The timing of the waxing and wining of the East Asian summer monsoon during the Holocene is still under debate. In present study, we present the high-resolution grain-size and LOI records from a well-dated mud/peat profile to reveal the lake-wetland transition in the Sanjiang Plain and discuss its significance to Holocene monsoon evolutions. The results show that the shallow-water lakes have developed in low-lying areas of the plain before 4600 yr BP, corresponding to the Holocene monsoon maximum. Thereafter, the wetlands began to initiate with the extinction of the paleolakes, marking a lake-shrinking stage with the relative dry climate. Considering the prevalent monsoon climate in the Sanjiang Plain, we suggest the lake-wetland transition at 4600 yr BP indicate a sharp decline of the summer monsoon rather than the basin infilling process. Such a remarkable monsoon weakening event has been widely documented in northern China, and we associated it with the ocean–atmosphere interacting processes in low-latitude regions.
The East Asian monsoon as an integral component of atmospheric circulation system plays a significant role in global hydrologic and energy cycles (Kutzbach, 1981; Ding et al., 1995). In China, monsoon climate, especially monsoon-associate precipitation has been widely considered as a key driver for evolution of biocommunity, maintenance of living environment and progress of human civilization in populous regions of East Asia (Wang et al., 2005; Chen et al., 2006; Zhao et al., 2011). So, it is an interesting question as to study the monsoon changes and their impacts on geo-biosphere, not only for the geologic past, but also for the prediction of future changes in terrestrial ecosystems.
During last decades, a large quantity of paleoclimate records with various proxies has been investigated to reveal Holocene monsoon variation (Hong et al., 2003; Xiao et al., 2004; Zhou et al., 2005; Chen et al., 2006). It was initially proposed that the strongest monsoon with a maximum precipitation induced by peak summer isolation occurred in early Holocene, and this hypothesis has been confirmed by a series of records from cave deposits, lake sediments, eolian deposits and peat accumulations (Hong et al., 2003; Li et al., 2004; Xiao et al., 2004; Jiang et al., 2008; Makohonienko et al., 2008). Comparing with the records in low-mid latitude regions, a few records from higher-latitude locations also suggest a much enhanced summer monsoon prevailed during mid-Holocene (Hong et al., 2001; Xiao et al., 2004; Jiang et al., 2006; Wen et al., 2010), and both early- and mid-Holocene (Zhou et al., 2002; Feng et al., 2004). Although a time-transgressive model has been developed to interpret these space–time discrepancies (An et al., 2000), there are still a few records showing no accordance with the model, especially for those from the mid-high latitude regions, which suggest a much later monsoon maximum (Hong et al., 2001; Jiang et al., 2006; Makohonienko et al., 2008; Wen et al., 2010). Therefore, a comprehensive and integrative view of East Asian monsoon evolution during Holocene is still under debate. Solutions to this problem require high-quality proxy records from more climatically sensitive and regionally representative locations.
Comparing with the low-mid latitude locations of the existing records on paleomonsoon evolution, the Sanjiang Plain located in northeast China, is more sensitive to climate changes for its mid-high latitudes and monsoon marginal locations (Sun and Chen, 1991; An, 2000). In this paper, we present a well-dated peat/mud profile in the Sanjiang Plain, to reconstruct wetland developing history during the Holocene and discuss its significance to the East Asian monsoon evolutions.
The Sanjiang plain (129
In addition to the warm and wet climate, such an area of low-relief is
favorable for development of wetlands (Ma et al., 1993). A recent
survey shows that over 70 % of the plain has been dominated by
freshwater wetlands, and it has been well known for the largest area
of the freshwater wetlands in China (Song et al., 2008). Most of these
wetlands are situated in shallow depressions originated from the
glacial movements during the Last Glacial stage. During the
Quaternary, the sand, mud and peat deposits exceeding 15
The studied profile HE (47
10 bulk samples were collected according to lithological changes of
the HE and dated with an accelerator mass spectrometry (AMS) system at
Xi'an Institute of Earth Environment, Chinese Academy of Sciences. The
AMS
A total of 160 samples were prepared for loss-on-ignition (LOI), and
grain-size analysis respectively. For LOI analysis, the samples with
a volume of 2
It has been widely adopted that the grain-size distribution of
unimodal clastic deposits follows the lognormal distribution, and the
lognormal distribution function can give sufficient accuracy in
describing unimodal grain-size distributions (Sun et al., 2002; Qin
et al., 2005; Xiao et al., 2013). In the present study, the grain-size
components of individual polymodal distributions were identified,
fitted and partitioned using the lognormal distribution function
method described by Qin et al. (2005), which is expressed as follows,
The fitting residual is calculated as follows,
Fitting experiments on a certain sample are accomplished when the residual error reaches its minimum. Numerical partitioning of the unimodal components of a measured polymodal distribution can be achieved simultaneously through lognormal distribution function fitting because the parameters and the distribution functions of each component are determined during fitting. The modal sizes and relative percentages of each component are given as soon as the fitting is accomplished.
The age-depth model of the HE indicates that the studied profile
covers the last 8600
The fitting and partitioning of the lognormal distribution function
suggests that the polymodal grain-size distributions of samples from
the studied profile is composed of four unimodal distributions,
representing four grain-size components. According to the dominant
range of modal size of each unimodal distribution, four components are
designated C1 through C4 from fine to coarse modes in this study. As
shown in Fig. 5, the four components can be easily determined for the
lacustrine sections, comparing two components of C1 and C3 for the
peat layers. The modal sizes of the components C1, C2, C3 and C4 vary
primarily within ranges of 0.5–1.8, 2.5–8.6, 25.6–38.6 and
386.2–514.8
It has been widely accepted that the grain-size depositional process
is closely linked to its sedimentary environment and the multimodal
characters of the deposits represent different transport or
depositional processes (McLaren and Bowles, 1985). In present study,
the grain-size components of the HE were partitioned using a lognormal
distribution function. The results show that the lacustrine deposits
contain four distinct unimodal grain-size distributions named as C1
through C4 with their modal sizes ranging 0.5–1.8, 2.5–8.6,
25.6–38.6 and 386.2–514.8
Although the components of both the C1 and C3 have widely distributed
in the studied profile HE (Fig. 6), only the C3 as the nearshore
suspension component is sensitive to the environment changes. As shown
in Fig. 7, comparing the other three components, the modal grain-size
of the C1 shows only very slight variations during last
8600
The lithofacies of local strata is one of the most direct indicators
for paleo-environment. As shown in Figs. 4 and 8, the blackish-grey
oozy mud layers have accumulated during the lower part of the studied
profile. The layers are also characterized by low and stable values of
accumulation rate (0.5
Such a striking environmental change was further supported by the
grain-size records. Based on the variations of C3 percentages of the
HE, the local hydraulic conditions during the Holocene were
tentatively reconstructed. During stage before 4600
Considering the prevalent monsoon climate in the Sanjiang plain, the locally environmental variations in the Plain must be potentially linked with the monsoon variations during the Holocene. While in addition to climate changes, the local environmental changes may also be influenced by lake-infilling process, hydrological changes or tectonic-induced changes, so we need to consider all these factors before interpreting the monsoonal implication of the lake-wetland transition in the Sanjiang Plain.
During the development of the lakes, the depositional process will lead to a gradual adoption to terrestialization, and eventually the extinction of the lake (Fang, 1991). This infilling action is usually accepted as a long-term and stable process on thousand-year time scale (Rhodes et al., 1996). Similarly, the tectonic activities are also a slow process and take gradually effects on local environment over much longer time scale (Jiang et al., 2008; Yang et al., 2011). Additionally, local hydrological changes, especially the channel changes of the neighboring rivers, may generate a direct effect on the extinction of a lake. However, the studied lake was broadly surrounded by low hills, and there are no big rivers affecting the basin directly. Therefore, all the three factors discussed above have only played a negligible role for the rapidly shrinking of the paleolake over several decades.
In comparison, the climate changes may exert a more important
influence for the sudden drying event at 4600
Such a remarkable monsoon decline have been well documented in cave
deposits, lake sediments, eolian deposits and peat accumulations in
monsoon regions of China (Hong et al., 2003; Li et al., 2004; Xiao
et al., 2004; Shen et al., 2005; Jiang et al., 2008; Sun and Huang,
2006; Wang et al., 2005). For the northeastern China, the alternations
of sand accumulation and paleosol development in desert regions are
regarded as the direct indicators for the monsoon circulations in the
geological past. As the soil development requires a much wetter/warmer
climate and better vegetation cover comparing with the drier climate
during the aeolian sand accumulation, in this context, the
alternations of aeolian sand and paleosols are mainly controlled by
the changes of summer monsoon strength. During the interval before
4600
The East Asian monsoon is primarily controlled by the thermal contrast
between the Asian landmass and tropic Pacific Ocean, and it is
characterized by the seasonal alterations of northward-moving moist
summer monsoon air and a northern mass of cooler winter monsoon air
(An, 2000). Modern meteorological studies indicate that the moisture
brought by the East Asian summer monsoon to the eastern China derives
from low latitudes of the west Pacific where is characterized by
a large pool of warm surface water, having the warmest global SST
Based on the high-resolution LOI and grain size records from a well
dated peat/mud profile in the Sanjiang Plain, a detailed environmental
change has been well reconstructed during the last
8600
This study was financially supported by the National Basic Research Program (No. 2012CB956100), the National Natural Science Foundation of China (No. 41 201 083, 41 271 209 and 41 271 106), and the Knowledge Innovation Program of Chinese Academy of Science (No. KZCX2-EW-319).
AMS radiocarbon dates of samples from the HE in the Sanjiang Plain.
Characteristics of the four grain-size components recognized on the polymodal distritutions of samples.
Digital elevation model of the Sanjiang Plain. The solid triangle in black color indicates the sampling site. In inset figure, the current northern limit (dashed line) of the East Asian Summer monsoon with its direction indicated by the arrows, the locations of the Sanjiang Plain (highlighted in black area) and the HLB profile (solid circles) mentioned in the text are shown.
Climate diagrams showing monthly temperature and precipitation in the Sanjiang Plain. All data were from climate normal for the period 1957–2000 at meteorological stations in the Sanjiang Plain.
A simplified configuration of the Quaternary strata in the studied basin with the location of HE profile.
Lithology, age model
Representative frequency curves of the grain-size distributions of samples from the studied profile. Noting that the upper four curves are from the peat sections, while the lower four curves are from the lacustrine layers. In each diagram, the number with the unit “cm” marks the depth of the sampled horizon.
Frequencies of the modal sizes of the four grain-size components, C1 through C4, in samples from the HE. Noting that the C2 and C4 components only constrained in a minor proportion of the total samples.
SDs of the percentages of the four grain-size components, C1 through C4, in samples from the HE. Noting that the C1 shows no apparent variation while the C3 exhibits the widest fluctuations responding to the climate changes.
The accumulation rate (
Schematic figures indicating the decline of East Asian summer monsoon played a driving role in lake-wetland transition during Holocene.
Schematic map showing the monsoon climate affecting China and the location of the modern Western Pacific warm pool (modified from Sum and Huang, 2006). Zone I is controlled by both the Indian monsoon and the east Asian monsoon; Zone II is mainly controlled by the east Asian monsoon; Zone III is controlled by the westerlies; Zone IV is located in the Tibetan Plateau with different climate. Noting that the East Asian summer monsoon plays an important role in transport of humid air masses from the western Pacific (especially from the warm pool) to the mid-high latitude regions of China, which is primarily controlled by the low-latitude solar radiation.