Two ice cores were retrieved from high elevations (
Data from remote sensing and in situ observations suggest that glacier shrinking has been prevailing over the Tibetan Plateau (including the Himalayas hereafter) in the past decades (e.g., Liu et al., 2006; Kang et al., 2010; Fujita and Nuimura, 2011; Bolch et al., 2012; Kääb et al., 2012; Yao et al., 2012; Neckel et al., 2014), raising major concerns over the impact on water supplies to some 1.4 billion people in Asia (Immerzeel et al., 2010), and on global sea level rise (Jacob et al., 2012; Gardner et al., 2013; Neckel et al., 2014). It has been estimated that glacier retreat has been occurring in more than 82 % of the total glaciers in the region (Liu et al., 2006), and since the 1970s, glacier areas have reduced by several percent (about 4.8 %) in the central Tibetan Plateau (Ye et al., 2006) and up to 20 % in the northeastern marginal regions of the Tibetan Plateau (Cao et al., 2010; Pan et al., 2012). In situ stake observations have also confirmed a continuously negative mass balance during the last decade in the region (Yao et al., 2012; Zhang et al., 2014). However, quantitative changes in the glacier ice volume, a key parameter for assessing retreating glaciers' impact on water supply or sea level rise, remain poorly known due to the lack of in situ measurements of glacier thickness through time. Although remote sensing techniques have provided some assessments of glacier thickness globally, especially in the last decade, the application of these techniques to the Tibetan Plateau region is rather limited, due to the complexity of the regional topography (Jacob et al., 2012; Kääb et al., 2012; Gardner et al., 2013; Neckel et al., 2014).
Based on the lack of distinctive marker horizons of atmospheric
thermonuclear bomb testing (e.g., beta radioactivity,
With an average elevation of over 4000 m a.s.l., the Tibetan Plateau is home to the largest volume of glacier ice outside the polar regions (Grinsted, 2013). The Tibetan Plateau blocks mid-latitude westerlies, splitting the jet into two currents that flow to the south and north of the plateau. The plateau is also a major forcing factor on the intensity of the Asian monsoons. The southern and central Tibetan Plateau is climatically influenced primarily by the Indian monsoon during the summer monsoon season, and the westerlies during the non-monsoon season (Bryson, 1986; Tang, 1998).
Location map of the glacier ice cores Geladaindong (GL) and Nyainqêntanglha (NQ) on the Tibetan Plateau. Also shown are the locations of the Naimona'nyi ice core by Kehrwald et al. (2008), Xiaodongkemadi glacier, Zhadang glacier, the meteorological stations, and Lake Nam Co.
Two ice cores were retrieved as part of the Sino–US cooperation expedition
(Fig. 1). The Nyainqêntanglha ice core (30
The Nyainqêntanglha ice core was transported in a frozen state to the
Climate Change Institute at the University of Maine, USA, whereas the
Geladaindong ice core was transported to the State Key Laboratory of Cryospheric Sciences of
the Chinese Academy of Sciences, Lanzhou, China. The cores were sectioned at
3 to 5 cm intervals in a cold (
All of the samples were measured for
Total Hg concentration was analyzed following the US EPA method 1631 using a
Tekran® 2600 at the Ultra-Clean Trace Elements
Laboratory at the University of Manitoba, Canada, or
Jena® MERCUR in a metal-free Class 100 laminar
flow hood, placed in a Class 1000 cleanroom laboratory at the Key Laboratory
of Tibetan Environment Changes and Land Surface Processes. Field blank
samples were collected during each sampling and their Hg concentrations were
always lower than 0.3 ng L
The
The tritium profiles of the Geladaindong (GL) and Nyainqêntanglha (NQ) ice
cores compared with tritium in precipitation in the Northern Hemisphere. Error
bars for the ice core samples are shown, but in most cases are only about
half of the symbol size. To enable direct comparison, both the Geladaindong
and precipitation tritium records are decay-corrected to the date of
Geladaindong ice core drilling (October 2005). The record of tritium in
precipitation (upper axis) shows the Ottawa precipitation record
(International Atomic Energy Agency, 2013, WISER database:
Some of the most prominent global stratigraphic markers recorded in ice
cores over the last century are radionuclides (e.g.,
To further test this hypothesis, we analyzed
In contrast, the Geladaindong ice core exhibited a classic
Dating of the Geladaindong ice core by annual layer counting based
on the seasonal cycles of
The Geladaindong ice core dating by counting annual layers is in agreement
with previous work in the region. Based on snow pit records in the Guoqu
glacier, Mt. Geladaindong, Zhang et al. (2007) reported that Ca
One assumption in dating by counting annual layers backwards from the AD 1963 nuclear bomb horizon to AD 1982 is that there was annual net ice accumulation during this period. Uncertainties in the chronology will thus rise should there be no net accumulation in one or some of the years due to ice melt. This does not appear to be the case for the Geladaindong ice core, as the annual variation patterns and amplitudes in the main ion concentrations were similar upward and downward from the AD 1963 layer, suggesting no occurrence of strong melt (Fig. 4). Furthermore, the air temperatures were much lower before the 1980s than those in the last 3 decades according to the data observed from the nearby meteorological stations such as Amdo. Indeed, the continuously deficit mass balance (cumulative negative mass balance) has only been reported since the 1990s in the central Tibetan Plateau (e.g., Xiaodongkemadi glacier, near to the Geladaindong region; Yao et al., 2012), as well as in the northern neighboring region (e.g., Glacier No. 1, Tienshan Mts.; Zhang et al., 2014), due to dramatic warming in recent decades. Therefore, we might suggest that the mass loss of the coring site occurred mainly from the 1990s in the central Tibetan Plateau.
To further investigate whether the lack of net ice accumulation at the Geladaindong site has occurred since the 1980s, we examined the profile of Hg in the ice core. Although naturally occurring in the Earth's crust, Hg emission (especially the gaseous elemental mercury) into the atmosphere has been greatly enhanced coinciding with the rise in anthropogenic activities (e.g., mining, burning of fossil fuels). Mercury has a lifetime of approximately 0.5–2 years (Holmes et al., 2010) and can be transported globally via atmospheric circulation. Hg profiles in ice cores from high (Fäin et al., 2008) and mid-latitude (Schuster et al., 2002) regions have matched the general chronological trends of global atmospheric Hg emissions or global industrial Hg use. As atmospheric transport is essentially the only transport pathway for anthropogenic Hg to the Tibetan Plateau, due to the region's high altitudes and minimal to nonexistent local industrial activities (Loewen et al., 2007), ice cores from the region could provide a useful indicator for atmospheric Hg concentrations, as demonstrated by Hg profiles in snowpacks overlying the glaciers across the plateau (Loewen et al., 2007; Zhang et al., 2012).
Comparisons of Hg records from the Geladaindong (GL) ice cores with those from Lake Nam Co sediments (Li, 2011), as well as with known history of regional (Asia and former USSR) and global Hg production (Hylander and Goodsite, 2006).
As shown in Fig. 5, the Hg profile in the Geladaindong ice core, with the
uppermost layer dated to around AD 1982, matches the atmospheric Hg
depositional chronology established from sediment records in Nam Co (Li,
2011), a large alpine lake (4710 m a.s.l.) on the Tibetan Plateau, as well
as the history of regional and global Hg production (Hylander and Goodsite,
2006), showing low and stable background levels prior to
The lack of recent deposition of mass (ice) at the Nyainqêntanglha and
Geladaindong glaciers, as well as at the Naimona'nyi Glacier (Kehrwald et
al., 2008), suggests that the melting and/or loss of the accumulation area
of glacier occurred in at least these three ice coring regions of the
Tibetan Plateau. Although there is a consensus that glaciers in the Tibetan
Plateau are largely retreating (Yao et al., 2004, 2012; Bolch et al., 2012;
Neckel et al., 2014),
Due to a lack of precipitation data at the coring sites, we cannot directly
quantify the annual ice loss in these high-altitude glaciers. The annual
precipitation data from local lower elevation meteorological stations are
444 mm at Damxung (50 km southeast of Nyainqêntanglha but at an elevation of
4300 m a.s.l.) and 467 mm at Amdo (120 km south of Geladaindong but at an
elevation of 4800 m a.s.l.) (Fig. 1). These data suggest that the glaciers
have experienced a net loss of at least several hundred millimeters each
year (mm w.e. yr
In situ observed annual and cumulative mass balance for the Xiaodongkemadi glacier in the Tanggula Mts. (Yao et al., 2012) and the Zhadang glacier in the Nyainqêntanglha Mts. (Qu et al., 2014).
In situ observational data using mass balance stakes close to our coring
sites have only been available for a short time period in the recent past (Kang et
al., 2009; Yao et al., 2012; Qu et al., 2014). Mass balance measurements of
Xiaodongkemadi glacier (80 km south of Mt. Geladaindong, Fig. 1), started in
1989, showed slightly positive mass accumulation until the mid-1990s, then
changed to a net mass loss over time (Yao et al., 2012) (Fig. 6). During the
period AD 1995–2010, the cumulative mass loss reached 5000 mm with an annual
mass loss rate of about 300 mm w.e. A much higher mass loss rate was
observed in situ at Zhadang glacier (5 km east of the Mt. Nyainqêntanglha,
Fig. 1) in the southern Tibetan Plateau; over the period AD 2005–2011 mass
loss rate at this glacier averaged at approximately 1200 mm w.e. yr
Calculated annual net mass balance (mm w.e. yr
Calculated annual net mass balance (mm w.e. yr
In order to further assess whether intensive melting could happen at
high elevations of the coring sites, a degree-day model (DDMs) was applied
to estimate glacier melt at the two ice core sites. DDMs can determine the
daily quantity of snow/ice melt (
Variations in annual positive cumulative temperature at the Nyainqêntanglha and Geladaindong ice core sites, annual precipitation amount at Damxung and Amdo station, and the estimated cumulative net mass balance based on a degree-day model (DDM) at the two ice core sites during AD 1966–2013. (Tr1, Tr2, and Tr3 as listed in Tables 1 and 2; dashed lines represent the average of the annual positive cumulative temperature before and after AD 1990).
As shown in Fig. 7, there is a statistically significant (
Meteorological data suggest dramatic warming has occurred in the Tibetan
Plateau since the late 1980s and that the magnitude of warming is
much greater than that in the low-elevation regions (Kang et al., 2010).
This warming has resulted in a continuous negative mass balance (or mass
loss) of glaciers during the last decade, ranging from the Himalayas to the north
of the Tibetan Plateau except for the northwestern Tibetan Plateau (e.g.,
Yao et al., 2012; Bolch et al., 2012; Gardelle et al., 2012; Neckel et al.,
2014). In recent years, the altitude of the equilibrium line for some of the
observed glaciers has risen beyond the highest elevations of the glaciers;
that is, there is no more net accumulation area and subsequently the entire
glacier is becoming an ablation area (Yao et al., 2012). Although glacier mass
balance varies depending on climate change and geographical conditions as
shown on the Tibetan Plateau (e.g., Yao et al., 2012; Bolch et al., 2012),
our
We suggest that the glaciers on the southern to the central Tibetan Plateau might be melting faster than previous data have shown (Liu et al., 2006; Jacob et al., 2012; Gardner et al., 2013). Ice losses on such a large scale and at such a fast rate could have substantial impacts on regional hydrology and water availability (Immerzeel et al., 2010), as well as causing possible floods due to glacier lake outbursts (Richardson and Reynolds, 2000; Zhang et al., 2009). Furthermore, the loss of glacier accumulation area warns us that recent climatic and environmental information archived in the ice cores is threatened and rapidly disappearing in the mid and low latitudes. As such, there is an urgent need to collect and study these valuable ice core records before they are gone forever.
S. Kang was the lead scientist of the entire
project and F. Wang was the principal investigator of the mercury
sub-project. S. Kang and F. Wang wrote the first draft of the manuscript,
with inputs from all other co-authors. U. Morgenstern did the tritium
measurement and interpretation. M. Schwikowski did the
This work was funded by the Global Change Research Program of China (2013CBA01801), the National Natural Science Foundation of China (41121001, 41225002, and 41190081), the US National Science Foundation (ATM 0754644), and the Natural Science and Engineering Council (NSERC) of Canada. We thank all members of the 2003 Nyainqêntanglha and 2005 Geladaindong expeditions. Edited by: A. Klein