Climatic Significance of Tree-Ring Δ18o in the Qilian Mountains

Chemical Geology 268 (2009) 147–154 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Climatic significance of tree-ring δ18O in the Qilian Mountains, northwestern China and its relationship to atmospheric circulation patterns Xiaohong Liu a,⁎, Xuemei Shao b, Eryuan Liang c, Tuo Chen a, Dahe Qin a, Wenling An a, Guobao Xu a, Weizhen Sun a, Yu Wang a a State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Lanzhou 730000, China b Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China c Laboratory of Tibetan Environment Changes and Land Surface Processes (TEL), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China article info abstract Article history: It is important to understand the history and dynamics of climate in a transitional region between areas with Received 28 April 2009 different atmospheric circulation patterns, where the vegetation and ecosystems are vulnerable to Received in revised form 26 July 2009 environmental change. We investigated variations in the long-term oxygen isotope composition (δ18O) in Accepted 8 August 2009 tree rings of Qinghai spruce (Picea crassifolia) and their relationships to climatic parameters in the arid Qilian Editor: R.L. Rudnick Mountains of northwestern China from 1870 to 2006. We found that the mean temperature from the previous November to the current February was significantly and positively correlated with the tree-ring δ18 Keywords: O values. The temperature effect, (the positive relationship between the temperature and the 18 18 Oxygen isotopes precipitation δ O value) can explain the connection between temperature and the tree-ring δ O values. Tree rings Due to pooling of the earlywood and latewood into yearly tree-ring samples, it appears that the cellulose Temperature effects δ18O may be influenced by isotopically nonhomogeneous water sources and climatic conditions during the Atmospheric circulation patterns previous and current growing seasons. Subtle shifts and amplitude deviations in cellulose δ18O, which abruptly became more positive around 1977–1978, may be attributed to the shifting climatic regime in China and to temperature variations, respectively. Our results illustrated the potential for investigating climatic or atmospheric circulation patterns based on oxygen isotope records in tree rings in regions near the interface between different large-scale synoptic circulations. © 2009 Elsevier B.V. All rights reserved. 1. Introduction and Stuiver, 1981; Saurer et al., 1997; Rebetez et al., 2003; Liu et al., 2004; Danis et al., 2006). Tree-ring isotopic data are a powerful tool for reconstructing The East Asian monsoon determines the key environmental climatic conditions where temperature and precipitation are primary conditions in China. The amount of precipitation in east Asia is mainly driving factors (see McCarroll and Loader, 2004, and the references controlled by the strength of the summer monsoons, which bring moist therein). Oxygen isotope ratios (δ18O) in rainfall are positively air from the Pacific and Indian Oceans over the continent, yielding correlated with atmospheric temperature (e.g., Dansgaard, 1964; precipitation (An, 2000). In northwestern China, the climate in most Jouzel et al., 1997). Although other parameters such as continentality areas is also influenced by the westerly flow (Wang et al., 2003, Tian or atmospheric circulation patterns also play a role in determining the et al., 2003; Qian et al., 2007). The oscillations between periods of dry δ18O value (Rozanski et al., 1993; Jouzel et al., 1997), the temperature and wet climate in northern China, as well as the transitions between at the location of the precipitation is widely recognized to be the most these states, are linked to interactions between the westerly flow and important parameter that determines the relative proportions of 18O the East Asian monsoon flow (Qian et al., 2007). The transitional region and 16O at middle and high latitudes (Rozanski et al., 1993; Jouzel between two monsoonal flows is very vulnerable to changes in these et al., 1997). Such climatic information is therefore contained in the flows. The climate in this region is influenced by the position of the δ18O value of precipitation and may therefore be recorded in the δ18O convergence interface between the East Asian monsoon and the values in tree annual growth rings after trees take up the water (Burk westerly flow, which changes from year to year and from decade to decade; as a result, climate anomalies will occur if the interface position moves northward or southward as a result of this flow interaction. ⁎ Corresponding author. State Key Laboratory of Cryospheric Sciences, Cold and Arid The activities of the atmospheric circulations in China determine the Regions Environmental and Engineering Research Institute, Chinese Academy of isotopic composition of the local precipitation (Wei and Lin, 1994; Tian Sciences, Donggang West Road No. 320, Lanzhou 730000, China. Tel.: +86 931 496 7342; fax: +86 931 827 1124. et al., 2001), and therefore affect the oxygen isotope composition of E-mail address: [email protected] (X. Liu). plant cellulose (Liu et al., 2004). Recent studies have also suggested the 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.08.005 148 X. Liu et al. / Chemical Geology 268 (2009) 147–154 great potential of using the oxygen isotopic composition in tree-ring site (38°31.4′N, 100°14.8′E) is at Dayekou in the middle of the Qilian cellulose to infer changes in North American atmospheric circulation Mountains (Fig. 1), which is about 15 km from Zhangye city. The patterns (Feng et al. 2007) and variability in the intensity of the North climate in the middle of the Qilian Mountains is dominated by the American monsoon (Roden and Ehleringer, 2007). westerly flow and partly affected by the East Asian monsoon. The tree- The Qilian Mountains (~99°E to 103°E, 37°N to 39°N) are located at ring cores were collected from the dominant Qinghai spruce (Picea the northeastern edge of the Tibetan Plateau, at the convergence of crassifolia), a tree species that is sensitive to environmental change the Qinghai–Xizang (Tibet) plateaus, Inner Mongolia and Xinjiang (Gou et al., 2005; Liang et al., 2006; Liu, et al., 2007, 2008b). Qinghai plateaus, and the Loess Plateau (Fig. 1). Annual precipitation in the spruce generally grows on thin soils. The sampled trees ranged in the Qilian Mountains ranges from 84.7 to 410.3 mm based on the data elevations from 2920 to 3040 m. For cross-dating, at least 20 from local meteorological stations (Eastern station: at Wushaoling; dominant trees were selected and two cores were taken per tree at western station: at Jiuquan). The annual pan evaporation varies breast height using 5-mm increment borers (Haglöf, Mora, Sweden). between 1041.2 and 1234.2 mm, and the annual relative humidity An additional 15 cores were obtained using 10-mm increment borers averages about 57% (Fu and Che, 1990). The climates of the eastern, from different trees for isotopic analyses. western, and middle regions of the Qilian Mountains are controlled by Many rings were narrow or had indistinct latewood, so to avoid the East Asian monsoon, the westerly flow, and the interaction of the interpretation errors, we used all the wood from each year for the two flows, respectively (Tang et al., 2007). In this ecologically isotopic analyses. After cross-dating, we selected eight cores without vulnerable semi-arid region, interdecadal climate shifts have impor- obvious damage from different trees and cut them into sub-samples tant effects on the regional ecosystems. Non-exchangeable hydrogen with a 1-year resolution under a binocular microscope. We then or oxygen isotopes in tree-ring cellulose can potentially allow us to pooled the samples from the same year. The pooled annual samples reconstruct these climatic changes (Liu et al., 2008a) and shifts in the were first milled, and then α-cellulose was extracted using a method region's climatic system (Feng et al., 2007). based on those of Green (1963) and Loader et al. (1997). To measure In the present paper, we report the results of a study of the δ18O the oxygen isotope compositions, we loaded 0.14 to 0.16 mg of α- values in tree rings from forests in the middle of the Qilian Mountains of cellulose into silver capsules, and determined the ratio using a High arid northwestern China, and discuss the climatic significance of this Temperature Conversion Elemental Analyzer coupled to a Finnigan data and the potential correlations between δ18O in tree rings and shifts MAT-253 mass spectrometer (Thermo Electron Corporation, Bremen, in the atmospheric circulation patterns. We hypothesized that the tree- Germany) at the State Key Laboratory of Cryospheric Sciences, ring δ18O values would record climatic information related to the water Chinese Academy of Sciences. The oxygen isotope analyses were sources (precipitation and soil water) from the previous winter, spring, repeated four times for each annual cellulose sample, from which we and summer from which the trees absorbed their water (Saurer et al., calculated the mean values. The 18O/16O ratios were expressed as 1997; Robertson et al., 2001; Treydte et al., 2006; Danis et al., 2006). δ18O, which represents the per mil deviation relative to the Vienna Standard Mean Ocean Water standard. We measured the ratio for a 2. Methods benzoic acid working standard with a known δ18O value (IAEA-601, 23.3‰) every seven measurements to monitor the analytical precision The study area is located in the Qilian Mountains on the and to calibrate the samples for analytical accuracy. The analytical northeastern margin of Tibetan Plateau. The location of the sampling uncertainty was less than 0.3‰ (1σ). Fig. 1.

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