The Pliocene climate and its driving mechanisms have
attracted substantial scientific interest because of their potential as an
analog for near-future climates. The late Miocene–Pliocene red clay sequence
of the main Chinese Loess Plateau (CLP) has been widely used to reconstruct
the history of interior Asian aridification and the Asian monsoon. However,
red clay sequences deposited on the planation surface of the Tibetan Plateau
(TP) are rare. A continuous red clay sequence was recently discovered on the
uplifted Xiaoshuizi (XSZ) planation surface in the Maxian Mountains,
northeastern (NE) TP. In this study, we analyzed multiple climatic proxies
from the XSZ red clay sequence with the aim of reconstructing the late
Miocene–early Pliocene climate history of the NE TP and to assess regional
climatic differences between the central and western CLP. Our results
demonstrate the occurrence of minimal weathering and pedogenesis during the
late Miocene, which indicates that the climate was arid. We speculate that
precipitation delivered by the paleo East Asian summer monsoon (EASM) was
limited during this period and that the intensification of the circulation of the westerlies
resulted in arid conditions in the study region. Subsequently,
enhanced weathering and pedogenesis occurred intermittently during
4.7–3.9 Ma, which attests to an increase in effective moisture. We ascribe
the arid–humid climatic transition near
The Pliocene, including the Zanclean (5.33–3.60 Ma) and Piacenzian
(3.60–2.58 Ma) stages, is one of the most intensively studied intervals of
the pre-Quaternary in climate change research. The Zanclean climate was
generally warm and wet and is often used as an analog for near-future climate
conditions in terms of carbon dioxide levels, ranging from 280–415 ppm
(Tripati et al., 2009; Pagani et al., 2010), and comparable temperatures in
the tropical region (Herbert et al., 2010, 2016). On the other hand, the
Zanclean was markedly different from today, although several critical changes
in thermohaline and atmospheric circulation towards modern conditions were
occurring (Haug et al., 2005; Lawrence et al., 2006; Chaisson and Ravelo,
2000). For example, the early Pliocene global mean temperature was
approximately 4
East Asia is one of the key regions for studying the aridification of the Asian interior and the Asian monsoon evolution, which are tightly linked to the uplift of the TP, regional climate change, and the evolution of global temperature and ice volume (An et al., 2001; Ding et al., 2001; Li et al., 2008; Clift et al., 2008; Nie et al., 2014; Ao et al., 2016; Sun and Liu, 2006; Sun et al., 2017; Chang et al., 2013; Liu et al., 2014). Previous research has revealed that red clay was widely deposited across the CLP since the late Miocene, indicating that Asian aridification was enhanced (Guo et al., 2001; Song et al., 2007; An et al., 2014; Ao et al., 2016; Li et al., 2017). In the eastern and central CLP, where the climate is dominated by the East Asian monsoon, paleontological evidence, mineral magnetic parameters, and geochemical records from the red clay indicate dry climatic conditions during the late Miocene but generally wet climatic conditions during the early Pliocene (Wang et al., 2006; Guo et al., 2001; Wu et al., 2006; Song et al., 2007; Sun et al., 2010; An et al., 2014; Ao et al., 2016). The most controversial climatic change occurred during the interval from 4.8–4.1 Ma, for which climate reconstructions using different proxies indicate conflicting paleoenvironmental trends. For example, field observations and pollen records indicate an intensified summer monsoon intensity, but low magnetic susceptibility values are more consistent with arid rather than wet climatic conditions (Ding et al., 2001; Ma et al., 2005; Song et al., 2007; Sun et al., 2010). It is thought that dissolution of ferrimagnetic minerals and iron reduction resulting from high precipitation significantly affected the climatic significance of magnetic susceptibility records during this period (Ding et al., 2001). In addition to the East Asian monsoon, the westerlies also had an impact on the climate of East Asia; however, the patterns of climate change in the westerlies-dominated regions were different from the eastern and central CLP during the early Pliocene. Geochemical, stratigraphic, and pollen evidence from the Qaidam and Tarim basins has demonstrated that aridification intensified since the early Pliocene (Fang et al., 2008; Sun and Liu, 2006; Sun et al., 2017; Chang et al., 2013; Liu et al., 2014). Although the general climatic trends of the main CLP and central Asia during this period are well-recorded, paleoclimatic changes in the NE TP, which is at the junction of the zones of westerly and monsoonal influences, remain unclear. Therefore, determining the climatic conditions of the NE TP during the early Pliocene not only improves our understanding of the pattern of regional climate change, but may also provide insights into the responses of the paleo-EASM and the westerlies to TP uplift and changes in the global climate system.
A continuous red clay sequence was recently discovered on the uplifted XSZ planation surface in the NE TP and has been dated via high-resolution magnetostratigraphy (Li et al., 2017). Due to its specific geographical location, the XSZ red clay provides the opportunity to reveal the late Miocene–early Pliocene climate history of the NE TP and to determine the climatic differences between the central and western CLP. In this study, we measured multiple climatic proxies from the late Miocene–Pliocene XSZ red clay core. Our aims were to construct a detailed record of precipitation, chemical weathering, and pedogenesis during 6.7–3.6 Ma and to determine the pattern of regional climate evolution and its possible causal mechanisms.
The XSZ planation surface is located in Yuzhong County in the western Chinese
Loess Plateau (Fig. 1). The main XSZ planation surface is at an altitude of
2800 m in the Maxian Mountains where it has truncated Precambrian gneiss.
The Maxianshan are rejuvenated mountains, which protrude into the broad
Longzhong Basin; they are located within a climatically sensitive zone
because of the combined influences of the Asian monsoon and the northern
branch of the midlatitude westerly circulation system. The planation surface
is mantled by over 30 m of loess and over 40 m of red clay. Our previous
bio-magnetostratigraphic study demonstrates that the red clay sequence
covering the XSZ planation surface is dated to
Location of the study area and atmospheric circulation patterns.
The XSZ core (35.8115
Photos of the XSZ planation surface and the red clay.
Samples for grain-size, carbonate content, and magnetic susceptibility
measurements were taken at 5 cm intervals, and samples for geochemical
analysis were collected at 25 cm intervals. Samples for grain-size
measurements were pretreated with 10 %
We used the coefficient of variation (CV) to measure the variability of the
records: the higher the CV, the more variable the record. The CV is defined
as
Profiles of the various environmental proxies are illustrated in Fig. 3.
Notably, there is evidence for a relatively wet interval from
Variations in carbonate content, major element concentration, minor
element concentration, magnetic susceptibility, and grain size for the XSZ red
clay section (6.7–3.6 Ma). Yellow shading indicates
Average values and coefficients of variation of the geophysical and geochemical data for the XSZ section.
The carbonate content of the entire core fluctuates from
1.6 %–39.2 % with an average of 15.9 %. From 42–16 m, the
average carbonate content is high (17.1 %) and the carbonate content
decreases upwards. The contrast in the carbonate content between the
Correlation coefficients for geochemical data for the XSZ section.
Results of a
significance test for the correlations among CaO
The variations in
The clay content (< 2
The carbonate content of aeolian sediments can be readily remobilized and
deposited in response to changes in precipitation and evaporation intensity; thus, it
is sensitive to changing climatic conditions. Previous studies
demonstrated that the carbonate content of loess–red clay sequences of the
CLP varies with precipitation (Fang et al., 1999; Sun et al., 2010). The
carbonate is mainly derived from a mixture of airborne dust (Fang et al.,
1999). Soil micromorphological evidence from Lanzhou loess demonstrates
that the carbonate grains in loess are little altered, whereas those in
paleosols have undergone a reduction in size as a result of leaching and
reprecipitation as secondary carbonate in the lower
Chemical weathering intensity is generally evaluated by the ratio of mobile
(e.g., K, Ca, Sr, and Na) to nonmobile elements (e.g., Al and Rb). In general,
Sr shows analogous geochemical behavior to Ca and is readily released into
solution and mobilized in the course of weathering; by contrast, Rb is
relatively immobile under moderate weathering conditions due to its strong
adsorption to clay minerals (Nesbitt et al., 1980; Liu et al., 1993). Thus,
the
In the red clay–loess sequence of the CLP, magnetic parameters and the clay
content are well-correlated and are thus regarded as proxies for EASM strength
(Liu et al., 2004). Aeolian particles usually have two distinct magnetic
components consisting of detrital and pedogenic material, respectively (Liu
et al., 2004).
Pedogenesis results in enhanced secondary clay formation (Sun and Huang,
2006); however, not all of the clay particles are derived from in situ
pedogenesis, but rather are inherited from aeolian transport and deposition.
Clay particles can adhere to coarser silt and sand particles (Sun and Huang,
2006). In the western CLP, the coarse silt (> 40
Spectrum analysis results of the XSZ red clay section,
Comparison of the paleoclimatic record of the XSZ red clay section
with climate records from elsewhere.
The power spectral analyses of carbonate content and
King (1996) proposed that non-orbital cycles may originate from harmonic effects or interactions of the orbital cycles, while Lu (2004) ascribed them to unstable dust depositional processes followed by varying degrees of pedogenesis in paleosol units. In the XSZ section, the deposition rate is low and uneven, which potentially resulted in the incomplete preservation of the paleoclimatic signal, especially for the relatively short precession cycles. In addition, pedogenesis and postdepositional compaction would also weaken the orbital signals and produce spurious cycles. Moreover, the carbonate content at various depths is affected by leaching, which means that the record integrates soil polygenetic processes, thus obscuring orbital forcing trends related to precipitation amount. Therefore, we speculate that an uneven and low deposition rate, combined with compaction and leaching processes, may have weakened the orbital signals and may be responsible for the presence of non-orbital cycles in the XSZ section.
To investigate the post-6.7 Ma frequency domain evolution of the climate
signals in the XSZ section, we filtered the carbonate content and
We used the proxies of pedogenesis and chemical weathering to reconstruct the
late Miocene and early Pliocene climatic history of the Xiaoshuizi planation
surface. During the late Miocene, relatively high carbonate values with
minor fluctuations indicate that the climate was dry, and low
Comparison of late Miocene–Pliocene paleoclimatic records from
Asia.
Coeval pollen, mollusk, and magnetic records from the central and eastern CLP
also indicate generally dry and cold climatic conditions (Wang et al., 2006;
Wu et al., 2006; Nie et al., 2014). However, the principal difference is
that at the XSZ site, the arid climate was relatively stable, while the
climate of the central and eastern CLP was interrupted by several humid
stages. For example, two humid stages (6.2–5.8 and 5.4–4.9 Ma) are
recorded by the magnetic susceptibility of the red clay in the central and
eastern CLP but are absent in the magnetic susceptibility record at the XSZ
site (Fig. 7). Notably, the 41 kyr filtered component of
thermo-humidiphilous species from Dongwan was damped in the late Miocene (Li
et al., 2008). Similarly, the amplitude of the orbital periodicities,
filtered from the XSZ carbonate content and
The especially damped response of the wet–dry climatic oscillations in the
western CLP to obliquity forcing may indicate that the influence of the
paleo-EASM in the western CLP was negligible. It is widely known that the
summer monsoon intensity decreases from southeast to northwest across the
CLP. A regional climate model experiment demonstrated that the modern East
Asian summer monsoon was not fully established in the late Miocene and had
only a small impact on northern China (Tang et al., 2011). A weak
paleo-EASM intensity from 7.0–4.8 Ma was revealed by
hematite
The simultaneous reduction in amplitude of the 41 kyr filtered components
from the western CLP and the deep-sea
During the early Pliocene, the proxy evidence indicates that the previously
arid climate of the XSZ area became humid from
Palynological and terrestrial mollusk records from the central CLP also
indicate relatively humid conditions during the early Pliocene (Wang et al.,
2006; Wu et al., 2006). Magnetic susceptibility records from the central and
eastern CLP are similar to those from the XSZ section in that both
magnitude and variability are high during the early Pliocene. From
4.1–3.9 Ma, the increased magnetic susceptibility indicates that humid
climatic conditions prevailed across the entire CLP (Fig. 7). Evidently, when
precipitation amount peaked in the vicinity of the XSZ section during
4.60–4.25 Ma, the magnetic susceptibility values at Xifeng, Lingtai, and
Chaona were low. However, a record of
In summary, a wet climate prevailed across the CLP in the early Pliocene. At
the same time, the hematite
Ding (2001) proposed that the uplift of the TP to a critical elevation
resulted in an enhanced summer monsoon system during 4.8–4.1 Ma. TP uplift
was shown to have had profound effects on the EASM in terms of its initiation
and strength, as well as in changing the distribution of the band of high
precipitation in East Asia (Li et al., 1991, 2014; An et al., 2001). A
detailed modeling study demonstrated that the uplift of the northern TP
mainly resulted in an intensified summer monsoon and increased precipitation
in northeast Asia (Zhang et al., 2012). From 8.26–4.96 Ma, massive
deltaic conglomerates were widely deposited and the sediment deposition rate
increased, indicating the uplift of the Qilian Mountains (Song et al.,
2001). At the same time, the Laji Mountains underwent pronounced uplift by
thrusting at
The occurrence of a humid climate across the CLP was synchronous with the
gradual closure of the Panama Seaway (Keigwin, 1978; O'Dea et al., 2016).
Nie (2014) proposed that the freshening of eastern equatorial and North
Pacific surface water, resulting from the closure of the Panama Seaway since
4.8 Ma (Haug et al., 2001), led to sea ice formation in the North Pacific
Ocean, which enhanced the high-pressure cell over the Pacific and increased
the strength of southerly and southeasterly winds. However, there was a
warming trend in the Northern Hemisphere from 4.6 Ma (Haug et al., 2005;
Lawrence et al., 2006). The gradual closure of the Panama Seaway resulted in
the reorganization of surface currents in the Atlantic Ocean. Notably, the
Gulf Stream was enhanced and began to transport warm surface waters to high
northern latitudes, thus strengthening the Atlantic meridional overturning
circulation and warming the Arctic (Haug and Tiedemann, 1998; Haug et al.,
2005). Three independent proxies from an early Pliocene peat deposit in the
Canadian High Arctic indicate that Arctic temperatures were 19
The continuous late Miocene–Pliocene red clay sequence preserved on the planation surface in the NE Tibetan Plateau provides the opportunity to elucidate the history of the Asian monsoon in the western CLP. Multi-proxy records from the XSZ section, together with other paleoclimatic records from the CLP, reveal the major patterns of climatic change from 6.7–3.6 Ma. During the late Miocene, both the amount and variability of precipitation over the XSZ section were small; however, they were much greater in the central and eastern CLP. Thus, the paleo-EASM had little influence on the climate of the western CLP at this time. During the early Pliocene, records from the XSZ section indicate that both the amount and variability of precipitation increased from 4.7–3.9 Ma. The climate was characterized by abrupt increases in the seasonality of precipitation, which attests to a major northwestward extension and enhancement of the summer monsoon. Multiple paleoclimatic proxies clearly show that the strongest summer monsoon occurred during 4.60–4.25 Ma. The expansion of the paleo-EASM may have been caused by warming of the high northern latitudes in response to the closure of the Panama Seaway during the early Pliocene.
The data related to this study is available online at
: JL and CS designed research. All authors contributed to field and environment work and discuss the paper. Tingjiang Peng modified the article. XL wrote the paper.
The authors declare that they have no conflict of interest.
We thank Ai Song, Jia Liu, Shanpin Liu, and Jun Zhang for the drilling operation and Fengxia Yu for her early experiment work. We thank Jan Bloemendal for modifying and polishing the language on an earlier version of the paper. We especially thank Ran Feng, Lin Li and three anonymous reviewers for their suggestions and comments that have helped improve the paper. This work was supported by the National Natural Science Foundation of China (grants 41330745 and 41401214) and the Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research (LCP201602). Edited by: Ran Feng Reviewed by: Lin Li and three anonymous referees