Carbon isotope composition of long chain leaf wax n ...

Organic Geochemistry 83-84 (2015) 190–201

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Carbon isotope composition of long chain leaf wax n-alkanes in lake sediments: A dual indicator of paleoenvironment in the Qinghai-Tibet Plateau Weiguo Liu a,b,⇑, Hong Yang c,⇑, Huanye Wang a,d, Zhisheng An a, Zheng Wang a, Qin Leng c a

State Key Laboratory of Loess and Quaternary Geology, IEE, CAS, Xi’an 710075, China School of Human Settlement and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, China Laboratory for Terrestrial Environments, College of Arts and Sciences, Bryant University, Smithfield, RI 02917, USA d University of Chinese Academy of Sciences, Beijing 100049, China b c

a r t i c l e

i n f o

Article history: Received 12 December 2014 Received in revised form 10 March 2015 Accepted 12 March 2015 Available online 3 April 2015 Keywords: Long-chain n-alkanes d13C values Aquatic plants Terrestrial plants Lake Qinghai

a b s t r a c t The carbon isotope composition (d13C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C3 and C4 photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting d13C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C27 and C29) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. 26.7‰ to 15.7‰ for C29) similar to that of terrestrial C4 plants. Thus, the use of d13C values of sedimentary C27 and C29 n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C4 plants. For sedimentary C31, however, the contribution from submerged plants was minor, so that the d13C values for C31 n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the d13C values of sedimentary C27 and C29 n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C4 vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the d13C values of sedimentary long chain n-alkanes (C27, C29 and C31) may carry different environmental signals. While the d13C values of C31 were a reliable proxy for C4/C3 terrestrial vegetation composition, the d13C values of C27 and C29 n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Long chain leaf wax n-alkanes (C27–C33) with a strong odd/even predominance are a major component of the cuticle surface of higher plants (Eglinton and Hamilton, 1967). They can be easily transported from a terrigenous environment to marine and lake sediments, along with plant remains, via riverine or wind erosion (Rieley et al., 1991; Schefuß et al., 2003a; Smith and Freeman,

⇑ Corresponding authors at: State Key Laboratory of Loess and Quaternary Geology, IEE, CAS, Xi’an 710075, China (W. Liu), Laboratory for Terrestrial Environments, College of Arts and Sciences, Bryant University, Smithfield, RI 02917, USA (H. Yang). E-mail addresses: [email protected] (W. Liu), [email protected] (H. Yang). http://dx.doi.org/10.1016/j.orggeochem.2015.03.017 0146-6380/Ó 2015 Elsevier Ltd. All rights reserved.

2006; Castañeda et al., 2009; Feakins and Sessions, 2010; Sinninghe Damsté et al., 2011; Garcin et al., 2012). In lakes and oceans, the autochthonous organic matter produced mainly by algae and cyanobacteria was considered to have contributed only a low concentration of n-alkanes dominated by short and mid chain homologues to the n-alkane pool (Cranwell et al., 1987; Meyers, 2003). Hence, long chain n-alkanes (C27–C31) in the sediments are considered to have originated primarily from terrigenous higher plants (Rieley et al., 1991; Pagani et al., 2006; Castañeda and Schouten, 2011; Sun et al., 2013). Due to their relative resistance to degradation, long chain n-alkanes are generally well preserved in lake sediments and can inherit carbon isotope signals representative of the carbon fixation pathway of their source plants (Castañeda and Schouten, 2011). Therefore, the

W. Liu et al. / Organic Geochemistry 83-84 (2015) 190–201

d13C values of long chain n-alkanes from ancient sediments have been routinely used for reconstruction of the relative abundance of terrestrial C4 vegetation, as well as of the environment/climate at a given geological time (e.g. Street-Perrott et al., 1997; Huang et al., 2001; Schefuß et al., 2003b; Lane et al., 2011; Sinninghe Damsté et al., 2011; Chu et al., 2014). Some recent studies have, however, shown that emergent aquatic plants can also produce long chain n-alkanes (Ficken et al., 2000; Aichner et al., 2010a), while submerged macrophytes produce n-alkanes dominated by mid-chain (C23 and C25) homologues (Ficken et al., 2000; Mead et al., 2005; Aichner et al., 2010a). Interestingly, submerged macrophytes are especially abundant in certain lacustrine environments (Ficken et al., 2000; Aichner et al., 2010a; Liu et al., 2013). In these cases, they may contribute a significant amount of organic carbon (OC) as well as mid-chain n-alkanes to the lake sediments (Ficken et al., 2000; Aichner et al., 2010a,b; Liu et al., 2013). The assessments based on a simple binary isotopic model revealed an average contribution of up to 60% (mean 40%) to total OC (TOC) and up to 100% (mean 66%) to mid-chain n-alkanes from submerged macrophytes in lakes on the Qinghai-Tibetan Plateau (QTP; Aichner et al., 2010a). Little attention has, however, been paid to the question of whether or not and to which extent submerged macrophytes may contribute to the sedimentary long chain n-alkane pool in these lakes. Without a clear understanding of the origin of long chain n-alkanes in lake sediments, the reconstruction of terrigenous vegetation and climate change based on the d13C values of sedimentary long chain leaf wax n-alkanes might not be completely reliable. Lake Qinghai is the largest lake on the QTP, with a water depth of ca. 30 m. Numerous paleoclimate records from its sediments have been published during the past decade (e.g. Shen et al., 2005; Liu et al., 2013; Thomas et al., 2014; Wang et al., 2014). Therefore, the lake is an ideal site for addressing this issue via both modern process investigations and ancient record investigations. In this study, we systematically investigated the distribution and d13C values of long chain n-alkanes derived from terrigenous sources (including surrounding plants and nearby surface soils), aquatic macrophytes at various water depths and surface lake sediments from nearshore and offshore settings in the lake and small lakes nearby. In addition, we analyzed long chain n-alkane d13C values in an ancient sediment core retrieved from the deposition center of the lake, spanning the past 12 ka. Our objective was to investigate the possible contribution of aquatic plants to long chain n-alkanes in the lake in order to provide further insight into the mechanism controlling the d13C values of long chain n-alkanes in the sediments. Our results should be useful for the proper application of sedimentary n-alkane d13C values as an indicator of changes in terrigenous plant composition (such as C4/C3 vegetation) and in other paleoclimate reconstruction studies using compound specific carbon isotope values of leaf wax components.

191

coordinates, water depth (if from the lakes), concentration, distribution and d13C values of n-alkanes for the samples are listed in Table 1. The sedimentary core (Core 1F) was retrieved from the deposition center of the southwestern sub-basin of the lake in 2005 using the ICDP GLAD800 drilling system (An et al., 2012). The upper 5.15 m, which contained sediments spanning ca. 12 ka according to the age model of An et al. (2012), were analyzed. In the laboratory, sediment, soil and macrophyte samples were freeze-dried before extraction. Macrophytes were extracted via sonication with dichloromethane (DCM; 4 15 min), whereas soils and sediments were finely ground and extracted ultrasonically using DCM/MeOH (9:1, 4 15 min). The extracts were dried under N2 in a water bath. The hydrocarbon fractions containing the n-alkanes were obtained using silica column chromatography via elution with hexane. Quantification of n-alkanes was determined for all samples with an Agilent 6890 gas chromatography (GC) instrument (Agilent HP1-ms column: 60 m 0.32 mm i.d., 0.25 lm film thickness) with flame ionization detection. Injection was performed in split mode with a GC inlet temperature of 310 °C. The flow rate of carrier gas was 1.2 ml/min. The temperature program was: 40 °C (1 min) to 150 °C at 10 °C/min and then to 315 °C (held 20 min) at 6 °C/min. Peak areas from individual n-alkanes were compared with those of an external standard (n-C21, n-C25, n-C27, n-C29 and n-C31 alkanes) with known amounts of individual n-alkanes to calculate the concentration of n-alkanes for each sample. Compound specific stable carbon isotope analysis was performed with a TRACE GC instrument coupled via a combustion reactor to a Thermo Finnigan DeltaV Plus isotope ratio mass spectrometer at the Institute of Earth Environment, Chinese Academy of Sciences. Each sample was pre-concentrated to an optimum volume to ensure an injection volume of 1 ll, generating a 4–6 V signal for C29. The temperature program was as for GC analysis. Isotope values were measured against calibrated CO2 reference gas and are reported in per mil (‰) vs. Vienna Peedee Belemnite (VPDB). The precision (0.3‰) of the system was evaluated via routine measurement using the standard mixture of n-alkanes after every four injections. The reproducibility for duplicate analysis of selected samples was < 0.4‰ for C29 (Table 1). Calculation of Paq was based on the relative proportion of midchain (C23, C25) to long chain (C29, C31) n-alkane homologues (Ficken et al., 2000):

Paq ¼ ðC23 þ C25 Þ=ðC23 þ C25 þ C29 þ C31 Þ:

ð1Þ

It was formulated to reflect the non-emergent aquatic macrophyte input to lake sediments relative to that from the emergent aquatic and terrigenous plants.

3. Results 3.1. Ecological information around and in the lakes

2. Material and methods Terrigenous plant and aquatic macrophyte, soil and sediment samples were collected from Lake Qinghai, its satellite lakes and their catchment areas (Fig. 1 and Table 1). Surface sediments from various locations and water depths were collected using a grab sampler, with aquatic plants carefully removed. The aquatic plants were washed out directly on board with lake water. The samples were transported to the laboratory on ice. Surface soils were collected from the uppermost layer (0–2 cm) from Mt. Laji to the south of the lake. For each soil sample, three randomly collected specimens were mixed to make one composite sample representing that location. Terrigenous plant samples were placed in paper bags and air dried during the days following the sampling. The

Located at nearly 3200 m above sea level on the northeastern QTP, the closed basin Lake Qinghai is the largest saline lake in China. Under a cold highland climate regime, the terrestrial flora of its catchment is characterized by alpine meadows and steppes (Duan and Xu, 2012). Detailed information regarding terrestrial vegetation of the Lake Qinghai region was obtained from our field botanical survey and sampling via the quadrant method (2 m 2 m) at two typical sites within the study area – one in an alpine meadow and the other in a steppe (Table 2). Bulk isotope analysis of these terrigenous plants indicated that Cleistogenes squarrosa from Site 2 (steppe) was the only C4 plant species, accounting for < 7% of the total biomass for the site. No C4 plant was found at Site 1 (alpine meadow). This result is consistent with

192


Fig. 1. Map showing sampling sites within Lake Qinghai catchment. Blue lines indicate rivers and arrows represent mountains. Surface sediments and aquatic macrophytes were collected using a grab sampler. Core 1F was retrieved from the depo-center of the southwestern sub-basin of the lake in 2005 using the ICDP GLAD800 drilling system (An et al., 2012). The sampling sites for soils are shown in Table 1 (for interpretation of the references to colour, the reader is referred to the web version of this article).

published data (Duan and Xu, 2012), indicating a C3 dominant modern terrestrial vegetation in the Lake Qinghai catchment. Aquatic vegetation was surveyed at various water depths in Lake Qinghai and Lake Gahai by using an unmanned submerging video imaging system. Extensive submerged macrophytes were discovered to cover the lake bottom (Fig. 2). Two genera of flowering plants, Potamogeton L. and Ruppia L., dominate the shallow water (< 9 m), whereas the green alga Cladophora Kützing was found to cover the sediment surface in offshore settings extensively (Fig. 2b). 3.2. Concentration of long chain n-alkanes from different types of macrophytes In general, the possible sources for n-alkanes in the sediments of Lake Qinghai and its satellite lakes include C3 plant-dominated terrigenous vegetation, the green alga Cladophora and submerged plants. The concentration of long chain n-alkanes varied substantially for different types of source macrophytes (Table 1 and Fig. 3). Terrigenous plants had a high concentration of long chain n-alkanes (avg. 47 ± 44, 173 ± 137 and 134 ± 99 lg/g for C27, C29 and C31, respectively), so they could potentially contribute significantly to the pool of sedimentary long chain n-alkanes. Notably, C27 and C29 were also abundantly produced by submerged plants (Fig. 3; n-C27, 44 ± 39 lg/g; n-C29, 14 ± 12 lg/g; n-C31, 1 ± 0.8 lg/ g), suggesting that, in addition to a terrigenous source, some long chain n-alkanes in the sediments might have also originated from these aquatic plants, particularly in shallow lake water where these plants are abundant. For Cladophora, however, only a small amount of long chain n-alkanes was found (avg. 0.6 ± 0.6,

0.5 ± 0.4 and 0.3 ± 0.1 lg/g for C27, C29 and C31, respectively). The concentrations of Cladophora C27 and C29 are over an order of magnitude lower than that of submerged plants. In fact, many Cladophora samples had an extremely low concentration of n-alkanes, too low to be detected, so these samples are not reported. 3.3. Distributions of long chain n-alkanes in samples from within the modern Lake Qinghai region The relative abundances of long chain (C27, C29 and C31) n-alkanes also exhibited distinct differences between different sources within the Lake Qinghai catchment (Fig. 4a–c). As chain length decreased, the relative abundance of individual n-alkanes generally decreased for the terrigenous source but increased for aquatic plants (Fig. 4a–c). This is in agreement with previous investigations of aquatic macrophytes (Ficken et al., 2000; Mead et al., 2005; Aichner et al., 2010a). For example, the proportion (%) of C31 (relative to the sum of C23–C31) from terrigenous sources ranged from 18–70%, significantly higher than the C27 values (2–37%) and those for aquatic plants (Cladophora, 6–20%; submerged plants, < 6%). However, for submerged plants we noticed that the average C27 and C29 values were 18% and 7%, respectively, much higher than that of C31 (avg. 1%, Table 3). In addition, the biomarker ratio for evaluating submerged plant input to lake sediments, the Paq index (Ficken et al., 2000; Aichner et al., 2012), based on the relative abundances of n-alkanes, also distinguished the different sources (Fig. 4 and Table 3). The average value was 0.90 for submerged plants, higher than that for Cladophora (0.49) and much higher than the values for terrigenous plants and soils (0.10). This is because n-alkanes produced by

193


Table 1 Sample locations, distribution of n-alkanes, their concentrations and d13C values of n-C27, n-C29 and n-C31 for different types of modern samples within Lake Qinghai catchment (errors for d13C values are for duplicate analyses). n-Alkane d13C (‰)

n-Alkane concentration (ng/g)

Sample #

Longitude

Latitude

Water depth (m)

Sample type

Paq

C72%

C29%

C31%

C27

C29

C31

C27

C29

C31

QHP13-1-1 QHP13-3-1 QHP13-6-1 QHP13-7-1 GHP-13H QHP05-24H QHP05-11H

36°560 3200 36°550 5300 36°530 0400 36°540 0700

99°410 5600 99°400 1500 99°400 4400 99°420 3100

0.76 0.49 0.68 0.42 0.32 0.43 0.34

0.22 0.15 0.21 0.18 0.15 0.16 0.16

0.15 0.18 0.13 0.23 0.24 0.21 0.32

0.07 0.19 0.06 0.21 0.18 0.19 0.17

33.8 ± 0.2 28.0 33.8 33.0 31.7 29.3 28.5

26.2 ± 0.1 31.6 26.9 34.6 32.2 32.4 32.8

29.3 ± 0.4 31.8 30.9 34.1 32.0 33.7 33.2

1035 198 492 206

492 212 238 185

99°370 1900 100°340 4700

Cladophora Cladophora Cladophora Cladophora Cladophora Cladophora Cladophora

1539 166 772 163

36°540 4900 37°020 2400

5.5 6.4 6 9.9 4.9 0.5 0.5

QHP05-13 GHP12-5 GHP12-6 28D-p QHP13-2-1 QHP13-4-1 QHP13-5-2 QHP13-5-3 QHP13-6-2 QHP13-12-2 GHP13-1-1 GHP13-2-1 GHP13-3-1 GHP13-4-1 GHP13-8-2 GHP13-9-1

37°060 5100 37°010 700 37°010 1400 36°350 0600 36°570 1200 36°560 1600 36°540 1600 36°540 1600 36°530 0400 36°570 4100 37°000 2700 37°000 3700 37°000 5000 37°010 0700 37°020 0900 37°010 2100

100°220 0700 100°350 1200 100°350 2400 100°300 3000 99°410 3600 99°390 0800 99°380 3300 99°380 3300 99°400 4400 99°490 5300 100°340 2800 100°340 4900 100°350 0800 100°350 2700 100°330 3200 100°350 3300

0.5 8.5 6.0 14.0 3.2 3 3.5 3.5 6 0.5 8.5 8.5 8.5 6.5 7.5 1.5

Submerged Submerged Submerged Submerged Submerged Submerged Submerged Submerged Submerged Submerged Submerged Submerged Submerged Submerged Submerged Submerged

plant plant plant plant plant plant plant plant plant plant plant plant plant plant plant plant

0.77 0.94 0.96 0.71 0.92 0.91 0.91 0.91 0.89 0.91 0.88 0.94 0.93 0.89 0.90 0.95

0.35 0.14 0.13 0.22 0.18 0.11 0.10 0.15 0.21 0.21 0.22 0.17 0.16 0.23 0.22 0.12

0.08 0.04 0.04 0.17 0.06 0.06 0.06 0.09 0.07 0.05 0.08 0.05 0.05 0.07 0.07 0.04

0.06 0.00 0.00 0.04 0.00 0.02 0.02 0.04 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00

15.1 25.6 24.1 25.4 26.0 21.8 20.6 ± 0.1 20.5 24.2 17.8 23.7 24.2 23.8 ± 0.0 22.0 24.9 ± 0.0 21.6

15.7 25.2 23.9 26.7 26.2 22.8 21.3 ± 0.1 21.0 25.1 19.2 24.4 24.8 24.2 ± 0.0 22.5 25.2 ± 0.2 22.9

14.3

45360 2331 7425 1800 30105 29885 30150 42750 56550 145800 69000 62400

14340 1362 4125 1125 10485 7454 10950 12450 18750 45900 21900 19300

1200 438 1650 533 1020 1292 0 0 1200 2400 0 2400

QHS13-1S QHS13-2S QHS13-3S QHS13-4S QHS13-5S QHS13-6S QHS13-7S QHS13-8S QHS13-9S QHS13-10S QHS13-12-1S GHS13-0S GHS13-1S GHS13-2S GHS13-3S GHS13-4S GHS13-5S GHS13-6S GHS13-7S GHS13-8S GHS13-10S GHS13-11S

36°560 3200 36°570 1200 36°550 5400 36°560 1600 36°540 1600 36°530 0400 36°540 0700 36°530 2900 36°530 0900 36°560 2400 36°570 4100 37°010 1900 37°000 2700 37°000 3700 37°000 5000 37°010 0700 37°030 1000 37°020 5500 37°020 3800 37°020 0900 37°010 2100 36°330 5400

99°410 5600 99°410 3600 99°400 1600 99°390 0800 99°380 3300 99°400 4400 99°420 3100 99°450 1600 99°590 5300 99°550 0100 99°490 5300 100°350 3700 100°340 2800 100°340 4900 100°350 0800 100°350 2700 100°330 0400 100°330 0400 100°330 1000 100°330 3200 100°350 3300 100°440 0900

5.5 3.2 6.4 3 3.5 6 9.9 12.5 24.2 12.2 0.5 0.5 8.5 8.5 8.5 6.5 0.5 2.5 4.5 7.5 1.5 0.5

Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface

sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment

0.28 0.33 0.26 0.31 0.34 0.23 0.22 0.19 0.19 0.23 0.26 0.38 0.15 0.22 0.19 0.20 0.27 0.23 0.20 0.19 0.33 0.43

0.15 0.14 0.14 0.15 0.14 0.14 0.15 0.13 0.13 0.14 0.15 0.13 0.14 0.16 0.16 0.16 0.18 0.16 0.14 0.15 0.16 0.16

0.25 0.23 0.26 0.23 0.23 0.29 0.29 0.29 0.29 0.28 0.30 0.22 0.31 0.30 0.31 0.28 0.26 0.27 0.28 0.30 0.26 0.23

0.28 0.28 0.29 0.28 0.28 0.30 0.29 0.33 0.33 0.29 0.26 0.29 0.34 0.29 0.30 0.30 0.27 0.27 0.31 0.31 0.23 0.18

31.1 ± 0.6 29.7 ± 0.2 32.6 ± 0.3 29.5 ± 0.3 28.5 ± 0.2 31.0 ± 0.2 34.0 ± 1.1 33.4 32.0 ± 0.5 32.9 31.9 27 ± 0 30.9 ± 0.0 29 ± 0.2 29.8 ± 0.1 29.5 ± 0.0 30.3 ± 0.1 28.9 ± 0.2 29.8 ± 0.2 29.5 29.5 ± 0.5 27.1 ± 0.3

32.0 ± 0.2 31.5 ± 0.1 32.9 ± 0.3 32.0 ± 0.4 31.5 ± 0 33.0 ± 0.1 33.1 ± 0.3 32.7 32.6 ± 0.3 32.7 32.2 30.9 ± 0.2 32.8 ± 0.2 32.3 ± 0.1 32.8 ± 0.0 32.3 ± 0.3 32.5 ± 0.1 32.4 ± 0.1 31.6 ± 0.2 32.1 31.3 ± 0.3 29.6 ± 0.2

32.0 ± 0.1 31.8 ± 0.2 32.8 ± 0.3 32.6 ± 0.4 32.1 ± 0.1 32.9 ± 0.1 32.7 ± 0.3 32.5 32.5 ± 0.2 32.7 33.3 31.1 ± 0.1 33.0 ± 0.2 33.0 ± 0.1 33.1 ± 0.1 31.9 ± 0.2 32.4 ± 0.0 31.4 ± 0.2 31.3 ± 0.0 32.7 33.4 ± 0.5 32.8 ± 0.1

690 315 410 240 570 645 420 345 885 338 420 810 893 1550 1573 900 855 578 503 915 115 105

1160 510 760 370 930 1305 825 780 1920 675 840 1373 2033 2860 3127 1650 1230 960 1005 1808 185 150

1320 615 850 450 1125 1335 840 885 2220 690 735 1748 2250 2740 3064 1763 1275 960 1095 1853 164 113

EHP12-5(3)H EHP12-7(1)H EHP12-7(2)H EHP12-8(1)H EHP12-8(2)H GCP12-5(1)H GCP12-5(2)H GCP12-6-1(1)H GCP12-6-1(2)H GCP12-6-2(1)H LTEM12-10 LTEM12–11 LTEM12-12 LTEM12-13 LTEM12-14 LTEM12-15 LTEM12-16

36°320 4600 36°320 4300 36°320 4300 36°320 4300 36°320 4300 37°110 5500 37°110 5500 37°120 1500 37°120 1500 37°120 1500 36°160 3300 36°140 4700 36°120 2300 36°100 2500 36°220 0600 36°220 4400 36°230 4000

100°430 14’’ 100°430 1600 100°430 1600 100°430 1600 100°430 1600 100°060 4600 100°060 4600 100°060 5100 100°060 5100 100°060 5100 101°320 5600 101°330 2900 101°330 3700 101°350 0800 101°280 4600 101°300 3600 101°330 5200

0.05 0.06 0.02 0.11 0.01 0.05 0.06 0.11 0.06 0.06 0.13 0.11 0.22 0.05 0.17 0.16 0.21

0.13 0.09 0.08 0.14 0.09 0.12 0.02 0.37 0.02 0.12 0.16 0.15 0.36 0.08 0.12 0.16 0.21

0.61 0.24 0.45 0.29 0.62 0.47 0.21 0.33 0.23 0.45 0.26 0.41 0.31 0.47 0.24 0.27 0.26

0.18 0.56 0.42 0.45 0.23 0.33 0.70 0.20 0.68 0.35 0.40 0.30 0.21 0.35 0.40 0.36 0.31

33.2 31.1 30.2 35.1 32.0

35.7 35.3 34.5 33.9 33.5 33.4 33.3 32.9 31.6 30.9 34.1 34.8 34.1 29.5 33.5 33.5 34.9

34.9 31.2 35.0 35.1 31.2 33.8 33.8 31.3 33.9 33.4 32.7 34.0 34.0 29.2 33.0 34.0 35.7

90970 12723 46830 81823 49521 21537 3820 139735 13005 10589 146 109 295 71 156 400 256

433622 35179 257311 169460 351115 85431 51710 124427 183988 38937 269 343 343 468 353 808 383

131765 80439 134873 131513 129902 60883 172040 77430 389318 31002 418 249 237 346 594 1068 461

Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous Terrigenous

plant plant plant plant plant plant plant plant plant plant soil soil soil soil soil soil soil

submerged plants are dominated by mid-chain (C23 and C25) n-alkanes (Ficken et al., 2000; Mead et al., 2005; Aichner et al., 2010a) while the n-alkanes of terrigenous plants are dominated by long chain (C29 and C31) homologues. Therefore, the Paq index

34.4 32.9 33.6 34.4 33.1 33.2 32.3 34.2

26.6 25.4 21.5 ± 0.0 22.5 21.6

could also aid in evaluating the contribution from different n-alkane sources for lake sediments in the region. As shown in Table 3 and Fig. 4, the C29 and C31 values for surface lake sediments (avg. 27% and 29%, respectively) were lower than

194


Table 2 Terrestrial plant samples from two typical sites in Lake Qinghai catchment and their biomass contribution, bulk carbon isotope composition and inferred photosynthetic pathway (NB Cleistogenes squarrosa is the only C4 plant). Taxon

Dry weight (g)

d13C VPDB (‰)

Photosynthetic pathway

Site 1 Alpine meadow (36°310 25.300 , 100°380 30.000 )

Adenophora stenanthina Anaphalis lactea Astragalus membranaceus Delphinium pylzowii Gentiana straminea Kobresia capillifolia Kobresia humilis Koeleria litvinovii Oxytropis ochrocephala Polygonum viviparum Potentilla longifolia Roegneria nutans Saussurea pulchra Stellera chamaejasme Stipa breviflora Stipa regeliana