Implications of variations in δ18O and δD in precipitation at Madoi in the eastern Tibetan Plateau

Quaternary International 313-314 (2013) 56e61

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Implications of variations in d18O and dD in precipitation at Madoi in the eastern Tibetan Plateau Wei Ren a, b, *, Tandong Yao a, Xiaoxin Yang a, Daniel R. Joswiak a a

Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China b Graduate University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 24 May 2013

Event-based rain and snow samples were collected from May 2009 to April 2010 at Madoi in the eastern Tibetan Plateau, and the stable isotopic compositions (d18O and dD) of these samples were analyzed. The local meteoric water lines (LMWLs) based on individual samples and monthly weighted means were dD ¼ 8.37d18O þ 11.86 (R2 ¼ 0.97, n ¼ 175) and dD ¼ 8.80d18O þ 20.16 (R2 ¼ 0.98, n ¼ 11), respectively. Although fluctuations of the isotopic compositions in precipitation throughout the year were high, monthly amount-weighted means revealed seasonal signals. According to the monthly averaged data, the d18O values decreased distinctly from June to July, indicating that summer monsoon circulation can significantly influence precipitation in this region. The high d-excess values from May to June and from October to November were probably the result of local moisture recycling. Apart from moisture sources, the d18O and d-excess values in precipitation were also affected by subcloud evaporation/sublimation, which could be inferred from their relationships with different temperature and relative humidity ranges. The results of this study augment the understanding of the water cycle in the eastern Tibetan Plateau. Ó 2013 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Stable isotopic compositions of oxygen and hydrogen in precipitation (d18O and dD) are widely applied in hydrological and climatic studies (Dansgaard, 1964; Gat and Carmi, 1970; Gat, 1996; Araguás-Araguás et al., 2000). Stable isotopic compositions in precipitation reflect various processes associated with equilibrium and kinetic isotopic fractionation, such as the evaporation of moisture sources, transport trajectory, condensation processes, and the evaporation of raindrops. In the last two decades, isotopic investigations in precipitation over the Tibetan Plateau and adjacent regions have contributed notably to an understanding of regional water cycle (Yao et al., 1999; Tian et al., 2001a; Zhang et al., 2002; Kurita and Yamada, 2008; Gao et al., 2011). In summer, two kinds of air masses dominate over the Tibetan Plateau: the humid marine air masses (monsoonal moisture) from the Indian Ocean, and the continental moisture evaporated from inland water bodies and

* Corresponding author. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, 16 Lincui Road, Chaoyang District, Beijing 100101, China. E-mail address: [email protected] (W. Ren). 1040-6182/$ e see front matter Ó 2013 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2013.05.026

soils. The former prevails in the south, becoming progressively weaker in the northward direction, and the latter prevails in the north. In winter, the whole Tibetan Plateau is controlled by westerlies. Moreover, in reality, the precipitation at any given site is not only influenced by advected moisture, but also by local processes, particularly local moisture recycling and subcloud evaporation. Research on the processes controlling stable isotopic compositions and variability in precipitation is helpful for a better understanding in this region of moisture sources and the water cycle, as well as local climate conditions. Based on previous studies in the Tibetan Plateau and adjacent regions, this paper analyzes and discusses stable isotopic compositions in precipitation at Madoi, in the eastern Tibetan Plateau, for which little data currently exist. The aim is to identify whether monsoon circulation influences precipitation and how local climate conditions influence the stable isotopic signal in precipitation there. In this study, two local meteoric water lines (LMWLs) are presented, based on individual samples and monthly weighted means, respectively. The isotopic data during the study period imply monsoonal water is an important component for precipitation in summer, and local recycled water generally dominates during the late-spring, early-summer and autumn months. The

W. Ren et al. / Quaternary International 313-314 (2013) 56e61

57

variations of isotopic compositions in precipitation are the incorporation of different moisture sources and subcloud evaporation/ sublimation. The results of this study facilitate an understanding of the water cycle in the eastern Tibetan Plateau.

10 0 -10

δ18O (‰)

18

δ Ο

-20

2. General setting and method 0 -100 -200

20 Temperature

0 -10

20

-20

15 Precipitation

10 5 0

/1 /2/1 /3/1 /1 /9/1 10/1 11/1 12/1 /1 /5/1 /1 /6/1 /7/1 0/1 / 0 9 / / 9 9/8 9/5 9 0 0/4 0 200 200 200 200 200 2009 2009 2009 201 201 201 201 201

Date 18

Fig. 2. Temporal variations of d O, dD in precipitation, surface temperature, precipitation amount during the period May 2009eApril 2010 at Madoi.

for dD. The amount-weighted averaged d18O and dD for all precipitation events at Madoi were 12.2& and 86.4&, respectively.

3. Results

4. Discussion

The precipitation amount throughout the study period from May 2009 to April 2010 at Madoi was 451 mm. A total of 175 samples were collected, including 107 rain and 68 snow samples. The accumulation of rainfall was 345 mm, while the accumulation of snowfall was 106 mm. The surface air temperature and precipitation displayed clear seasonality, with maxima in summer and minima in winter (Fig. 2). Stable isotopic compositions for all precipitation samples during the study period showed a wide range (Fig. 2): from 26.7& to 6.9& for d18O and from 209.5& to 60.1&

4.1. d18O, dD and d-excess in precipitation

Fig. 1. Map showing the locations of Madoi (circle) and some nearby stations used in this study (triangles).

10

T(°C)

Madoi 4300 m asl) is located in the eastern Tibetan Plateau (Fig. 1). Based on a database from 1953 to 2010, annual precipitation there is around 320 mm, with the majority falling in the summer months (JuneeSeptember) and the least in the winter months (DecembereFebruary). The annual average temperature here is about 4  C, ranging from 7.7  C in July to 16.2  C in January. It is also located at the headwaters of the Yellow River, and there are two large lakes nearby (the nearest only about 30 km away). In this study, event-based rain and snow samples were collected at the meteorological station of Madoi from May 2009 to April 2010. Once the rain had stopped, samples were immediately stored in sealed 15 ml high-density polyethylene bottles. Snow was collected, melted at room temperature, and then processed following the same method as the rainfall samples. All samples were stored in a cool environment until measurement. The corresponding meteorological parameters (including precipitation amount, surface air temperature and relative humidity) during each precipitation event were recorded. d18O and dD were measured by cavity ringdown laser spectroscopy (CRDS) using a L1102-i Picarro water isotope analyzer in the Key Laboratory of Tibetan Environment Change and Land Surface Processes, Chinese Academy of Sciences. The precision was 0.1& for d18O and 0.5& for dD. Results are reported as relative to the standard VSMOW (Vienna Standard Mean Ocean Water).

-30 δD

δD (‰)

98.26 E,

P(mm)

(34.92 N,

Although the stable isotopic data were only for one year in this study, comparison between Madoi and nearby stations can provide an overview of stable isotopic compositions in precipitation there. Table 1 shows the stable isotopic compositions in precipitation at Madoi and some nearby stations (locations are shown in Fig. 1). Lhasa, Tuotuohe and Delingha provide a general representation of locations in the southern, central and northern Tibetan Plateau, respectively, while Lanzhou represents more northeastern locations. The averaged d18O value at Madoi was close to that at Tuotuohe, which is located in the transition region between the monsoon domain (Indian monsoonal air masses dominant in summer) in the south and the westerlies domain (continental air masses dominant in summer) in the north of the Tibetan Plateau (Tian et al., 2001a). The d-excess value is defined as d ¼ dD  8d18O (Dansgaard, 1964), which is mainly related to the kinetic fractionation at the source region of the vapor water. Globally, the d-excess in precipitation is 10&, resulting from the evaporation from oceans at an average relative humidity of 85% (Merlivat and Jouzel, 1979). High d-excess values are found to occur under the interaction of air masses and water bodies in relatively dry settings (Gat and Carmi, 1970; Machavaram and Krishnamurthy, 1995; Peng et al., 2010). Moreover, the inland evaporation from water bodies or soils is also capable of producing high d-excess values (Gat and Matsui, 1991; Froehlich et al., 2008). As shown in Table 1, d-excess at Madoi was much higher than at Lhasa and Lanzhou, implying the occurrence of local moisture recycling, consistent with the intense local moisture recycling from soils (Peng et al., 2012) and water bodies in this region (two nearby large lakes, as shown in Fig. 1).

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W. Ren et al. / Quaternary International 313-314 (2013) 56e61

Table 1 Amount-weighted isotopic data in precipitation at Madoi and some nearby stations Station

Observation period

d18O (&)

dD (&)

d-Excess (&)

Reference

Lhasa

1993e1999(a),1986e1992(b)

17.38(a), 14.5(b)

109.1(b)

7.1(b)

Tuotuohe Madoi Delingha Lanzhou

1991e1999 May 2009eApr 2010 1991e1999 1985e1987, 1997e1999

12.0 12.2 7.7 5.9

e 86.4 e 39.8

e 11.2 e 7.6

Tian et al., 2003; GNIP data Tian et al., 2003 This work Tian et al., 2003 GNIP data

Note: (a) represents data from Tian et al. (2003) and (b) represents data from the Global Network of isotopes in Precipitation (GNIP) dataset.

If the precipitation form is considered, the amount-weighted averaged d18O, dD and d-excess for snow samples were 11.1&, 72.6& and 15.8&, respectively, while for rain samples they were 12.6&, 90.8& and 10.1&, respectively. Theoretically, for a single moisture source, the d18O should be much lighter for snow because of the lower condensation temperature, but the d-excess of snow is expected to be higher because of additional kinetic fractionation associated with the formation of ice from super-cooled vapour (Jouzel and Merlivat, 1984; Peng et al., 2007). In this study, much higher d-excess but slightly higher d18O values for snow samples were observed compared to rain samples. The slightly lower d18O for rain samples was probably because a large part of the rain in summer was derived from some moisture source with low d18O values (i.e. monsoonal moisture, as described below). 4.2. LMWL A LMWL calculated as a regression of d18O and dD in individual precipitation events at Madoi (Fig. 3) is:

dD ¼ 8:37d18 O þ 11:86



R2 ¼ 0:97; n ¼ 175



(1)

This equation is close to the global meteoric water line (GMWL):

dD ¼ 8.17d18O þ 11.27 (Rozanski et al., 1993). However, the LMWL based on monthly amount-weighted means shows a significant difference:

dD ¼ 8:80d18 O þ 20:16



 R2 ¼ 0:98; n ¼ 11

(2)

It is well known that small-amount precipitation samples, which are subject to some subcloud evaporation especially in dry conditions, would accordingly influence and produce a lower

100 Individual evetnts LMWL (individual) Monthly weighted means LMWL (monthly)

50

δD (‰)

0 -50 -100 -150 -200 -30

-25

-20

-15

-10

-5

0

5

10

δ18O (‰) Fig. 3. Plot of d18O versus dD in precipitation from May 2009 to April 2010 at Madoi. The circles represent individual samples and the rectangles represent the monthly amount-weighted means.

LMWL slope (Peng et al., 2007). According to Fig. 3, the discrepancy between Eqs. (1) and (2) was apparently caused by those samples falling off the regression trend. These samples likely suffered some degree of evaporation and the monthly amount-weighted means masked these outliers. Based on the isotopic data of monthly precipitation from the GNIP dataset, the LMWL for Lhasa was dD ¼ 8.08d18O þ 12.37 (R2 ¼ 0.98, n ¼ 42), and for Lanzhou it was dD ¼ 7.01d18Oþ1.53 (R2 ¼ 0.94, n ¼ 39). Both the slope and intercept in Eq. (2) were significantly higher than those for the GMWL and the two GNIP stations above. In general, a high LMWL slope can be caused by low temperature. According to a simple Rayleigh Fractionation model (Majoube, 1971; Araguás-Araguás et al., 2000), the mean annual temperature of 1.1  C during the observation period at Madoi would produce a theoretical dDed18O slope of 9.6 (calculation shown in the supplemental material). On the other hand, local moisture recycling could lead to a high slope and intercept (Gat and Matsui, 1991; Victoria et al., 1991; Tian et al., 2001b; Rhodes et al., 2006), and moisture recycling, which also can be reflected by high d-excess values as described above, could help to explain the higher slope and intercept for Madoi. Furthermore, the d18OedD correlation equation for snow samples was dD ¼ 8.39d18O þ 14.34 (R2 ¼ 0.96, n ¼ 68), while for rain samples it was dD ¼ 8.43d18O þ 11.02 (R2 ¼ 0.97, n ¼ 107). The slopes in these two equations were very close, but the intercept was higher in the former, consistent with the higher d-excess for snow samples.

4.3. Seasonal variations As shown in Fig. 2, the d18O and dD in precipitation exhibited high degree of fluctuations throughout the year, implying the alternating influences of different moisture sources. However, a pronounced seasonality was observed by using the monthly amount-weighted means (Fig. 4a). A distinct decrease of d18O was found from June to July (decreasing by about 10&), and this was greatly different from that in the typical westerlies domain (including the northern Tibetan Plateau and central Asian) where d18O values rise progressively with increasing temperatures (with a maximum in summer and minimum in winter; the so-called “temperature effect”) (Tian et al., 2001a; 2008; Pang et al., 2011). For the relationship between d18O and temperature at Madoi, the equation for individual samples was dD ¼ 0.36d18O  12.12 (n ¼ 175, r ¼ 0.31, p < 0.01), and for monthly means it was dD ¼ 0.57d18O  14.12 (n ¼ 11, r ¼ 0.67, p < 0.01). Although positive correlations can still be found for both individual and monthly data, the relationships were weak (low r values). This was probably the result of different moisture sources, which accordingly produce high fluctuations in d18O and then weaken the “temperature effect”. Previous studies have revealed that lower d18O values of precipitation during summer in the southern Tibetan Plateau are commonly indicative of monsoonal activity (Tian et al., 2001a; 2003; Kurita and Yamada, 2008; Yang et al., 2012). The distinct decrease of d18O in early summer at Madoi provides evidence that the precipitation there is significantly affected by the monsoon circulation during this observation period.

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8 4

-10 -15

0

-20

-4

-25

18

δ O T

-30

30

85

b

20

80 75

10

70

0

65

-8

-10

-12

-20

May Jun Jul Aug Sep Oct Nov Jan Feb Mar Apr

60

d-excess RH

Relative humidity(%)

δ18O (‰)

-5

12

d-excess (‰)

a

Temperature (°C)

0

59

55 50

May Jun Jul Aug Sep Oct Nov Jan Feb Mar Apr

Fig. 4. (a) Monthly temperature and weighted-averaged d18O values in precipitation; (b) Monthly relative humidity and weighted-averaged d-excess values in precipitation from May 2009 to April 2010 at Madoi.

4.4. Relationship between stable isotopic compositions and temperature and relative humidity ranges The d18O values co-varied with the d-excess values, with both having generally increased and then decreased with temperature (Fig. 5). According to the Rayleigh distillation model, precipitation d18O increases but d-excess decreases with condensation

temperature (Clark and Fritz, 1997; Pang et al., 2011; Bershaw et al., 2012). The snow samples at temperatures <10  C were characterized by the lowest d18O value, but the averaged d-excess was 9.3&, the lowest among samples <0  C. The winter and early spring samples suffering some degrees of sublimation as discussed above were indeed responsible for the low averaged d-excess value here. Another remarkable characteristic was that both the highest d18O and d-excess occurred for samples at 0e5  C, and this was probably associated with pre- and post-monsoon moisture recycling (Bershaw et al., 2012). This can be seen in Fig. 4a, which shows temperatures of 0e5  C occurred in late spring and early autumn. By contrast, when temperatures rose, d18O decreased distinctly from 7.2& for samples at 0e5  C to about 13& for samples at 5e 10  C, and d-excess decreased distinctly from 19.6& to 9.5& correspondingly, implying the effects of monsoonal moisture with relatively lower d18O and d-excess values in summer. Moreover, the moisture recycling may still be reflected by samples at temperatures >10  C, because the decreases of d18O and d-excess were compensated by recycled water with generally higher d18O and dexcess simultaneously. Further investigation considered the relationship between isotopic values with relative humidity. From Fig. 6, the d18O can be distinguished into three groups. For samples collected at relative humidity below 80%, the d18O values were inversely proportional to relative humidity and a reduction of subcloud evaporation/sublimation can be the reasonable cause. For samples within the relative humidity range of 80e85%, the d18O rose, indicating the decreasing tendency was changed by some other controlling factors such as recycled moisture with enriched heavy isotopes. Furthermore, the d18O varied slightly for samples at relative humidity >85% and this was related to overwhelming recycled moisture. In contrast, precipitation collected within the relative humidity range of <60% was

20

δ18O d-excess

-10

15

-15

10

-20

5

-25

d-excess (‰)

-5

δ18O (‰)

The d-excess parameter in precipitation is usually used to identify the moisture sources. In the Tibetan Plateau, the inland recycled water is characterized by high d-excess values compared to the monsoon precipitation derived from warm and humid ocean surfaces (Tian et al., 2001a; Kurita and Yamada, 2008; Bershaw et al., 2012). Fig. 4b shows a clear seasonal pattern for monthly averaged d-excess. The d-excess values were high from May to June (mean 18.5&) and from October to November (mean 14.5&), suggesting recycled water was an important component of the regional water budget during these two periods. The moisture recycling was associated with the meteorological conditions. From Fig. 4a and b, both the d18O and d-excess values from May to June were higher than those from October to November. Considering the higher temperature but lower relative humidity in the period from May to June, these results may suggest that the moisture recycling from May to June was more intense. This was also reflected by a positive correlation between d18O and temperature from May to June (d18O ¼ 0.89T  8.33, n ¼ 46, r ¼ 0.64, p < 0.01), but no apparent relationship was observed from October to November (d18O ¼ 0.02T  16.84, n ¼ 22, r ¼ 0.02, p > 0.05). By contrast, during the three months from July to September, the d-excess dropped to about 9.8&, also consisted with the impact of monsoon precipitation in the Tibetan Plateau. Furthermore, this value of 9.8& was higher than that of summer precipitation in the southern Tibetan plateau (as low as 4e5&, Kang et al., 2002; Tian et al., 2007), and this can be attributed to the evaporated flux along the monsoon trajectory and the mixing of the inland recycled water over Madoi. On average, the three months from January to March showed a mean temperature of 9.5  C and a lower relative humidity of 73% compared to other months (Fig. 4b). The precipitation was very low and there was a total of only eight snow samples (4.4 mm) collected during this time. As in other regions in/or near the Tibetan Plateau in cold months, the averaged d18O at Madoi was extremely low (mean 22.0&). However, these samples showed the lowest dexcess values, with an averaged value of 2.2&. Very low d-excess values in the small amount of winter snowfall were also observed at Delingha and Lhasa in the northern and southern Tibetan Plateau (Tian et al., 2001c). Although there is no firm explanation up to now, this may arise from some local physical processes. It is more likely that sublimation, which decreases d-excess values (Stichler et al., 2001; Schmidt et al., 2005), could be the cause, since the relative humidity was significantly lower during these precipitation events (Fig. 4b).

0 <-10

-10~-5

(6)

(11)

-5~0

0~5

5~10

>10

(25)

(37)

(76)

(20)

Temperature (°C) Fig. 5. Weighted average d18O and d-excess values versus temperature for precipitation collected from May 2009 to April 2010 at Madoi. Number of samples is shown in parentheses.

W. Ren et al. / Quaternary International 313-314 (2013) 56e61

20

δ18O d-excess

-4

15

δ18O (‰)

-6 -8

10

-10 5

d-excess (‰)

60

-12 0

Partnership Program for Creative Research Teams of Chinese Academy of Sciences (KZCXZ-YW-T11).We are very grateful to the members of the Madoi meteorological station whose hard work made this study possible, to Dr. Shenghai Li and Dongmei Qu for processing and measuring the samples. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quaint.2013.05.026.

-14

<60 60-65 65-70 70-75 75-80 80-85 85-90 >90 (10)

(11)

(13)

(30)

(43)

(35)

(27)

(5)

Relative humidity (%) Fig. 6. Weighted average d18O and d-excess values versus relative humidity for precipitation collected from May 2009 to April 2010 at Madoi. Number of samples is shown in parentheses.

characterized by the lowest d-excess values (mean < 1&), whereas the precipitation within the humidity range of >90% exhibited the highest d-excess values (mean > 19%). Two effects could be the causes: (1) for samples at relative humidity <70%, the d-excess increased remarkably with a reduction of subcloud evaporation/ sublimation; (2) moisture recycling led to higher d-excess values especially at relative humidity >85%. For the medial parts at relative humidity of 70e85%, which were consistent with the majority of precipitation events from July to August (the most typical monsoon period), the monsoon precipitation, with relatively lower d18O and d-excess values, could be responsible for the transitions in d18O and d-excess. 5. Conclusion This study examined variations in stable isotopic compositions in precipitation at Madoi, in the eastern Tibetan Plateau. The annual averaged d18O in precipitation showed a transition between the south and north of the Tibetan Plateau, and d-excess values there showed signal of inland recycling by comparing with some nearby stations. Based on individual or monthly averaged data, the LMWL was defined as dD ¼ 8.37d18O þ 11.86 or dD ¼ 8.80d18O þ 20.16. The fluctuations of individual isotopic compositions were high throughout the year, but clear seasonal variations of monthly weighted-averaged d18O and d-excess reflected changes in moisture sources: monsoonal moisture can be readily observed through the distinct decrease of d18O values in early summer, and high d-excess values indicated recycled water is a potentially significant component of precipitation in pre- and post-monsoon periods. Although the results reveal that monsoon circulation influences precipitation at Madoi in summer, the higher d-excess in the monsoon period (JulyeSeptember) indicate its impact is much weaker than in the southern Tibetan Plateau. Finally, the variations of d18O and dexcess in precipitation are the result of the incorporation of moisture sources and subcloud evaporation/sublimation, which can be inferred from their relationships with different temperature and relative humidity ranges. Therefore, combining d18O and d-excess values in precipitation provides an effective means for tracing the affecting processes, and future work is necessary to determine the detailed effects of different moisture sources on stable isotopic compositions in precipitation at the synoptic scale in this region. Acknowledgements This study is supported by the National Nature Science Foundation of China (Grant 41190081, 41101021) and the International

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