South-central Tibetan Plateau Research Articles

Annual to seasonal glacier mass balance in High Mountain Asia derived from Pléiades stereo images: examples from the Pamir and the Tibetan Plateau

Abstract. Glaciers are crucial sources of freshwater in particular for the arid lowlands surrounding High Mountain Asia. To better constrain glacio-hydrological models, annual, or even better, seasonal information about glacier mass changes is highly beneficial. In this study, we evaluate the suitability of very-high-resolution Pléiades digital elevation models (DEMs) to measure glacier mass balance at annual and seasonal scales in two regions of High Mountain Asia (Muztagh Ata in Eastern Pamirs and parts of western Nyainqêntanglha, south-central Tibetan Plateau), where recent estimates have shown contrasting glacier behaviour. The average annual mass balance in Muztagh Ata between 2019 and 2022 was −0.07 ± 0.20 m w.e. a−1, suggesting the continuation of a recent phase of slight mass loss following a prolonged period of balanced mass budgets previously observed. The mean annual mass balance in western Nyainqêntanglha was highly negative for the same period (−0.60 ± 0.15 m w.e. a−1), suggesting increased mass loss rates compared to the approximately previous 5 decades. The 2022 winter (+0.13 ± 0.24 m w.e.) and summer (−0.35 ± 0.15 m w.e.) mass budgets in Muztagh Ata and western Nyainqêntanglha (−0.03 ± 0.27 m w.e. in winter; −0.63 ± 0.07 m w.e. in summer) suggest winter- and summer-accumulation-type regimes, respectively. We support our findings by implementing the Sentinel-1-based Glacier Index to identify the firn and wet-snow areas on glaciers and characterize the accumulation type. The good match between the geodetic and Glacier Index results supports the potential of very-high-resolution Pléiades data to monitor mass balance at short timescales and improves our understanding of glacier accumulation regimes across High Mountain Asia.

Late Quaternary modeling and paleoclimate evolution of the Mount Taibai glacier (Qinling Mountains, China) constrained by cosmogenic radionuclide exposure dating

Glacial landforms on Mount Taibai, the main peak of the Qinling Mountains, China, are investigated to improve understanding of Quaternary palaeoclimate evolution in this key region of China's geographical north and south junction. Cosmogenic radionuclide 10Be exposure dating and glacier-climate modeling are used to reconstruct glacier evolution during the last glaciation in the Eryehai Valley, on the slopes of Mount Taibai. Chronological results indicate that the earliest initiation of the Mount Taibai glaciation was earlier than the global Last Glacial Maximum (gLGM). The glacier subsequently advanced further reaching its greatest extent during the gLGM between ∼23.50 ka and 21.76 ka. After this time, the glacier receded rapidly, before disappearing completely between ∼17.9 ka and 17.7 ka. We also analyzed a compilation of 229 recalculated 10Be exposure ages from the mountain ranges on the eastern Tibetan Plateau (TP), as well as in the Jaggang Mountains on the south-central TP, and found that a few mountain ranges on the eastern TP experienced two distinct millennial-scale moraine building events during the gLGM. Using glacier-climate modeling, temperature was calculated to have fallen from between ∼9.0 and 9.4 °C during the gLGM, with precipitation being 60–80% of the present-day value. If precipitation during the rapid retreat of the glacier between ∼21.76 ka and 18.10 ka were to be consistent with present trends, then the temperature would have needed to increase by a total of ∼1.3 °C during this period, with a mean rate of increase of 0.4 °C/ka. Combining our findings with other paleoclimate proxies, we infer that the main reason that there was no further glacial advance after the gLGM on Mount Taibai was the lower altitude of its peak relative to the Equilibrium-line Altitude (ELA). Our results provide the paleoclimatic conditions of the last glaciation in this critical area of China's geographical north and south junction, and deepen the understanding of how the East Asian Summer Monsoon (EASM) influences glaciers on the TP.

Holocene hydrological history of a Tibetan glacier-fed lake Taro Co in response to climate change

Lake hydrological fluctuations on the Tibetan Plateau (TP) are highly sensitive to global climate change, which can be reconstructed directly by dating paleoshorelines representing past lake-levels. In this study, we reconstructed the Holocene lake-level history of Taro Co lake system (Taro Co, Zabuye Salt lake and Lagkor Co) on the southwestern TP, based on seventeen optically stimulated luminescence ages of paleoshorelines. The results showed that lake-levels were at the broad highstands of ∼40 m above modern lake-level (amll) from 11.6 to 7.0 ka, during which the three lakes were integrated and formed a mega paleolake. After 7.0 ka, the mega paleolake fluctuantly contracted, and Lagkor Co and Zabuye Salt Lake probably became separated at ∼6.9 and ∼3.4 ka, respectively. Seven rapid millennial low lake stands were identified at 11.3–11.2 ka (14 m amll), 9.9–8.7 ka (< 28 m amll), 8.2–7.9 ka (6 m amll), ∼6.9 ka (5 m amll), ∼3.0 ka (8 m amll), 2.1–1.1 ka (< 6 m amll) and ∼0.4 ka (6 m amll). The large magnitude lake-level declines indicated that they reflected effective moisture in response to global millennial-scale climate events, e.g., the 11.1, 10.3, 9.4, 8.2 and 7.2 ka events in the early and middle Holocene, and the 4.2 ka and Little Ice Age events in the late Holocene. Holocene lake-level fluctuations were simultaneous with that of other lakes on the south-central TP. Comparisons with other paleoclimatic records suggested that the Holocene hydrological history of Taro Co was mainly dominated by both monsoonal precipitation and glacial meltwater supply, as well as evaporation and basin topography. Differences in meltwater supply probably led to different lake-level change histories for endorheic lakes in the lake system. In the future warming and wetting, Tibetan endorheic lakes would keep expanding, hence Taro Co lake system would be formed again through hydrological connections.

The Validation of Soil Moisture from Various Sources and Its Influence Factors in the Tibetan Plateau

The tempo-spatial continuous soil moisture (SM) datasets of satellite remote sensing, land surface models, and reanalysis products are very important for correlational research in the Tibetan Plateau (TP) meteorology. Based on the in situ observed SM, AMSR2, SMAP, GLDAS-Noah, and ERA5 SM are assessed at regional and site scales in the TP during the non-frozen period from 2015 to 2016. The results indicate that SMAP and ERA5 SM (AMSR2 and GLDAS-Noah SM) present an overestimation (underestimation) of the TP regional average. Specifically, SMAP (ERA5) SM performs best in Maqu and south-central TP (Naqu, Pali, and southeast TP), with a Spearman’s rank correlation (ρ) greater than 0.57 and an unbiased root mean square error (ubRMSE) less than 0.05 m3/m3. In Shiquanhe, GLDAS-Noah SM performs best among the four SM products. At the site scale, SMAP SM has relatively high ρ and low ubRMSE values at the most sites, except the sites at the Karakoram Mountains and Himalayan Mountains. The four SM products show underestimation in different degrees at Shiquanhe. The ρ values between AMSR2 SM and rainfall are the highest in most study subregions, especially in Naqu and Pali. For the other SM products, they have the highest positive correlations with a normalized difference vegetation index (NDVI). Besides, land surface temperature (LST) has significant negative (positive) correlations with SM products in the summer (other seasons). Through the multiple linear stepwise regression analysis, NDVI has negative (positive) impacts on SM products in the spring (other seasons), while LST shows the opposite conditions. NDVI (rainfall) is identified as the main influencing factor on the in situ observed, SMAP, GLDAS-Noah, and ERA5 (AMSR2) SM in this study. Compared to previous studies, these results comprehensively present the applicability of SM products in the TP and further reveal their main influencing factors.

Orbital-scale hydroclimate variations in the southern Tibetan Plateau over the past 414,000 years

Speleothem oxygen isotope (δ18O) constitutes one of key proxies to reconstruct the Indian and East Asian summer monsoon (ISM and EASM) hydroclimate history on a wide range of timescales. Here, we use three new well-dated speleothem δ18O records from Tianmen cave, in the south-central Tibetan Plateau (TP), together with previous records from the same cave, to reconstruct a composite Tianmen record spanning over the past 414,000 years. The Tianmen δ18O record follows Northern Hemisphere summer insolation (NHSI), broadly coherent with speleothem δ18O records from both ISM and EASM regimes. In comparison with previous model simulations and modern hydroclimate analyses, the Tianmen δ18O was interpreted primarily as a proxy indicating the ISM intensity. The Tianmen δ18O record comprises both high and low NHSI time periods, which reveals an exceptional large amplitude (∼14‰) of δ18O variations at the precession band, possibly resulted from a combination effect of precipitation amount, moisture source, transport pathway, high elevation and rainout. Besides, a broad consistency on orbital-scale variability among Tianmen δ18O, δ13C and trace element records is found, suggesting that the ISM intensity regulates the effective rainfall at the cave site as well. Notably, Tianmen speleothem growth occurred merely during warm-wet interglacial periods, even during low NHSI times near the precession maximum, and in contrast, hiatuses persistently occurred throughout cold-dry glacial periods. This pattern is similar to that found in travertine formations in the region, both suggesting that the interglacial temperature might be a prerequisite for speleothem/travertine formations in the region, while the strong ISM precipitation in turn provides a sufficient condition. Our new Tianmen record, combined with a large set of speleothem records from both Asian Westerlies and Asian monsoon domains, gives further evidence to reinforce a previous hypothesis: a consistent temporal pattern in the δ18O of precipitation on orbital and millennial scales over the vast Asian monsoon and Westerlies regimes. As such, in order to reconcile the precipitation δ18O variations between speleothem and TP ice cores, we argue that the Guliya ice-core chronology may have a basal age of ∼70 ka BP, which is supported by recent dating results from the Guliya ice cap.

Direct dating of lithic surface artifacts using luminescence.

Archaeological surface assemblages composed of lithic scatters comprise a large proportion of the archaeological record. Dating such surface artifacts has remained inherently difficult owing to the dynamic nature of Earth-surface processes affecting these assemblages and because no satisfactory chronometric dating technique exists that can be directly applied to constrain the timing of artifact manufacture, discard, and thus human use of the landscape. Here, we present a dating approach based on optically stimulated luminescence (OSL)-OSL rock-surface burial dating-and apply it to a lithic surface scatter in Tibet. We generate OSL burial ages (age-depth profiles) for each artifact, outline the methodological complexities, and consider the artifact burial ages in the context of local-scale Earth-surface dynamics. The oldest age cluster between 5.2 and 5.5 thousand years is likely related to quarrying activities at the site and thus represents the oldest chronometric age constraints for human presence on the south-central Tibetan plateau.

Delineation of a Quaternary Aquifer Using Integrated Hydrogeological and Geophysical Estimation of Hydraulic Conductivity on the Tibetan Plateau, China

Groundwater is the most unexplored element of the hydrologic cycle on the Tibetan Plateau (TP) due to harsh climate conditions. This study aims at delineating and characterizing the unexplored Zhanongtang–Ganmanong aquifer, situated in the Zhagu subcatchment of the Nam Co catchment, south-central TP. Multiple hydrogeophysical and lithological in situ field and laboratory methods are applied: depth-to-water-table measurements, grain size analysis, hydraulic empirical and field methods to estimate hydraulic conductivity (K), and analysis of electrical resistivity tomography profiles. Integration of these methods revealed the existence of a Quaternary hydrostratigraphic unit that was found to be unconsolidated, laterally heterogeneous and homogeneous over depth. The results revealed consistent K ranges of three K zones, which is in accordance with local lithology. The K ranges are applicable to other locations within the Nam Co catchment with similar lithology as in the study area without further field experiments. Permafrost was found to be absent in the study area ranging from 4730 m a.s.l. to 5200 m a.s.l. altitude. These results provide insight into the hydrogeological conditions of the TP and are useful for conceptual and numerical groundwater flow modeling to predict future changes of water fluxes and water budgets caused by climatic change, especially in remote areas.

A note on the lake level variations of Nam Co, south-central Tibetan Plateau from 2005 to 2019

A note on the lake level variations of Nam Co, south-central Tibetan Plateau from 2005 to 2019

Effects of the South Asian summer monsoon anomaly on interannual variations in precipitation over the South-Central Tibetan Plateau

An anomalous South Asian summer monsoon (SASM) system could generate a large anomaly in precipitation and hydrological disasters in the SASM-prevailing area, as widely reported for the Indian Peninsula. However, how the SASM system influences the precipitation anomaly over the South-Central Tibetan Plateau (SCTP) is largely unknown. In this study, we (a) analyze the influences of the early and late onset (demise) of the SASM on the interannual variations in precipitation over the SCTP during 1979–2015; and (b) illuminate the underlying mechanisms and asymmetric effects with regard to the onset and demise of the SASM by analyzing the characteristics of water vapor transport and moisture budgets in this region. Results indicate that the precipitation anomaly over the SCTP is dominated by the cyclonic and anticyclonic water vapor transport associated with the anomalous SASM activities, causing moisture convergence and divergence in this region. The topographic effect in the southeastern Tibetan Plateau (SETP) and southwestern Tibetan Plateau (SWTP) further strengthens the anomaly in water vapor transport in vertical direction and contributes to the precipitation anomaly through moisture convergence and divergence. The anomalous SASM and topography exhibit asymmetric effects between the onset and demise as well as between the early and late onset (demise) of the SASM. They cause 23.41%, 15.91%, and 1.96% difference in precipitation between the early and late SASM-onset years, and 13.05%, 21.50%, and 29.86% difference in precipitation between the early and late SASM-demise years in the SETP, central Tibetan Plateau (CETP), and SWTP, by regulating the horizontal and vertical thermodynamic and dynamic processes. The results help improve our understanding of the SASM-precipitation relationship over the SCTP and guide the prediction of precipitation and alleviation of water-related disasters in the region and its surroundings that are home to billions of people in Asia.

Revisiting growth and decline of late Quaternary mega-lakes across the south-central Tibetan Plateau

Lake basins record critical interactions between the hydrosphere, cryosphere and atmosphere. Their sediments and shoreline landforms archive timing, duration, and intensity of past climatic variability and environmental impacts over a variety of timescales. Thousands of lake systems spread across the internally drained Tibetan Plateau today, of which many are only the salty remains of much more expansive paleolakes in the past. This study presents new shoreline ages integrated with quantitative digital topographic analysis and regional geomorphic evidence to reconstruct the sizes and extents of late Quaternary rift lake systems for the south-central Tibetan Plateau. This study presents optically stimulated luminescence (OSL) and the first K-feldspar post-infrared infrared (pIRIR) stimulated luminescence ages for Zabuye Caka and Dawa Tso and tests if these lakes were once part of a singular Pleistocene mega-lake established across four N–S rift systems. Our new results show that two large but separate paleolake systems developed in the latest Pleistocene-earliest Holocene and no lake larger than ∼6460 km2 has existed since at least 41 ka in the Lunggar region. Early Holocene paleolakes expanded up to 7× modern (+220–335 km3) with two to four-fold asymmetric lake expansion in the west compared to the east. Findings corroborate earlier investigations, implying that dramatic runoff increase and/or evapotranspiration reduction prompted rapid earliest Holocene lake expansion. Precipitation variability, in combination with permafrost degradation, temperature and wind strength, governed paleolake moisture balance. Open-system lake behavior and evolving drainage configurations across complex topography explain delayed onsets of lake regression and punctuated lake decline. This suggests abrupt Holocene climate shifts are not necessarily required for punctuated lake decline. Lastly, unique geomorphological and paleoclimatic similarities between Tibetan and East African rift lakes highlight the potential for some Tibetan paleolakes to function as natural climate amplifiers over both short and geologic timescales.

Moisture Sources for Wintertime Intense Precipitation Events Over the Three Snowy Subregions of the Tibetan Plateau

AbstractWintertime intense precipitation events often lead to severe snow disasters. In this study, a Lagrangian approach is employed to examine the evaporative moisture sources for wintertime intense precipitation events over the three snowy subregions of the Tibetan Plateau (TP) during 1979–2016, including the western TP (WTP), south central TP (SCTP), and southeastern TP (SETP). More than 80.0% of the moisture for intense precipitation over each subregion originates from terrestrial areas. Although prevailing westerly winds dominate above the TP and its surrounding areas during winter, half of the precipitation over the three subregions is supplied by evaporation from the south (i.e., the Indian Peninsula). Specifically, evaporation from the Indian Peninsula contributes 68.0%, 65.0%, and 45.0% of the moisture for intense precipitation over the WTP, SCTP, and SETP, respectively. The two primary oceanic moisture source regions for intense precipitation are the Arabian Sea and the Bay of Bengal, playing complementary roles in supplying moisture. The relative contributions of the Arabian Sea to intense precipitation over the WTP, SCTP, and SETP are 9.2%, 6.9%, and 1.1%, while those of the Bay of Bengal are 1.1%, 12.1%, and 8.6%. Southerly winds downstream of a cyclonic anomaly over the Indian Peninsula are crucial for the low‐level moisture transport from the south to the Himalayan foothills. Under the combined effects of orographic lifting and favorable large‐scale circulation patterns, moisture ascends further into the three subregions. Changes in the position and intensity of the cyclonic anomaly are particularly crucial to facilitating moisture contributions from the key source regions.

Past the climate optimum: Recruitment is declining at the world's highest juniper shrublines on the Tibetan Plateau.

Alpine biomes are climate change hotspots, and treeline dynamics in particular have received much attention as visible evidence of climate-induced shifts in species distributions. Comparatively little is known, however, about the effects of climate change on alpine shrubline dynamics. Here, we reconstruct decadally resolved shrub recruitment history (age structure) through the combination of field surveys and dendroecology methods at the world's highest juniper (Juniperus pingii var. wilsonii) shrublines on the south-central Tibetan Plateau. A total of 1,899 shrubs were surveyed at 12 plots located in four regions along an east-to-west declining precipitation gradient. We detected synchronous recruitment with 9 out of 12 plots showing a gradual increase from 1600 to 1900, a peak at 1900-1940, and a subsequent decrease from the 1930s onward. Shrub recruitment was significantly and positively correlated with reconstructed summer temperature from 1600 to 1940, whereas it was negatively associated with temperature in recent decades (1930-2000). Recruitment was also positively correlated with precipitation, except in the 1780-1830 period, when a trend toward wetter climate conditions began. Warming-induced drought limitation has likely reduced the recruitment potential of alpine juniper shrubs in recent decades. Ongoing warming without a simultaneous increase in precipitation is expected to further impair recruitment at the world's highest juniper shrublines and alter the dynamics and competitive balance between woody plant species throughout these alpine biomes.

May–June drought reconstruction over the past 821 years on the south-central Tibetan Plateau derived from tree-ring width series

Knowledge of drought variability and their possible mechanisms during the past hundred years is still limited in the mountainous region of south-central Tibetan Plateau (TP). In this study, a long-term tree-ring width chronology dating back to 1190 CE was combined using 328 increment cores from the Nagqu region. Based on the relationships between this tree-ring width chronology and climate data, we reconstructed May–June self-calibrated Palmer Drought Severity Index (scPDSI) for the past 821 years (1190–2010 CE). Additional comparisons with other available precipitation or drought reconstructions were conducted. We further investigated the influence of the South Asian summer monsoon (SASM) on the drought variability in our study region. Results indicated that our tree-ring width chronology contained stable drought signal in the early summer season (May–June). During the past 821 years, the longest dry and wet periods lasted for 116 and 90 years, respectively, based on a 21-year Fast Fourier transform filter. Specifically, longer than ten years’ dry periods prevailed during 1211–1245 CE, 1280–1358, 1421–1471, 1500–1571, 1580–1598, 1650–1691, 1782–1807 and 1867–1982; while wet intervals occurred in 1190–1210 CE, 1246–1279, 1359–1420, 1472–1499, 1599–1649, 1692–1781, 1808–1866 and 1983–2010. Generally consistent dry and wet intervals across the southern TP were found by comparisons with other available datasets during their common periods. Interestingly, we detected an unstable influence of the SASM on the May–June drought variability in our study region, at least for the past three and a half centuries. This study therefore gives a new perspective of drought variability as well as their relationships with the SASM over a long-term period on the south-central TP.

Investigating changes in lake systems in the south-central Tibetan Plateau with multi-source remote sensing

Lakes in the Tibetan Plateau are considered sensitive responders to global warming. Variations in physical features of lake systems such as surface area and water level are very helpful in understanding regional responses to global warming in recent decades. In this study, multi-source remote sensing data were used to retrieve the surface area and water level time series of five inland lakes in the south-central part of the Tibetan Plateau over the past decades. Changes in water level and surface area of the lakes were investigated. The results showed that the water level of three lakes (Puma Yumco, Taro Co, Zhari Namco) increased, with expanding surface area, while the water levels of the other two lakes (Paiku Co, Mapam Yumco) fell, with shrinking area. The water levels of the lakes experienced remarkable changes in 2000–2012 as compared with 1976–1999. Spatially, lakes located at the southern fringe of the Tibetan Plateau showed consistency in water level changes, which was different from lakes in the central Tibetan Plateau.

Potential forcing mechanisms of Holocene lake-level changes at Nam Co, Tibetan Plateau: Inferred from the stable isotopic composition of shells of the gastropod Radix

The timing of lake-level fluctuations on the Tibetan Plateau and their relationship with climatic changes is still under debate, and the main reason for this is the lack of suitable archives for reconstructing the paleohydrology and paleoclimatology of the lakes. Here, we present the results of analyses of the shell geochemistry of Radix sp. from an exposed terrace of Nam Co lake on the south-central Tibetan Plateau. Optically stimulated luminescence (OSL) dating reveals that deep-water lacustrine sediments formed between ca. 4.4 and 2.2 ka, suggesting a high and stable lake level significantly above the present. The results of Sr/Ca, δ13C and δ18O analyses of the fossil shells of Radix sp. indicate that during the mid- to late-Holocene, lake-level variations in Nam Co were mainly controlled by variations in the Indian Summer Monsoon. A trend of decreasing evaporation also played an important role. Comparison with other results suggests a consistent pattern of mid- to late-Holocene lake-level changes across a large area of the Tibetan Plateau and adjacent regions to the south, which had a similar causal mechanism. Finally, our results indicate that fossil shells of the gastropod Radix sp. of the lakes on the Tibetan Plateau are a valuable archive for reconstructing the regional paleohydrology and paleoclimatology.

Multiproxy paleoaltimetry of the Late Oligocene-Pliocene Oiyug Basin, southern Tibet

The stable isotope compositions of carbonate and organic samples from the Oiyug basin in southern Tibet allows for model calculations of the Oligocene to Pliocene paleoelevation of the south central Tibetan Plateau. We measured the oxygen isotope composition of pedogenic and lacustrine calcite, dolomite, and siderite, and the hydrogen isotope composition of n-alkanes from plant waxes to reconstruct the δ18O and δD values of Oiyug basin paleometeoric water. Calculated water isotope values from Oiyug basin carbonate and organic samples, respectively, are in close agreement, suggesting the preservation of an unaltered paleometeoric water isotopic signal in these archives. Late Oligocene-middle Miocene paleoelevation estimates from groundwater/pedogenic calcite and lacustrine dolomite indicate basin elevations of 4.1 km +1.2/−1.6 km. Plant-wax n-alkanes δD and lacustrine-siderite δ18O compositions of middle Miocene (∼15 Ma) samples indicate paleoelevations of 5.1 km +1.3/−1.9 km. This estimated elevation is similar to the 5.4 km paleoelevation estimate based on fossil-floral physiognomy from the same stratigraphic interval. Calculated late Miocene-Pliocene paleoelevation estimates derived from the δ18O composition of lacustrine marls and carbonate/siderite concretions, as well as the δD from plant wax n-alkanes indicate a mean elevation of 5.5 km +1.4/−2.0 km at ∼5 Ma. Although calculated mean paleoelevations for the Oiyug basin all fall within the errors associated with the model calculations, the close agreement of the different paleoelevation proxies provides an additional degree of confidence in the fidelity of the calculated paleoelevations. Calculated paleoelevations indicate a possible increase in Oiyug basin elevations of ∼1.4 km between the early and late Miocene. Given the modern Oiyug basin elevation of ∼4.3 km, study results allow for a possible >1 km decrease in elevation since the early Pliocene. These findings, in conjunction with other Tibetan paleoaltimetry studies, are consistent with tectonic models supporting high elevations of the Tibetan Plateau since the initiation of India-Asia collision during the Eocene, and subsequent late Cenozoic extensional collapse.

Late quaternary hydrological changes at Tangra Yumco, Tibetan Plateau: a compound-specific isotope-based quantification of lake level changes

Lake level fluctuations are important features in paleo-reconstructions that can be linked to past climate changes. Closed-basin lakes on the south-central Tibetan Plateau record the balance between monsoonal precipitation and evaporative loss. To date, most studies provide only qualitative estimates of past hydrological changes. For the first time, we applied hydrogen isotopes (δD) of aquatic and terrestrial n-alkanes to quantify lake volume changes of Tangra Yumco and compared values to newly calculated lake volumes of Nam Co. We also used carbon isotopes of sedimentary n-alkanes to reveal past climate and environmental conditions around the lake. The water volume of Tangra Yumco changed by 146 km3 in the past 17.42 cal ka. Lake volume increased in two steps, the first mainly initiated by glacial meltwater input after 16 cal ka BP, and to a minor extent by first strengthening of the Asian Summer Monsoon (ASM). The second increase was caused by intensified summer monsoon precipitation at 11.45 cal ka BP. After 8.0 cal ka BP, lake volume decreased because of arid conditions, until an increase at 0.8 cal ka BP that was probably linked to a wet spell during the Little Ice Age. The lake level changes of Tangra Yumco and Nam Co were affected simultaneously by the ASM, although their amplitudes differed because of different local conditions, such as basin morphology or the blocking effect of neighboring mountains, which influences the rainout of air masses and wind-induced evaporation.

Biogeochemical evidence for freshwater periods during the Last Glacial Maximum recorded in lake sediments from Nam Co, south-central Tibetan Plateau

Improved knowledge of deglaciation pro- cesses during the termination of the Last Glacial Maximum on the Tibetan Plateau can provide impor- tant information for understanding deglaciations in climate-sensitive high-altitude ecosystems. Little, however, is known about this time interval because most lacustrine sediment records from the Tibetan Plateau are younger than 19,000 years. This study focused on a lake sediment record from Nam Co, south-central Tibetan Plateau, covering the interval from *23.7 to 20.9 cal ka BP. We analysed the distribution and compound-specific hydrogen isotope composition (dD) of sedimentary n-alkanes, as well as the bulk sediment TOC, TN, d 13 Corg and d 15 N composition, to infer lake system development. Pro- nounced changes in environmental conditions between *21.6 and 21.1 cal ka BP, as well as between 23.1 and 22.5 cal ka BP (Greenland Inter- stadial 2), were inferred from increased aquatic n- alkane amounts and decreased dDn-C23 values within these time intervals, respectively. Freshwater inputs, which most likely resulted from enhanced glacier melting, causedthese changes. Our results suggestthat mountain glacier retreat on the Tibetan Plateau started earlier than previously assumed. The required energy for thawing was probably provided by temperature changes caused by reorganization of atmospheric circulation,whichhasalsobeenrecorded inGreenland ice records.

Water balance observations reveal significant subsurface water seepage from Lake Nam Co, south-central Tibetan Plateau

Summary Hydrological observations including precipitation, runoff, evaporation and lake level were performed in detail in the basin of the Lake Nam Co in the southern-central Tibetan Plateau from 2007 to 2011. Two runoff coefficients at differing catchment scales were obtained and applied to derive the surface water inflow to the lake from the non-glaciated areas. Total runoff was partitioned into contributions from glaciated and non-glaciated areas at two additional catchment scales. The runoff from the non-glaciated areas amounted to 1400 mm lake depth during the observation period from May 18 to October 17, 2008, during which the most complete set of observations was obtained. Neglecting groundwater contributions, this amount accounted for 55–65% of the total water input to the Lake Nam Co. The precipitation contribution and the meltwater inflow from the glaciers accounted for 23–28% and 7–22% of the total water input to the lake, respectively. The water input was found to be 810–1220 mm lake depth (1.63 × 109–2.47 × 109 m3) more than the output of the lake during a 5-month period in 2008. Similar results were obtained for the years 2007 and 2009–2011. As there is no surface outflow from the lake, the large water imbalance can only be explained by water seepage, corresponding to an average outflow of 120–190 m3 s−1 or 5–8 mm lake water depth per day. It is likely that the main pathway for lake water seepage is through the sub-surface fault system, which is known to exist in the area.

Tree-ring derived millennial precipitation record for the south-central Tibetan Plateau and its possible driving mechanism

Knowledge of Asian monsoon variability remains limited because of sparse instrumental data available only for short time series. Here, an updated tree-ring width record covering the period ad 1037–2009 was developed for the south-central Tibetan Plateau (TP). Correlation analysis revealed a significant relationship ( r = 0.71) between the tree-ring index and annual (previous July to current June) precipitation series for the instrumental period 1963–2008, which accounts for 50.41% of the rainfall variability. Based on a linear regression model, the longest available regional precipitation history was reconstructed. Spatial correlation between tree ring width and annual precipitation data from previous July to current June indicates that the reconstruction is representative of precipitation changes on the south-central TP. Regional wet conditions occurred during ad 1095–1161, 1376–1403, 1414–1446, 1518–1537, 1549–1572, 1702–1757, 1848–1878 and 1891–1913, while dry periods were identified during ad1189–1242, 1256–1314, 1329–1357, 1470–1491, 1573–1623, 1636–1686, 1761–1821, 1823–1847, 1879–1890 and 1931–1985. The negative correlation between our reconstructed precipitation and India monsoon rainfall series indicates the seesaw pattern over northern and southern monsoon Asia. It is suggested that solar radiation-induced sea surface temperature (SST) anomalies over the tropical Pacific influence regional rainfall patterns. The degree of this influence has been stable at the multidecadal scale during the past 1000 years.