Spatial variability and the controlling mechanisms of surface sediments from Nam Co, central Tibetan Plateau, China

Sedimentary Geology 319 (2015) 69–77

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Spatial variability and the controlling mechanisms of surface sediments from Nam Co, central Tibetan Plateau, China Junbo Wang a,b,⁎, Liping Zhu a,b, Yong Wang a, Jianting Ju a, Gerhard Daut c, Minghui Li a a b c

Key Laboratory of Tibetan Environment Changes and Land Surface Processes (TEL), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China Institut für Geographie, Friedrich-Schiller-Universität Jena, D-07743 Jena, Germany

a r t i c l e

i n f o

Article history: Received 10 October 2014 Received in revised form 7 January 2015 Accepted 8 January 2015 Available online 12 February 2015 Editor: Dr. J. Knight Keywords: Geochemical proxies Spatial variability Lake level Controlling mechanisms Nam Co Tibetan Plateau

a b s t r a c t Sixty-six surface sediments were retrieved from Nam Co, central Tibetan Plateau, to evaluate the spatial variability of their distributions and controls across the entire lake. Grain size distribution, total carbon, total nitrogen, total sulfur, 210Pb and 137Cs activities, and carbon and oxygen isotopes of carbonate of the bulk sediments were analyzed, and the correlations among these variables, as well as water depth, were calculated. The results showed distinct spatial variability for all variables. The grain size distribution provides adequate information to reflect water energy levels within the lake. Based on these factors, the accumulation zone of Nam Co was distinguished from the erosional and transportation zones. The results indicate that the deep area (N 80 m) of the central main basin and the centre of the eastern small basin serve as the accumulation zones in Nam Co. Water depth is the most important factor influencing the distribution of the surface sediments because all variables show different distribution patterns in shallow and deep areas. Additionally, river input, sediment focusing, grain size effects, and heterogeneous physicochemical features of the lake water, as well as possible currents within the lake, also play different roles that affect surface sediment characteristics. Water depth noticeably affects grain size and the δ13C and δ18O values of carbonate sediment, implying that these proxies could be used as indicators of lake level change. These findings are of significance for palaeoenvironmental interpretations when using these proxies. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Sediment is not evenly distributed over the bed of most lakes, and various factors exist that influence the depositional processes in lakes (Sly, 1978; Downing and Rath, 1988; Anderson, 1990a). Theoretically, fine sediment particles are more prone to be distributed in deep water areas, whereas coarse materials dominate shallow regions due to the different energy levels within a lake with distance from sediment input point (Håkanson, 1982). As a result, a lake can typically be divided into three sedimentary zones: an erosional zone, a transportation zone, and an accumulation zone (Håkanson and Jansson, 1983). Based on some early studies, the term “sediment focusing” was introduced to describe the phenomenon that, in general, allows for more material to accumulate in deeper than in shallower areas of lakes. As a result, models have been constructed to predict the sediment focusing and redistribution in small lakes (e.g., Hilton, 1985; Hilton et al., 1986).

⁎ Corresponding author at: Key Laboratory of Tibetan Environment Changes and Land Surface Processes (TEL), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. E-mail address: [email protected] (J. Wang).

http://dx.doi.org/10.1016/j.sedgeo.2015.01.011 0037-0738/© 2015 Elsevier B.V. All rights reserved.

Many studies have evaluated the spatial variability of surface sediments in lakes with regard to aspects such as heavy metal distribution and sediment transport (Moore, 1980; Onyari and Wandiga, 1989), diatom assemblages (Anderson, 1990b, 1990c; Zalat and Vildary, 2005), geochemical records or fossil-fuel-derived pollutants (Hilton and Gibbs, 1984; Rose et al., 1998; Korsman et al., 1999), and pollen distribution (Debusk, 1997; Zhao et al., 2006). Vogel et al. (2010) analyzed the composition of recent sediments in Lake Ohrid (Albania/ Macedonia) and suggested that sedimentation in this lake is controlled by geological catchment features, anthropogenic land use and water currents. In Laguna Potrok Aike, southern Patagonia, Argentina, the lake's wind-driven internal currents are believed to be the main influence on the surficial sediment distribution pattern (Kastner et al., 2010). Some mechanisms affecting sedimentary processes have also been distinguished, such as turbidity currents from delta foreslopes that affect deep water depositional pattern (Lamoureux, 1999). Reynoldson and Hamilton (1982) suggested that wind action could disturb bottom sediment, resulting in homogeneous sediments, whereas other studies suggested wind was a forcing factor for uneven sediment distribution patterns (Odgaard, 1993; Whitemore et al., 1996). Bacterial oxidation (Hilton and Gibbs, 1984), benthic animals' activity (Downing and Rath, 1988), industrial distribution and land use conditions (Boyle

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et al., 1998; Rose et al., 1998; Gälman et al., 2006) are all influencing factors that can affect the distribution of sediments. Nam Co, the second largest lake in Tibet (Tibet Autonomous Region, China), has been considered an important site for both palaeoenvironmental changes and modern limnological studies in the past several years. Focusing on recent and long-term environmental changes, several cores have been retrieved from Nam Co, and palaeoenvironmental records or lake level fluctuations have been reconstructed for the Holocene period or more recent stages (Zhu et al., 2008, 2010a; Daut et al., 2010; Frenzel et al., 2010; Mügler et al., 2010; Li et al., 2011a; Kasper et al., 2012, 2013; Doberschütz et al., 2014). Wang et al. (2009) completed a survey of lake bathymetry and modern water physicochemical conditions. These studies have provided useful information for understanding the environmental evolution and the modern status of Nam Co. However, investigations of this lake are still insufficient with respect to the modern sedimentation patterns and related processes. Li et al. (2011b, 2012) and Wang et al. (2012a) illustrated the spatial distribution of rare earth elements, monohydrocalcite and organic matter and argued for their utility in palaeoenvironmental change studies. Nevertheless, the water energy level of Nam Co and the controlling factors of sediment distribution were still not sufficiently discussed. The present study focuses on the spatial variability of surface sediments in Nam Co based on their physical and chemical properties like grain size, organic and inorganic carbon component, total sulfur, radionuclide activities (210Pb, 137Cs) as well as oxygen and carbon isotopes of carbonate. The main aim is to map lake surface sediments and to elucidate the controlling mechanisms on sediment distribution

patterns. Moreover, the environmental significance of different proxies commonly used for lake palaeoenvironmental reconstruction will be discussed. 2. Materials and methods 2.1. Study area Nam Co is a closed lake formed by Himalayan tectonic activity. It is located in the central part of the Tibetan Plateau (90°16′–91°03′E, 30°30′–30°55′N, Fig. 1) at an altitude of approximately 4718 m a.s.l. The lake and its drainage area were 1920 km2 and 10,610 km2, respectively, in the 1970s. Thus, the replenishment coefficient (the ratio of drainage area to lake area) of Nam Co is 5.53 (Guan et al., 1984). The rivers sourced by precipitation and meltwater from modern glaciers in the Nyainqentanglha range to the southeast of Nam Co form the main water supply. More than 60 rivers flow into the lake during the summer season, and most of them are distributed along the western and southern shores (Wang et al., 2010). The largest rivers originate from the Nyainqentanglha range to the southwest, whereas almost no rivers flow in from the north (Fig. 1). The bathymetric survey showed a large and flat deep-water area in the central part of the lake, where the water depth is N95 m and the deepest recorded point is ~ 99 m (Wang et al., 2009, Fig. 1). Based on the calculated area from remote sensing images and water depth data, the water volume is estimated to be 783.23 × 108 m3 and 863.77 × 108 m3 for the years 1971 and 2004, respectively. Over the same period, the lake area increased from 1920 km2 to 2015.38 km2

Fig. 1. Map of Nam Co showing the isobaths, the surface sediment sampling sites, and some main river inflows. The upper small maps show the location of Nam Co in the Tibetan Plateau and the catchment.

J. Wang et al. / Sedimentary Geology 319 (2015) 69–77

(Zhu et al., 2010b). Thus, the average depth of Nam Co can be calculated as 40.8 m and 42.9 m in 1971 and 2004, respectively. The subaqueous slopes at the southern and northern shore areas are rather steep. In contrast, the underwater morphologies of the eastern and western parts have low gradients, dominantly formed by subaqueous fans. Climatically, Nam Co is located in the monsoon-influenced transition zone between semi-humid and semi-arid areas and has an annual mean temperature and precipitation of approximately 0 °C and ~ 450 mm, respectively. The rainy and dry seasons are distinct in this area, and the precipitation that falls during the warm season (May–September) accounts for 91% of the total annual precipitation (Guan et al., 1984; Ma et al., 2012). The mean annual wind speed is 4 m s − 1 , and the maximum wind speeds occur in January. The dominant wind direction is from southeast to west (135°–270°) throughout the year (You et al., 2007). 2.2. Sampling During two field campaigns (August 2007 and September 2008), a total of 66 surface samples was obtained from across the entire lake using a modified Van Veen grab sampler as previously presented in Li et al. (2011b, 2012) and Wang et al. (2012a, 2012b) (Fig. 1). The water depth of sampling sites ranged from 7.0 to 97.5 m (Table 1). From the grab samples, the uppermost 2 cm layer was subsampled to ensure that only modern processes are reflected in this study. Most samples were fine clay with a brown-gray or yellowish color, but some samples had a higher content of silt or even some sand. 2.3. Analysis The grain size distribution was measured using a Laser Diffraction Particle Size Analyzer (Beckman Coulter LS 13320, CA, USA. Size range: 0.375–2000 μm) after treatment with 10% HCl and 15% H2O2 to remove carbonates and organic matter, respectively. The total carbon (TC), total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) contents were determined using an Elemental Analyzer (Vario EL II, Elementar Analysensysteme GmbH, Germany). The accuracy of the carbon, nitrogen and sulfur (CNS) analyses was ± 0.02–0.05%. The total inorganic carbon (TIC) content was calculated from the difference between the TC and the TOC. Grain size and CNS analysis were completed in Department of Geography, Jena University, Germany. Radionuclide (210Pb, 137Cs) activities were measured by well-type high-purity germanium gamma spectrometry (ORTEC GWL-120-15, TN, USA) in the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. Cylindrical centrifugal tubes were filled with samples and sealed with parafilm for at least 3 weeks to allow radioactive equilibration before measurement (Appleby, 2001). Each sample was measured for ~22 h, and the excess 210Pb activity (210Pbex) was calculated from the total 210 Pb minus the 226Ra activity. The oxygen and carbon isotope ratios of the bulk carbonates were determined by an MAT 253 with an accuracy of ±0.2‰ (Finnigan, Germany) in the Laboratory of Stable Isotope and Geochemistry, Chinese Academy of Geological Sciences. The results of oxygen and carbon isotope ratios were expressed as per mill (‰) relative to the Vienna-Peedee Belemnite (V-PDB). In total, 66 samples were analyzed, except for the oxygen and carbon isotopes, which had 61 samples.

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3. Results 3.1. Grain size distribution Grain size of lake sediments is of great importance in studies of palaeoenvironmental change, usually providing information on catchment influences, lake level changes, density currents, and other factors. The mean grain size distribution shows a distinct variability in Nam Co (Table 1). Its distribution over the lake clearly correlates with water depth (Fig. 2). In the shallow lakeshore areas of both subbasins, the mean grain size is large, whereas in the central basin areas, the mean grain size is small. The mean grain size and the grain size standard deviation (SD) both show variations with water depth in almost identical patterns. In shallow areas, fine to very coarse sediments appear, while in deeper areas, only fine-grained sediments are found (Fig. 2). The boundary is at an approximately 60 m water depth, below which the variation of mean grain size markedly decreases with increasing water depth. More precisely, below an 80 m water depth, a much more homogeneous distribution with a mean grain size of 10.6– 21.9 μm is detected, reflecting very low water energy and gentle dynamics. Spatially, both mean grain size and their sorting coefficient (SD) show similar and clear variability across the lake (Fig. 3). The most impressive feature is that the coarsest materials (mean grain size up to ~ 400 μm) appear in the southwestern part of the lake and has the poorest sorting (the largest SD value of 390 μm). In this area, there is a decrease of mean grain size away from the lake shore (Fig. 3A). SD of grain size has the same trend, showing sorting increases with increased water depth (Fig. 3B). This pattern clearly reflects the strong influence of river input on the deposited materials in the lake bottom. However, the largest value of mean grain size (437 μm with SD of 306 μm) was detected in the eastern small basin at a water depth of 30 m. For most areas of Nam Co, mean grain size shows a typical regime of decreasing continuously from littoral zone to pelagic areas. In central areas of both sub-basins, the mean grain size and SD are similar. 3.2. CNS content variability In Nam Co, CNS content shows explicit variability. TIC (average 3.98%) is more than TOC (average 1.07%) in the total carbon composition. TN (average 0.19%) and TS (average 0.13%) display much lower content compare with TOC. C/N ranges from 2.56 to 7.00 with an average of 5.55 (Table 1). The CNS contents of surface sediments display notable variations across the entire lake with respect to water depth (Fig. 4). Based on the distributions, the TOC, TIC and TN show similar patterns generally in a quasi “V” shape. This pattern indicates that both low and high concentrations of carbon and nitrogen appear in shallow areas (lakeshore areas), and with increasing water depth, the ranges of concentrations become narrower until an approximately 80 m depth (open lake areas), below which most proxies are in very limited ranges with medium values for all the samples. The TS and the C/N ratios have patterns that are different from the others. The TS of most samples is low, while only a few samples have relatively high values. The overall TS distribution displays a slight decreasing trend with increasing water depth. The C/N ratio presents an opposite pattern of that of TS: the ratio increases with increasing water depth. Despite

Table 1 Statistics of the surface sediment variables from Nam Co (n = 61 for δ13CV-PDB and δ18OV-PDB, n = 66 for the other variables, SD = standard deviation; CV = coefficient of variation.).

Minimum Average Maximum SD CV

Depth (m)

Mean grain size (μm)

TOC (%)

TIC (%)

TN (%)

TS (%)

C/N

210

7.0 53.8 97.5 / /

10.62 70.44 437.50 92.59 1.31

0.10 1.07 2.26 0.47 0.44

0.68 3.98 7.29 1.21 0.30

0.04 0.19 0.41 0.08 0.41

0.05 0.13 0.56 0.09 0.68

2.56 5.55 7.00 0.78 0.14

7.15 161.35 271.23 56.18 0.35

Pbex (Bq/kg)

137

Cs (Bq/kg)

1.96 26.36 48.95 13.49 0.51

δ13CV-PDB (‰)

δ18OV-PDB (‰)

2.8 3.73 4.5 0.39 0.11

−10.6 −6.85 −5.3 1.18 −0.17

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Fig. 2. Mean grain size (Mean) and standard deviation (SD) of surface sediments versus water depth in Nam Co.

Fig. 3. Contour map (filled shading) showing mean grain size (A) and standard deviation (B) of surface sediments (values in μm) combined with bathymetric information (isobathic line with depth data) in Nam Co (other legends see Fig. 1).

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Fig. 4. Total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), total sulfur (TS) and TOC/TN ratios (TOC/TN) of surface sediments versus water depth in Nam Co (partly from Wang et al. (2012a)).

the dissimilar distribution patterns, both the TS and the C/N ratios are as homogenous as the carbon and nitrogen values at water depths greater than 80 m (Fig. 4). Wang et al. (2012a) illustrated the organic matter distribution of surface sediments in Nam Co. The results showed that TOC and TN show similar spatial distribution patterns and the highest values appeared in the central area of the eastern sub-basin. In the open area of the main basin, TOC and TN concentration displayed an increasing trend from shallow to deep areas and the lowest values were found in the southwestern part of the lake. However, the spatial distribution of C/N showed a different regime, in that relatively high values appeared in the western area of the main basin except for the southwest corner (for more detailed results and discussion, see Wang et al. (2012a)). As for inorganic carbon, it is indicated that the lowest values are in the southwest part of the lake and relatively low concentrations were detected along the entire south bank to the east until the end of the main basin (Fig. 5A). TIC values show an increasing trend with increased water depth from south to north, and high concentrations appear in the deepest part of the main basin and the north bank area of the whole lake, as well as most parts of the eastern small basin. The highest TIC concentrations were found in the northwestern corner and southeastern area of the small sub-basin. Compared with the other variables, TS content shows no clear spatial distribution pattern. High values appear in a small area of the eastern part of the lake. For the main basin, TS shows an increasing trend from south to north, and relatively high values were found in littoral zones in the west and north (Fig. 5B). 3.3. 210Pbex and 137Cs activities distribution As two radioactive nuclides, the activities of 210Pbex (unsupported Pb) and 137Cs are commonly used to determine recent chronology in core sediment within lake (Appleby, 2001). Therefore, their spatial distributions in a lake are of significance when determining their utility as a reliable dating tool. In the Nam Co surface sediments, the activities of 210Pbex and 137Cs show remarkable variability (Table 1). The distributions of both 210Pbex and 137Cs activities display similar positive correlations to water depth (Fig. 6). Low activities only appear in relatively shallow areas, while higher activities appear in the deeper areas. Both radionuclide activities are enhanced with increased water depth. 210

3.4. Carbon and oxygen isotopes of carbonates In Nam Co, large variabilities in the carbonate δ13CV-PDB and δ18OV-PDB values of the 61 surface sediments have been observed (Table 1). The distributions of δ13CV-PDB and δ18OV-PDB values in the lake display rather similar patterns; both increase with increasing water depth (Fig. 7). Broader ranges of values exist in shallower areas than in deeper areas. The δ13CV-PDB and δ18OV-PDB values are quite homogeneous in the areas where the water depths are N80 m. 4. Discussion 4.1. Sediment distribution and water energy level in Nam Co The accumulation zone is of great interest to palaeoenvironmental change studies because it is the least influenced by wind-driven turbulence and other processes, allowing for the accumulation of fine-grained sediments in the accumulation zone. These sediments provide the most complete, continuous, and most reliable record of past environmental change (Smol, 2008). Therefore, to conduct palaeolimnological investigations in a lake, one of the most important tasks is to distinguish the accumulation zone. The grain size of lake sediments is mainly influenced by water energy levels in a lake fine grain size material that generally dominates the open water areas, whereas coarser deposits dominate the shallow regions (e.g., Sly, 1978; Håkanson, 1982; Last, 2001; Xiao et al., 2012). The grain size distribution in Nam Co shows a distribution pattern affected by processes that lead to deposition at greater depth. Coarse sediments dominate the littoral zones, and fine particles are mostly detected in the pelagic area of the two sub-basins (Fig. 3). The grain size distribution implies that energy level diminishes with increasing water depth. The highest energy level appears in the southwestern part of the lake, with dominantly coarse particles, which could be attributed to the large river inflows in this area. That the grain size decreases continuously from the river mouth to the deeper area indicates that the influence of river inflow in this area could extend to a water depth of ~60 m (the maximum distance is beyond 20 km) (Fig. 3). The medium energy level (rather coarse sediments) is found in several small littoral parts of the eastern basin. However, unlike the

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Fig. 5. Contour map (filled shading) showing total inorganic carbon (A) and total sulfur (B) distributions in surface sediment (values in %) combined with bathymetric information (isobathic line with depth data) in Nam Co (other legends see Fig. 1).

Fig. 6. 210Pbex and 137Cs activities of surface sediments versus water depth in Nam Co.

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Fig. 7. Carbon (δ13CV-PDB) and oxygen (δ18OV-PDB) isotope ratios of carbonate from surface sediments versus water depth in Nam Co.

highest energy area, no river flows in there. The mean grain size increases to a maximum at approximately 30 m of water depth (Fig. 2). Most of these samples with large grain sizes are located in the medium energy level areas, indicating that only coarse particles stay in these places. The possibility is that the fine sediments were not deposited in or they could be eroded and transported elsewhere by other forces. In either case, the water circulation, internal lake currents, or turbulence could be responsible for this distribution. However, the evidence is still insufficient to identify the forcing mechanism. The lowest energy level, reflected by the dominance of homogeneous fine particles, is found in the small internal area of the eastern sub-basin and a large area of the central main basin (Fig. 3). These areas can be distinguished as the accumulation zone, whereas the other areas are dominated by erosional and transportation effects. However, to establish the limit between areas of erosion and transportation is very diffuse and difficult, both in theory and in practice, as Håkanson (1982) has argued. The sediments in the open areas of the main basin (generally similar areas with water depths N80 m) and the central area in the eastern subbasin display homogeneous fine grain sizes, reflecting low energy environments. Consequently, these areas are regarded as the accumulation zones of Nam Co. 4.2. Sediment focusing at the bottom of Nam Co From the overall data set, it is difficult to find a visible relationship among the TOC, the TN and the water depth. The TOC and TN exhibit high correlation coefficients, implying that TN is most likely controlled by TOC, but both variables show almost no correlations with water depth (Table 2). However, Wang et al. (2012a) described the TOC and TN distributions in the whole lake and found distinctly higher concentrations of TOC and TN in the accumulation areas than in the other zones. Moreover, higher values were observed in the eastern subbasin than in the open areas of the main basin. The n-alkanes distribution shows a similar pattern. This pattern implies that the distribution of organic matter in Nam Co is controlled to some extent by the sediment focusing mechanism. The distribution pattern of rare earth

element (REE) concentrations shows the highest values in the open area of the main basin, suggesting that the sediment focusing processes affect REE in the main accumulation area of Nam Co (Li et al., 2011b). However, the TIC and TS are not concentrated in the deep areas. The TIC exhibits no relationship with water depth across the entire data set (Table 2) and generally presents higher values in the northwestern and eastern areas near river mouths. The lowest values appear in southwestern areas where the largest river inputs are (Fig. 5A). In Nam Co, the inorganic carbon mainly consists of authigenic calcite (Wang et al., 2010, 2012b; Li et al., 2012). The distribution pattern of TIC implies that the rivers discharge low salinity water, which is not favorable for calcite precipitation within the lake. Wang et al. (2010) investigated the main ionic composition of rivers around Nam Co and found that both Ca2 + and HCO− 3 showed remarkable variabilities in the rivers, low concentrations were detected in the southwestern area and high concentrations appeared in northwestern and eastern areas which well matched the distribution pattern of TIC. In northern area of Nam Co where no river input, TIC also show relatively high concentration in surface sediment, which could be ascribed to evaporation (Wang et al., 2010). The TS is a unique component in terms of spatial distribution, low contents and high variations in all the surface samples (Table 1). These factors indicate that there is little sulfuric material such as sulfides and sulfates from the catchment delivered to the lake. Relatively high TS contents are observed in shallow areas, especially in the littoral zone of the eastern sub-basin and the medium water energy level in the northwestern littoral zone. The relatively low contents are widely distributed in the open lake area of the main basin, especially along the southern flank of the lake (Figs. 4, 5B). Different terrestrial inputs of sulfides and sulfates could be the main influence. This hypothesis is partly supconcentrations in the river inputs around the lake ported by the SO2− 4 (Wang et al., 2010). However, the negative correlation between the TS and water depth leads to the inference that the TS is not prone to transport to the pelagic zones from the littoral zones. More importantly, the TS distribution is most likely also affected by sulfur cycling in the lake associated with hydrogen sulfide releases due to different redox water environments (Goldman and Horne, 1983; Wetzel, 2001).

Table 2 Correlation coefficients between the variables of the surface sediments from Nam Co (n = 66, ** Correlation is significant at the 0.01 level (2-tailed); * Correlation is significant at the 0.05 level (2-tailed)).

TOC (%) TIC (%) TN (%) TS (%) C/N Mean grain size (μm) 210 Pbex (Bq/kg) 137 Cs (Bq/kg) Depth (m)

TOC (%)

TIC (%)

TN (%)

TS (%)

C/N

Mean grain size (μm)

1.00 0.31* 0.97** 0.42** 0.59** −0.61** 0.67** 0.71** 0.14

1.00 0.32** 0.38** 0.23 −0.06 0.21 0.11 0.08

1.00 0.45** 0.40** −0.55** 0.66** 0.62** 0.08

1.00 0.11 0.08 −0.11 −0.15 −0.49**

1.00 −0.53** 0.51** 0.66** 0.37**

1.00 −0.71** −0.74** −0.48**

210

Pbex (Bq/kg)

1.00 0.77** 0.60**

137

Cs (Bq/kg)

1.00 0.67**

Depth (m)

1.00

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4.3. Sedimentary processes and influence mechanisms in Nam Co There are various external and internal factors influencing the sedimentary processes in a lake, and most of them are believed to be lake specific (e.g., Sly, 1978; Kastner et al., 2010; Vogel et al., 2010). Lake basin morphology (or water depth distribution) is likely one of the most common factors in many lakes, and it influences the sedimentary processes in lakes to different extents through different water energy levels and transport distances from the catchment (e.g., Murase and Sakamoto, 2000; Vogel et al., 2010; Shanahan et al., 2013). In Nam Co, most variables exhibited spatial variability associated with water depth, especially the proxies most likely controlled by sediment focusing. It is obvious that water depth is the general and main controlling factor for variable distributions in Nam Co (Figs. 2–7). The δ13C values of carbonate in the surface sediments of Nam Co correlate well with water depth (r = 0.69, n = 61), i.e., the δ13C values are higher in deeper areas, indicating that the δ13C values of carbonate from Nam Co lake sediments could be used to reflect lake level change. The δ18O values of carbonate show a similar correlation with water depth, with more positive values in deeper areas (r = 0.57, n = 61). In general, the oxygen isotope composition of the minerals is only controlled by the temperature and lake water isotopic composition (Leng and Marshall, 2004). Therefore, it is argued that, the δ18O values of carbonate in sediments could reflect temperature changes as well as lake level. This is similar to Lake Qinghai, for which Liu et al. (2009) argued that the δ18O values increased as water depth increased. The TOC, TN, 210Pbex and 137Cs have negative correlations with mean grain size, i.e., higher values with finer particles and lower values with coarser particles. It is assumed that these variables are partly controlled by the grain size effect during transport and deposition (e.g., Moalla, 1997; Boyle, 2001; Zhang et al., 2002; Wang and Zhu, 2008). REE concentrations also possess higher values in central areas where the grain size is finer (Li et al., 2011b). As a very large and deep lake, the physicochemical features of Nam Co water may differ in different areas, both horizontally and vertically (Wang et al., 2009). The spatial variability of the δ13CV-PDB and δ18OV-PDB values of carbonate in the surface sediments displays remarkable inhomogeneity across the lake, and may be attributed to different dissolved inorganic carbon and oxygen isotopic compositions of the lake water. As the main phase of carbonate in the surface sediment, monohydrocalcite was found to be irregularly distributed in Nam Co. This pattern was also explained as the result of heterogeneous features of the lake water (Li et al., 2012). Wang et al. (2012a) argued that the dissimilar concentrations of organic matter in the surface sediments may be influenced by heterogeneous water quality parameters in different areas, especially the comparison between the river mouth and the open areas. Through the monitoring of the water quality at multiple stations, the spatial heterogeneity of the lake water has been preliminarily described, including water temperature, pH, and electric conductivity (Wang et al., 2009). Currents from the river mouths, as well as internal lake currents, are generally an important factor superposed on the basic sedimentary processes in a lake. In Nam Co, there is a narrow belt with coarse sediments in the eastern small basin with water depths of b30 m. This is most likely the result of internal currents within Nam Co. As a whole, the water depth, river inputs, sediment focusing, grain size effects, heterogeneous physicochemical features of the lake water, as well as possible internal currents, are the main controlling factors that influence the spatial distribution of surface sediments in Nam Co, and consequently have different impacts that must be considered when using proxies for palaeolimnological investigations. 5. Conclusions Based on our analysis of 66 surface sediments retrieved from Nam Co, the accumulation zones of this lake were determined according to

the grain size of the sediments. The results indicated that the deep area (with an approximate water depth of N 80 m) of the central main basin and the centre of the eastern small basin act as the accumulation zones. Sediment focusing and grain size effects were observed in several proxies from the lake sediments. The water depth, river inputs, grain size effects, heterogeneous physicochemical features of the lake water, and possible internal currents within the lake affect surface sediment characteristics. Among these factors, water depth plays the most important role and correlates with grain size and δ13C/δ18O values of sedimentary carbonate, implying that these proxies could be used as indicators of lake level changes associated with past environmental changes. Acknowledgments We are grateful to Dr. Xiao Lin, Dr. Xiaolin Zhen and Dr. Ping Peng for their assistance in the field. We also thank Ms. Kati Hartwig from the Institute of Geography, Jena University, and Mr. Shaopeng Gao from the Institute of Tibetan Plateau Research, Chinese Academy of Sciences, who completed most of the analyses. This work was jointly supported by the National Natural Science Foundation of China (Grant No. 41071123), the National Basic Research Program of China (Grant No. 2012CB956100) and the CADY project from BMBF of Germany (Grant No. 03G0813F). Thanks are also given to one anonymous reviewer and Dr. Jasper Knight for the constructive and helpful comments which greatly improved the manuscript. References Anderson, N.J., 1990a. Spatial pattern of recent sediment and diatom accumulation in a small, monomictic, eutrophic lake. Journal of Paleolimnology 3, 143–160. Anderson, N.J., 1990b. Variability of diatom concentration and accumulation rates in sediments of a small lake basin. Limnology and Oceanography 35, 497–508. Anderson, N.J., 1990c. Variability of sediment diatom assemblages in an upland, windstressed lake (Loch Fleet, Galloway, S.W. Scotland). Journal of Paleolimnology 4, 43–59. Appleby, P.G., 2001. Chronostratigraphic Techniques in Recent Sediments. In: Last, W.M., Smol, J.P. (Eds.), Tracking environmental change using lake sediments. Basin analysis, coring, and chronological techniques 1. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 171–204. Boyle, J.F., 2001. Inorganic Geochemical Methods in Palaeolimnology. In: Last, W.M., Smol, J.P. (Eds.), Tracking environmental change using lake sediments. Physical and geochemical methods vol. 2. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 83–142. Boyle, J.F., Mackay, A.W., Rose, N.L., Flower, R.J., Appleby, P.G., 1998. Sediment heavy metal record in Lake Baikal: natural and anthropogenic sources. Journal of Paleolimnology 20, 135–150. Daut, G., Mäusbacher, R., Baade, J., Gleixner, G., Kroemer, E., Mügler, I., Wallner, J., Wang, J., Zhu, L., 2010. Late Quaternary hydrological changes inferred from lake level fluctuations of Nam Co (Tibetan Plateau, China). Quaternary International 218, 86–93. Debusk, G.H., 1997. The distribution of pollen in the surface sediments of Lake Malawi, Africa, and the transport of pollen in large lakes. Review of Palaeobotany and Palynology 97, 123–153. Doberschütz, S., Frenzel, P., Haberzettl, T., Kasper, T., Wang, J., Zhu, L., Daut, G., Schwalb, A., Mäusbacher, R., 2014. Monsoonal forcing of Holocene paleoenvironmental change on the central Tibetan Plateau inferred using a sediment record from Lake Nam Co (Xizang, China). Journal of Paleolimnology 51, 253–266. Downing, J.A., Rath, L.C., 1988. Spatial patchiness in the lacustrine sedimentary environment. Limnology and Oceanography 33, 447–458. Frenzel, P., Claudia, W., Xie, M., Zhu, L., Schwalb, A., 2010. Palaeo-water depth estimation for a 600-year record from Nam Co (Tibet) using an ostracod-based transfer function. Quaternary International 218, 157–165. Gälman, V., Petterson, G., Renberg, I., 2006. A comparison of sediments varves (1950–2003 AD) in two adjacent lakes in northern Sweden. Journal of Paleolimnology 35, 837–853. Goldman, C.R., Horne, A.J., 1983. Limnology. McGraw–Hill, New York. Guan, Z., Chen, C., Ou, Y., Fan, Y., Zhang, Y., Chen, Z., Bao, S., Zu, Y., He, X., Zhang, M., 1984. Rivers and Lakes in Tibet. Science Press, Beijing, pp. 1–238 (in Chinese). Häkanson, L., 1982. Bottom dynamics in lakes. Hydrobiologia 91, 9–22. Häkanson, L., Jansson, M., 1983. Principles of Lake Sedimentology. Springer-Verlag, Berlin. Hilton, J., 1985. A conceptual framework for predicting the occurrence of sediment focusing and sediment redistribution in small lakes. Limnology and Oceanography 30, 1131–1143. Hilton, J., Gibbs, M.M., 1984. The horizontal distribution of major elements and organic matter in the sediments of Esthwaite Water, England. Chemical Geology 47, 57–83. Hilton, J., Lishman, J.P., Allen, P.V., 1986. The dominant processes of sediment distribution and focusing in a small, eutrophic, monomictic lake. Limnology and Oceanography 31, 125–133.

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