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Early Pliocene paleo-altimetry of the Zanda Basin indicated by a sporopollen record
Palaeogeography, Palaeoclimatology, Palaeoecology 412 (2014) 261–268
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Early Pliocene paleo-altimetry of the Zanda Basin indicated by a sporopollen record Fuli Wu a,⁎, Mark Herrmann b, Xiaomin Fang a a b
Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China BFU-Buero für Umwelttechnologien GmbH, Gelnhausen, Germany
a r t i c l e
i n f o
Article history: Received 22 December 2013 Received in revised form 4 August 2014 Accepted 7 August 2014 Available online 19 August 2014 Keywords: Early Pliocene Zanda Basin Sporopollen record Dark coniferous forest Paleo-altimetry
a b s t r a c t Whether the uplift of the Tibetan Plateau (TP) led to the origin and development of the Indian monsoon is still in dispute. A helpful indicator would be greater knowledge about the chronological development of paleoaltimetry. The Zanda Basin, located on the SW margin of the TP, forms one of the most extensive Cenozoic sedimentary basins in Tibet. The modern climate of this area is controlled by the Indian Monsoon circulation system. It is thus an appropriate region for the study of paleo-altimetry, in addition to research into the relation between the TP and the Indian Monsoon. Sporopollen analyses of early Pliocene lacustrine strata in the Zanda Basin show that gymnosperm pollen is dominant (av. 77%) and mainly composed of Picea (av. 27.2%) and Abies (av. 24.2%); fern spores occupy a lesser proportion (av. 17.6%), while angiosperm pollen including temperate, sub-tropical broadleaved trees, shrubs and herbs, accounts for only a small proportion of the assemblage. This sporopollen assemblage suggests that sub-alpine dark coniferous forests were distributed around the basin and grow in a cool and wet climate, rather than in a modern alpine steppe environment. These characteristics indicate that the Zanda Basin's altitude throughout the early Pliocene was most probably lower than it is at present. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Whether or not the uplift of the TP has played a decisive role in the origin and development of the East Asian monsoon (EAM) system remains highly contentious. Early studies suggested that the TP stimulated the formation of the EAM when it reached a certain height, after which the EAM became stronger as the TP continued to rise (e.g. Li and Fang, 1999; An et al., 2001). However, in recent years, a number of studies have suggested that the TP may have reached its modern elevation during the Oligocene (Garzione et al., 2000; Rowley et al., 2001; DeCelles et al., 2007; Quade et al., 2007; Rowley and Garzione, 2007; Polissar et al., 2009; Xu et al., 2013; Ding et al., 2014). Given such diverse views, the dynamic relation between the origin and development of the EAM and the uplift of the TP has become a bone of contention (Guo et al., 2002; Jia et al., 2003). In particular, scientists have yet to reach a consensus on whether the uplift of the TP had the same influence on the formation and development of the Indian monsoon as it may have done on the EAM (Boos and Kuang, 2010; An et al., 2011; Wu et al., 2012; Qiu, 2013). The key to solving this problem is to establish when the TP reached a certain height and/or when it reached its present-day altitude. Sediments in the highly-elevated Zanda Basin, located immediately north of the Himalaya in SW Tibet, display well-preserved, continuous stratigraphic sequences, as well as yielding abundant fossil ⁎ Corresponding author. Fax: +86 10 8409 7079. E-mail address: [email protected] (F. Wu).
http://dx.doi.org/10.1016/j.palaeo.2014.08.006 0031-0182/© 2014 Elsevier B.V. All rights reserved.
remains (Qian, 1999; Deng et al., 2011, 2012; XM Wang et al., 2013; Y Wang et al., 2013). Today, the climate in this area is controlled by the circulation of the Indian Monsoon. Interpretation of the Zanda Basin's paleo-altitude is of crucial relevance to any reconstruction of the development of the Indian Monsoon. Recently, Deng et al. (2012) estimated that the altitude of the Zanda Basin at 4.6 Ma was ~4000 m, a value deduced from a well-preserved skeleton of a three-toed horse (Hipparion zandaense) which is certified to have lived in alpine steppe habitats. Accordingly, Deng et al. concluded that the SW TP reached its present-day elevation during the early Pliocene. Sporopollen analysis of samples collected from the same lacustrine strata in which the Hipparion fossil was found allows a chronological reconstruction of the paleo-altitude of the Zanda Basin. 2. Regional setting and age of the studied section The ~10,000 km2 Zanda Basin is located in SW Tibet (~80°E, 31°N) between the Himalayan and TransHimalayan ranges to the S and N, with the Gurla Mandhata and Leo Pargil domes to the E and W, respectively (Fig. 1). Most of the basin lies at altitudes between 4200 and 4500 m. The surrounding mountains reach heights of 5000–6000 m above sea level (asl). The Zanda Basin exhibits features typical of an alpine temperate to cold temperate climate: dry, cold winters and cool summers (J. G Li and Zhou, 2001; J Li and Zhou, 2001). Its modern vegetation is characterized by sub-alpine desert grassland mixed with a small proportion of shrubs and grasses, e.g. Stipa plareosa mixed with
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Fig. 1. Simplified geological map of the Zanda Basin and surrounding areas (after Kempf et al., 2009).
Artemisia, Ephedra, Ceratoides latens and Salsola (The comprehensive scientific expedition to the Qinghai–Xizang Plateau, 1985). The sediments of the Zanda Formation, ca. 800 m thick, are thought to be of late Neogene age. The determination of the age of this formation has been the focus of many scientific paleomagnetic studies. Zhang et al. (1981) estimated that the aforementioned Zanda Basin sediments occurred between ~ 6 Ma and ~ 1.5 Ma. Qian (1999) dated this process from ~ 7 to ~ 1 Ma. Wang et al. (2008) inferred an age range from 9.5 to 2.6 Ma, based on magnetostratigraphic data taken from a section in the southern part of the Zanda Basin. Saylor et al. (2009) assigned an age range from ~9.2 to b 1 Ma to the Zanda Basin sediments, based on a combination of magnetostratigraphic data from two subsections in the central Zanda Basin. Deng et al. (2011) re-evaluated the strata where fossils were found and established a new chronological model based on the above results from paleomagenetic data results, dating the above-mentioned three-toed horse fossils to 4.6 Ma. Recently, XM Wang et al. (2013) generated a more detailed chronological model based upon various types of mammalian fossils and the magnetostratigraphic results mentioned above, they also dated the initiation of the sediment sequence to about 6.4 Ma, similar to the results of Deng et al. (2011, 2012). Our sporopollen analysis of the lacustrine strata was focused on the middle of the Zanda formation, at depths of ca. 200 m to 500 m, similar to those investigated by Wang et al (2008). This section lies west of Zanda county, and at the depth of 292 m in the section (79°45′32.7″E, 31°25′39.24″ N), the altitude is 3800 m asl. And the age of our studied part of this section is between 4.8 and 3.6 Ma, based on Deng et al. (2011, 2012) and a new interpretation by XM Wang et al. (2013), Y Wang et al. (2013) (see Fig. 2). 3. Material and methods Fifty two lacustrine samples were taken from the middle of the Zanda Basin section (Fig. 2). Each sample from clayey to silty calcareous marl layers was 2 cm thick; 200 g was taken from each sample following standard palynological processing protocols, including treatment with HCl and HF (e.g., Kaiser and Ashraf, 1974). Prior to chemical processing, Lycopodium tablets were added to facilitate the calculation
of palynomorph concentrations. A × 400 magnification microscope was routinely used for identification and counting of sporopollen. For tiny, almost indiscernible types, a ×1000 magnification was preferred. Most of the laboratory work was undertaken at the Institute of Geosciences, University of Tübingen. 4. Result of palynologic analysis Most of the studied samples yielded rich, well-preserved palynomorph assemblages, with the identified sporomorphs representing about 71 taxa, including arboreal taxa, e.g. Pinus, Picea, Abies, Cedrus, Tsuga, Podocarpus, Tilia, Betula, Melia, Juglans, Quercus, Ulmus, Carya, Pterocarya, Castanea, as well as scrubby and herbaceous taxa, e.g. Compositae (Anthemis-, Aster-, Saussurea-, and Taraxacum-type), Artemisia, Gramineae, Rosa, Ericaceae, Ephedra, Chenopodiaceae, and some fern spores, including Lycopodiaceae, Pteridaceae, and Polypodiaceae. Some algal remains (e.g. Pediastrum, Botryococcus) were also detected (Plate 1). As shown in Fig. 3, the sporopollen spectra are particularly characterized by a high abundance of gymnosperms, a few fern spores and sporadic angiosperm pollen. The maximum proportion of gymnosperms reaches ca. 96%, with an average of 77%. The gymnosperm plants principally include Pinus (2–51.2%, av. 27.2%), Picea (up to 54.3%, av. 24.2%), Abies (3.2–50.76%, av. 21%), and Podocarpus (av. 5.2%) (Fig. 3). Tsuga, Cedrus and TCT (Taxodiaceae/Cupressaceae/Taxaceae) are occasionally seen in some samples. Pteridophytes dominate the palynomorph assemblage in several samples. Their maximum abundances reach 64%, with an average of 17.6%. The main fern taxa are Polypodiaceae (av. 4.4%), Pteridaceae (av. 4%) and Lycopodiaceae (av. 3%) (Fig. 3). Angiosperms account for only a small percentage of the total assemblage, with a maximum proportion of 10%, and an average of 2.1%. Species representing arboreal plants are relatively abundant, but their proportion percentages within pollen spectra are extremely low, with Betula, Carya, Alnus, Tilia, etc. appearing only occasionally. The main components of scrubby and herbaceous taxa are the Compositae (av. 1.1%) and the Cyperaceae (av. 2.6%). Artemisia, Gramineae, and Ericaceae occur sporadically within this section (Fig. 3).
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Fig. 2. Correlation of published paleomagnetic sections (Deng et al., 2011); the blue line indicates section we studied and from which samples were collected for sporopollen analysis. The age of our section spans from 4.8 to 3.6 Ma; the age of the Hipparion zandaense fossil (red star mark) is 4.6 Ma.
Concentrations of algal remains are quite high, with maximum values of 5064 s/g (specimens/gram of sediment) and average values of 1692 s/g (Fig. 3).
5. Discussion 5.1. The early Pliocene ecologic environment of the Zanda Basin The sporopollen assemblage of the lacustrine strata in the Zanda Basin is characterized by high percentages of gymnosperm pollen, e.g. Picea, Abies and Pinus, and low percentages of angiosperm pollen. This is supported by previous studies (Li and Liang, 1983; J. G Li and Zhou, 2001; Zhu et al., 2006 & 2007; Yu et al., 2007). For example, according to the results provided by Li and Liang (1983), gymnosperms are principally by Picea, Abies and Pinus, which account for more than half of the total percentage of woody plants, with additional Tsuga, Cedrus and Podocarpus also accounting for a certain proportion. Dicotyledonous woody plants account for less than 20% in the whole assemblage, and are composed mainly of Alnus, Juglans, Betula, Ulmus, Acer and Salix. Shrubs and herbs are composed of fewer species, but predomiantly Rosa, Lonicera, Chenopodiacea, Compositae, Polygonum, Caryophyllaceae, Cruciferae and Gramineae, and sporadic aquatic Cypera as well as Sparganium. The features of the assemblage described by Li and Liang (1983) are similar to those outlined in this study.
Many studies of the representation of Picea/Abies-pollen in modern sporopollen assemblages indicate that such assemblages are highly indicative of autochthonous vegetation (Li and Yao, 1990; Li, 1991; Xu et al., 1995, 1996, 2005; Xiao et al., 2011), although some studies show that various factors such as violent air motions, geography (Li, 1991) or river flow (Wu et al., 2013) may affect Picea/Abies dispersal to some extent. According to our results, the average percentage of Picea and Abies pollen exceeds 20%, suggesting that spruce and fir forests developed widely in the Zanda Basin. On the TP today, Picea and Abies are the main representatives of sub-alpine dark coniferous forests, and are principally distributed in the Himalaya, Nyainqentanglha and Hengduan Mountains (Committee of China Forest Compiler, 2000; Zhang and Wang, 2007). According to this analysis, the zonal vegetation in the Zanda Basin during early Pliocene times should have been subalpine dark coniferous forests as indicated by our sporopollen record. Small amounts of angiosperm pollen in the assemblage indicate that scattered grassland may have grown on the sunny slopes of the surrounding mountains as well as in the drier areas in the Zanda Basin itself, with sporadic subtropical or temperate broadleaved trees in the valleys or close to lakes. Another explanation for the sporadic subtropical or temperate indicators could be the long-distance transport of pollen by air motion originating in warmer, southern regions. Samples with high algal concentrations are interpreted as reflecting more humid conditions and, presumably, an increase in the area and depth of the lake water. A plausible reason for this would be an increase in precipitation
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Plate 1. I–III, Picea; IV–VI, Abies; VII, Cedrus; VIII–XII, Podocarpus; XIII, Pinus; XIV–XV, Tsuga; XVI–XVII, Multiporopollenites (Juglans?) ; XVIII, Ericaeceae; XIX, Pteridaceae; XX, Polypodiaceae; XXI, Selaginellaceae; XXII, Osmundaceae; XXIII, Sphagnum; XXIV, Pediastrum; XXV, Juglans; XXVI, Ulmus; XXVII, Carya; XXVIII–XXIX, Gramineae; XXX, Lonicera; XXXI, Quercus; XXXII– XXXIII, Anthemis-type (Compositae); XXXIV, Aster-typer (Compositae); XXXV, Chenopodiaceae.
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Fig. 3. Sporopollen percentages of the principal taxa and concentration in the Zanda section display a higher proportion of gymnosperm pollen in the remainder section, and are dominated especially by Pinus, Picea and Abies.
rates caused by a strong Indian Summer Monsoon reaching the Zanda Basin and the surrounding mountains. 5.2. Early Pliocene paleo-altimetry of the Zanda Basin Current approaches to the reconstruction of paleo-altimetry rely largely upon the study of variations of temperature with altitude (e.g. Ghosh et al., 2006), isotopes (Chamberlain and Poage, 2000; Garzione et al., 2000; Rowley et al., 2001; Quade et al., 2007, 2011; Xu et al., 2013), atmospheric pressure (Sahagian et al., 2002), enthalpy (e.g. Spicer et al., 2003), pCO2 (e.g. McElwain, 2004), fossils (Deng et al., 2011, 2012), theδD of leaf-waxes (Chikaraishi and Naraoka, 2003; Bi et al., 2005; Liu et al., 2006; Sachse et al., 2006), and sporopollen assemblages, e.g. sporopollen to elevation transfer functions (Lü et al., 2011) and co-existence with elevation (Song et al., 2010; Sun et al., 2014). Each approach has its own strengths and weaknesses and, for this reason, we aim to render only a semi-quantitative palaeo-altitude inferred from the ecological habits of typical plants as represented in our pollen spectra. We have reached the conclusion that the paleoaltimetry of the Zanda Basin did not reach present altitudes, and did not exceed 3600 m even during the early Pliocene, if the following factors are taken into consideration: i. Evidence taken from modern sporopollen studies. Today, 18 species of Picea (such as Picea purpurea, Picea likiangensis, Picea crassifolia, and Picea asperata) grow in the mountainous regions of the TP (Committee of China Forest Compiler, 2000; Lü et al., 2008). Their uppermost height limit is about 4100 m asl in the south and about 2000 m asl in the north. At present, 22 species of Abies (such as Abies ernestii, Abies georgei and Abies pindrow) grow at lower altitudes in China, and another 12 species are found at elevations ranging from 2000 to 4000 m asl along the margins of the TP (Lü et al., 2008). Furthermore, pollen results obtained from TP surface soil samples show that both Picea and Abies have a growth optimum at altitudes between 2500 m and 4000 m asl, and their highest pollen concentration is distributed in soil samples at heights of around 3200 m asl (Lü et al., 2008).
ii. Evidence from fossil oxygen isotopes. As shown in Fig. 4, we know that sub-alpine dark coniferous forests grew on the TP in early Pliocene times. The present-day climatic conditions of this region are described as follows: mean annual temperature (MAT) is generally above 4 °C (Fig.4a), mean annual precipitation (MAP) is about 400–800 mm (Fig. 4b) and the warmest month mean temperature (WMMT) is about 10–14 °C (Fig.4c), as verified by other studies (e.g. Li and Chou, 1979, 1984; Lü et al., 2008; Li et al., 2012). Recently, δ13C values of enamel samples have pointed to a C3 vegetation that dominated the Zanda Basin during the early Pliocene, suggesting that the annual precipitation at that time was likely to be about 200–400 mm higher than it is today (200 mm), with paleotemperatures reaching 15 ± 7 °C between 4.2 and 3.1 Ma (XM Wang et al., 2013; Y Wang et al., 2013). Data derived from a fossil bone-based oxygen isotope temperature proxy (XM Wang et al., 2013; Y Wang et al., 2013) are thought to represent the warm season temperatures (Breecker et al., 2009; Passey et al., 2010; Quade et al., 2011). Such estimated climatic conditions for the early Pliocene times are similar to the present-day growing conditions required by dark coniferous forests. iii. Evidence taken from heat condition. Temperature and precipitation are two important factors affecting the distribution of coniferous forests. At present, MAT is 0 °C in Zanda County (3700 m asl) (Climatic Data Center, National Meteorological Information Center, China Meteorological Administration). Therefore, the difference between today's mean local temperature and that needed for the growth of dark coniferous forests (4 °C) is 4 °C. Ocean temperatures during the Pliocene warm period (ca. 5–3 Ma) were about 2–3 °C higher than today's, as suggested by marine paleo-temperature records (e.g. Lear et al., 2000; Ravelo et al., 2004), and temperature change due to climate change (caused by a warmer ocean) at high altitudes is probably greater than at sea level (Bradley et al., 2006). These warmer temperatures during the Pliocene might have compensated for the ca. ~ 4 °C difference in temperature required for the growth of dark coniferous forests in the Zanda Basin during the early Pliocene. In fact, the present altitude of the dark coniferous forest near Zanda at present is about 3000–3600 m asl
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(The comprehensive scientific expedition to the Qinghai–Xizang Plateau, 1985). Accordingly, it may be assumed that the paleoaltitude of the Zanda Basin during the early Pliocene was not higher than 3600 m asl, although there may have been some higher mountains surrounding the basin within which some animals such as H. zandaense may have lived.
6. Conclusion Sporopollen analysis of the lacustrine strata in the Zanda Basin shows a predominance of gymnosperm pollen (mainly Picea and Abies); angiosperms are represented by Ulmus, Betula, Carya, and Castanea pollens as well as Compositae. These pollen spectra imply
Fig. 4. Climatic conditions on the TP (Climatic Data Center, National Meteorological Information Center, China Meteorological Administration) and the distribution of dark coniferous forests (Zhang and Wang, 2007), where MAT is at or above 4 °C (a), MAP is ca. 400–800 mm (b) and WMMT is ca. 10–14 °C (c).
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Fig. 4 (continued).
sub-alpine dark coniferous forest vegetation with a wet and cool climate within the Zanda Basin during the early Pliocene. Taken into account the growing conditions required for this type of vegetation and the known reconstructed warm Pliocene climatic conditions, the paleo-altimetry of the Zanda Basin probably did not exceed an altitude of 3600 m asl, and hence did not achieve its present-day altitude before or during the early Pliocene. Acknowledgments Financial support has been provided by the German Science Foundation (DFG) within the framework of the TiP priority program (BL 436/51, BL 436/5-2), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB03020103, XDB03020400), the (973) National Basin Research Program of China (2013CB956400), and the National Natural Science Foundation of China (41272184 & 41321061). We thank the editor Thierry Corrège and four anonymous reviewers for their helpful suggestions and comments and Professors Bill Isherwood and Edward A Derbyshire for their help with improvement of the English. Many thanks are also due to Dr. Congrong An for his assistance in drawing the diagrams. References An, Z., Kutzbach, J.E., Prell, W.L., Porters, S.C., 2001. Evolution of Asian monsoon and phased uplift of the Himalaya–Tibetan plateau since Late Miocene times. Nature 411, 62–66. An, Z., Clemens, S.C., Shen, J., Qiang, X., Jin, Z., Sun, Y., Prell, W.L., Luo, J., Wang, S., Xu, H., Cai, Y., Zhou, W., Liu, X., Liu, W., Shi, Z., Yan, L., Xiao, X., Chang, H., Wu, F., Ai, L., Lu, F., 2011. Glacial–Interglacial Indian summer monsoon dynamics. Science 333, 719–723. Bi, X., Sheng, G., Liu, X., Li, C., Fu, J., 2005. Molecular and carbon and hydrogen isotopic composition of n-alkanes in plant leaf waxes. Org. Geochem. 36, 1405–1417. Boos, W.R., Kuang, Z.M., 2010. Dominant control of the South Asian monsoon by orographic insulation versus plateau heating. Nature 463, 218–223. Bradley, R.S., Vuille, M., Diaz, H.F., Vergara, W., 2006. Threats to water supplies in the tropical Andes. Science 312, 1755–1756. Breecker, D.O., Sharp, Z.D., McFadden, L.D., 2009. Seasonal bias in the formation and stable isotopic composition of pedogenic carbonate in modernsoils from central New Mexico, USA. Geol. Soc. Am. Bull. 121, 630–640.
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Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo
Early Pliocene paleo-altimetry of the Zanda Basin indicated by a sporopollen record Fuli Wu a,⁎, Mark Herrmann b, Xiaomin Fang a a b
Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China BFU-Buero für Umwelttechnologien GmbH, Gelnhausen, Germany
a r t i c l e
i n f o
Article history: Received 22 December 2013 Received in revised form 4 August 2014 Accepted 7 August 2014 Available online 19 August 2014 Keywords: Early Pliocene Zanda Basin Sporopollen record Dark coniferous forest Paleo-altimetry
a b s t r a c t Whether the uplift of the Tibetan Plateau (TP) led to the origin and development of the Indian monsoon is still in dispute. A helpful indicator would be greater knowledge about the chronological development of paleoaltimetry. The Zanda Basin, located on the SW margin of the TP, forms one of the most extensive Cenozoic sedimentary basins in Tibet. The modern climate of this area is controlled by the Indian Monsoon circulation system. It is thus an appropriate region for the study of paleo-altimetry, in addition to research into the relation between the TP and the Indian Monsoon. Sporopollen analyses of early Pliocene lacustrine strata in the Zanda Basin show that gymnosperm pollen is dominant (av. 77%) and mainly composed of Picea (av. 27.2%) and Abies (av. 24.2%); fern spores occupy a lesser proportion (av. 17.6%), while angiosperm pollen including temperate, sub-tropical broadleaved trees, shrubs and herbs, accounts for only a small proportion of the assemblage. This sporopollen assemblage suggests that sub-alpine dark coniferous forests were distributed around the basin and grow in a cool and wet climate, rather than in a modern alpine steppe environment. These characteristics indicate that the Zanda Basin's altitude throughout the early Pliocene was most probably lower than it is at present. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Whether or not the uplift of the TP has played a decisive role in the origin and development of the East Asian monsoon (EAM) system remains highly contentious. Early studies suggested that the TP stimulated the formation of the EAM when it reached a certain height, after which the EAM became stronger as the TP continued to rise (e.g. Li and Fang, 1999; An et al., 2001). However, in recent years, a number of studies have suggested that the TP may have reached its modern elevation during the Oligocene (Garzione et al., 2000; Rowley et al., 2001; DeCelles et al., 2007; Quade et al., 2007; Rowley and Garzione, 2007; Polissar et al., 2009; Xu et al., 2013; Ding et al., 2014). Given such diverse views, the dynamic relation between the origin and development of the EAM and the uplift of the TP has become a bone of contention (Guo et al., 2002; Jia et al., 2003). In particular, scientists have yet to reach a consensus on whether the uplift of the TP had the same influence on the formation and development of the Indian monsoon as it may have done on the EAM (Boos and Kuang, 2010; An et al., 2011; Wu et al., 2012; Qiu, 2013). The key to solving this problem is to establish when the TP reached a certain height and/or when it reached its present-day altitude. Sediments in the highly-elevated Zanda Basin, located immediately north of the Himalaya in SW Tibet, display well-preserved, continuous stratigraphic sequences, as well as yielding abundant fossil ⁎ Corresponding author. Fax: +86 10 8409 7079. E-mail address: [email protected] (F. Wu).
http://dx.doi.org/10.1016/j.palaeo.2014.08.006 0031-0182/© 2014 Elsevier B.V. All rights reserved.
remains (Qian, 1999; Deng et al., 2011, 2012; XM Wang et al., 2013; Y Wang et al., 2013). Today, the climate in this area is controlled by the circulation of the Indian Monsoon. Interpretation of the Zanda Basin's paleo-altitude is of crucial relevance to any reconstruction of the development of the Indian Monsoon. Recently, Deng et al. (2012) estimated that the altitude of the Zanda Basin at 4.6 Ma was ~4000 m, a value deduced from a well-preserved skeleton of a three-toed horse (Hipparion zandaense) which is certified to have lived in alpine steppe habitats. Accordingly, Deng et al. concluded that the SW TP reached its present-day elevation during the early Pliocene. Sporopollen analysis of samples collected from the same lacustrine strata in which the Hipparion fossil was found allows a chronological reconstruction of the paleo-altitude of the Zanda Basin. 2. Regional setting and age of the studied section The ~10,000 km2 Zanda Basin is located in SW Tibet (~80°E, 31°N) between the Himalayan and TransHimalayan ranges to the S and N, with the Gurla Mandhata and Leo Pargil domes to the E and W, respectively (Fig. 1). Most of the basin lies at altitudes between 4200 and 4500 m. The surrounding mountains reach heights of 5000–6000 m above sea level (asl). The Zanda Basin exhibits features typical of an alpine temperate to cold temperate climate: dry, cold winters and cool summers (J. G Li and Zhou, 2001; J Li and Zhou, 2001). Its modern vegetation is characterized by sub-alpine desert grassland mixed with a small proportion of shrubs and grasses, e.g. Stipa plareosa mixed with
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Fig. 1. Simplified geological map of the Zanda Basin and surrounding areas (after Kempf et al., 2009).
Artemisia, Ephedra, Ceratoides latens and Salsola (The comprehensive scientific expedition to the Qinghai–Xizang Plateau, 1985). The sediments of the Zanda Formation, ca. 800 m thick, are thought to be of late Neogene age. The determination of the age of this formation has been the focus of many scientific paleomagnetic studies. Zhang et al. (1981) estimated that the aforementioned Zanda Basin sediments occurred between ~ 6 Ma and ~ 1.5 Ma. Qian (1999) dated this process from ~ 7 to ~ 1 Ma. Wang et al. (2008) inferred an age range from 9.5 to 2.6 Ma, based on magnetostratigraphic data taken from a section in the southern part of the Zanda Basin. Saylor et al. (2009) assigned an age range from ~9.2 to b 1 Ma to the Zanda Basin sediments, based on a combination of magnetostratigraphic data from two subsections in the central Zanda Basin. Deng et al. (2011) re-evaluated the strata where fossils were found and established a new chronological model based on the above results from paleomagenetic data results, dating the above-mentioned three-toed horse fossils to 4.6 Ma. Recently, XM Wang et al. (2013) generated a more detailed chronological model based upon various types of mammalian fossils and the magnetostratigraphic results mentioned above, they also dated the initiation of the sediment sequence to about 6.4 Ma, similar to the results of Deng et al. (2011, 2012). Our sporopollen analysis of the lacustrine strata was focused on the middle of the Zanda formation, at depths of ca. 200 m to 500 m, similar to those investigated by Wang et al (2008). This section lies west of Zanda county, and at the depth of 292 m in the section (79°45′32.7″E, 31°25′39.24″ N), the altitude is 3800 m asl. And the age of our studied part of this section is between 4.8 and 3.6 Ma, based on Deng et al. (2011, 2012) and a new interpretation by XM Wang et al. (2013), Y Wang et al. (2013) (see Fig. 2). 3. Material and methods Fifty two lacustrine samples were taken from the middle of the Zanda Basin section (Fig. 2). Each sample from clayey to silty calcareous marl layers was 2 cm thick; 200 g was taken from each sample following standard palynological processing protocols, including treatment with HCl and HF (e.g., Kaiser and Ashraf, 1974). Prior to chemical processing, Lycopodium tablets were added to facilitate the calculation
of palynomorph concentrations. A × 400 magnification microscope was routinely used for identification and counting of sporopollen. For tiny, almost indiscernible types, a ×1000 magnification was preferred. Most of the laboratory work was undertaken at the Institute of Geosciences, University of Tübingen. 4. Result of palynologic analysis Most of the studied samples yielded rich, well-preserved palynomorph assemblages, with the identified sporomorphs representing about 71 taxa, including arboreal taxa, e.g. Pinus, Picea, Abies, Cedrus, Tsuga, Podocarpus, Tilia, Betula, Melia, Juglans, Quercus, Ulmus, Carya, Pterocarya, Castanea, as well as scrubby and herbaceous taxa, e.g. Compositae (Anthemis-, Aster-, Saussurea-, and Taraxacum-type), Artemisia, Gramineae, Rosa, Ericaceae, Ephedra, Chenopodiaceae, and some fern spores, including Lycopodiaceae, Pteridaceae, and Polypodiaceae. Some algal remains (e.g. Pediastrum, Botryococcus) were also detected (Plate 1). As shown in Fig. 3, the sporopollen spectra are particularly characterized by a high abundance of gymnosperms, a few fern spores and sporadic angiosperm pollen. The maximum proportion of gymnosperms reaches ca. 96%, with an average of 77%. The gymnosperm plants principally include Pinus (2–51.2%, av. 27.2%), Picea (up to 54.3%, av. 24.2%), Abies (3.2–50.76%, av. 21%), and Podocarpus (av. 5.2%) (Fig. 3). Tsuga, Cedrus and TCT (Taxodiaceae/Cupressaceae/Taxaceae) are occasionally seen in some samples. Pteridophytes dominate the palynomorph assemblage in several samples. Their maximum abundances reach 64%, with an average of 17.6%. The main fern taxa are Polypodiaceae (av. 4.4%), Pteridaceae (av. 4%) and Lycopodiaceae (av. 3%) (Fig. 3). Angiosperms account for only a small percentage of the total assemblage, with a maximum proportion of 10%, and an average of 2.1%. Species representing arboreal plants are relatively abundant, but their proportion percentages within pollen spectra are extremely low, with Betula, Carya, Alnus, Tilia, etc. appearing only occasionally. The main components of scrubby and herbaceous taxa are the Compositae (av. 1.1%) and the Cyperaceae (av. 2.6%). Artemisia, Gramineae, and Ericaceae occur sporadically within this section (Fig. 3).
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Fig. 2. Correlation of published paleomagnetic sections (Deng et al., 2011); the blue line indicates section we studied and from which samples were collected for sporopollen analysis. The age of our section spans from 4.8 to 3.6 Ma; the age of the Hipparion zandaense fossil (red star mark) is 4.6 Ma.
Concentrations of algal remains are quite high, with maximum values of 5064 s/g (specimens/gram of sediment) and average values of 1692 s/g (Fig. 3).
5. Discussion 5.1. The early Pliocene ecologic environment of the Zanda Basin The sporopollen assemblage of the lacustrine strata in the Zanda Basin is characterized by high percentages of gymnosperm pollen, e.g. Picea, Abies and Pinus, and low percentages of angiosperm pollen. This is supported by previous studies (Li and Liang, 1983; J. G Li and Zhou, 2001; Zhu et al., 2006 & 2007; Yu et al., 2007). For example, according to the results provided by Li and Liang (1983), gymnosperms are principally by Picea, Abies and Pinus, which account for more than half of the total percentage of woody plants, with additional Tsuga, Cedrus and Podocarpus also accounting for a certain proportion. Dicotyledonous woody plants account for less than 20% in the whole assemblage, and are composed mainly of Alnus, Juglans, Betula, Ulmus, Acer and Salix. Shrubs and herbs are composed of fewer species, but predomiantly Rosa, Lonicera, Chenopodiacea, Compositae, Polygonum, Caryophyllaceae, Cruciferae and Gramineae, and sporadic aquatic Cypera as well as Sparganium. The features of the assemblage described by Li and Liang (1983) are similar to those outlined in this study.
Many studies of the representation of Picea/Abies-pollen in modern sporopollen assemblages indicate that such assemblages are highly indicative of autochthonous vegetation (Li and Yao, 1990; Li, 1991; Xu et al., 1995, 1996, 2005; Xiao et al., 2011), although some studies show that various factors such as violent air motions, geography (Li, 1991) or river flow (Wu et al., 2013) may affect Picea/Abies dispersal to some extent. According to our results, the average percentage of Picea and Abies pollen exceeds 20%, suggesting that spruce and fir forests developed widely in the Zanda Basin. On the TP today, Picea and Abies are the main representatives of sub-alpine dark coniferous forests, and are principally distributed in the Himalaya, Nyainqentanglha and Hengduan Mountains (Committee of China Forest Compiler, 2000; Zhang and Wang, 2007). According to this analysis, the zonal vegetation in the Zanda Basin during early Pliocene times should have been subalpine dark coniferous forests as indicated by our sporopollen record. Small amounts of angiosperm pollen in the assemblage indicate that scattered grassland may have grown on the sunny slopes of the surrounding mountains as well as in the drier areas in the Zanda Basin itself, with sporadic subtropical or temperate broadleaved trees in the valleys or close to lakes. Another explanation for the sporadic subtropical or temperate indicators could be the long-distance transport of pollen by air motion originating in warmer, southern regions. Samples with high algal concentrations are interpreted as reflecting more humid conditions and, presumably, an increase in the area and depth of the lake water. A plausible reason for this would be an increase in precipitation
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Plate 1. I–III, Picea; IV–VI, Abies; VII, Cedrus; VIII–XII, Podocarpus; XIII, Pinus; XIV–XV, Tsuga; XVI–XVII, Multiporopollenites (Juglans?) ; XVIII, Ericaeceae; XIX, Pteridaceae; XX, Polypodiaceae; XXI, Selaginellaceae; XXII, Osmundaceae; XXIII, Sphagnum; XXIV, Pediastrum; XXV, Juglans; XXVI, Ulmus; XXVII, Carya; XXVIII–XXIX, Gramineae; XXX, Lonicera; XXXI, Quercus; XXXII– XXXIII, Anthemis-type (Compositae); XXXIV, Aster-typer (Compositae); XXXV, Chenopodiaceae.
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Fig. 3. Sporopollen percentages of the principal taxa and concentration in the Zanda section display a higher proportion of gymnosperm pollen in the remainder section, and are dominated especially by Pinus, Picea and Abies.
rates caused by a strong Indian Summer Monsoon reaching the Zanda Basin and the surrounding mountains. 5.2. Early Pliocene paleo-altimetry of the Zanda Basin Current approaches to the reconstruction of paleo-altimetry rely largely upon the study of variations of temperature with altitude (e.g. Ghosh et al., 2006), isotopes (Chamberlain and Poage, 2000; Garzione et al., 2000; Rowley et al., 2001; Quade et al., 2007, 2011; Xu et al., 2013), atmospheric pressure (Sahagian et al., 2002), enthalpy (e.g. Spicer et al., 2003), pCO2 (e.g. McElwain, 2004), fossils (Deng et al., 2011, 2012), theδD of leaf-waxes (Chikaraishi and Naraoka, 2003; Bi et al., 2005; Liu et al., 2006; Sachse et al., 2006), and sporopollen assemblages, e.g. sporopollen to elevation transfer functions (Lü et al., 2011) and co-existence with elevation (Song et al., 2010; Sun et al., 2014). Each approach has its own strengths and weaknesses and, for this reason, we aim to render only a semi-quantitative palaeo-altitude inferred from the ecological habits of typical plants as represented in our pollen spectra. We have reached the conclusion that the paleoaltimetry of the Zanda Basin did not reach present altitudes, and did not exceed 3600 m even during the early Pliocene, if the following factors are taken into consideration: i. Evidence taken from modern sporopollen studies. Today, 18 species of Picea (such as Picea purpurea, Picea likiangensis, Picea crassifolia, and Picea asperata) grow in the mountainous regions of the TP (Committee of China Forest Compiler, 2000; Lü et al., 2008). Their uppermost height limit is about 4100 m asl in the south and about 2000 m asl in the north. At present, 22 species of Abies (such as Abies ernestii, Abies georgei and Abies pindrow) grow at lower altitudes in China, and another 12 species are found at elevations ranging from 2000 to 4000 m asl along the margins of the TP (Lü et al., 2008). Furthermore, pollen results obtained from TP surface soil samples show that both Picea and Abies have a growth optimum at altitudes between 2500 m and 4000 m asl, and their highest pollen concentration is distributed in soil samples at heights of around 3200 m asl (Lü et al., 2008).
ii. Evidence from fossil oxygen isotopes. As shown in Fig. 4, we know that sub-alpine dark coniferous forests grew on the TP in early Pliocene times. The present-day climatic conditions of this region are described as follows: mean annual temperature (MAT) is generally above 4 °C (Fig.4a), mean annual precipitation (MAP) is about 400–800 mm (Fig. 4b) and the warmest month mean temperature (WMMT) is about 10–14 °C (Fig.4c), as verified by other studies (e.g. Li and Chou, 1979, 1984; Lü et al., 2008; Li et al., 2012). Recently, δ13C values of enamel samples have pointed to a C3 vegetation that dominated the Zanda Basin during the early Pliocene, suggesting that the annual precipitation at that time was likely to be about 200–400 mm higher than it is today (200 mm), with paleotemperatures reaching 15 ± 7 °C between 4.2 and 3.1 Ma (XM Wang et al., 2013; Y Wang et al., 2013). Data derived from a fossil bone-based oxygen isotope temperature proxy (XM Wang et al., 2013; Y Wang et al., 2013) are thought to represent the warm season temperatures (Breecker et al., 2009; Passey et al., 2010; Quade et al., 2011). Such estimated climatic conditions for the early Pliocene times are similar to the present-day growing conditions required by dark coniferous forests. iii. Evidence taken from heat condition. Temperature and precipitation are two important factors affecting the distribution of coniferous forests. At present, MAT is 0 °C in Zanda County (3700 m asl) (Climatic Data Center, National Meteorological Information Center, China Meteorological Administration). Therefore, the difference between today's mean local temperature and that needed for the growth of dark coniferous forests (4 °C) is 4 °C. Ocean temperatures during the Pliocene warm period (ca. 5–3 Ma) were about 2–3 °C higher than today's, as suggested by marine paleo-temperature records (e.g. Lear et al., 2000; Ravelo et al., 2004), and temperature change due to climate change (caused by a warmer ocean) at high altitudes is probably greater than at sea level (Bradley et al., 2006). These warmer temperatures during the Pliocene might have compensated for the ca. ~ 4 °C difference in temperature required for the growth of dark coniferous forests in the Zanda Basin during the early Pliocene. In fact, the present altitude of the dark coniferous forest near Zanda at present is about 3000–3600 m asl
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(The comprehensive scientific expedition to the Qinghai–Xizang Plateau, 1985). Accordingly, it may be assumed that the paleoaltitude of the Zanda Basin during the early Pliocene was not higher than 3600 m asl, although there may have been some higher mountains surrounding the basin within which some animals such as H. zandaense may have lived.
6. Conclusion Sporopollen analysis of the lacustrine strata in the Zanda Basin shows a predominance of gymnosperm pollen (mainly Picea and Abies); angiosperms are represented by Ulmus, Betula, Carya, and Castanea pollens as well as Compositae. These pollen spectra imply
Fig. 4. Climatic conditions on the TP (Climatic Data Center, National Meteorological Information Center, China Meteorological Administration) and the distribution of dark coniferous forests (Zhang and Wang, 2007), where MAT is at or above 4 °C (a), MAP is ca. 400–800 mm (b) and WMMT is ca. 10–14 °C (c).
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Fig. 4 (continued).
sub-alpine dark coniferous forest vegetation with a wet and cool climate within the Zanda Basin during the early Pliocene. Taken into account the growing conditions required for this type of vegetation and the known reconstructed warm Pliocene climatic conditions, the paleo-altimetry of the Zanda Basin probably did not exceed an altitude of 3600 m asl, and hence did not achieve its present-day altitude before or during the early Pliocene. Acknowledgments Financial support has been provided by the German Science Foundation (DFG) within the framework of the TiP priority program (BL 436/51, BL 436/5-2), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB03020103, XDB03020400), the (973) National Basin Research Program of China (2013CB956400), and the National Natural Science Foundation of China (41272184 & 41321061). We thank the editor Thierry Corrège and four anonymous reviewers for their helpful suggestions and comments and Professors Bill Isherwood and Edward A Derbyshire for their help with improvement of the English. Many thanks are also due to Dr. Congrong An for his assistance in drawing the diagrams. References An, Z., Kutzbach, J.E., Prell, W.L., Porters, S.C., 2001. Evolution of Asian monsoon and phased uplift of the Himalaya–Tibetan plateau since Late Miocene times. Nature 411, 62–66. An, Z., Clemens, S.C., Shen, J., Qiang, X., Jin, Z., Sun, Y., Prell, W.L., Luo, J., Wang, S., Xu, H., Cai, Y., Zhou, W., Liu, X., Liu, W., Shi, Z., Yan, L., Xiao, X., Chang, H., Wu, F., Ai, L., Lu, F., 2011. Glacial–Interglacial Indian summer monsoon dynamics. Science 333, 719–723. Bi, X., Sheng, G., Liu, X., Li, C., Fu, J., 2005. Molecular and carbon and hydrogen isotopic composition of n-alkanes in plant leaf waxes. Org. Geochem. 36, 1405–1417. Boos, W.R., Kuang, Z.M., 2010. Dominant control of the South Asian monsoon by orographic insulation versus plateau heating. Nature 463, 218–223. Bradley, R.S., Vuille, M., Diaz, H.F., Vergara, W., 2006. Threats to water supplies in the tropical Andes. Science 312, 1755–1756. Breecker, D.O., Sharp, Z.D., McFadden, L.D., 2009. Seasonal bias in the formation and stable isotopic composition of pedogenic carbonate in modernsoils from central New Mexico, USA. Geol. Soc. Am. Bull. 121, 630–640.
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