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Morphodynamics and lake level variations at Paiku Co, southern Tibetan
Plateau, China
Bernd Wünnemann
a,b,
⁎
,DadaYan
a,b
,RenCi
c
a
School of Geographic and Oceanographic Sciences, Nanjing University, China
b
Institute of Geographical Sciences, Freie Universität Berlin, Germany
c
Geography and Resource Department, Science Faculty, Tibetan University, Lhasa, China
abstractarticle info
Article history:
Received 23 January 2015
Received in revised form 1 July 2015
Accepted 2 July 2015
Available online 6 July 2015
Keywords:
Lake–catchment interaction
Lake level
Late Pleistocene
Holocene
Proxy records from lakes on the Tibetan Plateau are commonly used to infer monsoon-related climatic changes
during the late Quaternary. Specificinfluences of catchment processes and their interaction with the lake basin
are seldom utilized. Based on morphological field investigations, supported by remote sensing analyses in
combination with radiocarbon-dated sediment data from lacustrine sequences along paleoshorelines and
terraces, we can demonstrate that close relationships exist between glacier dynamics, fluvial–alluvial fan/terrace
formation and lake level and lake area changes of Paiku Co, southern Tibet. Our results show that the formation of
large-scale, fluvial–alluvial fans (F1) predates the maximum advance of the Xixiabangma glaciers. The latter
formed a distinct terminal moraine complex north of the present glaciers during the local LGM (LL GM) at
42–21 cal ky BP. A younger fan generation (F2) developed from the LLGM to the late Holocene, which was
accompanied by lake level fluctuations with a generally decreasing trend. The highest morphologically traceable
lake level at 4665 m asl existed prior to 2 5 cal ky BP and induce d a potential overflow to the neighboring
Langqiang Co and Pengqu River. A high level also existed during the LLGM, followed by a minor decline until
ca. 15 cal ky BP, owing to reduced meltwater discharge under cold and dry climatic conditions. A return to the
previous level during the late-glacial/early Holocene period between 11.9 and 9.5 cal ky BP is likely caused by
climate warming, increased meltwate r discharge, and enhanced Indian Summer Monsoon (ISM) moisture
supply. Afterwards, Paiku Co shrank gradually toward its present level, while the youngest fan (F3) generation
evolved as individual small-sized bodies under ephemeral discharge conditions from the mid-Holocene to the
present.
The formation of four terrace levels (T4-1) is likely the result of sequential incision into the fan generations with a
mean erosion rate of 50 cm/ky, caused by lake level lowering. Tectonic impact cannot be completely ruled out.
Since 1976, the glaciers lost ca. 15% in area, accompanied by lake area loss of ~3.7% between 19 72 and June
2014. Seasonal lake level variations of about 1–2 m in height occur in response to summer monsoon rainfall.
Our data show a close interaction between glacial dy namics, fluvial processes, terrace formation, and water
budget changes throughout the last 25 cal ky BP in response to the well-known, insolation-driven, ISM-
effective moisture supply during the late-glacial and Holocene period. Temperature-driven meltwater dynamics
were the controlling factors for variations in water balance of Paiku Co.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Investigation into the history of lake basins on the Tibetan Plateau
has become a major concern because we know that lakes, functioning
as local sinks, are able to preserve multiple data about hydrological
cycles, lake/catchment ecology, and sedimentary processes for the
period of the lake's existence. Commonly, records from lake-bottom
sediments are applied to identify the lake's evolution in response to
climate change. However, an increasing number of publications also in-
dicates that alterations in the depositional environment of a lake basin
might not only be linked to local or even global climatic variations but
also to nonclimatic influences, such as tec tonic activity, changes in
local catchment morphology, and human impact (Coulthard et al.,
2005; Dietze et al., 2010; Zhang et al., 2012; Ghimire and Higaki,
2014). However, little is known about how and to what extent catch-
ment processes are linked with dynamic processes in the lake basin or
how quickly lakes respond to changes in onshore regions.
The closed Paiku Co (Lake Paiku) is a good example to use in order to
demonstrate the relationship between geomorphic processes and
changes in water balances of the lake over time. Previous studies within
the catchment revealed a complex distribution of glacier advances
Geomorphology 246 (2015) 489–501
⁎ Corresponding author at: Nanjing University, China and Frei e Universität Berl in,
Germany.
E-mail addresses: bwuenne@nju.edu.cn, wuenne@zedat.fu-berlin.de
(B. Wünnemann).
http://dx.doi.org/10.1016/j.geomorph.2015.07.007
0169-555X/© 2015 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Geomorphology
journal homepage: www.elsevier.com/locate/geomorph
(Guo, 1974)andfluvial–alluvial formations, which are believed to have
formed during the entire Quaternary history from the early Pleistocene
to the Holocene (Zhu et al., 2008). According to Liu et al. (2011) howev-
er, the maximum glacier advances toward the lake basin of Paiku Co oc-
curred between 42.1 and 22.3 ky BP (Laqu I stage) and 18.6–14.8 ky BP
(Laqu II stage), associated with a depression of the equilibrium line alti-
tude (ELA) by 380–400 m. Sixteen lacustrine units around the lake basin
were m entioned by Zhu et al. (2008), of which four units can be
assigned to terraces formed by sequential river incision and perhaps at-
tributed to tectonic uplift. Han et al. (2009) studied the Bangong Forma-
tion (late Pleistocene) in more detail. Based on an ESR and U/Th-dated
fluviolacustrine stacked record comprising terraces T1–T4, they con-
clude d that the lake experienced change s from a shallow stage
(127–56 ky) to a high phase between 56 and 31 ky and a return to a
shallow lake from between 31 and 15 ky. Bian et al. (2013) used the
same chronology and reported alterations in the vegetation cover
from widespread broad-leaved and coniferous mixed forests to wood
grassland for the period 127–31 ky BP. Variations between alpine grass-
land (31 and 15 ky) and forest steppe assemblages were recorded for
the period after 11 ky. The authors assigned these changes to climate
shifts between warm–cold and wet–dry conditions. However, Huang
(2000) reported a different climate and vegetation record from a similar
terrace level (T3) at an unknown location for the time period between
13 and 5 ky BP (uncalibrated ages). The record displays climate-driven
vegetation changes within a generally steppe-dominated environment
under arid conditions with h igher moisture availability during the
early Holocene, roughly matching climate inferences from ostracod as-
semblages reported by Peng (1997).
The history of lake level/area fluctuations in Paiku Co during the past
decades in response to summer monsoon influence was recorded by Dai
et al. (2013) for the period 2003–2011. Based on remote sensing analy-
ses, the results indicate a general declining trend in lake area and water
level, similar to other lakes in southern Tibet but in contrast to many
lakes on the Tibetan Plateau that experienced rising or stable lake levels
between 1972 and 2010 (Li et al., 2014; Song et al., 2014). The plateau-
wide pattern of lake change was mainly attributed to precipitation var-
iations. Rapidly rising lakes, however, were explained by the potential
contribution of permafrost melt (Li et al., 2014), whereas glacier melt
played a minor or even negligible role, except for the region at around
33°N, where meltwater supply from glaciers favored a faster lake level
rise during the 2000s (Song et al., 2014).
Most of the data from Paiku Co relied on single or stacked records as
a base for climate and environmental reconstructions. To date, a com-
prehensive consideration of linkages between various morphodynamic
processes, sedimentary records, and lake level variations in Paiku Co has
not been well established.
In this study, we try to shed more light on the morphological aspects
of erosional/depositional processes along the southern onshore regions
of Paiku Co, taking into account the spatial distribution pattern of the
glaciers along the Xixiabangma Mountain range and associated fluvial/
alluvial terrace formations. We hypothesize that catchment processes
are strongly linked to the lake's water balances (shorelines) from the
l
ate Quaternary to the present.
2. Geological setting
The closed Paiku Co basin (ca. 28°25′– 29°07′N; 85°20′– 86°05′E,
Fig. 1) is located at the Tethyan Himalaya, between the High Himalaya
and the Lhasa Block (Aoya et al., 2005), 20–30 km south of the
Yarlung–Tsangpo River, and was formed along the active Jilong–Tingri
fault zone. Deng and Liu (1998) argued that glaciofluvial deposits
from the north-slope glaciers of the Xixiabangma Mountains blocked
the Menqu River valley and formed the Paiku Co barrier lake. The lake
area comprises 270 km
2
(June 2014) and covers only 9.75% of the entire
catchment (2768 km
2
). The southern and western subcatchments are
characterized by the steeply ascending Xixiabangma Mountains
reaching elevations above 7200 m, with the highest peak of Mt.
Xixiabangma at 8046 m asl. However, mountain peaks within the
Paiku Co catchment rise up to 6500 m asl. The area above 5000 m asl
is glaciated.
The northern and eastern parts of the catchment are dominated by
the Malashan Mountains and a wide alluvial plain east of the lake
basin, merging with alluvial plains in the northern foreland of the
Xixiabangma range. The plains incline toward the lake from roughly
4900 m to 4585 m asl.
The northern and eastern subcatchments are occupied by the Ceno-
zoic Paiku composite leucogranitic pluton in the Malashan gneiss dome
with heights b 5500 m. This pluton consists of tourmaline leucogranite,
two-mica granite and garnet-bearing leucogranite, which is considered
to have formed some 28 Ma ago (Gao et al., 2013). It is surrounded by
pelitic and c alcareous schists, mainly from the Jurassic period. Older
rocks from the Devonian, Carboniferous, and Permian periods only
occur occasionally within the catchment of Paiku Co.
Extended al luvial/fluvial fans form a broad fringe between the
mountains and the lakeshore. As observed in satellite images the shapes
of these fans are superimposed by numerous lines of former lake levels,
which also can be found along the steeper parts around Paiku Co.
Seven larger drainage systems crossing the southern, southeastern,
and southwestern slopes enter the lake basin, of which only one system
contributed water (glacier meltwater) during fieldwork conducted in
June 2014. A sm aller ephemeral drainage system with several
paleoshoreline features occurs at the northeastern side of the lake
(Fig. 1).
The lake consists of two subbasins, of which the northern basin is
expected to be N 90 m deep (Andrew C.G. Henderson, Newcastle Univer-
sity, UK, pers. comm., 2014), whereas the maximum water depth in the
Fig. 1. (A) Overview map of the Paiku Co catchment and lake basin with locations of the
study sites (PC0-20), mentioned in the text (red triangles). (B) Location of the study site
within China
10
Be cosmogenic nuclide exposure ages: LI*, LI/LII* for Laqu I and Laqu II
stages, after Liu et al. (2011), LII** = corresponding to Laqu II stage, after Kong et al.
(2011). (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
490 B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
southern basin does not exceed 50 m. The salinity is about 0.5 mS cm
− 1
with a mean pH value of 9.24 (Hu et al., 2014). Annual mean precipita-
tion, mainly falling during the summer monsoon period, accounts for
431 mm; the mean annual temperature is − 0.9 °C (Hu et al., 2014).
3. Methods
During fieldwork conducted in June 2014, a detailed geomorpholog-
ical mapping of the southern and southwestern parts of the lake and its
subcatchment was carried out. For the position and elevation of individ-
ual sites, we applied a standard GPS to fix the geographical coordinates.
The elevation was measured by a Thommen altimeter device, later
cross-checked by Google Earth elevation data. All sampling sites are
listed in Table S1 (Supplementary data). Two naturally exposed sections
(PC-4 within the third terrace level on the southern slope of the lake and
PC-12 close to a shoreline position southwest of the lake) (Fig. 1)were
analyzed in more detail and sampled at 5-cm intervals from the bottom
to the top. Other sites were inspected for the identification of fluvial and
lacustrine deposits. Individual samples from wetland a nd lake sedi-
ments at selected sites were taken for radiocarbon dating.
In the two sections PC-4 and PC-12, Loss on Ignition (LOI) was deter-
mined. Total organic matter (OM) was achieved by burning dried and
powdered samples at 550 °C for 2 h in an oven. Total carbonate was cal-
culated on the same sample by the loss of CO
2
at a burning temperature
of 880 °C for 2 h. The respective results are presented in Figs. 4 and 5.
A total of 15 samples including modern plants and gastropod shells
(Radix sp.) from the lake shore were dated by accelerator mass spec-
trometry (AMS) at the BETA Analytic Laboratory, Miami, Florida, USA,
using bulk organic matter, depicted plant remains, and shell carbonate
(Tabl e 1). All ages were calibrated by using the Calib 6.1.1 program
(Reimer et al., 2009) and reported as 2σ weighted mean ages in cal y
BP or cal ky BP.
The fieldwork was supplemented by remote sensing analysis to
identify variations in g lacier and lake area, the spatial distribut ion
of paleoshore lines, and te rrace generations along the southern a nd
southwestern subcatchments. We used available MSS, Landsat 4/5,
Landsat 7 ETM+, and Landsat 8 satellite data (Earth Expl orer:
http://earthexplorer.usgs.gov/) from diffe ren t ye ars and months for
the periods between November 1972 a nd July 2014. The comparison
of lake area changes was based on images free of clo uds and tempo-
rary snow cover. In total, 25 images could be used. For every individual
image, we produced shape files of the lake and glacier area by ArcGIS 9.3
and calculated the lake area (Table S2, Supplementary data). Changes of
the glacier area are based on images from September 1977, 1991, 2001,
and 2013 (Table S2, Supplementary data). None of the other images
allowed a clear differentiation between the glacier ice area and tempo-
rary snowfields.
4. Results and discussion
4.1. Chronological frame
As lacustrine deposits are widespread in the southern catchment of
Paiku Co, we collected individual samples from various paleoshorelines
and from two sections located within the terrace level T2 (PC-4) and be-
tween shorelines S9 and S8, roughly 7 m above the present lake level
(PC-12).
Chlorophyll-containing aquatic plants from the southern part of the
modern lakeshore yielded an age of 126 ± 50 cal y BP (PC-10), thus in-
dicating minor contamination by older carbon source s. Conversely, a
fossil Radix shell in close proximity to the dated plant was washed
ashore and dated to 1938 ± 59 cal y BP. Although the time at which
the gastropod died cannot be defined exactly, we have to assume a con-
siderable reservoir error of a maximum of 1900 years. Whether or not
this reservoir effect can be applied to all fossil lacustrine sediments in
onshore regions remains an open question, but it may serve as a poten-
tial error. As reservoir effects in lake sedim ents can vary spatially,
e.g., reports on Qinghai Lake (Henderson et al., 2010), Hala Lake
(Wünnemann et al., 2012), and other locations (Hou et al., 2012), we
have to assume similar conditions here, taking into account the fac t
that erosion of old lake de posits from onshore regions down to the
Paiku Co basin may contribute to this error to an unpredictable extent.
Hence, we did not include any reservoir erro r in any of the reported
ages.
Our dated sediments cover the period from N 25 cal ky BP (at the
highest position of lacustrine sediments at shoreline or near-shoreline
locations) to the present. Lake sediments between shorelines at 44 m
above the present lake level (4629 m asl, PC-15) date to 24,779 ±
221 cal y BP (Table 1). Three samples from near-surface deposits close
to shoreline locations date to 15.4, 13.7 and 11.9 cal ky BP (PC-20, PC-
11, PC-0, Table 1) respectively.
The two sections PC-4 and PC-12 provide reasonable age-depth rela-
tionships, comprising the period from ca. 19.6 to 9.5 cal ky BP and 21.4–
8.0 cal ky BP, respectively. Only two samples from a shoreline position at
ca. 4620 m asl refer to mid-Holocene lake deposits (6.0 cal ky BP, PC-13;
2.9 cal ky BP, PC-14). We were not able to determine sediment ages ex-
ceeding the dating limit of the radiocarbon method. Our reported ages
from terrace levels T2 and T 3 are not in line with ESR/U–Th ages of
the stacked record T1–4 that provided ages between 130 and ~11 ky
(Zhu et al., 2008; Han et al., 2009).
4.2. Glaciers and glacial deposits
Glaciers only occur in the southern part of the Paiku Co catchment
be
longing to the Xixiabangma Mountains. Their area was calculated as
Table 1
Radiocarbon-AMS dating results from Paiku Co sediments.
Sample ID Depth (cm) Lab no. Dated material Conv. age Dev. +/− 13C/12C Cal y BP 2σ Weighted mean
age cal y BP
Dev. +/−
From To
PC-0 Beta-386502 Bulk OM 10,190 40 − 22.9 12,035 11,755 11,904 158
PC-4.1 440 Beta-386503 Plant remains 16,260 50 − 22 19,720 19,520 19,659 191
PC-4.2 655 Beta-386504 Plant remains 8510 30 − 20 9535 9480 9508 29
PC-4-3 650 Beta-386505 Bulk OM 9030 30 − 20 10,235 10,185 10,020 28
PC-7 0 Beta-386506 Carbonate (gastropod) 1990 30 0.8 1995 1880 1938 59
PC-10 0 Beta-386507 Modern plant 90 30 − 7.9 145 20 126 50
PC-11 20 Beta-386508 Bulk OM 11,890 40 − 21.6 13,770 13,590 13,678 103
PC-12.1 20 Beta-386509 Bulk OM 17,350 70 − 22.3 21,095 20,745 20,933 251
PC12.0 50 Beta-398574 Bulk OM 17,710 60 − 22.1 21,175 21,700 21,438 263
PC-12.2 115 Beta-386510 Bulk OM 7930 30 − 21.7 8815 8635 8784 71
PC-12.3 230 Beta-386511 Bulk OM 7270 30 − 22.2 8170 8010 8090 75
PC-13 10 Beta-386512 Bulk OM 5200 30 − 2.5 5995 5910 5953 42
PC-14 0 Beta-386513 Carbonate (gastropod) 2810 30 1.4 2970 2850 2922 78
PC-15 50 Beta-386514 Bulk OM 20,570 80 − 22.3 25,055 24,490 24,779 331
PC-20 70 Beta-386515 Bulk OM 12,920 40 − 20,3 15,585 15,285 15,441 198
491B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
109.06 km
2
(Oct. 2013) and covers ca. 3.9% (4.67% in 1971, Fig. 2,
Table S2, Supplementary data) of the entire catchment. Seven glacier
tongu es extend from the northern slopes of the mountain ranges
down to a maximum of 5110 m asl. Based on the simple summit-to-
toe method (Louis, 1955), the modern equilibrium line altitude (ELA)
is located at 5750 m asl, comparable with the values obtained by the
THAR method (Liu et al., 2011). The glacier tongues are accompanied
by distinct lateral and terminal moraines that border the modern and
past glaciers. The most distinctive moraine complex is located between
4665 and 5004 m asl as a set of moraine ridges, terminating up to 11 km
north of the modern glaciers, as reported by Guo (1974) and Liu et al.
(2011). These ridges indicate the amalgamation of all individual glaciers
into a widespread (piedmont) foreland glaciation. The red line in Fig. 2
marks the maximum advance of the former glaciers, which most likely
occurred between N 24 and 19 cal ky BP, matching the well-known glob-
al LGM (26.5–20 cal ky BP; Clark et al., 2009). Accordin g to Liu et al.
(2011) however, a maximum extent may have already occurred earlier
(between 42.1 and 20.1 ky BP) as indicated by one of the six
10
Be terres-
trial cosmogenic nuclide exposure ages of the moraine deposits near to
the maximum glacier extent (Fig. 2). The local LGM (LLGM) in the Paiku
Co region therefore could have experienced a longer time span of max-
imum glacier advances than had already occurred during marine iso-
tope stage (MIS) 3. This phase has been assigned to the Laqu I stage.
Younger exposure ages of the Laqu II stage range between 18.6 and
14.8 ky BP (Fig. 2; Liuetal.,2011) and indicate that the glaciers did
not retreat far.
Taking the maximum extent of the glaciers into account, a local ELA
depression of roughly 350 m was calculated ~50 m lower than the value
reported by Liu et al. (2011). However, Guo (1974) described at least
three different phases of glaciation during the early, mid-, and late Pleis-
tocene in the region that was separated by interglacial periods, accord-
ing to his morphostratigraphic description. We were not able to confirm
his results, as our findings only display morphodynamic processes of the
late glacial period.
Younger moraine ridges encompass the individual glacier tongues
showing glacier advances between a few hundred meters and 2 km
north of the modern glaciers. We assume that they represent advances
during the Neoglacial period, in particular the Little Ice Age (LIA) or even
during the last 100 years. The proglacial lakes in front of the glaciers de-
veloped afterward and would therefore be younger (Fig. 2).
4.3. Fluvial fan F1 and related terrace T4
According to our field investigations and remote sensing analysis,
three different glaciofluvial/alluvial fan gen erations were identified.
The oldest generation (F1 in Fig. 2) formed a large gravelly fan along
the southern side of Paiku Co from the oldest moraine belt and related
outwash centers (major glaciofluvial drainages) toward the lake basin,
terminating in a distinct paleoshoreline at 4665 and 4655 m elevation,
respectively (Figs. 2 and 3). From a morphological point of view, this
fan complex F1 is linked to the glacier dynamics of the nearby mountain
ranges, as it originates from glaciated valleys of the Xixiabangma
Fig. 2. Geomorphological map of Paiku Co basin showing the distribution of fluvial–alluvial fans and the glacier area of 2013 with major moraine stages and selected shorelines. Sites are
marked with red triangles and obtained radiocarbon ages. Circles refer to the location of
10
Be cosmogenic nuclide exposure ages, LI = Laqu-I stage, LLII = Laqu-II stage, after Li et al. (2011)
and Kong et al. (2011). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
492 B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
Mountain range. It is covered largely by moraine material of LLGM age.
Consequently, the F1 complex south of Paiku Co had already started to
form prior to this maximum glacier extent.
On the southwestern side of the lake (Fig. 2), three larger fans devel-
oped at the formerly glaciated valley outlets and formed individual bod-
ies composed of gravel and sand. Their formation most likely occurred
contempora neously with the fan complex south of the lake because
they evolved at similar heights, and they are also bordered by a paleo-
shoreline at 4655 m elevation. Together the fans form the highest and
oldest terrace level T4 at 55–80 m above the present lake level (ca.
4640–4665 m asl). This level was formed by subsequent fluvial incision
into the fan body. A transect across the fan from the foot of the mountain
to the lake was laid through the fan into the lake. The measured heights
are shown in Fig. 3B.
Fig. 4 provides insight into the sedimentary composition and struc-
ture of the fan F1 on the southeastern side of the lake where the river
has cut into the gravel by more than 10 m. Three generations of fan for-
mation could be distinguished that indicate multiple phases of gravel,
sand, and silt deposition in different time periods: (i) a prograding
delta formation with oblique foreset bedding at the bottom of the section
(unit 1) indicating an early fan development in a near-shoreline position;
(ii) a second generation of complex oblique delta formation (unit 2), sep-
arated from the lower sequence by a clear unconformity. It displays hor-
izontally bedded gravels, sand, and silt layers that turn to oblique
bedding toward the lake basin (e.g., Pettijohn et al., 1987); and (iii) a
ca. 1-m-thick, horizontally bedded topset of coarse gravels (unit 3).
With respect to a chronological framework of this fan formation, we
can refer to a cosmogenic
10
Be/
26
Al dating result from the top gravels of
the terrace (unit 3 at 4680 m elevation), which yielded an age of 18.7 ky
(Kong et al., 2011), thus indicating its terminal formation at the end of
the LLGM, corresponding with the
10
Be exposure ages of the Laq u II
stage (Fig. 2; Liu et al., 2011). Hence, the two lower units 1 and 2 are
older and were formed during a fluvially dominated time prior to the
LLGM, as can also be assum ed for the F1 complex farther southea st.
The time of formation can be deduced from silt-sized lake sediments
at section PC-15 (200 m distance from a cliff, 44 m above the present
lake level), of which a sample from a 50-cm depth dates to 24,779 ±
331 cal y BP (Table 1). Hence, we conclude that the respective lake sed-
iments were deposited after the formation of unit 2 and before unit 3
within the fan F1.
The upper two fluvial sequences (units 2 and 3) end abruptly at a
steep head cliff that faces toward the lake basin. They were cut off by
Fig. 3. (A) Geomorphological map of the southwestern part of the Paiku Co basin with fan generations (F1–3), shorelines (S1–10), cross section P–P′, sites (black squares), calibrated AMS-
ages of dated sites and the location of Fig. 4. (B) Cross section P–P′ from terrace T4 down to the lake with position, elevation, and ages of shorelines, terrace levels, and cliff position.
493B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
the formation of the cliff during a lake highstand at about 4635 m eleva-
tion (50 m above the present lake level). A distinct abrasion platform
composed of shoreline gravels above fine-grained lacustrine sediments
at the foot of the cliff is evidence of erosional processes along the former
lakeshore.
The formation of the cliff occurred during a lake highstand at ca .
12 cal ky BP, as revealed by lacustrine sediments in section PC-0 south
of the la ke basi n (Fig. 1, T able S1 in Supplementary data) at exactly
the same elevation of the cliff foot.
After ca. 12 cal ky BP, the fan was subject to strong incision by en-
hanced fluvial activity and a lowered erosional base of several meters
down to its present shape. An initial erosion process may have already
started earlier at the end of the LLGM (ca. 19 cal ky BP), after deposition
of the topset gravels of terrace T4.
Lake shrinkage may have been the major trigger inducing erosion,
although tectonic uplift of the mountain range and/or subsidence of
the lake basin cannot be ruled out. Indicators of tectonic impact
(e.g., offsets, ruptures, deformation of strata) within the sedimentary
units of terrace 4 could not be found. Taking the exposed ca. 10-m -
high section and its topmost age into account, a mean incision rate of
53 cm/ky was calculated. Comparable erosion rates of 60–80 cm/ky
along the foothills of the high mountains in southern Tibet were report-
ed by Vance et al. (2003).
4.4. Fluvial fan F2 and related terrace T3
A younger generation of fans (F2 in Fig. 2) only occurs in the south-
western and western subcatchment of the lake basin. These fluvial fans
evolved along major drainage systems that originated from the glaciers.
They developed along the incised older fans most likely after the LLGM
and occur in much more limit ed sizes and shapes than those of F1
(Figs. 2 and 3A). This fan generation remained active during successive
periods of lake level decline, as is revealed by cuts thro ugh paleo-
shorelines (Fig. 3A). Most of the fans of F2 terminate at a distinct
paleosh oreline at 4602 m elevation, indicat ing that they were active
until the lake level of Paiku Co fell below 4600 m elevation. Only the
fan farther east remained active and extended down to 4590 m eleva-
tion (ca. 5 m higher than at present; Fig. 3A). Farther to the east, larger
gravelly fans belonging to generation F2 did not develop. Instead, wide-
spread lacustrine deposits indicate that this area was almost completely
covered by the former lake, which prevented fluvial fan development
and related deposi tional and/or erosional processes, except along
small-sized drainages, which successively evolved with the shrinkage
of the lake.
The related terrace level T3 (4635–4640 m; ca. 50 m a.l.l.), may serve
as an excellent example of how this terrace is composed: lacustrine de-
posits consisting of sandy-silt to silty-clay layers, rich in carbonate and
fossil remains (ostracods, plenty of gastropods, plant rem ains) were
found in section PC-0 above a thick sequence of fluvial deposits. They
date to 11,904 ± 158 cal y BP (Fig. S1, Supplementary data; Table 1)
and thus confirm a 50-m higher lake level of Paiku Co at that time.
This terrace level corresponds in height with the cliff foot mentioned
above.
4.5. Fluvial fan F3 and related terraces T2 and T1
The youngest flu vial fans (F3 in Fig. 2) only developed along the
major drainages after a period of fluvial incision, caused by a lowered
erosion base, triggered either by lake level decline, tectonic uplift of
the source area, and/or subsidence of the lake basin. Most of the individ-
ual small-sized fans terminate close to the paleoshoreline at 4602 m asl
(Fig. 3A) and thus indicate that their active formation ended after the
lake level dropped below this elevation. Only three fans extend to the
modern shoreline and remain active during seasonal water discharge.
The sediment composition of T2 at the southern side of Paiku Co
could be studied at section PC-4 (Figs. 1 and 5
), roughly 42 m a.l.l.
(4
627 m elevation; Fig. 3B). The ca. 7.5-m-long profile consists of fluvial
gravels and sand (0–430 cm in height), followed by a 3-m-thick se-
quence of silty clay, alternating with thin silt layers, containing ostra-
cods and plant remains. Modern roots from the vegetation at the top
have penetrated the entire section. The fossil plant remains are associat-
ed with iron hydroxide (goethite) horizons and occur at heights of 440,
625, and 650 cm (Fig. 5). Organic matter (OM) matter varies between
1.4% and 7.2% (mean: 3.85%). A higher OM level (6–7%) is rela ted to
layers with enriched plant remains. Total carbonate is generally low
and ranges between 1.1% and 4.5% CO
3
, except for the lower part of
the lacustrine sequence with values between 8.8% and 15.1% (~ 10.4–
17.8% CaCO
3
). Three samples of plant remains, taken from the bottom
of the lacustrine sequence at heights of 440, 635, and 665 cm date to
19,659 ± 191 cal y BP, 9508 ± 29 cal y BP, and 10,020 ± 28 cal y BP,
respectively (Fig. 5, Table 1). According to the chronology, we infer
close to a 10,000-year time sp an of variable lacustrin e depositional
Fig. 4. East–west profile through terrace T4 at the southwestern part of the lake basin, with subscription of sedimentary units and position of former lake levels. The position of the
10
Be
cosmogenic nuclide exposure age after Kong et al. (2011) is located east of the section.
494 B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
conditions. Periods of shallow water and near-shore environments are
indicated by enriched or ganic matter, whereas low organic content
and clayey deposits point to deeper water conditions. Shallow littoral
conditions apply to the time intervals at around 19, 15, and 10 cal ky
BP, whereas the periods between reflect opposite conditions. The inter-
polated age of the high OM at a height of 550 cm correlates with com-
parable fossil-rich sediments in section PC-20 at a slightly higher
elevation farther south (Fig. 2). They were dated to 15,441 ± 198 cal y
BP (Table 1) and also reflect shallow water depositional environments.
Furthermore, remnants of clayey lacustrine deposits at site PC-11 locat-
ed within the fluvial fan of generation F2 close to paleoshoreline S6 at
4610 m eleva tion (N 25 m a.l.l.; Fig. 3A,B) were dated to 13,678 ±
103 cal y BP and matches deeper water conditions, as can be inferred
by the respective sediments from section PC-4.
The ca. 2-m-deep fluvial incision along the drainages after the for-
mation of terrace level T1 (Fig. 5C) may be the result of lake level decline
since the late Holocene, according to calculated incision rates at terrace
level T4 southwest of the lake basin.
4.6. Paleoshorelines
Paiku Co is surrounded by numerous paleoshorelines. Most of them
are not very visible in the field and are much better represented in sat-
ellite images. Only a few shorelines appear as elongated ridges of a few
decimeters in height, but they show well-developed abrasion platforms
containing scattered patches of water-polished and flattened pebbles
above lacustrine fine-grained sediments. The latter contain plenty of
gastropods (Radix sp.) and ostracods (six genera) extending as far as
the shorelines c an be traced. The most distinct paleoshorelines and
their spatial distribution patterns are marked in Figs. 2 and 3. Several
of the higher shorelines only occur as remnants along fan generations
F1 and F2, as they were partly eroded by fluvial activity.
Along t he s outhwestern onsho re region of Paiku Co we identified
seven shoreline ge nerations, which can be distinguishe d by
(i) elongated sandy–silty ridges and shoreline pebbles, (ii) a step-
like pattern from the lowest to the highest shoreline, and (iii) associ-
ationwithanabrasionplatform(Fig . 3A). The highest one (S1) oc-
curs ca. 80 m (4665 m asl) a.l.l. This shoreline can be easily traced
around the lake perimeter bordering the distal parts of the oldest flu-
vial fans (F1). Kong et al. (2011) placed t he highest shoreline at
4680 m asl, which would be high enough to induce an overflow to-
ward Langqiang Co and the Pengqu River outside the Paiku Co catch-
ment (Fig. 1). The two lakes, however, were already connected at a
lake level of 4665 m asl, but the lake would have drained toward
Nepal, if its level had ever exceeded the threshold of 4680 m asl.
A lower shoreline is located ca. 50 m a.l.l. (4635 m elevation). It cor-
responds to lake sediments at section PC-0, which date to ca. 12 cal ky
BP and thus indicates a high lake level during the late-glacial and early
Holocene transition.
Lower shorelines follow a step-like pattern down to the lake with
close proximity to each other because of the steep slopes around the
lake (Fig. 3B). On the southern side of the lake however, the distance be-
tween the individual shorelines can exceed several hundred meters.
Prominent shorelines occur at 65–63 m (S2), 50–
47 m (S3), 44 m
(S
4), 39–38 m (S5), 26–24 m (S6), 18–16 m (S7), 10 m (S8), 7–5m
(S9) and 4–3 (S10) m above the present lake level (Figs. 2, 3). Several
more can be traced in between, which have morphologically obscure
features. Shorelines 7 and 8 are associated with cliff-like slopes, indicat-
ing a longer period of shoreline development in that area.
Lacustrine fine-grained sediments from the near surface at site PC-
13 (Fig. 3A), between shorelines S6 and S7 (4602–4612 m asl), were
Fig. 5. Section PC-4, 4627 m asl at terrace level T2 south of Paiku Co. (A) Photo and lithology description; (B) LOI data (OM and CO
3
); (C) Overview photo of the location with terrace levels
T2 and T1.
495B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
dated to 5953 ± 42 cal y BP and indicate N 20 m higher lake level at that
time. The dated Radix sp. gastropod from the nearby site that was embed-
ded in reworked sediments, however, is much younger (2922 ± 78 cal y
BP) and may have been washed ashore during a younger lake phase that
had also reached this level. Considering a potential reservoir error of
roughly 1900 years, the shell would have lived ca. 1000 years ago.
Past and modern erosion from the upper shorelines down to the lake
by rainfall-induced surface flow resulted in a gully-like incision, deep in
parts, into the lacustrine sequences. The strongest incisions occur
between shorelines S6 and S9. As a result, older sediments were trans-
ported into the lake and may be a reason that the chronology of a pre-
viously retrie ved sediment core from Paiku Co (Zhu Liping, ITPCAS
Beijing; A. Henderson, Newcastle Univ. U K, pers. comm.) could not
provide a consistent age-depth model.
4.7. Near-shoreline sediments (section PC12)
Section PC-12 at a cliff position above shoreline S9, ca. 7 m a.l.l.
(4592 m asl; Fig. 6) was exposed by gully erosion. The 2.3-m-high pro-
file displayed lacustrine deposits, consisting of dark gray compact clayey
silt at the bottom (0–35 cm), followed by light brownish layers of silty
clay (35–169 cm) with strong oxidation features in the upper sequence
and crumbly dark gray silty clay from 169 cm to the top (230 cm). Mod-
ern roots penetrated the top 60 cm of the section. All sequences
contained plenty of ostracods (six genera). Radix sp. gastropods only oc-
curred occasionally in the upper two units. Sediment composition and
fossil remains are indicative of deposition in a lacustrine environment.
However, the crumbly structure of the top unit is most likely a result
of freeze–thaw effects under water-saturated conditions that destroyed
the original layering of this sequence. A similar phenomenon of freeze–
thaw effects influencing the sediment structure has been previously
reported (Wünnemann et al., 2010). They als o influence the shift of
chemical components and microbiological processes (e.g., Mountfort
et al., 2003; Sawicka et al., 2010).
Organic matter (OM) varies between 2.5% and 7% (mean: 4.91%)
with the highest values at 30 c m, arou nd 1 00 and 215 cm in heigh t
(Fig. 6), and lowest at a height of 50–60 cm (2.5%) and between 135
and 180 cm (4–4.5%). Total carbonate (shown as CO
3
) variations behave
opposite to those of OM and range between 14.5% and 23.5% (mean:
18.44%; Fig. 6). The carbonate content is markedly higher than that of
section PC-4. The highes t value in the lower part of the profile
(50–60 cm in height, 24% CO
3
) is attributed to very high abundances
of ostracod shells. The lowest values (14.5% CO
3
, ~24.2% CaCO
3
) occur
in the bottom sediments of section PC-12.
Fig. 6. Section PC-12, 4592 m asl southwest of Paiku Co. (A) Photo of the section; (B) lithology and LOI data; (C) overview photo of the location.
496 B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
Two AMS dates on bulk organic matter at the boundary between
units 1 an d 2 yielded calibrated ages of 20,933 ± 251 and 21,438 ±
263 cal y BP, respectively (Fig. 6, Table 1) and testify to the depositional
time of the sediments during the LLGM, despite potential reservoir er-
rors. Two further samples from heights of 115 and 230 cm date to
8784 ± 71 and 8090 ± 75 cal y BP. The very homogenous sediments
point to a continuous sedimentation proces s from the LLGM to the
early Holocene. The uppermost age might be influenced by the mixing
of old and younger sediments because of freeze–thaw effects and thus
must be considered with so me cau tion. This is corroborated by the
fact that younger sediments ca. 13 m above this section at site PC-13
(see above) were not found at this site.
4.8. Glacier and lake area variations over the last 43 years
According to our satellite image-based reconstruction, the glacier
area within the Paiku catchment increased by 0.31% between 1971
and 1977 after which period it shrank by 15.73% until October 2013
(Fig. 7A). This significant area loss over the last 37 years is marked by
the shrinkage in length and size of all glacier tongues, indicating a re-
markable loss of ice volume within the ablation zone of each individual
glacier. Similar trends were also reported from other regions on the Ti-
betan Plateau. They were interpreted as a response to the global climate
warming trend during the last decades (Yao et al., 2004, 2012; Li et al.,
2010; Song et al., 2014).
Younger paleoshorelines below 4592 m elevation are traceable in
satellite images and by visual inspection, although they are morpholog-
ically not well deve loped. They represent fluctuating lake levels and
area variations of Paiku Co during the last 42 years (Figs. 2, 7B and S2;
Table S2, Suppl. data). Since September 1977, the lake area shrank grad-
ually from a nearly 280-km
2
size to about 269.87 km
2
until June 2014
(Table S2 Suppl. data), which corresponds to an overall loss of 3.44%,
based on the reference area of November 1972. However, between
1972 and 1977 a slight increase in area of ca. 0.2% was accompanied
by a lake level rise that reached the shoreline at 4590 m elevation and
indicates a ca. 5-m higher lake level th an at present (Fig. S2 Suppl.
data). This period corresponds with a stable glacier net mass balance
at that time interval.
A strong area loss of 1.84% occurred afterward until November 1991,
followed by slight fluctuations with a generally increasing trend until
September 2001. A second stepwise phase of area loss resulted in the
largest shrinkage of 3.61% (June 2010) during the last 42 years. At that
time the lake level was even lower than it is today. The subsequent pe-
riods after December 2010 indicate a relatively stable lake area at the
level of June 2014. However, seasonal fluctuations are a controlling fac-
tor for small-scale variations in area and lake level. Smaller areas and
thus lower lake levels coincide with the pre-monsoon period (winter–
spring) of reduced rainfall and lower discharge rates from the glaciers.
Conversely, increases in lake area and lake level correspond with pe-
riods of enhanced monsoonal precipitation and glacier meltwater flux
(summer–autumn). In our records, this effect applies to the years
2009–2012 and corroborates previous investigations on seasonal lake
level fluctuations between 2003 and 2011, displaying a general trend
toward area loss (Dai et al., 2013). Similar trends of area loss at Nam
Co have been reported by Zhu et al. (2010). Owing to the relatively
flat offshore regions south-southeast of Paiku Co, lake area cha nges
are more obvious here than at all other regions around the lake.
4.9. Morphodynamic linkages and climatic implications
Our investigations along the southern and southwestern onshore re-
gions of Paiku Co indicate a close relationship between various catch-
ment processes and the lake's wa ter budget, covering the time
between the late Pleistocene and the present. Fig. 8 outlines the links
between glacier dynamics, fan development, erosional processes and
related terrace formation, and lake level variations throughout the late
Quaternary.
Fig. 7. Glacier and lake area changes in the Paiku Co Basin. (A) Glacier area changes 1971–2013; (B) lake area changes 1972–2014.
497B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
Although the Xixiabangma glacier area within Paiku Co catchment is
relatively small (3.9% of the total catchment), it plays an important role
in terms of morphodynamics and related mass flux to ward the lake
basin. The advances and retreats of the glaciers are directly linked
with the formation of huge outwash fans (generation F1) that definitely
evolved prior to the maximum advance of the glaciers in Paiku Co catch-
ment. Hence, the major alluvial bodies south of the lake basin are con-
sidered to be older than the LLGM and may have formed during a
potentially more moist period of the marine isotope stage 3 (MIS 3) or
even during older periods.
The accumulation of the fluvial sediments requires a much higher
erosional base compared with that of today to enable deposition instead
of erosion. This requirement is fulfilled by a high lake level and maxi-
mum extent of Paiku Co (Figs. 8 and 9) that facilitated the amalgamation
of the lake with Langqiang Co in the upper reaches of the Pengqu River
and also its potential outflow toward Nepal (Kong et al., 2011). Al-
though the highest paleoshorelines could not be dated (so far), we as-
sume that they represent lake levels predating the LLGM by several
thousand years (N 25 ky BP; Fig. 8). The calculated lake area was
700 km
2
, exceeding the modern lake area by nearly 2.6 times (Fig. 9).
The maximum extent of the glaciers toward a connected piedmont
(foreland) glaciation can be traced by a terminal moraine complex
north of the modern glaciers (Fig. 2), which was deposited on the
older fluvial–alluvial gravels (F1). We assume that this glacier advance
reflects colder but also wet climate conditions that favored ice volume
growth. An increased water availability at that time may be related to
effective moisture supply by the Indian Summer Mon soon (ISM), a s
was also revealed by the Wulu cave record and loess–pa leosol se-
quences in southwest China (Han et al., 2010; Duan et al., 2014).
However, the influence of a westerly controlled moisture supply may
have played an important role, enabling the growth of the glaciers during
the cold seasons and increased meltwater flux during summers. Accord-
ing to Liu et al. (2011), the maximum glacier extent down to 4680 m asl
falls into the period of the LLGM (42–21 cal ky BP), comprising at least the
time interval from the MIS3 to MIS2. Analyses of numerous exposure
ages from glacial deposits in combination with ELA depression calcula-
tions indicate that many glacier advances, but by no means all of them,
terminated during that time period (Owen et al., 2005; Heyman, 2014).
The reconstructed local ELA depression of about 350 m within the
Paiku Co catchment corroborates the calculated mean ELA depression
during the global LGM (Liuetal.,2011;Heyman,2014).
Unlike other lake sites on the Tibetan Plateau, the maximum ice
advance corresponds with ~65-m higher lake level of Paiku Co that ex-
tended close to the outer moraine belt (Fig. 2). Subsequent lake shrink-
age induced erosion into the fan F1 and resulted in the formation of
terrace level 4. Taking into account the 15-m lower shoreline S2 (50 m
above the present lake level; Fig. 8) and respective lake deposits in sec-
tion PC-15 with an age of 24.5 cal ky BP, the lake still reached a high
level d uring the LLGM. Its area was ~1.6 times larger than t oday's
(Fig. 9), but shrank afterward down to 4629 m asl (shoreline S4 in
Fig. 8).
We suggest that the meltwater flux to the lake was reduced, owing
to shorter and colder summer periods of seasonal ice/snow melt. Fur-
thermore, local rainfall may also have diminished. The fine-grained la-
custrine deposits roughly 40 m above the present lake as well as the
ca. 19 cal ky BP old lacustrine deposits in section PC-4 are strong evi-
dence for a still higher lake level, although the water balance of Paiku
Co was slightly negative during this time interval.
Fig. 8. Summary graph showing the relationship between glacier development (moraines), fan formation and terrace formations, shorelines, and lake level changes against elevation and
time.
498 B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
This finding contradicts the general opinion of cold and dry climate
conditions during the LGM (e.g., Herzschuh, 2006) being the major trig-
ger for very low lake levels on the Tibetan Plateau (e.g., Nam Co: Daut
et al., 2010; Kasper et al., 2015; Qinghai Lake: Coleman et al., 2007;
Lake Ximencuo: Zhang and Mischke, 2009;HalaLake:Yan and
Wünnemann, 2014). Some lakes, however (such as Zhabuye Lake,
Zhacang Caka and Zigetang Co) experienced h igh to medium-high
lake status records too, according to Yu et al. (2001). High lake levels
of Toson Lake near Qinghai Lake at arou nd 30 ky BP are reported by
Fan et al. (2012).
We assume that lakes with relatively high lake levels, such as Paiku
Co, were mainly controlled by the contribution of meltwater from the
glaci ers or large snowfields as the main drivin g force. This would
imply that the summer periods were warm enough to enable high melt-
water flux from larger ice/snow bodies toward the lake basin. If it is be-
lieved that a monsoon-derived effective moisture supply during thi s
period was weak or absent, the necessary moisture supply (such as
snow) must have been derived from the westwind system to feed gla-
ciers and snowfields.
One the other hand we must consider that Paiku Co already experi-
enced the highest lake levels prior to the LLGM and afterwards experi-
enced continuous shrinkage until the late glacial period. The relatively
low magnitude of shrinkage is most likely caused by high meltwater
supply that could partly compensate the annual water loss by a negative
P/E ratio.
The glacier in the Paiku Co catchment, however, started to retreat
slightly after ca. 24 cal ky BP, as is indicated by moraines of the Laqu II
stage (18.6 and 14.8 ky BP: Liu et al., 2011). The corresponding lake
level reached 4650 m asl (shoreline S2; Figs. 3Band8). Gravels formed
the thin topset at the terrace T4 outcrop, which dates to 18.7 ky BP
(Kong et al., 2011). The subsequent decline down to 4629 m asl (shore-
line S4, Fig. 8) is considered to have been the major trigger for continued
erosion/incision as well as the initial formation of the younger fan gen-
eration F2l. Tectonic impact by uplift of the surrounding mountains or
subsi dence of the lake basin may have played a minor role or have
been absent.
Based on the 12 cal ky BP old lacustrine deposits in section PC-0, ca.
50 m a.l.l., we infer a lake level rise and return to a previous level during
the LLGM (S3 in Fig. 3B, Fig. 8). It was triggered by climate amelioration
and a strengthened influence of the ISM moisture supply during the late
glacial period. Glacier melt and local rainfall induced a lake level rise of
at least 6–7 m, or possibly even higher. The 13.8 ky BP old peat deposits
at a slightly higher position (Deng and Liu, 1998) support our assump-
tion of climate warming and a more positive precipitation–evaporation
(P/E) ratio. A warm and moist phase accompanied by increased water
inflow between 14 and 13 cal ky and after 10.8 cal ky BP with a peak
in the high lake level at 9.4 cal ky BP was also repor ted for Nam Co
(Doberschütz et al., 2014; Kasper et al., 2015).
The high lake level at Paiku Co lasted for at least ca. 2500 years as in-
dicated by the 9.5 cal ky BP old sediments at the top of section PC-4.
Fig. 9. Paleolake level and area changes of Paiku Co. (A) Area and lake level changes at selected time intervals between N 25 cal ky BP and the present; (B): bar chart of area changes for
selected lake levels.
499B. Wünnemann et al. / Geomorphology 246 (2015) 489–501
Apart from increased monsoonal rainfall during the early Holocene, gla-
cier melt may have substantially contributed to the overall water bud-
get of Paiku Co, resulting in a strong glacier retreat (Fig. 8)inthatperiod.
The fan F2 remained active throughout the Holocene and formed
delta-like distal ends that terminated at shorelines S7 and S9 (Fig. 8).
The latter one dates to 1977.
After ca 9.5 cal ky BP the lake level dropped again, favored the forma-
tion of terrace T3, and formed the lower paleoshorelines, as has also
been reported from the Tangra Yumco (Ra des et al., 2014). The lake
area shrank to 372 km
2
(4602 –4605 m elevation, shoreline S7 in
Figs. 8 and 9A) until ca. 6 cal ky BP, which was still 1.2 times larger
than it is at present (Fig. 9B). Owing to the lack of age control for shore-
lines and respective sediments, we are not able to reconstruct possible
lake level fluctuations and related climate variations during the early
to mid-Holocene period between 9 and 6 ky BP. Taking into account
that the reported lake highstand of Tso Kar Lake in Ladakh, northwest
India between 8 and 7 cal ky BP was lin ked to enhanced glacier melt
(Wünnemann et al., 2010), similar conditions could have occurred at
Paiku Co, too. The overall trend of lake shrinkage however, is deduced
from the paleoshorelines, thus indicating variable water budgets of
Paiku Co (Figs. 3A, 8) during the Holocene.
The youngest phase of fan development (F3) is restricted to local
drainages. It started to form in the mid-Holocene (Fig. 8), while individ-
ual small-sized fans developed later during the last few thousand years
along temporarily active spillways of the larger drainage complexes, but
they did not reach the modern lake. They indicate a reduced and only
ephemeral water supply to the lake basin, as occurs today. Only three
of the fans are still active while remaining connected to the modern
lake (Fig. 3A). The timing and causes of incision remain unclear, but it
seems likely that this phase is also related to lake level shrinkage.
Shortly before or around ca. 6 cal ky BP (shoreline S7 in Fig. 8), a fur-
ther lake level decline of several meters induced erosion and the forma-
tion of terrace T1 (Fig. 8). This period of incision may be related to the
Neoglacial period of glacier advances because of a cooling trend and
an ISM-related reduction in effective moisture supply. According to a
lake record at Par u Co, northeast of Lhasa (Bird et al., 2014), the ISM
tended toward drier conditions after 5.2 ky and after 4.8 cal ky at Nam
Co (Doberschütz et al., 2014), corroborating our assumption.
The glaciers within the Paiku Co catchment retreated dramatically
after the LLGM toward individual tongues. Oscillations during the Holo-
cene are marked by several lateral and terminal moraine ridges, which
indicate a successive loss of glacier volume and area. Prominent mo-
raine ridges encompassing the individual tongues several hundred me-
ters to 2 km apart from the modern stage may represent a short-term
advance during the Little Ice Age (LIA; Fig. 8), known as a period of
drier and climatic conditions. Inho mogeneous ice advances between
the fourteenth and seventeenth centuries on the Tibetan Plateau ( Xu
and Yi, 2014) indicate individual dynamic processes along the ice mar-
gins, depending on the subcatchments of each glacier system.
The total area of ice lo st within Paiku Co catchment accounts for
more than 15% during the period 1976–2013 and may be attributed to
the general trend of climate wa rming, rather than to a reduction in
monsoonal precipitation (Dai et al., 2013). This glacier loss is associated
with the reduction in lake area and water volume during the same peri-
od (Figs. 8,S2inSuppl.data),asisalsorevealedbydatacomparisonof
various lake records (Li et al., 2010) and remote sensing analyses of
lakes on the Tibetan Platea u (Liao et al., 2013; Li et al., 2014). As the
loss in glacier area did not result in increased meltwater flux, we assume
that the ice volume and area below the ELA (ablation zone) had already
become too small for an enhanced meltwater supply during the sum-
mer seasons to balance lake water loss by an overall negative P/E ratio
or even result in lake level rise, as reported for some lakes on the north-
ern Tibetan Plateau after A.D. 2000 (Song et al., 2014).
As a whole, the lake level variations seem to follow the general trend
of summer monsoon records (e.g., Dykoski et al., 2005; Wang et al.,
2005; Fleitmann et al., 2007), which indicate the insolation-driven
decline of the Indian summer monsoon effective moisture supply
t
hroughout the Holocene. Despite the influence of the ISM with respect
to effective moisture availability in the Paiku Co region, higher temper-
atures during the Holocene, notably during the thermal climatic opti-
mum (early- to mid-Holocene, ca. 8 and 7 ky BP) may have played an
important role in glacier retreat and thus reduction in meltwater supply.
This effect on the general water budget of Tibetan lakes still seems to be
underestimated. The contribution of meltwater to the overall budget of
the lake during the last decades, however, has declined.
Our current data do not allow us to draw a precise and highly re-
solved history of lake–catchment interaction throughout the late Pleis-
tocene and Holocene. Proxy data from the lake basin and a denser
database from the catchment are needed to improve the knowledge of
water budget variations in Paiku Co basin.
5. Conclusion
Our research results show a clear relationship between morpho-
dynamic processes in the Paiku Co catchment and lake level variations
during the late Pleistocene and Holocene.
The formation of three different fluvial–alluvial fans, associated with
four terrace levels, is directly linked with the glacier dynamics of the
Xixiabangma Mountains and with contemporary lake level variations
of Paiku Co. It is obvious that the oldest fan and the related highest rec-
ognizable lake level predates the maximum glacier advance and may
have formed under wet climatic conditions during MIS 3 or even in an
older period. The distinct maximum foreland glaciation most likely oc-
curred during the LLGM between 42 and ca. 20 cal ky BP, accompanied
by a high lake level of Paiku Co.
Periods of glaciofluvial erosion and contemporaneous terrace forma-
tion seem to be associated with lake level declines rather than tectonic
uplift and/or subsidence of the lake basin.
The post-LLGM lake shrinkage was succeeded by a lake level rise
after ca. 15 cal ky BP and a relatively stable high level during the early
Holoc ene, in accordance with climate amelioration and the well-
known pattern of the ISM effective moisture suppl y. Fluctuating lake
levels during the Holocene are documented by a series of paleo-
shorelines and the evolution of two fluvial fans, associated with three
terrace levels, and glacier fluctuations that follow the general trend of
the insolation-driven Holocene ISM pattern and thus a stepwise lake
level shrinkage toward its present stage.
The current trend of climate warming is well documented by the
retreat of the glaciers within the Paik u catchment and a fluctuating
but continuous lake level decline over the last 42 years.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.geomorph.2015.07.007.
Acknowledgments
This research was funded by the Deutsche Forschungsgemeinschaft
(DFG, Wu290-11-1). Nanjing University funded the AMS dating of
sediment samples. We are thankful to Zhou Jun, Yan Chongchong and
Lobsang Choephel, for their logistic support and assistance during our
fieldwork. We also acknowledge the help ful and valuable comments
and suggestions by the unknown reviewers and the editor Richard A.
Marston, who helped improve the manuscript.
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