Moisture Availability Over the Past Five Centuries Indicated by Carbon

Aeolian Research 11 (2013) 101–108

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Aeolian Research

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Moisture availability over the past five centuries indicated by carbon isotopes of Tamarix taklamakanensis leaves in a nebkha profile in the Central Taklimakan Desert, NW China ⇑ Lili Lang a,b, Xunming Wang a,b, , Ting Hua b, Caixia Zhang b a Department of Physical Geography and Global Change, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China b Key Laboratory of Desert and Desertification, Cold & Arid Regions Environmental & Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China article info abstract

Article history: We inferred moisture availability changes from 1555 to 2010 in the Central Taklimakan Desert (NW Received 19 August 2012 China) using Chinese tamarisk (Tamarix taklamakanensis) litter deposited within a nebkha that developed Revised 28 March 2013 in this region. The litter d13C trends revealed ﬂuctuations of moisture availability: From 1555 to 1785, Accepted 30 May 2013 moisture availability was higher than the long-term average, but from 1555 to 1570, 1585 to 1620, Available online 9 July 2013 1640 to 1660, 1675 to 1700, and 1730 to 1755, this region experienced periods with lower than average moisture availability. From 1785 to 1850, signiﬁcant moisture availability changes occurred, and the Keywords: region experienced the lowest moisture availability since 1555. From 1850 to the present, the d13C trends Moisture availability suggested that moisture availability increased. Our results also showed that although moisture availabil- Coppice dunes d13C ity in this region is controlled by precipitation and evaporation in the Central Taklimakan Desert, temper- Aridity ature variations in the adjacent Kunlun Mountains (whose glacier and snowmelt runoff are dominant Tarim Basin water sources for the study area) were potentially more important. Water from these mountains plays an important role in moisture availability due the region’s extremely low precipitation. In addition, although previous studies suggested that the Tarim Basin and its adjacent areas experienced a wet climate for the past several centuries, this suggestion was inconsistent with the reconstructed moisture availability changes in the Central Taklimakan Desert. The moisture availability in the Central Taklimakan was more closely related to temperature variations in the adjacent mountain regions. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction availability, and particularly by changes in groundwater depth (Cui and Shao, 2005; Liu et al., 2005), precipitation (Hu and Pan, The Central Taklimakan Desert, which is located in the central 1996; Zhang and Xia, 1997), and evaporation (Cao et al., 2003; to southern parts of the Tarim Basin, is surrounded by the Kunlun Kemp et al., 1997). The sparse vegetation plays an important role Mountains in the south and the Tianshan Mountains in the north in controlling sand transport and the formation of nebhkas (also (Fig. 1), creating an extremely arid area with annual precipitation referred to as nabkhas, coppice dunes, and vegetated dunes), which <30 mm and evaporation greater than 3800 mm (1999–2008 aver- are indicators of changes in the ecological environment in this re- aged). Due to the extremely arid climate, mobile megadunes have gion; these dunes form when there is sufficient soil moisture to developed in this region, and few vegetation species grow in the sustain the vegetation that anchors the dunes (Forman and Pier- interdune areas; these include Populus euphratica, Populus pruinosa, son, 2003; Forman et al., 2009; Qong et al., 2002; Xia et al., Phragmites communisi, Inula salsoloides, Hexinia polydichotoma, 2005). Although there have been recent advances in our under- Cynanchum kashgaricum, Apocynum venetum, Agriophyllum arenari- standing of climate change during the past several centuries um, Salsola ruthenica, Halogeton arachnoideus, Heliotropium mic- around the Tarim Basin (e.g., Li et al., 2006; Liu et al., 2011; Yao ranthum, Cistanche tubulosa, Calligonum spp., and Tamarix et al., 1997; Zhang et al., 2003), the absence of proxies with rela- taklamakanensis (He, 1997; Wang et al., 2002a). The formation of tively high resolution has made it difficult to reconstruct past cli- the vegetation communities is mainly controlled by moisture mate change and moisture availability, and the associated significance for changes in the ecological environment of the Cen- ⇑ Corresponding author. Address: Cold & Arid Regions Environmental & Engi- tral Taklimakan, so these phenomena remain poorly understood. neering Research Institute, Chinese Academy of Sciences, No. 320 West Donggang Nebkhas are the product of basic aeolian processes (e.g., Hesp Road, Lanzhou 730000, Gansu Province, China. Tel.: +86 931 4967491; fax: +86 931 and Mclachlan, 2000; Langford, 2000; Tengberg, 1995), and are 8273894. scattered in the interdune areas between mobile megadunes of E-mail address: [email protected] (X. Wang).

Fig. 1. Geomorphologic map of the Tarim Basin, showing the location of the Central Taklimakan Desert, rivers and bodies of water, the nebkha sampling site (Site F), and the locations of the sampling sites from other studies (A–G) that are discussed in the text. Sites A–C: Li et al. (2006) and Yuan et al. (2000, 2001). Site D: Chen et al. (2006, 2010). Site E: Liu et al. (2011). Site G: Yao et al. (1997) and Yang et al. (2004). the Central Taklimakan Desert (Wang et al., 2002a). In this region, nual temperature, precipitation, and evaporation were 12.3 °C, nearly all nebkhas develop with Chinese tamarisk (T. taklamakan- 27.8 mm, and 3835.4 mm, respectively. In this region, rainfall ensis) as the center for sand accretion. Because of the biological events mainly occur from April to June and are typically intense characteristics of this species, it deposits litter within the dune but brief downpours. Nebkhas with Chinese tamarisk as their nu- sediments, and variations in the litter d13C provide excellent prox- cleus form by stabilizing the sediment supply provided from the ies for reconstructions of the palaeoclimate and palaeoenviron- adjacent mobile dunes, and develop a nearly oval shape. They are ment in arid areas (e.g., Lipp et al., 1996; Xia et al., 2004, 2005; scattered within the interdune areas between the mobile megad- Yakir et al., 1994). Although the variations of d13C in surface organ- unes. As the vegetation traps blowing particles, the nebkha grows ic matter and live leaves of various plant species reflect changes in in height and length, and the tamarix roots extend downwards to regional moisture conditions (e.g., Stuiver and Braziunas, 1987; Su maintain contact with subsurface water below the interdune level. et al., 2000), in precipitation (Liu et al., 2004; Wang, 2003; Wang In the interdune area that we sampled, rough estimates over the et al., 2006; Wang and Han, 2001), or in groundwater depth (Wang past decade indicate that the vegetation cover on the nebkhas dur- et al., 2008, 2010), studies of these variations produce inconsistent ing the growing season ranges from 50% to 60%, versus 20% to 30% results because of the confounding effects of factors that vary during the other seasons (Wang et al., 2005). Although we cannot among regions, including how strongly the dominant species dis- ensure that no wind erosion occurred on the surfaces of the neb- criminate against different carbon isotopes. Nonetheless, varia- khas under these levels of vegetation cover, previous research sug- tions in d13C can, in principle, provide significant clues to gested that at a vegetation cover >14%, aeolian deposition is variations in the water supply within a given region (Diefendorf greater than or equal to aeolian erosion (e.g., Wiggs et al., 1995), et al., 2010). Therefore, to provide relatively high-resolution data which means that the nebkhas are likely to grow or remain stable. on changes in moisture availability in the Central Taklimakan, we In addition, based on the results of previous studies (i.e., Liu et al., analyzed d13C variations in the litter contained in a nebkha, and 2011), there have been no extreme arid events during recent cen- used the results to reconstruct moisture availability changes and turies in the Tarim Basin. On this basis, we hypothesize that the re- the evolution of the ecological environment for a period of nearly gional moisture conditions ensured that the vegetation cover at the five centuries. apex of the nebkhas during recent centuries has remained above 14%, that the nebkhas developed continuously since they first 2. Sampling site and analytical methods formed, and that the strata that resulted from their formation processes can be used to infer the environmental changes that oc- Our sampling site is located in the Central Taklimakan Desert curred in this region. (Fig. 1, site F). Meteorological records (1999–2008 averages, ob- On 21 October 2010, we sampled a nebkha located in an interd- tained about 20.7 km from the study site) show that the mean an- une area (38°51.6070N, 83°30.5540E). The nebkha was around 5 m L. Lang et al. / Aeolian Research 11 (2013) 101–108 103

Fig. 2. Photographs of the nebkha that was analyzed in this study: (A) before excavation, (B) after excavation. (C) diagram of the proﬁle.

tall, 14 m in diameter, and roughly oval in shape (Fig. 2A). Apart and only picked clean leaf litter for radiocarbon dating and carbon from T. taklamakanensis, no other plant species were growing on isotopic analyses. After cleaning the separated leaf litter samples the nebkha’s surface or in the adjacent interdune areas. Visual esti- with distilled water, we oven-dried them at 60 °C for 48 h, and mates of the vegetation cover were about 30% even in late October, then ground them into fragments smaller than 0.120 mm in size. at the end of the growing season. In this region, the dominant We then combusted 0.06–0.09 mg of each pretreated sample in winds are from the northeast, and the T. taklamakanensis cover pure oxygen to convert the organic matter into CO2 in a Flash was highest on the southern side of the dune. We therefore exca- EA1112 high-vacuum elemental analyzer (Thermo Finnigan, Wal- vated the dune to provide a clean vertical profile through the full tham, MA) equipped with a continuous-flow system (Thermo Finn- height on the northern side (Fig. 2B). From the crest of the dune to- igan). After the CO2 gas was purified and collected according to the wards the bottom, there were distinct cross-stratifications of litter manufacturer’s instructions, we measured the d13C values, which and aeolian sediments, especially in the uppermost parts of the we expressed as follows: profile (Fig. 2B and C). The litter remains mainly consisted of ÀÁÂÃ 13 3 leaves, at least in the upper section (0 to 172 cm), where well-pre- d C ¼ Rsample=Rstandard 1 10 ‰ served leaves can be visually identified. In addition, we found no where R and R are the 13C/12C ratios in the sample and bioturbations created by burrowing animals throughout the pro- sample standard in the Vienna Pee Dee Belemnite standard, respectively. We used a file, and therefore we concluded that the record was complete Delta Plus isotope-ratio mass spectrometer (Thermo Finnigan) for and undisturbed. However, during our field work, it was very diffi- this analysis, and the analytical error was less than 0.05‰. The cult to sample the sediments and litter within these layers. As a re- AMS dating was performed by the Xi’an AMS Center, Chinese Acad- sult, we had to collect mixed sediments and litter at 2-cm intervals emy of Sciences. The dating processes, instruments, and measure- throughout the profile. From the top of the dune to a depth of ment errors were described in detail by Zhou et al. (2007). The 270 cm, we continuously sampled the litter; however, the litter calculations of the 14C age used the methods of Stuiver and Polach was discontinuously present from 272 to 330 cm in depth, and (1977), and the ages were calibrated using version 6.1.0 of the Calib we found no litter below a depth of 332 cm. We collected a total software (Stuiver and Reimer, 1993) and the IntCal09 calibration of 241 samples to a final depth of 482 cm. In addition to the sedi- data set (Reimer et al., 2009). ments and litter within the nebkha, we also collected litter at depths of 160, 238, and 328 cm (Fig. 2C) to perform accelerator mass spectrometry (AMS) 14C dating following the methods de- 3. Results and Discussion scribed later in this section. After the T. taklamakanensis litter in each sample was separated Because we sampled the nebkha during the leaf-free season for from the bulk sediments by sieving, we carefully excluded plant T. taklamakanensis (at the end of October), we assumed that the roots and branches (to avoid contamination with modern carbon) surface litter was produced during the current year (2010). In 104 L. Lang et al. / Aeolian Research 11 (2013) 101–108

Table 1 238, and 328 cm were 1810, 1661, and 1359 (calibrated age), AMS radiocarbon dates for the sediment profile from the nebkha. respectively, suggesting that the nebkha originated around 1359 Sample Depth 14C age (a Calibrated range Calibrated age (Table 1). The net deposition rates (differences between deposition ID (cm) B.P.) (2r) (A.D.) (2r) (A.D.) and erosion) from 1810 to the present, from 1661 to 1810, and TK80 160 152 ± 33 1666–1953 1810 from 1359 to 1661 averaged 0.79, 0.52, and 0.30 cm/a, respectively. TK119 238 251 ± 34 1519–1802 1661 Therefore, the profile containing Chinese tamarisk litter had aver- TK164 328 581 ± 42 1297–1421 1359 age and maximum resolutions of 3.97 a and 2.54 a, respectively. The litter d13C values in the profile ranged between 26.10‰ and 21.67‰, and averaged 24.50‰ (Fig. 3). Because the litter addition, we established the chronology of the nebkha by means of only appeared continuously from the top of the dune to a depth linear interpolation between the adjacent calibrated radiocarbon of 270 cm (deposited in 1555), we will therefore only discuss the 13 ages. The AMS dating showed that the dates at depths of 160, variations in the litter d C trends from that time to the present.

Fig. 3. Trends in the litter d13C values from the sampled nebkha and the corresponding changes in moisture availability (smoothed using a 10-year running mean) from 1555 to 2010 in the Central Taklimakan Desert. Subperiods are labeled with the period number followed by the subperiod (e.g., 1S2 is subperiod 2 of period 1).

Fig. 4. Plan and proﬁle diagrams of the terrain between the Kunlun Mountains and the Central Taklimakan Desert, and the associated moisture supply patterns in the desert. Contour intervals represent elevations (m a.s.l.). Based on data from Zhang (1985). L. Lang et al. / Aeolian Research 11 (2013) 101–108 105

Fig. 5. Measured annual mean temperature and precipitation data from 1960 to 1998, and evaporation data from 1964 to 1998, and comparison with the litter d13C values. The annual values of the litter d13C were acquired by linear interpolation between measurements with a resolution of 2.54 a. The temperature and precipitation measurements represent the averages of the values at the Minfeng and Andehe meteorological stations (the stations nearest to the Kunlun Mountains and between those mountains and our sampling site). Due to the unavailability of data at the Andehe station, the evaporation data are only from the Minfeng station. Station locations are identiﬁed in Fig. 1.

Fig. 6. Comparisons of (a) the reconstructed temperatures inferred from the ice core d18O trends at the Guliya site (G in Fig. 1) with (b) the moisture availability changes in the Central Taklimakan Desert (smoothed using a 10-year running mean). The reconstructed temperature data are from Yao et al. (1997) and Yang et al. (2004).

From 1555 to the present, there were large variations in the litter 1785 to 1850, large fluctuations of d13C again occurred, with a d13C trends. Based on the average value throughout the profile mean of 23.91‰. From 1850 to 2010, d13C values were relatively since 1555, the litter d13C values experienced three overall stages. stable compared with the two previous periods, with a mean of From 1555 to 1785, significant fluctuations in the litter d13C values 24.58‰ that was more negative than the long-term average, occurred, but the litter d13C values generally became more nega- and the sub-stage from 1850 to 1885 reflected a state near the tive, with a mean of 24.71‰. During this stage, there were five long-term average. However, our results also show that d13C in- sub-stages with litter d13C values higher than the long-term aver- creased greatly from 1885 to 1935, with a mean of 24.32‰. age: from 1555 to 1570, 1585 to 1620, 1640 to 1660, 1675 to 1700, In the Central Taklimakan Desert, the roots of T. taklamakanensis and 1730 to 1755, with mean litter d13C values of 23.56‰, usually grow through the dune to reach the level of the interdune 24.35‰, 24.51‰, 24.49‰, and 24.38‰, respectively. From surface materials, and continue to grow deeper until they can 106 L. Lang et al. / Aeolian Research 11 (2013) 101–108

Fig. 7. Changes between wet and dry climates since 1550 around the Tarim Basin, and comparisons with the changes in moisture availability in the Central Taklimakan Desert. A, B, and C: reconstructed using tree-ring records in the Tianshan Mountains north of the study area (Li et al., 2006; Yuan et al., 2000, 2001). D: reconstructed using variations in carbonate contents and fractions >30 lm in lacustrine deposits (Chen et al., 2006, 2010). E: reconstructed using d13C variations (Liu et al., 2011). F: the moisture availability trends (present study). The locations of sites A–F are identiﬁed in Fig. 1. Horizontal lines represent long-term means for each series.

absorb moisture deep in the soil. The modern groundwater depth Mountains. The primary water sources for these seasonal rivers are in the interdune areas usually varies between 2 and 4 m, and is glacier and snowmelt runoff from the mountains (Fig. 4). Because mainly controlled by the underlying terrain (Fan et al., 2008; Li the precipitation in this region is extremely low (less than et al., 2007; Wang et al., 2002b) and by the incoming flows from 30 mm, long-term average annual value), temperature increases seasonal rivers that originate on the northern slopes of the Kunlun in the Kunlun Mountains may therefore be more important; this L. Lang et al. / Aeolian Research 11 (2013) 101–108 107 warming increases glacier and snowmelt runoff, thereby greatly a (with a range of 5 to 15 a), which again shows high consistency raising the groundwater table and changing the moisture availabil- between the temperature variations and the d13C trends. In areas ity in the Central Taklimakan Desert. around the Central Taklimakan Desert from 1550 to the present, In addition, based on the results of previous studies (e.g., Stuiver periods with high temperature occurred from 1580 to 1650, 1700 and Braziunas, 1987; Su et al., 2000), more negative d13C values in to 1800, and 1940 to the present. Our litter d13C trends also show the organic matter of plants indicate periods with higher moisture that from 1570 to 1780 and from 1940 to 2010, the Central Takli- availability. Thus, the results of our analysis of the litter d13C trends makan Desert had periods with high moisture availability, despite in the nebkha profile show that in this region, the moisture avail- several fluctuations during which moisture availability was lower ability from 1555 to the present experienced three major stages than the long-term average. In addition, regions around the Central and several minor stages. From 1555 to 1785, moisture availability Taklimakan Desert experienced a period of temperatures lower was higher than the long-term average, but from 1555 to 1570, than the long-term average from 1780 to 1940, and the litter 1585 to 1620, 1640 to 1660, 1675 to 1700, and 1730 to 1755, mois- d13C trends also show that this period was characterized by low ture availability was lower than the long-term average (substages moisture availability. 1S1 to 1S5 in Fig. 3). From 1785 to 1850, moisture availability de- Changes between wet and dry periods that did not play an creased, especially from 1815 to 1845 (2S1 in Fig. 3); during this important role in the moisture availability in the Central Taklima- period, the region experienced the lowest moisture availability kan were also identified by the reconstructions of water conditions since 1555. From 1850 to the present, the d13C trends suggested around the Tarim Basin. In most periods during the past five cen- generally increasing moisture availability, although there were turies, the regions around the Tarim Basin appear to have experi- two periods with lower moisture availability (3S1 and 3S2 in enced a wet climate, and this was not always consistent with the Fig. 3) during this period. moisture availability trends in the Central Taklimakan (Fig. 7). The resolution of the Chinese tamarisk litter profile after 1810 For instance, tree-ring proxies (Li et al., 2006) suggested that the was 2.54 years, which let us compare the d13C trends with mea- Tien Shan area experienced a wet climate in the 1700s and sured records of temperature, precipitation, and evaporation from 1900s, and a dry climate in the 1800s (Fig. 7A). Yuan et al. (2000, 1960 to the end of the 1990s (at Minfeng and Andehe; Fig. 5). Due 2001) reported that there were three or four wet/dry alternations to the uncertainty in the AMS 14C dating and the resolution limita- since the late 1600s in the Northern Tarim Basin (Fig. 7B and C). tions in the profile, it was not possible to perform direct year to Chen et al. (2006, 2010), Liu et al. (2011) also suggested a wet cli- year comparisons between the measured meteorological data mate in the northern and eastern edges of the Tarim Basin before and the litter d13C values. However, our results show that from 1900 (Fig. 7D and E). However, over the past five centuries, there 1960 to 1998, the temperature and precipitation variations fol- were significant differences between the moisture availability that lowed a similar pattern, whereas both followed an inverted pattern we reconstructed in the Central Taklimakan Desert and these other compared with the d13C trends. However, the mean annual precip- reconstructions. We deduced that in the Central Taklimakan Des- itation in the Central Taklimakan Desert was only 27.8 mm, with ert, although precipitation may be a viable candidate for a primary an annual range of 12.2 mm, compared with an annual evaporation factor influencing moisture availability to tamarisk growing on top that exceeded 3830 mm (1999–2008). Therefore, although high of dunes, precipitation (<30 mm annually) plays a weaker role than precipitation may improve the soil moisture availability, it may the groundwater depth in determining the regional moisture avail- not play a key role in determining moisture availability in this re- ability, since the groundwater depth is the dominant control on gion. In addition, comparing the litter d13C trends with the evapo- vegetation formation (Bruelheide et al., 2010; Mu, 1994; Thomas ration trends showed a coincident change between evaporation et al., 2006). As a result, variations in precipitation may not pro- and moisture availability. We deduce that this relationship reduce equally strong variations in the litter d13C value, which is sulted from temperature increases that increased the glacier and the proxy we used to represent the fluctuations in moisture avail- meltwater runoff from the adjacent Kunlun Mountains, which are ability in this region. In this extremely arid area, moisture avail- the dominant sources of groundwater in the Central Taklimakan ability is mainly determined by the groundwater depth, which is Desert. Unfortunately, records of groundwater depth variations in turn determined by the quantity of glacier and snowmelt runoff are unavailable for most of the study period because groundwater from the Kunlun Mountains, which is controlled by temperature depth monitoring stations were only established in this region in variations in the mountains. Therefore, rising temperatures in the the early 2000s. Thus, the issue of groundwater depth remains un- regions around the Tarim Basin may benefit the region’s ecological solved in this study. However, in the southern Taklimakan Desert environment, at least for as long as snow cover and glaciers are during the 1990s, the water resource can be hypothesized to have present in the surrounding mountains, and may thus help to con- increased because the flows of the region’s seasonal rivers in- trol desertification in this region. However, this may only be a creased with increasing temperature (e.g., Yao et al., 2004). short-term effect if high temperatures in the mountains reduce Temperature variations appear to have been the main factor snowfall and glacier accumulation. that controlled the moisture availability in the Central Taklimakan Desert. This hypothesis is supported by temperature reconstructions using the Guliya ice cores southwest of our study area (G in 4. Conclusions Fig. 1). Since 1550, periods of high moisture availability in the Cen- tral Taklimakan Desert generally occurred simultaneously with Since the 1550s, the Central Taklimakan Desert has experienced warm periods. Because fluctuations in the d18O values in the ice several fluctuations in moisture availability. From 1570 to 1780 cores were larger than those in the d13C values, there is a low-tem- and from 1940 to the present, this region experienced periods with perature period (1650–1700 A.D, shown by the yellow color in moisture availability higher than the long-term average, despite Fig. 6b) that did not correspond with the period of low moisture several periods with lower-than-average moisture availability. in the d13C trends during the same period. However, despite this From 1780 to 1850, the region experienced the lowest moisture exception, the temperature fluctuations generally corresponded availability during the study period. Temperature variations in to the trends in d13C. 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