mid Holocene, and weakened after mid Holocene (An et al., which culminated in a temperature optimum and developed 2000; Dong et al., 2010; Kutzbach, 1981; Wang et al., 2005; between 7.8 and 6.8 ka BP. During this period, the input of Yang et al., 2019; Zhang et al., 2018). However, the overall Atlantic water to the Barents Sea reached its maximum. Meyer trend of weakening ASM intensity during the Holocene was per- turbed by several abrupt climate events (Liu et al., 2013, 2015; Wang et al., 2001, 2005; Yang et al., 2010; Zhang et al., 2008). 1 Chongqing Key Laboratory of Karst Environment, School of In particular, two cold events that occurred in 8.2 and 4.2 ka BP Geographical Sciences, Southwest University, China were the most remarkably abrupt events during the entire Holo- 2Field Scientific Observation and Research Base of Karst cene (Bond et al., 1997; Liu et al., 2015; Mayewski et al., 2004; Ecoenvironments, Ministry of Natural Resources of China, China Staubwasser et al., 2003; Zhang et al., 2018). In the latest edi- 3Institute of Global Environmental Change, Xi’an Jiaotong University, China tion of the international chronological table (2018/2007) 4 Department of Earth Sciences, University of Minnesota, USA 5 released by the International Commission on Stratigraphy, 8.33 College of Geography Science, Nanjing Normal University, China and 4.2 ka BP were reported to be the Northgrippian stage, Corresponding author: which was the starting and ending points of the mid Holocene Yan Yang, Chongqing Key Laboratory of Karst Environment, School of (Walker et al., 2012, 2019). However, Wang et al. (2005) Geographical Sciences, Southwest University, Chongqing 400715, China. reported that the notable weak AM event at 7.2 ka BP, has not Email: [email protected] 2 The Holocene 00(0) Figure 1. The location of Shuiming Cave, Fengdu County, Chongqing. (a) Red circles depict cave locations: Shuiming Cave site (this study); Dongge Cave (Wang et al., 2005); Kotumsar Cave (Band et al., 2018); Mawmluh Cave (Berkelhammer et al., 2012); Qunf Cave (Fleitmann et al., 2003). (b) The specific location of the Shuiming Cave in China (modified after Chen and Li, 2018). et al. (2017) reported a detailed reanalysis of travertine from Materials and methodology the Chusang site (located on the central Tibetan Plateau approx- imately 80 km northwest of Lhasa near Chusang village), which Study site supported the existence of an early pre-agropastoral population SMY2 stalagmite were collected from Shuiming Cave (29°47′N, on high-elevation areas of the Tibetan Plateau between approxi- 107°47′E), which is located in the upper reaches of the Yangtze mately 7.4 and 8.4 ka BP. Zheng et al. (2018) suggested that sea River in central Chongqing City, Southwest China (Figure 1). levels prominently decreased at approximately 7.0 ka BP, Shuiming Cave is located in the valley of the lower reaches of whereas sediment supply in the drainage remained high; this the Longhe River, which is the first tributary of the south bank combination led to land formation and propagation. What are of the Yangtze River (Pu et al., 2009; Zhu et al., 2004). This the characteristics and driving mechanism of the weak ASM cave, which is natural, nearly undeveloped, and well preserved, event of 7.2 ka BP in the early middle Holocene? How does the belongs to the Xueyu Cave group in the Three Gorges Reservoir climate event influence the characteristics of human evolution area. The area where the Shuiming Cave is located has a sub- and its relationship with climate change in the suitable Holo- tropical humid monsoon climate, with an average annual pre- cene period? These important scientific questions have yet to cipitation of over 1000 mm, and a multi-annual average be answered. temperature of 16–18°C (Zhu et al., 2004). Shuiming Cave is During the Holocene, various proxy records in many parts of situated in the interaction zone between the southwest monsoon the world were used to study climate change. Speleothem geo- and the East AM in China, so it is sensitive to the monsoon cli- chemical records play an increasingly important role in shaping mate (Li and Li, 2018; Yang et al., 2019). the world’s understanding of past climates: among which, oxy- gen isotopes are currently the most widely used indicator(Cheng et al., 2009a; Liu et al., 2013; Neff et al., 2001; Wang et al., Sampling and experiments 2005; Zhang et al., 2018; Zhao et al., 2019). The factors influ- The SMY2 stalagmites used in this study are narrow on top and encing stalagmite δ13C are complex, and its application in paleo- wide at the bottom, with an oblique conical shape and sedimen- climate research is far less extensive than that of δ18O. However, tary deposition below 310 mm. The stalagmite samples comprise at climatically sensitive sites, records of speleothem δ13C have calcite and dense crystals without corrosion and recrystallization. been successfully used to track orbital to multi-decadal climate Before sampling, the stalagmite were cut, polished, and washed variability and soil CO2 production (Burns et al., 2003; Cruz with deionized water. A total of 12 samples were obtained by et al., 2006; Dorale et al., 1992; Genty et al., 2003; Liu et al., using a handheld carbide dental drill from the top to the bottom of 2016). Although trace elements are less widely used than δ18O the stalagmite at 15, 45, 60, 80, 110, 125, 155, 205, 245, 280, 305, and δ13C, these elements have an irreplaceable referential role in and 325 mm. A total of 12 subsamples for 230Th dating were paleoclimatic research. Numerous studies have shown that vari- drilled along the growth axis of SMY2 and then dated at the ability (temperature, humidity) between caves and overlying Department of Earth Sciences of the University of Minnesota and soil zones, especially in hydrological aspects, is well docu- the Institute of Global Environmental Change of Xi’an Jiaotong mented by stalagmite trace elements, thereby providing valu- University. 230Th ages were determined with Neptune multi-col- able paleoclimatic information (Carolin et al., 2019; Fairchild lector inductively coupled plasma mass spectrometry. The et al., 2000; Fairchild and Treble, 2009; Griffiths et al., 2016; researchers referred to Cheng (2000) and Cheng et al. (2013) for Johnson et al., 2006; Liu et al., 2013; Treble et al., 2003; Zhang analytical details and standard use, and Shen et al. (2002) for the et al., 2018; Zhou et al., 2009). chemical experiment. Speleothem records with precise age control and multi- The samples of δ18O, δ13C, and trace elements were jointly proxy records are critical to further testing the paleoclimate analyzed by the Chongqing Key Laboratory of Karst Environ- reconstructions of the mid Holocene.