Junbo Wang

Environmental changes since 8.4 ka reflected in the lacustrine core sediments from Nam Co, central Tibetan Plateau, China

The Tibetan Plateau induces and enhances the Asian monsoon that influences the plateau itself, East China and even the whole of Asia. Pursuing the changing monsoon history and its impact in this environment-sensitive area is a key for understanding the climatic changes. Here, we present the initial results from core sediments in Nam Co, central Tibet, to elucidate the Holocene environment changes in this area. The studied lake (Nam Co) is the second largest lake, with an elevation of 4718 m, water area of 2015 km2 and catchment area of 10 610 km2. By using echo-seismic profiling and hydro-echosounding, we investigated the bathymetric distribution of the whole lake. One 332 cm long PISTON core was successfully taken in the east part of the lake at 60 m water depth. Twelve AMS 14C dates from top to bottom of this core established a good time sequence. TOC, TN, n-alkanes, grain size, clay minerals, Fe/Mn, Sr/Ba and CaCO3 content were analysed with sampling intervals from 1 cm to 10 cm. Results showed that since 8400 yr BP, the environment changes in this region indicate three stages. The early stage (8400—6900 yr BP) was characterized by slight shifting from warm to cold with a cold-dry interval at 8100—7800 yr BP. The middle stage was from 6900 to 2900 yr BP, during which climate changed from warm-humid to cold-dry. However, around 2900 yr BP, the climate once was slightly cold but strongly dry. The late stage, from 2900 yr BP to present, was characterized by a general tendency toward cold-dry: the first temperature decline was around 1700—1500 yr BP, following which temperature increased once, before decreasing again around 600—300 yr BP, which might be the result of the `Little Ice Age.

Investigation of bathymetry and water quality of Lake Nam Co, the largest lake on the central Tibetan Plateau, China

Comprehensive field investigations have been conducted four times on Nam Co, central Tibet, from September 2005 to September 2008. Here, we present the preliminary results focusing on the bathymetric survey and water quality measurements. The isobathic map shows that Nam Co is a high-altitude, deep lake where a flat and large basin lies in the central part with a water depth of more than 90xa0m. Water depth data from the northwestern bank areas of Nam Co provide unquestionable evidence of rising water levels in the last 3 decades because of the formation of two small islands that were still peninsulas in the 1970s. Water quality measurements taken at 19 stations during three summer field campaigns (2006, 2007 and 2008) covering almost all of the lake areas showed that the temperature, pH, dissolved oxygen and electric conductivity of surface water are on average 11.43°C, 9.21, 8.90xa0mgxa0l−1 and 1,851xa0μSxa0cm−1, respectively. The surface water shows no obvious spatial variability among all the stations. Vertical fluctuations of profiles, however, display some differences in thermocline and related parameters, such as pH and dissolved oxygen. According to the vertical variations of water quality parameters, the water column in relatively deep lake areas of Nam Co could be divided into three layers with distinctly various features: the epilimnion is from the surface to about 18–20xa0m depth in which the parameters are homogeneous with higher temperature and abundant sunlight; the metalimnion ranged from 20–60xa0m where a thermocline develops; the deepest layer forms a cold and dark hypolimnion.

Pollen-inferred climate changes and vertical shifts of alpine vegetation belts on the northern slope of the Nyainqentanglha mountains (central Tibetan Plateau) since 8.4 kyr BP

Fossil pollen from Nam Co and modern pollen from altitudinal vegetation belts around the lake are investigated to reveal alpine vegetation succession in response to climate changes during the Holocene in the central Tibetan Plateau. The discriminant analysis on 37 topsoil samples shows that pollen samples from alpine steppe at lower elevations (<4800 m) and alpine meadow on upper slopes (4800–5200 m) can be distinguished by their pollen assemblages. Samples from alpine steppe contain more Artemisia (25.1%) and Poaceae pollen (11.5%), whereas those from alpine meadow are dominated by Cyperaceae pollen (>60%). Our result indicates that the pollen ratio of Artemisia to Cyperaceae (A/Cy) can be used as an indicator of the vertical shift of vegetation belts and temperature changes in the central Tibetan Plateau as suggested by previous studies. A history of the vertical shift of vegetation belts on the northern slope of Nyainqentanglha Mountains and climate changes since 8.4 kyr BP are thus recovered by 198 fossil pollen assemblages from a 332 cm core of Nam Co. Paleovegetation reconstructed from fossil pollen assemblages through discriminant analysis shows a general downward shift of altitudinal vegetation belts, suggesting a decline in the temperature trend since 8.4 kyr BP. This result is consistent with the reduction of A/Cy ratios. The fossil pollen record also reveals warm and wet climate during the early to mid Holocene, and cold and dry conditions during the late Holocene in the Nam Co area. A comparison of Holocene climatic reconstructions across the Plateau indicates that termination of maximum moisture at around 6–5.5 kyr BP in our record is associated with the southeastward retreat of the Southwest Monsoon.

Climate change on the Tibetan Plateau in response to shifting atmospheric circulation since the LGM

The Tibetan Plateau (TP) is primarily influenced by the northern hemispheric middle latitude Westerlies and the Indian summer monsoon (ISM). The extent, long-distance effects and potential long-term changes of these two atmospheric circulations are not yet fully understood. Here, we analyse modern airborne pollen in a transition zone of seasonally alternating dominance of the Westerlies and the ISM to develop a pollen discrimination index (PDI) that allows us to distinguish between the intensities of the two circulation systems. This index is applied to interpret a continuous lacustrine sedimentary record from Lake Nam Co covering the past 24u2009calu2009kyru2009BP to investigate long-term variations in the atmospheric circulation systems. Climatic variations on the central TP widely correspond to those of the North Atlantic (NA) realm, but are controlled through different mechanisms resulting from the changing climatic conditions since the Last Glacial Maximum (LGM). During the LGM, until 16.5u2009calu2009kyru2009BP, the TP was dominated by the Westerlies. After 16.5u2009calu2009kyru2009BP, the climatic conditions were mainly controlled by the ISM. From 11.6 to 9u2009calu2009kyru2009BP, the TP was exposed to enhanced solar radiation at the low latitudes, resulting in greater water availability.

Monsoonal forcing of Holocene paleoenvironmental change on the central Tibetan Plateau inferred using a sediment record from Lake Nam Co (Xizang, China)

This study focuses on Holocene monsoon dynamics on the central Tibetan Plateau (TP) inferred using a sediment record from Lake Nam Co. A high-resolution (decadal) multi-proxy approach, using geochemical, micropaleontological, and sedimentological methods was applied. Fifteen AMS-14C ages were used to establish the chronology, assuming a reservoir effect of 1,420xa0years. Our results point to a first strong monsoonal pulse at Lake Nam Co ~11.3xa0calxa0kaxa0BP, followed by colder and drier conditions until ~10.8xa0calxa0kaxa0BP. A warm and humid climate from ~10.8 to ~9.5xa0calxa0kaxa0BP is related to a strong summer monsoon on the central TP, triggering a lake-level highstand at ~9.5xa0calxa0kaxa0BP. Declining minerogenic input after ~9.5xa0calxa0kaxa0BP indicates less moisture availability until ~7.7xa0calxa0kaxa0BP. Following stable conditions between ~7.7 and ~6.6xa0calxa0kaxa0BP, a warm and wet climate is inferred for the time span from ~6.6 to ~4.8xa0calxa0kaxa0BP. A change towards drier conditions after ~4.8xa0calxa0kaxa0BP points to a weakening of the Indian Ocean Summer Monsoon on the central TP, further diminished after ~2.0xa0calxa0kaxa0BP. A short-term wet spell occurred from ~1.5 to ~1.3xa0calxa0kaxa0BP. By comparing the results derived from Lake Nam Co with several lacustrine records from the central, northern, and eastern TP, a similar but not synchronous pattern of monsoon-driven paleoenvironmental change is observed. Although the general trend of lake and catchment evolution on the TP during the Holocene is also reproduced in this record, pronounced spatial and temporal offsets with respect to distinct climate events were detected, suggesting periods of non-uniform moisture and temperature evolution.

Comparisons between the chemical compositions of lake water, inflowing river water, and lake sediment in Nam Co, central Tibetan Plateau, China and their controlling mechanisms

ABSTRACT The chemical composition of lake water and inflowing river water was investigated in Nam Co (lake), Tibetan Plateau, in September 2005. Lake water samples (n = 76) were collected at different depths along a south-north transect of the eastern part of the lake while water samples were collected from 69 rivers flowing into the lake; a sediment core was collected at 64 m midway along the water survey transect to investigate salt precipitation. Na+ and Ca2+ were the dominant cations, accounting for 76.2% and 60.6% of the lake and river water cations, respectively while was the dominant anion accounting for 70.8% and 93.4% of lake and river anions, respectively. CaCO3 precipitation from the water column decreased Ca2+ in the lake water, with the relative proportion of other ions increasing significantly. Evaporation-crystallization processes largely control Nam Co lake-water chemistry, while rock weathering is the dominant processes influencing the chemistry of river water; carbonate and silicate weathering are the major sources of ions in these rivers.

Ostracod-based environmental reconstruction over the last 8,400 years of Nam Co Lake on the Tibetan plateau

From a 332-cm long lacustrine core taken at 60xa0m water depth and 25 surface sediment samples taken at different water depth sites in Nam Co lake (4,718xa0m a.s.l.) in the middle-south part of the Tibetan Plateau, we identified nine species of ostracods (Crustacea: Ostracoda) belonging to six genera. Using lithological data, auto-ecological information of the recovered taxa and an ostracod-based transfer function for water depth reconstruction, we distinguished three main environmental stages over the past 8,400xa0years: during Stage I (8,400–6,800xa0yrxa0BP), the climate changed from warm-humid to cold-humid, and eventually to cold-arid. The water depth of the site was much lower than today and changed from an estimated 50 to 20–30xa0m. During Stage II (6,800–2,900xa0yrxa0BP), environmental conditions were again warm-humid, turning into a cold-arid episode. The lake water depth initially stayed much shallower than today, but then gradually deepened to around 50xa0m. At the earlier period of Stage III (2,900xa0yrxa0BP–present), the climate became again warm-humid from cold-arid status. There was a cold-dry event between 1,700 and 1,500xa0yrxa0BP, which intensified afterward while the surface run off weakened. Early in this stage, lake depth decreased slightly, but then it continuously deepened to 60xa0m. Our results revealed that central Tibet experienced wavily warm toward tendency in early Holocene, a shift from warm-humid to cold-dry conditions in the middle Holocene, and from warm-humid to cold-dry conditions in the late Holocene. They also show that ostracod assemblages are not only indicative of cold-warm conditions, but are also usable to imply the dry–wet status of a lake area by the inferring water depth variations. Finally, this study provides baseline data on (natural) climate change in this mountain region against which to compare global change impacts.

Spatiotemporal variations in volume of closed lakes on the Tibetan Plateau and their climatic responses from 1976 to 2013

To study the response of lakes to climate change, variations in volume of closed lakes larger than 50xa0km2 on the Tibetan Plateau (TP) were analyzed by integrating Shuttle Radar Topography Mission, Digital Elevation Model (SRTM DEM) and LANDSAT images. Total lake volume (LV) increased (net) by 102.64xa0km3 during 1976–2013 with an average annual rate of 2.77xa0km3xa0year−1; the rate increased dramatically to 7.67xa0km3xa0year−1 during 2000–2013. Four main patterns of LV change were discovered using k-means clustering analysis: (1) a slight increase in LV during 1976–2000 followed by a rapid increase in the southeastern part of the endorheic region, (2) an initial decrease in LV followed by an increase during 1990–2013 in the central and western parts of the endorheic region, (3) an initial decrease in LV followed by an increase during 2000–2013 in the northeastern part of the endorheic region, and (4) an overall decrease during the whole study period for the southern outflow region. Precipitation was the dominant factor affecting LV change. In particular, abnormally large amounts of precipitation during 2000–2013 resulted in a dramatic increase in LV. Temperature changes were positively correlated to LV change before 2000 and negatively after 2000. Glaciers have different influence on LV change based on the comparison between lakes with and without glacial melt input. The distinct regional patterns of change in LV indicate that the sensitivity of lakes to a changing climate varies depending upon the time period and spatial location. These findings have important implications for both the interpretation of past lake level records and prediction of future lake responses to climate change.

Sediment dynamics and hydrologic events affecting small lacustrine systems on the southern-central Tibetan Plateau – the example of TT Lake

A sedimentological, geochemical, micropaleontological, and palynological study of a lacustrine sediment record from the small TT Lake (southern-central Tibetan Plateau) shows that the background sedimentation was frequently interrupted by event-related deposits. These event-related deposits are interpreted as the result of hydrologic events that are triggered by above-average precipitation events. In total, 11 events were recorded in the TT Lake sequence. Two types can be differentiated: fluvial runoff events caused by precipitation that carried sediment in suspension into the lake and a sediment mass transport caused by torrential precipitation. The hydrologic events appear to be decoupled from long-term climate and environmental variations, but there is evidence that anthropogenic impact, in terms of pastoralism, might have favored the runoff events. The multi-proxy approach proved to be valuable and allowed for a detailed study of sedimentary processes within the lake and its watershed in order to assess their triggering processes and dynamics. The findings show the complexity of these sedimentary processes and their controlling factors, and the study aims to improve their understanding. This study is the first effort to investigate event-related deposits and sedimentary processes on the Tibetan Plateau and its triggering processes and dynamics by utilizing lacustrine sediment records.

Holocene lake level history of the Tangra Yumco lake system, southern-central Tibetan Plateau

Massive carbonate banks representing ancient lacustrine deposits are exposed in the catchment of the lake Tangra Yumco (southern-central Tibetan Plateau) and nearby lake Xuru Co. Nine sediment samples were taken below and above these lacustrine deposits to determine periods of changing lake level using optically stimulated luminescence (OSL) applying a multiple-aliquot regeneration (MAR) protocol. According to facies and stratigraphy, samples below the carbonate banks indicate a rising and samples from above a falling lake level. The results indicate that the rising lake level of Tangra Yumco passed the sampling location at 10.5 and 2.1 ka while a falling lake level passed the sampling location at 0.9 ka. The rising lake level of Xuru Co passed the sampling location at 7.9 and 1.7 ka and a falling lake level at 0.5 ka showing a similar trend as at Tangra Yumco. Combining these results with recalculated cosmogenic nuclide ages and previously published feldspar luminescence data allows the establishment of a Holocene lake level reconstruction for Tangra Yumco, which is unique for the southern-central Tibetan Plateau. The lake level of Tangra Yumco crested a lake level highstand of 181–183u2009m above the recent lake level prior to 8.5 ka and has generally fallen after 8.5 ka, with a minor lake level rise at 2.1 ka. Lake level variations at Tangra Yumco occur simultaneously with other lakes on the Tibetan Plateau indicating that variations were controlled by monsoonal dynamics with a moist early Holocene and a successive reduction of available moisture thereafter. The average rate of the lake level rise between 10.5 and 8.5 ka is at least 0.05u2009m a−1 (compared with a modern value of 0.38u2009m a−1), providing valuable insights in the monsoonal impact on lake level change on the southern-central Tibetan Plateau.