Last glacial–Holocene geochronology of sediment cores from a high-altitude Tibetan lake based on AMS 14C dating of plant fossils: Implications for paleoenvironmental reconstructions

Chemical Geology 277 (2010) 21–29

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Last glacial–Holocene geochronology of sediment cores from a high-altitude Tibetan lake based on AMS 14C dating of plant fossils: Implications for paleoenvironmental reconstructions Takahiro Watanabe a,⁎, Tetsuya Matsunaka b, Toshio Nakamura c, Mitsugu Nishimura b, Yasuhiro Izutsu b, Motoyasu Minami d, Fumiko Watanabe Nara a, Takeshi Kakegawa a, Junbo Wang e, Liping Zhu e a

Graduate School of Science, Tohoku University, 6-3 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8578, Japan School of Marine Science and Technology, Tokai University, 3-20-1 Orido, Shimizu, Shizuoka 424-0902, Japan c Center for Chronological Research, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8601, Japan d Department of Environmental Biology, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501, Japan e Institute of Tibetan Plateau, Chinese Academy of Science, No. 18 Shuangqing Road, Haidian District, Beijing 100085, China b

a r t i c l e

i n f o

Article history: Received 30 October 2009 Received in revised form 22 June 2010 Accepted 7 July 2010 Editor: B. Bourdon Keywords: Tibetan plateau Lake Pumoyum Co Radiocarbon dating δ13C Reservoir effect Old carbon effect

a b s t r a c t We obtained three sediment cores from a high-altitude lake (Lake Pumoyum Co; altitude, ~ 5020 m asl) on the southern Tibetan plateau for reconstruction of environmental changes during the last glacial–Holocene transition. In this study, we established the first reliable chronology for sediment cores from Lake Pumoyum Co, ca. 18.5 cal ka BP at the bottom, by 14C analyses of terrestrial plant residue concentrates (PRC, N 125 μm) and aquatic plant residues. The calibrated ages of the PRC fraction in the surface sediment were nearly modern (0.1 ± 0.1 cal ka BP), and the δ13C values (− 22‰ to − 24‰) were agreed well with those of modern terrestrial C3 plants. In addition, we estimated 14C reservoir ages of macrophyte remains from changes in their δ13C values. The major climate boundary layers in the cores (transitions to Bølling–Allerød, 14.5 ± 0.5 cal ka BP; Younger Dryas, 12.8 ± 0.1 cal ka BP; and Preboreal, ~ 11.6 cal ka BP) were confirmed by our new 14C chronology. The transition to the Bølling–Allerød warm phase from the last glacial (14.5 ± 0.5 cal ka BP) coincided with an obvious lithologic boundary (a rapid decrease in the abundance of the macrophyte remains) in the Lake Pumoyum Co sediment cores. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The Tibetan plateau (area, ~2.5×106 km2; altitude, ~4500 m on average) plays an important role in global climatic and environmental changes, especially Quaternary monsoon circulation, because of its topographic features (Ruddiman and Kutzbach, 1991; Kutzbach and Behling, 2004). Summer monsoon activity is under the control of a strong latitudinal temperature gradient between land and sea, reflecting insolation and environmental conditions in the area (Prell and Kutzbach, 1992). Therefore, paleoclimatic and environmental records from the Tibetan plateau provide important clues for understanding the Asian climate system (Fang et al., 2003; Owen et al., 2006; Zhang et al., 2006; Kaiser et al., 2007). Previous studies have documented environmental changes in the central, northern, and western Tibetan plateau (Lister et al., 1991; Fontes et al., 1993; Hui et al., 1996; Shen et al., 2005). However, only a few studies have investigated continuous climate records in lake ⁎ Corresponding author. Present address: Department of Geology, Liège University, Allée du 6 Août, B-4000 Liège, Belgium. Tel.: + 81 22 795 5903; fax: + 81 22 795 6675. E-mail address: [email protected] (T. Watanabe). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.07.004

sediment and ice cores over the last glacial period (~ 20 cal ka BP) on the Tibetan plateau (Wang et al., 2002; Thompson et al., 2006). In particular, the detailed environmental changes during the late Quaternary are still largely unknown in the southern plateau because of its very high altitude (~5000 m asl) and severe environmental conditions. Lake sediment cores from the southern Tibetan plateau provide novel and important clues that can reveal variations in both the past environment and biological activities. Radiocarbon (14C) analysis based on accelerator mass spectrometry (AMS) is a widely used and accepted dating method for the last ~ 50,000 years in sediment cores (Nakamura et al., 2003). For accurate chronology of sediment cores, 14C measurements of terrestrial plant residues, because direct fixation of atmospheric CO2 occurs during the lifetime of the plants, are preferable (Abbott and Stafford, 1996; Moreton et al., 2004; Morrill et al., 2006; Watanabe et al., 2007, 2009a). However, terrestrial plant residues are extremely rare in most Tibetan lake sediments because of the lack of vegetation cover in the area (Wang et al., 2002; Ji et al., 2005; Wu et al., 2006; Watanabe et al., 2008). Therefore, it has been necessary to estimate variations in the freshwater 14C reservoir effect (14C concentration of dissolved inorganic carbon) and the old carbon effect (caused by terrestrial

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organic material containing dead carbon derived from lake terrace or loess deposits) to date lake sediment layers. At present, annual precipitation in the southern Tibetan plateau is mainly derived from the Indian summer monsoon (Hren et al., 2009). The southern Tibetan plateau is highly sensitive to monsoon variability, because it is near the limit of monsoonal influence (Weiÿ and Gasseÿÿ, 1999; Shi, 2002; Zhang et al., 2006). Lake Pumoyum Co, the largest lake in the high-altitude region of the southern Tibetan plateau (N5000 m asl), has a sedimentary sequence at least 40 m thick (Iwashita et al., 2003). The first continuous sediment core (PY104PC; core length, 3.80 m) for reconstruction of climatic changes in the area was taken from the eastern part of Lake Pumoyum Co in April 2001. The second core was taken from the lake in September 2004 (PY409PC: core length. 3.88 m). Watanabe et al. (2008) has reported preliminary 14C dating results of plant residues in the PY104PC and PY409PC from Lake Pumoyum Co, and Wang et al. (2009) have discussed the environmental changes recorded in Pumoyum Co on the basis of the rough chronology of Watanabe et al. (2008). However, this preliminary 14C chronology insufficiently accounts for possible changes in the plant residue sources and any lake reservoir effect. Therefore, boundaries of major climate intervals, such as the Bølling– Allerød warm phase, have not yet been determined. In this study, fine (N125 μm) plant remains from the Holocene sediment layers of three sediment cores from Lake Pumoyum Co were concentrated by wet-sieving and picked out for dating in order to preclude an old carbon effect. The main objective of our study was to evaluate the resulting suite of 41 AMS radiocarbon ages to determine an accurate chronology for the cores. In this study, we propose a new age model for reconstruction of environmental and climate changes in southern Tibetan plateau during the last ca. 19 kyr based on high-timeresolution 14C data sets from these sediment cores. Because stable carbon isotopes (δ13C) have proved to be useful for deducing the sources of plant fragments in lake sediments (Morrill, 2004; Watanabe et al., 2004), we carried out both high-time-resolution 14C dating and δ13C

measurements of plant residues from the three sediment cores covering the period from the last glacial to the Holocene. 2. Materials and methods 2.1. Study area Lake Pumoyum Co is a freshwater lake on the southern Tibetan plateau formed by fault action (28°34′N, 90°24′E; altitude, ~5020 m asl; lake surface area, 281 km2; maximum water depth, 65 m; Fig. 1). Limnological and geological investigations of Lake Pumoyum Co were performed during the 2001–2006 China–Japan Scientific Research expeditions (Nishimura et al., 2003; Mitamura et al., 2003; Murakami et al., 2007; Zhu et al., 2010). During these expeditions, salinity and pH of the lake water were 0.4 g/l and 8.3–8.7, respectively (Murakami et al., 2007). A mean annual precipitation and evaporation on the area were 355 mm and N1770 mm, respectively (Wang et al., 2009). During the September 2004 expedition, the air temperature ranged between +20 °C and −5 °C. A mean annual temperature was from +2 to +4 °C (Ju et al., 2009). The Jiaqu River is the largest river flowing into Lake Pumoyum Co (72% of the inflow, Fig. 1), and it forms vast wetlands on the western side of the lake. The watershed area of Lake Pumoyum Co is about 1700 km2. In the present, nine rivers flow into Lake Pumoyum Co. Total inflow from the nine rivers into the lake was estimated to be about 860,000 m3/day, and the outflow rate was estimated as 960,000 m3/day (Murakami et al., 2007). Lake Pumoyum Co is located in a mountain basin of pre-Himalayas and belongs to Upper Triassic stratum. In the watershed of Lake Pumoyum Co, unconsolidated Quaternary deposits of glacial, fluvial and lacustrine origin are widely distributed (Takada and Zhu, 2003; Zhu et al., 2007; Ju et al., 2009; Zhu et al., 2010). In Lake Pumoyum Co, a zone characterized by Chara globularis and shells (e.g., Lymnaeidae) has been observed at 30–40 m water depth. The euphotic zone of the lake reaches 50 m water depth (Murakami et al., 2007). The present vegetation around Lake Pumoyum Co could be classified as follows: 1) Marshy and

Fig. 1. Map showing the location of Lake Pumoyum Co on the southern Tibetan plateau, and the coring sites (PY104PC, PY409G, PY409PC, and PS06-31).

T. Watanabe et al. / Chemical Geology 277 (2010) 21–29

swamp meadow, 2) Alpine meadow and alpine steppe, and 3) Alpine desert steppe. During the August 2006 expedition, the most common plant species around Lake Pumoyum Co were Arenaria bryophylla, followed by Halerpestes tricuspis, Androsace tapete, Carex aridula, Kobresia pygmaea and Phlomis rotata.

2.2. Core locations and sampling A 3.80-m-long continuous sediment core (PY104PC) was obtained with a piston corer from the eastern part of Lake Pumoyum Co in 2001 (28°33′56″N, 90°29′59″E; water depth, 46.5 m; Fig. 1). In 2004, a short gravity core (PY409G; core length, 0.44 m) and a piston core (PY409PC; core length, 3.88 m) were obtained from the deepest, central part of the lake (28°34′47″N, 90°28′80″E; water depth, 62.2 m; Fig. 1). Surface sediments (PS06-31) in the western, shallower part of the lake (28°36′21″N, 90°18′24″E; 33.6 m, water depth) were also sampled in 2006 with a gravity corer. Sediment layers below 220 cm depth in PY104PC were composed mainly of relatively large macrophytic plant residues (up to ~3 cm long and 45 dry wt.% of the sediment; mostly Potamogetonaceae) with an admixture of fine sand and sandy silt (Fig. 2). The macrophytic residues quickly disappeared between 220 and 210 cm core depth in PY104PC, and were replaced by silty clay and silt. All sediment layers in the PY409G and PY409PC cores, from the deep central basin (62.2 m water depth) of the lake, were composed mainly

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of silty clay and silt and no macrophytic residues were observed (Nishimura et al., 2007). Sediment samples were taken at 1 cm intervals, and their outer rims were removed to avoid contamination. The macrophyte fragments in the plant residue-rich layers of PY104PC (380–220 cm core depth) were picked out with tweezers. In the silty transitional layer, which contained only a small number of large plant fragments (220–210 cm depth), the plant residues were concentrated by wet-sieving and then picked out with tweezers (Table 1). The fine plant fragments in PY409G and PY409PC and in the silty to silty clay layers of PY104PC (210–52 cm core depth) were concentrated by wet-sieving through a 125-μm-mesh sieve, and then picked out with a tapering pipette. We refer to this material as the plant residue concentrate fraction (PRC fraction, Table 1). In PS06-31 (0–1 cm core depth), undecomposed plant fragments (up to ~1 cm in length) were picked out with tweezers. Living aquatic and terrestrial plants were also collected in and around Lake Pumoyum Co in August 2006 (28°36′N, 90°30′E). 2.3. Radiocarbon dating After sonication with pure H2O to remove adhering contaminants (sediment particles), the modern and fossil plant samples were treated with 1.2 M HCl and 1.2 M NaOH (AAA treatment, Nakamura et al., 2003). After the AAA treatment to remove any possible carbonate contaminant, the concentration of total organic carbon (TOC) in each sample was determined with an elemental analyzer (EA, NA-1500, FISONS). The

Fig. 2. Downcore profiles of a–c) conventional 14C ages and d–f) δ13C values of plant residues in the sediment cores from Lake Pumoyum Co. The lithologic composition of PY104PC is also shown to the right of the 14C and δ13C profiles. The results for the plant residue concentrate (PRC) fraction (N 125 μm, mainly terrestrial plants) from the PY409G, PY409PC, and the silty to silty clay layers of PY104PC (above 210 cm core depth) are shown with filled diamonds. The results for aquatic plant residues from the plant residue-rich layers and the silt transition layer in PY104PC (380–210 cm core depth) are shown with open circles. The shaded ovals in the PY104PC profiles indicate the results in the silty transition layer (220– 210 cm core depth).

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Table 1 Conventional 14C ages and calibrated ages for PY104PC, PY409G, PY409PC, and PS06-31. Core depth (cm)

Conventional 14C age (BP, ±1σ)

Calibrated age (cal BP)

Lab code (NUTA2-)

PY104PC PRC fraction (N 125 μm)a 52.4–59.0b 98.5–102.5 b 148.4–152.5 171.0–172.0 181.1–182.1b 185.2–186.2 190.2–191.2 195.3–196.3 198.3–199.3 202.3–203.3 206.4–208.4 Plant residues 211.4–212.4a 214.5–215.5 214.5–215.5a 217.5–218.5 217.5–218.5a 218.5–219.5b 228.6–229.6 242.7–243.7 247.8–248.8 260.9–261.9b 271.1–272.1 281.3–282.3 290.5–291.5 299.6–300.6 310.9–311.9b 320.0–321.0 329.2–330.2 331.3–332.3c 340.4–341.4d 346.6–347.6b 359.8–360.8 368.0–369.0b 378.4–380.0

4624 ± 27 5682 ± 56 7402 ± 82 8720 ± 45 l9625 ± 29 10,839 ± 116 10,669 ± 50 10,483 ± 51 10,811 ± 80 12,344 ± 91 12,095 ± 118

5311–5444 6403–6537 8163–8343 9560–9736 10,827–11,136 12,771–12,925 12,701–12,801 12,381–12,633 12,793–12,877 14,096–14,573 13,817–14,068

11,626 11,627 12,109 12,080 11,628 12,113 12,081 12,110 12,095 12,112 12,118

13,195 ± 56 13,035 ± 153 12,680 ± 53 12,912 ± 60 12,791 ± 56 12,951 ± 62 12,838 ± 52 13,077 ± 61 13,346 ± 52 13,500 ± 63 13,623 ± 49 13,778 ± 61 14,186 ± 60 14,290 ± 52 14,773 ± 68 14,593 ± 53 14,813 ± 66 14,873 ± 62 17,432 ± 87 14,898 ± 69 14,870 ± 76 15,558 ± 72 14,923 ± 67

15,436–15,790 15,161–15,641 14,857–15,109 15,097–15,372 14,981–15,210 15,138–15,427 15,023–15,263 15,274–15,608 15,648–16,016 15,847–16,240 16,016–16,398 16,202–16,606 16,717–17,130 16,880–17,306 17,765–18,054 17,475–17,871 17,815–18,438 17,978–18,472 20,410–20,714 18,007–18,479 17,968–18,477 18,790–18,905 18,033–18,482

12,096 13,252 12,714 12,094 12,114 11,145 12,718 12,097 12,717 11,146 12,721 12,115 12,098 12,722 11,148 12,723 12,116 13,246 12,724 11,149 12,117 11,150 12,100

PY409PC PRC fraction (N 125 μm)a 13.9–16.7b 48.1–50.9 81.5–84.2b 102.2–105.1 145.9–148.8b 165.7–168.7 208.3–211.2b 242.5–245.4 291.3–294.3b 338.4–341.4b 359.0–362.0b 374.7–377.7b 383.6–386.6

1705 ± 38 2675 ± 74 3002 ± 43 3271 ± 64 4019 ± 54 4577 ± 109 4814 ± 47 6267 ± 205 7022 ± 63 8533 ± 138 9007 ± 63 9114 ± 48 9115 ± 48

1557–1691 2742–2858 3081–3318 3410–3570 4420–4567 5048–5450 5478–5600 6951–7417 7794–7933 9320–9697 9958–10,244 10,220–10,371 10,221–10,371

12,086 12,708 12,122 12,709 12,087 12,712 12,123 12,713 12,088 12,089 12,090 12,092 12,093

PY409G PRC fraction (N 125 μm)a 5.0–8.0 39.0–42.0 TOC fractione 6.0–7.0

PS06-31 Plant residues 0.0–1.0 TOC fraction 0.0–1.0

modern and fossil plant samples remaining after the AAA treatment were combusted at 650 °C with CuO and Ag wires. The resulting CO2 was collected and purified in a vacuum line and subsequently reduced to graphite with an iron catalyst and hydrogen at 650 °C for 6 h. The 14C measurements were performed with a Tandetron Accelerator Mass Spectrometry system (AMS, Model-4130, HVEE) at the Center for Chronological Research, Nagoya University. The measured values (14C/ 12 C ratios) were corrected to reflect the “conventional 14C age” by simultaneous measurements of stable carbon isotopes (δ13C), also with the AMS system. The relative standard deviations of the 14C/12C ratios were generally between ±0.3% and ±0.5%. The HOx-II standard (NIST oxalic acid, SRM-4990C) was used as a 14C-concentration reference. Conventional 14C ages were converted to calendar years by using the INTCAL04 data set (Reimer et al., 2004). 2.4. Stable carbon isotope ratio measurements

63 ± 57 (pMC = 99.2 ± 0.7) 1099 ± 37

12,119 963–1054

12,121

2355 ± 34 (pMC = 80.6 ± 0.5)

13,253

140 ± 30 (pMC = 98.3 ± 0.5)

12,129

1204 ± 37 (pMC = 86.1 ± 0.5)

12,127

e

δ13C analyses of the fossil plant samples were carried out by using an isotope ratio mass spectrometer (IRMS, MAT-252, Thermo Finnigan) and a conventional dual inlet system. δ13C values of the living plants were measured using an EA interfaced to an IRMS via a Conflo III split interface. δ13C values were expressed as per mil (‰) relative to the Vienna-Peedee belemnite (VPDB). Standard deviations of the δ13C measurements for an organic compound standard (2,5-bis(5-tert-butyl-benzoxazol-2-yl)-thiophene, δ13C = − 26.7‰) were generally less than ±0.1‰. 3. Results and discussion 3.1. Conventional sediment cores

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C ages and sources of plant fragments in the

To establish age models for the sediment cores by 14C dating it is necessary to identify the sources of the plant residues. In this section, we evaluate the sources of the large plant residues and the PRC fractions from the sediment cores on the basis of their δ13C values. 3.1.1. Plant residues from 380 to 210 cm depth in PY104PC (plant residue-rich layers to the silty transitional layer) In the plant residue-rich layers of PY104PC (380–220 cm depth), conventional 14C ages of the plant residues, mainly Potamogetonaceae, ranged from 15.6 to 12.8 14C ka BP (Table 1, Fig. 2a). Their carbon content ranged from 396 to 575 mg C/g dry sample (497 mg C/g dry sample on average); these values are consistent with those of living plants in and around Lake Pumoyum Co (380–491 mg C/g dry sample, Table 2). The plant fragments in the plant residue-rich layers of PY104PC were extremely enriched in 13C (δ13C from − 2.7‰ to −7.2‰, Fig. 2d). These heavy isotope compositions clearly differed from those of living terrestrial plants around the lake (− 23.9‰ to − 28.1‰) and agreed well with those of living aquatic plants (Potamogetonaceae) in Lake Pumoyum Co (− 4.7‰ to −6.7‰; 1– 17 m water depth; Table 2). Smith and Walker (1980) reported 13C-enrichment of macrophytes grown in culture experiments, and Morrill (2004) reported 13Cenrichment of macrophytes in a freshwater lake in the central Tibetan plateau. The extremely heavy stable carbon isotope ratios of the Potamogetonaceae in Lake Pumoyum Co may be due to carbon fixation in a semi-closed system with limited dissolved inorganic Notes to Table 1: a These samples were collected by wet-sieving (opening, 125 μm) and picked out by pipette from the sediments. b 14 C dates are from Watanabe et al. (2008) and Wang et al. (2009). c Plant seeds (Potamogeton natans, Potamogetonaceae). d 14 C age of this sample was anomalously old. We have no explanation as to the cause. e 14 C ages of total organic carbon (TOC) are shown. pMC, percent modern carbon.

T. Watanabe et al. / Chemical Geology 277 (2010) 21–29 Table 2 TOC, δ13C values, and (28°36′N; 90°30′E).

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C contents of living plants in and around Lake Pumoyum Co

Sample

TOC (mg/g dry plants)

Living terrestrial plants (around the lake) August 2006 (this study) 08WT2, Eleocharis 464 palustris (Cyperaceae) a April 2001 04TPA, Androsace tapete 444 (Primulaceae) 04TPC, Arenaria bryophylla 430 (Caryophyllaceae) 04TPD, Stipa sp. (Poaceae) 431 04TPF, Elymus nutans Griseb. (Poaceae)

416

Living aquatic plants (in the lake, ca. 1–17 m August 2006 (this study) 08EA1, Ruppia rostellata 491 Koch (Potamogetonaceae) 08EA3-1, Ruppia rostellata 471 Koch (Potamogetonaceae) 08WA2, Ruppia rostellata 497 Koch (Potamogetonaceae) 08EA4-1, Chara globularis 380 Thuiller (Characeae) 08WA1-1, Chara globularis 381 Thuiller (Characeae) April 2001a 04APH, Chara globularis 404 Thuiller (Characeae)

δ13C organic (‰, VPDB)

14 C content (pMC, ± 1σ)

Lab Code (NUTA2-)

− 28.1 ± 0.6

106.9 ± 0.4

12,141

− 25.8 ± 0.2 − 23.9 ± 0.1 − 26.2 ± 0.3 − 25.8 ± 0.1

110.5 ± 0.2

11,609

109.5 ± 0.3

11,610

109.7 ± 0.3

11,611

107.3 ± 0.3

11,612

in water depth) − 6.7 ± 0.2 − 4.7 ± 0.1 − 5.9 ± 0.1 − 12.6 ± 0.2 − 15.2 ± 0.1

108.9 ± 0.4

12,167

108.9 ± 0.4

12,139

n.a.

–

108.1 ± 0.8

12,767

110.6 ± 0.8

12,764

− 15.1b

108.5 ± 0.3

11,613

Living aquatic plants (in the wetland, ca. 0.3 m in water depth) August 2006 (this study) 80.3 ± 0.5 466 − 14.4 08WA5, Potamogeton ± 0.1 pectinatus L. (Potamogetonaceae) 472 − 11.5 84.8 ± 0.5 08WA6, Potamogeton ± 0.2 pectinatus L. (Potamogetonaceae) 08WA8, Chara globularis 400 − 15.8 82.3 ± 0.5 Thuiller (Characeae) ± 0.4

12,156

12,158

12,159

n.a.: not analyzed. a 14 C contents are from Watanabe et al. (2008). b δ13C was measured by IRMS via conventional dual inlet.

carbon content. On the other hand, δ13C values of living Potamogetonaceae in the western wetlands of the Lake Pumoyum Co watershed (less than ~ 30 cm water depth) were clearly characterized by lighter values (−11.5‰ to −14.4‰, Table 2) than those of plants from the lake waters (−5.8‰ on average). The relatively lighter δ13C values of the macrophytes may indicate that they used CO2 produced by decomposition of organic materials in the surface sediments of the lake. In the Lake Pumoyum Co, organic fractions of Characeae were clearly depleted in 13C (down to −15.2‰) in comparison with those of Potamogetonaceae from the same area (Table 2). The 13C depletion may be due to kinetic isotope fractionation during photosynthetic assimilation of bicarbonate by proton pumping in Characeae (Hammarlund et al., 1997; Pentecost et al., 2006). These δ13C values suggest that the macrophyte remains (mainly Potamogetonaceae) in the plant residue-rich layers of PY104PC (δ13C = −4.4‰ on average) may have been from plants growing in the lake rather than on the wetland. In the silty transitional layer (220–210 cm core depth), δ13C values of plant residues decreased from − 7.2‰ to −9.8‰ (Fig. 2d). A large number of 13C-depleted Characeae fragments is probably not present in this layer, because the peculiar calcite incrustations of Characeae were not observed in the plant residue fraction. In addition, Morita

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(2007) inferred an enhancement of the Lake Pumoyum Co water level at the time of deposition of this transitional layer on the basis of a decrease in Chlorophyceae fossil contents, indicating that this decrease in δ13C probably was not caused by decomposition of organic materials in the lake under low water-level conditions, but that the 13C-depleted plant fragments in the transitional layer may be derived from the wetland. 3.1.2. PRC fractions in PY104PC sediments above 210 cm depth, and in PY409G and PY409PC Conventional 14C ages of the PRC fraction (N125 μm) in the three cores become progressively older with depth (Table 1 and Fig. 2a–c) except for those in sediments from 200 to 185 cm depth in PY104PC. The δ13C values of the PRC fractions from all three sediment cores varied from − 22.3‰ to −24.4‰ (Fig. 2d–f). This range of δ13C values is very close to that of living terrestrial plants in the area surrounding the lake (from −23.9‰ to − 28.1‰, Table 2). In addition, the δ13C value (− 23.4‰, in this study) of undecomposed plant fragments from the surface layer (PS06-31, 1–0 cm sediment depth, Fig. 1) in northwestern Lake Pumoyum Co also agreed with the values of the PRC fractions in the three cores. These δ13C values are typical of those of C3 plants (− 32‰ to − 20‰, about − 27‰ on average; Schwarz and Redman, 1987), indicating that the PRC fractions in the Lake Pumoyum Co sediment cores were likely composed mainly of terrestrial plant fragments from the area surrounding the lake. The TOC content of the PRC fractions from the sediment cores was 128 ± 63 mg/g dry sample. The relatively low TOC content of the PRC fractions compared with that of living terrestrial plants around the lake (437 ± 18 mg C/g dry sample on average, Table 2) was caused by the admixture of inorganic materials coarser than fine sand. Diatoms are the main primary producers in modern Lake Pumoyum Co, and about 6700 diatom frustules/mg sediment were found in the surface sediment (Murakami et al., 2007). However, diatom frustules in the PRC fractions were removed by the acid-alkaline-acid (AAA) treatment before the δ13C measurements in this study (see Materials and methods). Moreover, pollen grains are usually less than 125 μm in diameter, and the pollen content of the sediment cores was extremely low (about 50 grains/cm3 on average; Morita, 2007). Therefore, most organic materials, excluding terrestrial plant fragments, were probably removed by the wet-sieving and the AAA pretreatment. In the surface sediments (PS06-31), the 14C content of the plant residues (98.3 ± 0.5% modern carbon [pMC]) was higher than that of TOC (86.1 ± 0.5 pMC, Table 1). Similarly, the 14C content of the PRC fraction in PY409G (99.2 ± 0.7 pMC; average depth, 6.5 cm; Table 1) showed values near to the modern. By contrast, the 14C content of the TOC fraction from the top of core PY409G was obviously low (74.6 ± 0.4 pMC, Table 1). At the top of PY409G, the 14C age difference between the PRC fraction and TOC was up to 2300 years. This result might have been caused by a relatively large supply of terrestrial organic materials containing old carbon (“old carbon effect”; carbon derived from lake terrace, loess, or other strata with dead 14C). These results clearly indicate that age control based on 14C data sets of the PRC fraction in the sediment cores from Lake Pumoyum Co is closer from actual age than that based on TOC. 3.2. Age models of the sediment cores from Lake Pumoyum Co based on calibrated ages of the plant residues 3.2.1. PY104PC, 380–220 cm depth (aquatic plant residues, 18.5–15 cal ka BP, last glacial period) In the plant residue-rich layers of PY104PC (380–220 cm core depth), calibrated ages of the aquatic plant fossils were ~ 18.5– 15 cal ka BP, which correspond to the last glacial period (Fig. 3). 14 C ages of aquatic plants are generally influenced by the reservoir effect resulting from large inflows of melting glacier and groundwater, as well as by the dissolution of carbonate rock

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Fig. 3. a) Calibrated ages of the plant residues in PY104PC (380–180 cm core depth). The bold dashed line is the regression line for the calibrated ages of the aquatic plant residues (301–229 cm core depth). The shaded bar indicates the layers with 14C age reversal (220–210 cm core depth). b) δ13C values of the aquatic plant residues in PY104PC, and the fraction of aquatic plant fragments from the wetland (fwetland) in the total plant residue content of each layer in PY104PC. c) Calibrated ages of the plant residues in PY104PC (corresponding to the top part shown in panel a). The bold dashed line is the regression line for the calibrated ages of the aquatic plant residues (301–229 cm in core depth). The lithologic composition is also shown to the right of the profiles. The ages of the PRC fraction and of the aquatic plant residues are shown with filled diamonds and open circles, respectively. The ages of aquatic plant residues after correction for the reservoir effect are shown with open triangles. BA, Bølling–Allerød; YD, Younger Dryas.

such as limestone in the surrounding area (Hall and Henderson, 1991; Morrill et al., 2006). In most cases, reservoir ages of autochthonous organic matter in lake sediments are evaluated in relation to the 14C concentrations of living aquatic organisms (Hendy and Hall, 2006; Morrill et al., 2006). The 14C contents of living terrestrial plants around Lake Pumoyum Co (106.9– 110.5 pMC, Table 2) and living aquatic plants in the lake (ca. 1– 17 m in water depth, 108.1–110.6 pMC, Table 2) agreed well with those of tropospheric CO2 (~ 108 pMC, after Morrill et al., 2006), suggesting that in modern Lake Pumoyum Co, the 14C reservoir effect is negligible. As described above (Section 3.1.1), the sources of plant residues at 380–220 cm depth in PY104PC were lake macrophytes, as indicated by their δ13C values. Therefore, we propose that the most likely 14C reservoir age of the aquatic plant residues in these layers is nearly 0 year. In the lowest part of the plant residue-rich layer (380–311 cm depth in PY104PC), most of the calibrated ages were consistent within 1σ error throughout (18.5–17.5 cal ka BP, Fig. 3a). This part was composed of alternate plant residue-rich layers and silty clay to silty layers (Fig. 2). Therefore, each of these layers was probably deposited with a high sedimentation rate (more than 70 cm/kyr) during 18.5–17.5 cal ka BP. At 301–229 cm depth in PY104PC (17.3–15.0 cal ka BP), the linear sedimentation (LSR) rate was constant at 36.2 cm/kyr, indicating that stable sedimentary conditions during 17.3–15.0 cal ka BP might have succeeded the high sedimentation rate period, at least in the northeastern part of Lake Pumoyum Co.

3.2.2. PY104PC, 220–200 cm depth (aquatic plant residues and PRC fraction, 15–12.8 cal ka BP, Bølling–Allerød period) At 220–210 cm depth in PY104PC (aquatic plant residues, silty transition layer), a dating reversal was observed from 12.9 to 13.3 14 C ka BP (Fig. 3a). At present, the 14C contents of the living macrophytes in the wetland (less than ~ 30 cm water depth) on the western side of Lake Pumoyum Co are relatively low (80.3, 82.3, and 84.8 pMC, Table 2). We estimated the 14C reservoir ages in the western wetland to range from 1.2 to 1.7 kyr on the basis of the 14C content of living macrophytes (Table 2). The aquatic plant residues in the silty transition layer (220–210 cm core depth) may

be partly derived from the wetland, as suggested by their δ13C values (down to − 9.8‰, Fig. 2d, Section 2.2). Therefore, the dating reversal might be due to a relatively large influx of aquatic plant fragments showing a 14C reservoir effect from the wetland into the lake. To correct the 14C ages of the mixed plant fragments, we calculated the fraction of plant fragments from the wetland (fwetland) in the total plant residues of the cores using the following mass balance equations. 13

13

13

δ C plant residues = flake δ C lake + fwetland δ C wetland

ð1Þ

flake + fwetland = 1

ð2Þ

Average δ13C values in living plants from the lake and wetland were −5.8 ± 1.0‰ and − 13.9 ± 2.2‰, respectively (Table 2). The average fwetland values increased from ~0.2 to 0.5 in the transition layer (220–210 cm depth, Fig. 3b, Table 3). The uncertainties were calculated by error propagation. In this study, we tentatively corrected the 14C ages for the reservoir effect by using the fwetland values (Eq. (3)), and calibrated them with the INTCAL04 data set (Reimer et al., 2004). Average R values (14C concentrations) of the living plants

Table 3 δ13C values, fraction of plant fragments from wetland (fwetland), and corrected ages for the reservoir effect of the aquatic plant residues in the PY104PC (282–211 cm core depth). Depth (cm)

δ13C organic (‰, VPDB)

fwetland

Calibrated agea (cal BP)

Remarks

211.4–212.4b 214.5–215.5b 217.5–218.5b 218.5–219.5 242.7–243.7 247.8–248.8

− 9.8c − 9.8 − 9.7 − 7.8 − 7.2 − 7.1

0.49 ± 0.19 0.49 ± 0.19 0.48 ± 0.19 0.25 ± 0.14 0.17 ± 0.13 0.16 ± 0.13

13,600–14,800 13,000–14,000 13,100–14,100 14,000–15,000 14,400–15,400 15,000–15,900

Silty transition layer Silty transition layer Silty transition layer Silty transition layer Plant-rich interval Plant-rich interval

a b c

These ages were corrected for the 14C reservoir effect. These samples were collected by wet-sieving. This value was inferred from δ13C values of the layers below.

T. Watanabe et al. / Chemical Geology 277 (2010) 21–29

from the lake and wetland were 1.090 ± 0.010 and 0.825 ± 0.023, respectively (Table 2).   Reservoir ageðyrÞ = – T1 = 2 = ln2 × ½lnfðflake Rlake Þ

ð3Þ

+ ðfwetland Rwetland Þg – lnðRlake Þ The reservoir ages calculated with Eq. (3) were 500–1100 year in the transition layer. The reservoir-corrected age at the bottom of the silty transition layer (219 cm depth) was thus 14.5 ± 0.5 cal ka BP (Table 3), which corresponds to the boundary between the last glacial and the Bølling–Allerød warm phase (t-LG/BA). The relatively high content of plant fragments from the wetland in the silty transition layer (220–210 cm depth) might be due to (1) the disappearance of aquatic plant habitat in the deeper northeastern part of Lake Pumoyum Co as a result of an increase in the water level and/or (2) a large influx of plant fragments along with mountain glacier meltwater from the southern Tibetan plateau during t-LG/BA. The tLG/BA in Lake Pumoyum Co coincided with the sudden decrease in macrophyte remains (Figs. 2 and 3). This obvious lithologic boundary at 14.5 ± 0.5 cal ka BP can be used as a key layer when dating other sediment cores from Lake Pumoyum Co. At 220–200 cm depth in PY104PC, the reservoir-corrected ages of aquatic plant residues and the calibrated ages of the PRC fraction fluctuated between 15.0 and 12.9 cal ka BP with dating reversals, and the LSR was not constant. The age fluctuations might be due to climate amelioration and a large influx of clastic materials from the watershed as a result of the melting of mountain glaciers and increased precipitation. In addition, the age fluctuations might reflect the 14C plateau caused by changes in the atmospheric radiocarbon concentration during the Older Dryas (Kitagawa and van der Plicht, 1998; 2000). 3.2.3. PY104PC, 200–185 cm depth (PRC fraction, 12.8–11.6 cal ka BP, Younger Dryas) The calculated LSRs in PY104PC during the climate transition showed notable fluctuations. In these layers, we observed both extremely high LSRs and 14C age reversals (positive LSR anomaly, Fig. 4b). The positive LSR anomaly in PY104PC (10.8–10.5 14C ka BP, uncalibrated ages) coincided with the Younger Dryas (YD) rapid cooling event during the climate transition from the last glacial to the Holocene (van Campo and Gasse, 1993; Gasse and van Campo, 1994; Wang et al., 2001; Watanabe et al., 2009b). Zhou et al. (1996) and

27

Lehmkuhl and Haselein (2000) have suggested that the summer monsoon intensity decreased during the YD event on the Tibetan plateau. Climate deterioration and low precipitation on the Tibetan plateau, however, do not accord with the increased LSR during the YD. In addition, the positive LSR anomaly in Lake Pumoyum Co was not caused by increased deposition of aquatic plant remains. Previous studies have documented the fluctuation of past atmospheric radiocarbon concentrations (Δ14C; Bard et al., 1990; Stuiver et al., 1991; Hughen et al., 1998). Previous studies of sediment cores from the Cariaco Basin, Caribbean Sea (Hughen et al., 1998, 2000), and Lake Suigetsu, Japan, have revealed changes in Δ14C during the YD (Kitagawa and van der Plicht, 2000). The changes in conventional 14C ages in PY104PC during the climate transition from the last glacial to the Holocene parallel those in the Lake Suigetsu sediment cores (Fig. 4a, b). Therefore, the positive LSR anomaly in PY104PC may correspond to the “14C plateau” associated with the YD (12.8–11.6 cal ka BP). Indeed, the beginning of the LSR anomaly in PY104PC coincides with the transition from the Bølling–Allerød to the Younger Dryas (t-BA/YD; 12.8 ± 0.1 cal ka BP; Wang et al., 2001; Fig. 4c), and the top of the 14C plateau corresponds to the transition from the Younger Dryas to the Preboreal Holocene (t-YD/PB; 11.6 ± 0.2 cal ka BP; Alley et al., 1993; Hughen et al., 1998, 2000; Friedrich et al., 1999; Wang et al., 2001). The t-YD/PB sediment layer can thus be used as a key layer for absolute dating of the PY104PC sediment core. In this study, the calibrated age of the t-YD/PB in PY104PC was 12.8 ± 0.1 cal ka BP (Fig. 4c). This age should be adjusted to 11.6 cal ka BP (10.1 14C ka BP, t-YD/PB, Fig. 4c) on the basis of the age of the 14C plateau. Therefore, the corrected LSR based on the new calibrated ages (LSRcal) was 8.7 cm/kyr during 14–10 cal ka BP (Fig. 4c). 3.2.4. PY104PC above 185 cm depth, and PY409G and PY409PC (PRC fractions, after 11.6 cal ka BP, Holocene) The PRC fractions in the layers above 185 cm depth in PY104PC dated to after the climate transition from the Pleistocene to Holocene (after 11.6 cal ka BP). Moreover, 14C dating of the PRC fractions revealed that PY409G and PY409PC contained a continuous record for the last 10.4 cal ka BP (Fig. 5). In the PY409PC sediment core, the calibrated age at the core top (0 cm in depth), obtained by extrapolating the regression line for the ages of the PRC fractions, was 1.0 cal ka BP, (Fig. 5b). This relatively old age for the surface sediments of PY409PC may reflect the loss of the upper layer during piston core sampling, or reworking of the sediment layers. The calibrated age of the core bottom of PY409G (40.5 cm depth) was

Fig. 4. Age correction of PY104PC based on the 14C plateau in the Younger Dryas (YD). a) Plot of conventional 14C ages vs. varve ages during 14–8 cal ka BP from annually laminated sediments in Lake Suigetsu, Japan (Kitagawa and van der Plicht, 2000). b) Conventional 14C ages of the PRC fraction during the climate transition. The vertical shaded oval indicates the 14C plateau (YD). The upper solid horizontal line indicates the transition from the Younger Dryas to the Preboreal Holocene (t-YD/PB, ~ 11.6 cal ka BP), and the lower solid horizontal line indicates the transition from the Bølling–Allerød to the Younger Dryas (t-BA/YD, 12.8 ± 0.1 cal ka BP). c) Calibrated ages of the PRC fraction during the climate transition. The dashed line is the regression line for the calibrated ages of the PRC fraction during 14–10 cal ka BP. One point during the BA (202.8 cm depth) and two points during the YD (190.7 and 185.7 cm depth) were excluded from the regression calculation. The open diamond indicates the corrected age (185.7 cm depth) of the t-YD/PB.

28

T. Watanabe et al. / Chemical Geology 277 (2010) 21–29

Fig. 5. Calibrated ages of plant residues in a) PY104PC, b) PY409PC, and c) PY409G. The bold dashed lines are regression lines for the calibrated ages of the PRC fraction during the Holocene. t-LG/BA, transition from the last glacial to the Bølling–Allerød (14.5 ± 0.5 cal ka BP).

1.0 cal ka BP (Fig. 5a). Therefore, the sediments of the PY409G gravity core complement those of PY409PC because they cover the lost part of PY409PC. Similarly, the PY409G and PY409PC cores together cover the lost upper part of PY104PC (3.0 cal ka BP at the core top). Therefore, the three cores together contain a continuous record of the Holocene. In PY104PC, the average LSR after 10 cal ka BP (Fig. 3c), estimated using the calibrated ages of the PRC fractions, was 32.5 cm/kyr. In the two sediment cores from the deepest part of Lake Pumoyum Co (PY409G and PY409PC), the average LSRs were nearly the same (38.5 and 41.5 cm/kyr, respectively). The relatively high LSRs in PY409G and PY409PC compared with that in PY104PC might have been caused by a large inflow from the Jiaqu River into the western part of Lake Pumoyum Co (Fig. 1). In addition, the similar LSRs indicate stable sedimentary conditions in Lake Pumoyum Co during the Holocene. In this study, age dating of the PRC fractions showed that the Lake Pumoyum Co sediment cores have a high time resolution (about 25 years per 1 cm of sediment) and contain a continuous sedimentary record of the Holocene on the southern Tibetan plateau.

4. Conclusion We presented a new 14C chronology of three sediment cores from Lake Pumoyum Co for the period from the last glacial to the Holocene (~18.5 cal ka BP to the present). In the lower part of PY104PC (380– 220 cm in core depth), sediment layers were composed mainly of aquatic plant residues. The aquatic plant residues quickly disappeared between 220 and 210 cm in core PY104PC, and were replaced by silty clay and silt. The transition to the Bølling–Allerød (BA) warm phase from the last glacial (14.5 ± 0.5 cal ka BP) coincided with this obvious lithologic boundary (sudden disappearance of macrophytic remains). We established the chronology of the sediment cores after 14 cal ka BP by using terrestrial plant residues concentrated by wet-sieving (opening, 125 μm, PRC fraction). The calibrated age of the PRC fraction in the surface sediment was nearly modern (0.1 ± 0.1 cal ka BP). During the Younger Dryas (YD) cool period (12.8–11.6 cal ka BP), we observed a positive LSR anomaly in PY104PC, which may correspond to a 14C plateau caused by changes in the atmospheric 14C concentration during the YD. We used the 14C plateau layer as a key layer for age correction of the t-BA/YD (12.8 ± 0.1 cal ka BP) and the tYD/PB (~11.6 cal ka BP), and it can also be used for comparing environmental changes on the southern Tibetan plateau with fluctuations of monsoon intensities in other areas. The new 14C chronology proposed in this study clearly shows the major climate boundary layers during the last deglaciation in the sediment cores from Lake Pumoyum Co.

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