Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
1 Late Miocene-Pliocene climate evolution recorded by the red clay covered
2 on the Xiaoshuizi planation surface, NE Tibetan Plateau
3
4 Xiaomiao Li1, Tingjiang Peng1, Zhenhua Ma1, Meng Li1, Zhantao Feng1, Benhong Guo1, Hao Yu1, Xiyan
5 Ye1, Zhengchuang Hui1, Chunhui Song2, Jijun Li1, 3
6
7 1. MOE Key Laboratory of Western China's Environmental Systems, College of Earth and Environmental
8 Sciences, Lanzhou University, Lanzhou 730000, China
9 2. School of Earth Sciences, Key Laboratory of Western China's Mineral Resources of Gansu Province,
10 Lanzhou University, Lanzhou 730000, China
11 3. College of Geography Science, Nanjing Normal University, Nanjing 210023, China
12
13
14
15
16
17
18
19 *Corresponding author: Key Laboratory of Western China’s Environmental Systems
20 (Ministry of Education), College of Earth and Environmental Science, Lanzhou University,
21 Lanzhou 730000, China; E-mail address: [email protected](J.J. Li), Fax: +86-931-891-2724;
22 Tel.: +86-931-891-2724
1 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
23 Abstract
24 As an analogue for predicting the future climate, Pliocene climate and its driving
25 mechanism attract much attention for a long time. Late Miocene-Pliocene red clay sequence
26 on the main Chinese Loess Plateau (CLP) has been widely applied to reconstruct the history
27 of interior aridification and Asian monsoon climate. However, the typical red clay sequences
28 deposited on the planation surface of Tibetan Plateau are rare. Recently, continuous red clay
29 has been found on the uplifted Xiaoshuizi peneplain in the Maxian Mountains, northeastern
30 (NE) Tibetan Plateau (TP). To reconstruct the late Miocene-early Pliocene climate history of
31 NE Tibetan Plateau and to assess the regional differences between the central and western
32 CLP, multiple climatic proxies were analyzed from the Xiaoshuizi red clay sequence. Our
33 results demonstrate the minimal weathering and pedogenesis from 6.7 to 4.8 Ma, which
34 implicates that the climate was sustained arid. We speculate that precipitation delivered by
35 the paleo-Asian Summer Monsoon (ASM) was limited during this period, and instead the
36 intensification of the westerlies circulation resulted in arid condition in the study region.
37 Subsequently, enhanced weathering and pedogenesis occurred during the interval of 4.8-3.6
38 Ma, which attests to increasing effective moisture. Thus, we ascribe the obvious arid-humid
39 climate transition near 4.8 Ma to the palaeo-ASM expansion. Increasing Arctic temperatures,
40 the vast poleward expansion of the tropical warm pool into the subtropical regions and water
41 freshening in the subtropical Pacific in response to the closure of the Panamanian Seaway
42 may have been responsible for the thermodynamical enhancement of the palaeo-ASM system,
43 which permitted more moisture to be carried to the NE Tibetan Plateau.
44 Keywords: Late Miocene-earlyPliocene; Xiaoshui Peneplain; Red Clay; Palaeo-ASM;
2 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
45 Westerly Circulation
46
47 1. Introduction
48 The Pliocene, including the Zanclean (5.33-3.60 Ma) and Piacenzian (3.60-2.58 Ma)
49 stages, is one of the most intensively studied intervals of the pre-Quaternary. The Zanclean
50 climate was generally warm and wet and it is analogous to the present day in terms of (i)
51 land-sea distribution, (ii) orbital configuration, (iii) carbon dioxide levels ranging from 280-
52 380 ppm (Raymo et al., 1996; Fedorov et al., 2013), and (iv) comparable temperatures in the
53 tropic region. In addition, both the Holocene and Zanclean are transitional periods from cold
54 to warm climatic condition. For these reasons, the early Pliocene climate is often used as an
55 analogue for that of the Holocene and attracts much attention. On the other hand, Zanclean is
56 unique and some crucial transitions of the thermorhaline and atmospheric circulation towards
57 modern conditions were undergoing. Temperatures at the high northern latitudes were
58 considerably higher and therefore continental glaciers were almost absent from the Northern
59 Hemisphere (Ballantyne et al., 2010; Dowsett et al., 2010). The warm and wet climate
60 prevailed across the major continents and the warm Arctic is thought to have resulted from a
61 greenhouse effect caused by higher atmospheric moisture content (Abbot and Tziperman,
62 2008). The low meridional surface temperature gradient resulted in an “equable” climate
63 during this interval (Abbot and Tziperman., 2008; Fedorov et al., 2013). The east-west sea
64 surface temperature gradient in the tropical Pacific during this interval is also believed to be
65 low, which is tightly linked with El Nino Southern Oscillation (Lawrence et al., 2006).
66 However, debate persists on whether permanent El Nino–like conditions were sustained
3 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
67 during the Pliocene (Wara et al., 2005; Watanabe et al., 2011; Zhang et al., 2014).
68 Meanwhile, the most significant tectonic movements were the uplift of the TP (Li et al., 2015;
69 Zheng et al., 2000 ; Fang et al., 2005a, 2005b) and gradual closing of the Panama seaway
70 (Lunt et al., 2008; Haug et al., 1998, 2005). These tectonic movements resulted in major
71 changes in the global thermohaline and atmospheric circulation system which were thought to
72 make crucial preconditions for both appearing of ice sheet in northern hemisphere at ~3Ma
73 (Haug et al., 1998; Driscoll et al., 1998) and development of modern east-west
74 hydrographic gradient in the equatorial Pacific (Lawrence et al., 2006; Chaisson et al., 2000).
75 The ASM and meridional (westerlies) circulation systems, as major components of
76 atmospheric circulation, delivered moisture to Eurasia which might have prepared enough
77 moisture for long-term growth of ice sheet in northern hemisphere between 3 and 2 Ma
78 (Driscoll et al., 1998). Make clear the evolution of the palaeo-ASM and westerlies during
79 early Pliocene is critical to understanding formation mechanism of ice sheet at the Northern
80 high latitudes. Furthermore, the palaeo-ASM might be dynamically linked with the TP uplift,
81 changes in latitudinal and longitudinal heat gradients, global temperature and ice volume
82 during early Pliocene. Warm and wet climate background tends to yield wet climate condition
83 while reductions in the east-west sea surface temperature (SST) gradient in the tropical
84 Pacific results in a weakened summer monsoon (Wang et al., 2000). Several studies have
85 shown that a major atmospheric teleconnection links the ASM with both Arctic volume and
86 the TP uplift (Ding et al., 1990; Li et al., 1991; An et al., 2001; Clift et al., 2008; Sun et al.,
87 2015). Thus, it is crucial to make clear what the climate was like in East Asia under such
88 warm and equable climatic conditions in the Northern Hemisphere.
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89 Previous research has revealed that since the late Miocene, red clay widely deposited
90 across the CLP, indicating that the onset of interior Asian aridification related to the uplift of
91 the TP occurred (Guo et al., 2001; Song et al., 2007; An et al., 2014; Ao et al., 2016; Li et al.,
92 2017). Element, strata and pollen evidence from the Qaidam and Tarim basin demonstrated
93 that the aridification had intensified since early Pliocene (Fang et al., 2008; Sun et al., 2006a,
94 2017; Chang et al., 2013; Liu et al., 2014). In eastern and central CLP, palaeontological
95 evidence, mineral magnetic parameters and geochemical records from the red clay also
96 indicate a dry climate condition during late Miocene, however, aridification process was
97 interrupted by a long interval of wet climate during the early Pliocene (Wang et al., 2006;
98 Guo et al., 2001; Wu et al., 2006; Song et al., 2007; Sun et al., 2010; Ao et al., 2016). The
99 most controversial climate change occurred during the interval of 4.8-4.1 Ma, for which
100 climate reconstructions from different proxies reveal conflicting palaeoenvironmental trends.
101 For example, field observations and pollen records indicate an intensified monsoon system,
102 but low magnetic susceptibility values are more consistent with arid rather than wet climatic
103 conditions (Ding et al., 2001; Ma et al., 2005; Song et al., 2007; Sun et al., 2010). It’s thought
104 to be substantial gleying resulted from large amount precipitation which made magnetic
105 susceptibility invalid over this period (Ding et al., 2001). Obviously, climate changes in
106 westerlies dominated regions and monsoon dominated regions are discrepancy. The
107 inconsistent climate change may be related to different response of westerlies and the palaeo-
108 ASM to global climate changes and the TP uplift during early Pliocene. To clarify the
109 evolution and dynamic of westerlies and the palaeo-ASM, requires accurate paleoclimatic
110 reconstructions in the CLP, especially in the western CLP.
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111 Till now, early Pliocene paleoclimatic records from the western CLP red clay are
112 lacking. Recently, continuous red clay has been found on the uplifted Xiaoshuizi (XSZ)
113 peneplain in the NE Tibetan Plateau and well dated via high-resolution magnetostratigraphy
114 analysis (Li et al., 2017). The special gemorophological and climatic characteristic of the
115 Xiaoshuizi red clay makes it different from the main CLP red clay, and provides particular
116 opportunity to reveal the late Miocene-early Pliocene climate history in NE Tibetan Plateau
117 and discuss the climatic difference between the central and western CLP red clay. In this
118 study, multiple climatic proxies have been applied in the Xiaoshuizi late Miocene-Pliocene
119 red clay sequence. Then we reconstruct the detailed precipitation, chemical weathering and
120 pedogensis history in the Xiaoshuizi planation surface during the interval of 6.7-3.6 Ma.
121 Finally, the regional climate and possible mechanism have been further discussed.
122
123 2. Regional background
124 The XSZ planation surface locates in Yuzhong County in the western Chinese Loess
125 Plateau (Fig. 1). The main XSZ planation surface is at an altitude of 2800 m in the
126 Maxianshan Mountains where it has truncated Precambrian gneiss. The Maxianshan are
127 rejuvenated mountains which protrude into the broad Longzhong Basin, and are in a
128 climatically sensitive zone because of the combined influences of the Asian Monsoon and the
129 northern branch of the mid-latitude westerly circulation system. The planation surface is
130 mantled by over 30 m of loess and over 40 m of red clay. Our previous bio-
131 magnetostratigraphic study has demonstrated that the red clay sequence covered on the XSZ
132 peneplain is dated to ~6.9-3.6 (Ma Li et al., 2017), here we choose the Xiaoshuizi core to
6 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
133 discuss the regional climate because of its continuous deposit and whole timescale relative to
134 the Shangyantan core mentioned in Li et al (2017). Yuzhong County lies within the semi-arid
135 temperate climate zone at the junction of the eastern monsoon area, the arid area of northwest
136 china, and the Tibetan Plateau cold region. The East Asian Monsoon system and the westerly
137 circulation operate together. The mean annual temperature is about 6.7 °C and the
138 precipitation amount is 300-800 mm. The spatial distribution of precipitation is uneven,
139 decreasing from south to north in Yuzhong County. Precipitation amount increases with
140 elevation at a rate of 27 mm per 100 m, attaining a maximum of 800 mm.
141
142 3. Material and methods
143 The XSZ core (35.81154°N；103.8623°E and 2758.1 m above sea level) is composed of
144 42 m of pure red clay and ~3 m of red clay with an increasing angular gravel content. The red
145 clay is composed of brownish red and yellowish clay layers. The upper 20 m is impregnated
146 with many horizontal carbonate nodule horizons and most of these horizons underline the
147 brownish red layer; there are occasional carbonized plant root channels, elliptical worm
148 burrows and snail fossil fragments. Fe-Mn stains are more frequent in the brownish layers
149 than in the yellowish layers, which is also the case for horizons containing carbonized root
150 channels. The red clay over the Xiaoshuizi planation surface is similar to that of typical
151 eolian red clay in the CLP, all of which are characterized by many carbonate nodule-rich
152 horizons. Grain-size, carbonate content and magnetic susceptibility samples were taken at 5-
153 cm intervals, while samples for geochemical analysis were collected at 25-cm intervals. Each
154 sample age was modeled using linear interpolation to derive absolute ages, constrained by our
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155 previous magnetostratigraphy study. The grain-size distribution of samples was measured
156 with a Malvern Mastersizer 2000 with a detection range of 0.02-2000μm. Magnetic
157 susceptibility (χ) was measured using a Bartington MS2 meter and MS2B dual-frequency
158 sensor at two frequencies (470 Hz and 4700 Hz, designated χlf and χhf, respectively). Three
159 measurements were made at each frequency and the final results were averaged. The
160 frequency-dependent magnetic susceptibility (χfd) was calculated as χlf–χhf. Chemical
161 composition was measured using Panalytical Magix PW2403. The sample preparation
162 procedure for XRF analysis was as follows: first, the bulk sample was heated to 35℃ for 7
163 days, then each sample was ground to less than 75μm using an agate mortar, and finally about
164 4 g of powdered sample was pressed into a pellet with a borate coating using a semiautomatic
165 oil-hydraulic laboratory press (model YYJ-40). All the measurements were finished in the
166 MOE Key Laboratory of Western China's Environmental Systems, Lanzhou University. The
167 molar content of silicate Ca (CaO*) was calculated using the following equation: 10 P O 168 CaO *(mol) CaO(mol) CaCO (mol) * 2 5 3
3 M (P2O5 ) 169 The carbonate content was measured with a calcimeter using the volumetric method of
170 Avery and Bascomb (1974) in the Key Laboratory of Mineral Resources in Western China
171 (Gansu Province), Lanzhou University.
172
173 4. Results
174 Carbonate content
175 According to the fluctuations in carbonate content, the red clay sequence was divided
176 into two intervals: Interval -Ⅰis from 6.7-4.8 Ma, during which the carbonate content
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177 fluctuates from 3.8-39.2% with an average of 17.4%; the amplitude of fluctuations is small
178 and the carbonate content decreases upwards. From 5.4-4.9 Ma, the carbonate content
179 fluctuations are of greater amplitude than during 6.7-5.4 Ma. Interval -Ⅱ is from 4.8-3.6 Ma,
180 during which the carbonate content fluctuates from 1.6-39.1% with an average of 13.8%.
181 From 4.8-3.9 Ma there are several leaching-accumulation layers with <7% carbonate content
182 in the leached loess layers and >20% carbonate content in the accumulation layers.
183 Element geochemistry
184 The XSZ red clay consists mainly of SiO2, Al2O3, CaO and Fe2O3 with low
185 concentrations (<5%) of MgO, K2O, Na2O, Sr, Rb and Ba (Table 1). The variations in Al2O3
186 and K2O are synchronous and roughly opposite to that of CaO. The variations in CaO show
187 the same trend as carbonate content. When the carbonate content is high, CaO is high, while
188 Al2O3 and K2O are low. The contents of Al2O3 and K2O from 4.8-3.6 Ma are clearly higher
189 than those from 6.7-4.8 Ma. The variations in these element concentrations from 4.8-3.6 Ma
190 are also greater than those from 6.7-4.8 Ma. The changes in Sr are similar to those of CaO,
191 but opposite to those of Ba and Rb.
192 Magnetic susceptibility
-8 3 193 During intervalⅠ(6.9-4.8 Ma), χhf changes from 9.6-33.3×10 m /kg with an average of
-8 3 -8 3 -8 3 194 19.4×10 m /kg. χlf ranges from 11.4-36.1×10 m /kg with an average of 20.3×10 m /kg,
-8 3 -8 3 195 whilst χfd fluctuates from 0-2.8×10 m /kg with an average of 1.0×10 m /kg. During interval
-8 3 -8 3 196 Ⅱ(4.8-3.58 Ma), χhf ranges from 12.8-53.9×10 m /kg with an average of 25.4×10 m /kg, χlf
-8 3 -8 3 197 ranges from 13.56-59.0×10 m /kg with an average of 26.9×10 m /kg and χfd ranges from 0-
198 4.7×10-8 m3/kg with an average of 1.2×10-8 m3/kg. Clearly, the average values of the three
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199 parameters are larger during intervalⅡthan during interval Ⅰ; the amplitudes and durations
200 of the fluctuations of the three parameters during interval Ⅱare also larger and longer than
201 those during the interval Ⅰ. From 4.8-4.7 Ma, 4.6-4.25 Ma and from 4.1-3.9 Ma, the values
202 of the three parameters are high, and they exhibit peaks from 4.6-4.25 Ma.
203 Grain-size analysis
204 The average clay content (<2μm ) is 8.2% during interval Ⅰand 8.0% during interval Ⅱ.
205 The fluctuations in clay content are minor, except for maxima at about 5 Ma, 4.6 Ma and 4.2
206 Ma. The coarse silt component (>43μm), mainly carried by the East Asian Winter Monsoon,
207 exhibits a different trend to that of the clay content. From 6.7-4.8 Ma, the >43μm curve is
208 characterized by low values and high-frequency fluctuations, while after 4.8 Ma it exhibits
209 high values and long-duration fluctuations.
210
211 5. Discussion
212 5.1 Paleoenvironmental explanation of the proxies
213 The carbonate content of aeolian sediments is sensitive to varying climatic conditions,
214 and can be readily remobilized and deposited. Previous studies demonstrated that carbonate
215 in the loess-red clay sequence on the CLP records varies with precipitation (Fang et al., 1999;
216 Sun et al., 2010). The carbonate is mainly derived from a mixture of airborne dusts (Fang et
217 al., 1999). Soil micromorphological evidence from the Lanzhou loess demonstrates that
218 carbonate grains in loess are little altered, while those in the palaeosols have undergone a
219 reduction in size as a result of leaching and reprecipitation in the lower Bk horizons as
220 secondary carbonate (Fang et al., 1994, 1999). Furthermore, seasonal alternations between
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221 wet and dry conditions are thought to be a key factor in driving carbonate dissolution and
222 reprecipitation (Sun et al., 2010). Thus, changes in carbonate content are generally controlled
223 by the effective precipitation. When effective precipitation is high, carbonate leaching
224 increases, and vice versa. So the carbonate content is regarded as an effective precipitation
225 proxy for studying wet-dry oscillations as well as summer monsoon evolution (Fang et al.,
226 1999; Sun et al., 2010).
227 Chemical weathering intensity is generally evaluated by the ratio of mobile (i.e., K, Ca,
228 Sr and Na) to non-mobile elements (e.g., Al and Rb). In general, Sr shows analogous
229 geochemical behavior to Ca and is easily released into solution and mobilized in the course of
230 weathering, while Rb is relatively immobile under moderate weathering conditions due to
231 strong adsorption to clay minerals (Nesbitt et al., 1980; Liu et al., 1993). Thus, the Rb/Sr ratio
232 potentially reflects chemical weathering intensity. However, the initial Rb/Sr ratio can be
233 affected by the precipitation of secondary carbonate leached from overlying sediments during
234 pedogenesis (Chang et al., 2013; Buggle et al., 2011), which may limit its environmental
235 significance. The correlation between Sr and CaO* (silicate CaO) is significant in the XSZ
236 section, while the correlation between Sr and CaCO3 is not significant (99% confidence
237 interval). Thus we speculate that the Rb/Sr ratio mainly reflects the weathering intensity in
238 our studied samples (Fig. 4 e and f). In addition, previous study has proposed that the
239 K2O/Al2O3 ratio can also indicate the weathering intensity. Al2O3 is typically chosen to
240 measure the mobility of elements due to its high stability (Taylor et al., 1983), while K2O
241 (mainly produced by the physical weathering of potash feldspar) is easily leached from
242 primary minerals and then absorbed by secondary clay minerals with ongoing weathering
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243 (Yang et al., 2006; Liang et al., 2013). In the arid and semi-arid regions of Asia, K2O is
244 enriched in palaeosols compared to loess horizons (Yang et al., 2006), meaning that the
245 enrichment of K2O is positively related with the amount of secondary clay. Thus, to some
246 extent, K2O/Al2O3 reflects the amount of secondary clay and hence weathering intensity.
247 Generally, the K2O/Na2O ratio is used to evaluate the clay content in loess and is also a
248 measure of plagioclase weathering, avoiding biases due to uncertainties in separating
249 carbonate Ca from silicate Ca (Liu et al., 1993 ; Buggle et al., 2011). As the product of
250 plagioclase weathering, Na2O is easily leached by increasing precipitation. As mentioned
251 above, K2O is easily absorbed by secondary clay particles, meaning that high K2O/Na2O
252 ratio is indicative of intense chemical weathering.
253 In the red clay-loess sequence of the CLP, magnetic parameters and clay (<2 µm)
254 content are well correlated and thus are regarded as the proxies of the ASM strength (Liu et
255 al., 2004). Eolian particles usually have two distinct magnetic components consisting of
256 detrital and pedogenic material (Liu et al., 2004). χlf can reflect the combined susceptibilityof
257 both two components, but changes in χlf are dominantly affected by changes in the
258 concentration of pedogenic grains (Liu et al., 2004). Grain size distribution of pedogenic
259 particles confining within the superparamagnetic (SP) and single-domain (SD) grain size has
260 been proven to be steady (Liu et al., 2004, 2005). χfd can detect superparamagneticminerals
261 produced by pedogenesis and therefore the correlation coefficient between χlf and χfd can
262 measure the contribution of SP grains (<0.03 µm for magnetite) to the bulk susceptibility (Liu
263 et al., 2004; Xia et al., 2014). As shown in Figure 4A, χlf is positively correlated with χfd,
264 which means that the magnetic susceptibility of the XSZ red clay mostly reflects pedogenic
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265 enhancement of the primary eolian ferromagnetic content through the in situ formation of
266 fine-grained ferrimagnetic material. This means that the magnetic susceptibility of the red
267 clay on the XSZ planation surface reflects pedogenic intensity. Both the original and
268 pedogenic magnetic signals can be separated using a simple linear regression method (Liu et
269 al., 2004; Xia et al., 2014). We use this method to extract the original magnetic component
270 (χ0) and the pedogenic magnetite/maghemite component (χpedo). In this study, χfd explains 11%
271 of the susceptibility in terms of pedogenic magnetite/maghemite (χpedo = χfd / 0.11).
272 Pedogenesis results in enhanced secondary clay formation (Sun et al., 2006); however,
273 not all of the clay particles are derived from in situ pedogenesis, but rather are inherited from
274 aeolian transport and deposition. Clay particles can adhere to coarser silt and sand particles
275 (Sun et al., 2006b). In the western CLP, the coarse silt (>40 µm) content is regarded as a
276 rough proxy for the winter monsoon strength (Wang et al., 2002). Therefore, to eliminate this
277 signal from the primary clay particles, the <2 µm/>40 µm ratio is proposed to evaluate
278 pedogenic intensity. Furthermore, the similarity of the variations between the <2 µm/>40 µm
279 ratio and χpedo confirms that both proxies are sensitive to pedogenic intensity in the XSZ red
280 clay.
281 5.2 Time domain and frequency domain analysis of the carbonate content and χpedo
282 Power spectral analyses of carbonate content and χpedo show different dominant cycles
283 (Fig. 5). In detail, χpedo is concentrated in the eccentricity (100 ky), obliquity (41 ky) and
284 precession (21 ky) bands and another periodicities (71 ky and 27 ky) are also evident. In
285 contrast, the carbonate signal is concentrated in the precession (21 ky) and obliquity (41 ky)
286 bands, but it also exhibits even more prominent periodicities at 56 ky and 30 ky. Furthermore,
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287 Morlet wavelet transform analysis of both carbonate content and χpedo show that the orbital
288 signal increases since 4.8 Ma (Fig. 5 d).
289 As for the non-orbital cycles, King (1996) proposed that these may possibly originate
290 from harmonics or interactions of the orbital cycles, while Lu (2004) ascribed them to the
291 unstable dust deposition processes followed by varying pedogenesis in palaeosol units. Here
292 we speculate that they may be caused by the low deposition rate, which potentially resulted in
293 the incomplete preservation of the paleoclimatic signal, especially for short cycles of
294 precipitation change. Thus, the incomplete nature of the red clay time series may be
295 responsible for the presence of spurious cycles. In addition, the carbonate content at various
296 depths is affected by leaching which means that the record integrates soil polygenetic
297 processes, thus obscuring orbital forcing trends related to precipitation amount. Low
298 deposition rates, compaction and leaching processes would obscure the orbital cycles, and
299 spectral peaks that do not correspond to orbital cycles may reflect these processes.
300 To investigate the post-6.7 Ma evolution of the climate signals in the XSZ section in the
301 frequency domain, we filtered the carbonate content and χpedo time series at the 100, 41, and
302 21-kyr periods, using Gaussian band filters centered at frequencies of 0.01, 0.02439, and
303 0.04762, respectively, and compared them with the equivalent filtered components of the
304 stacked deep-sea benthic foraminiferal oxygen isotope record. Our results show that the
305 fluctuations of the three filtered components of both two proxies change rapidly from very
306 low amplitude from 6.7-4.8 Ma to a much larger amplitude from 4.8-4.1 Ma (Fig. 5). The
307 enhanced orbital-scale variability of the two proxies from 4.8-4.1 Ma implies an increased
308 seasonality and wet-dry contrasts. This shift is not observed in the earth orbital parameters
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309 but is observed in the filtered 41-kyr component of the stacked deep-sea benthic foraminiferal
310 oxygen isotope record (δ18O). This means that the increased contrast in wet-dry oscillations at
311 the XSZ site was not driven directly by changes in solar radiation intensity but rather was
312 linked with changes in ice volume or global temperature.
313 5.3 Late Miocene-Pliocene climate history revealed by the Xiaoshuizi red clay
314 5.3.1 Multiporxy evidence for the dry climate during the interval of 6.7-4.8 Ma
315 Based on the previous mentioned proxies of pedogenesis and chemical weathering, we
316 reconstruct the late Miocene and early Pliocene climatic history of the Xiaoshuizi peneplain,
317 NE Tibetan Plateau. As shown in Figure 6, we observe that a significant change recorded by
318 the most of the multiproxy (carbonate, Rb/Sr, K2O/Al2O3, χpedo) occurred near 4.8-4.7 Ma,
319 and therefore the climatic record was generally divided into two intervals. During interval I
320 (6.7-4.8 Ma), the relatively high carbonate values with minor fluctuations indicate that the
321 climate was dry and low Rb/Sr, K2O/Al2O3 and K2O/Na2O ratios support the weak chemical
322 weathering. Importantly, both the Rb/Sr and K2O/Na2O ratios show opposite trends with
323 carbonate content, meaning that low effective precipitation resulted in weak chemical
324 weathering intensity. Furthermore, the pedogenic proxies (<2 µm/>40 µm ratio, χpedo and χlf),
325 which characterised by low values with minor fluctuations, generally supports the weak
326 pedogenesis under the arid climate. Thus, during this interval the Xiaoshuizi climate was
327 relative arid, which characterized by weak chemical weathering and pedogenesis intensity.
328 However, subtle differences exist when these proxies detailed climate changes especially
329 when climate is relative wet. It is evident that the carbonate content decreases with increased
330 variation amplitude after 5.5 Ma, which is consistent with the cycles of carbonate nodules
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331 within paleososol horizons observed in the field (Li et al., 2017). It may be increased
332 precipitation which induced eluviation-redeposition of carbonate since 5.5 Ma. However,
333 from pedogenesis indicies we observe that the general arid climate was interrupted by two
334 enhanced pedogenesis events (occurred at 5.85-5.7 Ma and 5.5-5.35 Ma, respectively). The
335 subtle differences may result from different sensitivity of magnetic susceptibility and
336 carbonate content to precipitation variability when precipitation is low (Sun et al., 2010). In
337 addition, a record of mollusks from the western Liupanshan showed cold-aridiphilous species
338 dominating which also document the cold and dry climate condition on the western CLP
339 during late Miocene (Fig . 7 g ).
340 During this interval, pollen, mollusk and magnetic records from the central and eastern
341 CLP also indicate generally dry and cold climatic conditions (Wang et al., 2006; Wu et al.,
342 2006; Nie et al., 2014). However, the obvious difference is that the Xiaoshuizi arid climate is
343 relative stable, while the climate of central and eastern CLP was interrupted by several
344 obvious humid stages. For instance, two humid stages (6.2-5.8 Ma and 5.4-4.9 Ma) are
345 recorded by the magnetic susceptibility of red clay in the hinterland of the CLP, but are not
346 recorded by the Xiaoshuizi magnetic susceptibility (Fig. 7). It is worth noting that 41-kyr
347 filtered component of thermo-humidiphilous species from the Dongwan was damped in late
348 Miocene (Li et al., 2008). Similarly, the amplitude of the orbital periodicities, filtered from
349 the XSZ carbonate content and χpedo, are obviously damped from 6.7-4.8 Ma. However, the
350 three periodicities in Summer Monsoon Index from the central CLP show no obvious
351 difference between the late Miocene and Pliocene, but only a slight reduction in variability
352 after 4.2 Ma (Sun et al., 2010). Therefore, we agree that a dry climate prevailed on the CLP
16 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
353 during the interval of 6.7-5.2 Ma. The only difference is that the climate in the CLP
354 hinterland fluctuated more significantly than that of the Xiaoshuizi red clay.
355 The particularly damped response of the western CLP wet-dry oscillations to obliquity
356 forcing may indicate the palaeo-ASM had a negligible influence on the western CLP. It is
357 widely known that the summer monsoon intensity decreased from southeast to northwest
358 across the CLP. A regional climate model experiment demonstrated that the modern Asian
359 summer monsoon was not fully established in the late Miocene and had only a small impact
360 on the northern China (Tang et al., 2011). The weak palaeo-ASM intensity from 7.0-4.8 Ma
361 has been revealed by hematite/goethite and smectite/kaolinite ratios at ODP Site 1148 from
362 the South China Sea (SCS) (Fig 7 i and j). Therefore, we deduce that the Asian monsoon
363 was weak and put a small impact on the Xiaoshuizi climate. In addition, during late Miocene,
364 the TP was not intensively uplifted and thus it could not block the westerlies completely (Li
365 et al., 2015). Previous studies suggested that the red clay may have been transported by both
366 low-level northerly winds and upper-level westerlies (Sun et al., 2004; Vandenberghe et al.,
367 2004). This means the impact of the westerly circulation on the study region cannot be
368 ignored. Notably, pedogenesis proxies roughly parallel to the stacked deep- sea benthic
369 foraminiferal oxygen isotope curve (Fig. 6). It indicates when global temperature was low,
370 pedogenesis intensity increased. It is unreasonable if the precipitation was dominated by the
371 palaeo-ASM. Thus, we speculate from 6.7 to 4.8 Ma, the precipitation transported by the
372 palaeo- ASM was limited and the westerly circulation probably dominated the climate of our
373 study region.
374 The simultaneous reduction in amplitude of the 41-kyr filtered components from the
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375 western CLP and the deep sea δ18O record from 6.7-4.8 Ma likely indicates that the dry
376 climate was related to changes in global temperature and ice volume. Look around the globe,
377 a cooling climate would be witnessed in late Miocene. δ18O records from DSDP and ODP
378 sites show an increase of ~1.0‰ during the late Miocene which resulted from the increased
379 ice volume and the associated decrease in global temperature (Zachos et al., 2001). Records
380 from high latitude regions of the northern Hemisphere show continuously decreasing
381 temperatures and increasing ice volume during the late Miocene (Jansen and Sjøholm, 1991;
382 Mudieand Helgason, 1983; Haug et al., 2005). In the Quaternary, the dry climate prevailed
383 during glacial periods when global average temperature (especially in summer) was low.
384 Cool summers would have resulted in a small land-sea thermal contrast which in turn
385 weakened the palaeo-ASM in the late Miocene. Furthermore, the increased ice volume in the
386 Northern Hemisphere resulted in an increased meridional temperature gradient, thus
387 strengthening the westerlies and driving them southward. This would have prevented the
388 northwestward penetration of the Asian Summer Monsoon, which was also proposed as the
389 driving mechanism for a weak EASM in northern China during glacial periods (Sun et al.,
390 2015). Thus, the southward shift of the westerlies had a significant impact on the XSZ region.
391 Global cooling and the growth of polar ice-sheets reduced the amount of atmospheric water
392 vapor; thus, relatively little moisture was carried by the westerlies, producing a dry and stable
393 climate in the XSZ region. In conclusion, global cooling and increasing ice volume in the
394 Northern Hemisphere contributed to dry climatic conditions in the study region.
395 5.3.2 Humid climate with enhanced fluctuations during the interval of 4.8-3.6 Ma
396 During interval Ⅱ (4.8-3.6 Ma), the available proxy evidence indicates that the
18 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
397 Xiaoshuizi climate turns into humid condition from previous arid climate. The carbonate
398 content was low on average but with large fluctuations, indicating that the climate was
399 generally humid with increased dry-wet oscillations, especially during the interval of 4.8-3.9
400 Ma. Several obvious eluvial-illuvial cycles are observed from 4.8 to 3.9 Ma. The carbonate
401 content in the eluvial horizons was less than 10%, whereas in illuvial horizons it exceeded 30%
402 (Fig. 6). The emergence of high frequency cycles of carbonate eluviation-redeposition
403 indicates that seasonal precipitation was increased during this interval. Furthermore, the
404 variations of Rb/Sr and K2O/Na2O ratios are very similar to those of carbonate content, which
405 suggests that weathering intensity was related to precipitation amount. Generally, high <2 µm
406 / >40 µm ratio, χpedo and χlf correspond to large contrasts in carbonate content between eluvial
407 and illuvial horizons; thus, increased precipitation had a significant influence on enhanced
408 pedogenic intensity. From 4.8 to 3.9 Ma, high precipitation persisted and the weathering and
409 pedogenesis intensity were strong. The K2O/Al2O3 ratio also increased rapidly at about 4.8-
410 4.7 Ma and maintained relatively high values after 4.7 Ma. This may indicate that the overall
411 weathering intensity was sufficient to produce secondary clays, resulting in a spike in K2O
412 concentration. From 4.60-4.25 Ma, pedogenesis and weathering intensity reach the maximum,
413 as was precipitation intensity, which was manifested by enhanced eluviation and carbonate
414 accumulation. From 3.9 to 3.6Ma, precipitation decreased, and then weathering and
415 pedogensis intensity weakened, which may indicate that the Xiaoshuizi climate is generally
416 humid toward arid direction. Consisting with XSZ records, Dongwan mollusk records also
417 indicate the warm and wet conditions on the western CLP during early Pliocene (Fig. 7 h).
418 Palynological and terrestrial mollusk records from the central CLP also indicate
19 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
419 relatively humid conditions during early Pliocene (Wang et al., 2006；Wu et al., 2006). The
420 magnetic susceptibility records from the CLP hinterland exhibit similar characteristics to the
421 XSZ records that both the magnitude and variability of magnetic susceptibility are large from
422 4.8-3.6 Ma. From 4.1-3.9 Ma, the enhancement of magnetic susceptibility indicates that
423 humid climatic conditions prevailed across the entire CLP (Fig. 7). Obviously, when
424 precipitation amount peaked from 4.6-4.25 Ma in the XSZ section, the χlf values at Xifeng,
425 Lingtai and Chaona were low. However, the Lingtai Fe2O3 ratio record showed an
426 extraordinary high value corresponding to abundant clay coating over the interval of about
427 4.8-4.1 Ma and this interval was interpreted as the strongest ASM intensity in the CLP
428 since 7.0 Ma (Ding et al., 2001). In addition, the relative intensity of pedogenic alteration of
429 the grain-size distribution was the strongest during the interval from 4.8-4.2 Ma in the Lingtai
430 section (Sun et al., 2006c). Pollen assemblages at Chaona indicate a considerably warmer and
431 more humid climate from 4.61-4.07 Ma (Ma et al., 2005). These evidences indicate climate
432 from 4.6-4.25 Ma is warm and wet in the central CLP. Gleying has been implicated in
433 reducing the value of magnetic susceptibility as a record of precipitation during this period
434 (Ding et al., 2001). When soil moisture regularly exceeds the critical value, dissolution of
435 ferrimagnetic minerals occurs and the susceptibility signal is negatively correlated with
436 pedogenesis (Liu et al., 2003). This by itself indicates that precipitation was likely to have
437 been very high during this interval.
438 In summary, a wet climate prevailed across the CLP in early Pliocene. At the same time,
439 hematite/goethite ratio from the SCS also shows enhanced precipitation amount and
440 Smectite/Kaolinite ratio there shows increased seasonality at about 4.8Ma (Fig. 7 i and j),
20 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
441 which indicate the enhancement of palaeo-ASM (Clift et al., 2006, 2014). Thus, we regard
442 climate change of Xiaoshuizi as the result of expansion of the Palaeo-ASM expressed in its
443 intensity and reach during this interval.
444 The remarkably increased amplitude of the 41-kyr filtered components from XSZ and
445 the deep sea δ18O record at about 4.8 Ma indicates the expansion of palaeo-ASM may be
446 related to changes in global temperature and ice volume. Furthermore, decreasing input of ice
447 raft debris into subarctic northwest Pacific was synchronous with the expansion of palaeo-
448 ASM during early Pliocene (Fig. 6). In addition, from 4.8-4.7 Ma and 4.6-4.25 Ma, the high
449 values of the three pedogenic indices at the XSZ section indicate that strong pedogenic
450 intensity corresponded with high SSTs in the eastern equatorial Pacific (EEP). These
451 coincides imply that phases of enhanced precipitation may be correlated with changes in SST
452 and ice volume (or temperature) at northern high latitudes.
453 5.4 Possible mechanism for the paleo-ASM expansion during early Pliocene
454 Ding (2001) proposed that uplift of the TP to a critical elevation resulted in an enhanced
455 summer monsoon system during 4.8-4.1 Ma. The TP uplift was shown to have had profound
456 effects on the ASM initiation, having strengthened ASM intensity and changed the shape of
457 the precipitation band in East Asia (Li et al., 1991, 2014; An et al., 2001). A more detailed
458 modeling study demonstrated that the uplift of the northern TP mainly resulted in an
459 intensified summer monsoon and increased precipitation in northeast Asia (Zhang et al.,
460 2012). From 8.26-4.96 Ma, massive deltaic conglomerates were widely deposited and the
461 sediment deposition rate increased, indicating the uplift of the Qilian Mountains (Song et al.,
462 2001). At the same time, the Laji Mountains underwent a pronounced uplift by thrusting at
21 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
463 about 8 Ma, which resulted in the current basin-range pattern (Li et al., 1991; Fang et al.,
464 2005a; Zheng et al., 2000). However, geological and palaeontological records indicate that
465 the uplift of the eastern and northern margins of the TP was very small from late Miocene to
466 middle Pliocene (Li et al., 1991, 2015; Zheng et al., 2000; Fang et al., 2005a, 2005b). So we
467 speculate the TP uplift may be not the major contribution to the expansion of palaeo-ASM
468 occurred at ~4.8 Ma.
469 As mentioned above, the extremely wet climate across the CLP was synchronous with
470 the gradual closure of the Panama Seaway, which led to a larger reorganization of the global
471 thermohaline circulation pattern. Nie (2014) proposed that the freshening of the Eastern
472 Equatorial and North Pacific surface water, resulting from the closure of the Panama Seaway
473 since 4.8 Ma (Haug et al., 2001), led to sea ice formation in the North Pacific Ocean, which
474 enhanced the high-pressure cell over the Pacific and increased the strength of southerly and
475 southeasterly winds. However, there was a warming trend in the Northern Hemisphere at 4.6
476 Ma (Haug et al., 2005; Lawrence et al., 2006). The gradual closure of the Panama Seaway
477 resulted in the reorganization of surface currents in the Atlantic Ocean. In particular, the Gulf
478 Stream was enhanced and began to transport warm surface waters to high northern latitudes,
479 thus strengthening the Atlantic meridional overturning circulation and warming the Arctic
480 (Haug et al., 1998, 2005). This in turn resulted in higher global atmospheric water vapor
481 levels which promoted warm moist conditions during the Pliocene (Abbot and Tziperman,
482 2008; Dowsett et al., 2010). Three independent proxies from an early Pliocene peat deposit in
483 the Canadian High indicate that Arctic temperatures were 19 °C warmer during the early
484 Pliocene than at present (Ballantyne et al., 2010). Therefore, even freshening of the Pacific
22 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
485 led to sea ice formation in the North Pacific Ocean. However, this process would be delayed
486 (occurring during 3.2-2.7 Ma) and the extent of the sea ice in the early Pliocene was thus very
487 limited. In contrast, the warming of the northern high latitude region led to increases in
488 summer temperature in the mid-latitudes of Eurasia. On the other hand, equatorial SSTs
489 remained stable or cooled slightly (Brierley et al., 2009; Fedorov et al., 2013). This amplified
490 the land-ocean thermal contrast and was essential for enhancing the palaeo-ASM.
491 Furthermore, external heating derived from reduced planetary albedo also enhanced the
492 thermal contrast between the Pacific and Eurasian regions (Dowsett et al., 2010). On the
493 other hand, the unusually warm Arctic and small meridional heat gradient in the Northern
494 Hemisphere pushed the Intertropical Convergence Zone northward. This weakened the
495 westerly circulation and thus facilitated the northwestward expansion of the ASM.
496 Fig 6 indicated high values of pedogenic indices at the XSZ section correspond with
497 high SSTs in the EEP. It seems to be discrepancy with the modern ENSO cases (when the
498 EEP temperature is high, the precipitation amount of the western CLP is low). The
499 discrepancy may indicate sea-air interaction during early Pliocene is different from today.
500 From 4.8 to 4.0 Ma, the thermahaline circulation was reorganizing and creating a
501 precondition for the development of the modern equatorial Pacific cold tongue (Chaisson et
502 al., 2000). Some crucial changes linked with summer monsoon occurred. We noticed a vast
503 expansion of the western Pacific warm pool into subtropical regions occurred in early
504 Pliocene (Brierley et al., 2009; Fedorov et al., 2013). Temperatures at the edge of the warm
505 pool show a warming trend of ~2℃ from the latest Miocene to the early Pliocene (Karas et
506 al., 2011). The thermal state of the WEP warm pool significantly enhanced the summer
23 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
507 monsoon and its northward extension. In modern times, when the north of western pacific
508 warm pool was warm, the convection over and around the Philippine was enhanced.
509 Subsequently, the northern extent of the western Pacific subtropical high shifted northwards
510 from the Yangtze River valley to the Yellow River valley and moisture was introduced across
511 the entire CLP (Wang et al., 2000; Huang et al., 2003). Whether it is also the case for the
512 early Pliocene or not needs further researching. However, warming and freshening seawater
513 of subtropic Pacific would have been more readily evaporated which would have provided
514 enhanced moisture for the palaeo-ASM leading to increased rainfall across the CLP.
515 Thus, we deduce it may be warming of high northern latitudes, accompanied by the vast
516 poleward expansion of the tropical warm pool into the subtropical regions and freshening of
517 water in the subtropical Pacific facilitated the expansion of the palaeo-ASM during early
518 Pliocene.
519 6. Conclusions
520 Continuous late Miocene-Pliocene red clay preserved on the representative planation
521 surface in NE Tibetan Plateau provides particular opportunity to discuss the Asian monsoon
522 history. Multi-proxy records from the XSZ planation surface in the western CLP, together
523 with other palaeoclimatic records from the CLP, reveal two intervals of major climatic change
524 from 6.7 to 3.6 Ma. During the first interval (6.7-4.8 Ma), the XSZ records indicate that both
525 the amount and variability of precipitation were small; however, they were much greater in
526 the hinterland of the CLP. Thus, the palaeo-ASM had little influence on the climate of the
527 western CLP during this interval. During the second interval (4.8-3.6 Ma), the XSZ records
528 indicate that both the amount and variability of precipitation were large. From 4.8 and 3.6 Ma,
24 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
529 the climate was characterized by abrupt increases in the seasonality of precipitation, which
530 attests to a major northwestward extension and enhancement of the summer monsoon.
531 Obviously, multiple paleoclimatic proxies show that the strongest summer monsoon occurred
532 during the interval of 4.6-4.25 Ma. The expansion of palaeo-ASM may have been caused by
533 warming of the Arctic, the vast poleward expansion of the tropical warm pool into the
534 subtropical regions and freshening of water in the subtropical Pacific in response to the
535 closure of the Panamanian Seaway during early Pliocene.
536
537 Author contribution: Tingjiang Peng and Jijun Li supported fund and edited the article.
538 Zhenhua Ma provided age frame and participated in the most of field work. Meng Li,
539 Zhantao Feng, Benhong Guo, Xiyan Ye and Hao Yu participated in investigation and
540 experiment works. Chunhui Song and Zhengchuang guided and supervised the field work.
541 Competing interests: The authors declare that they have no conflict of interest.
542
543 Acknowledgements
544 We thank Ai Song for the field drilling and Fengxia Yu for her early experimental work.
545 This work was supported by National Natural Science Foundation of China (41330745,
546 41401214) and Key Laboratory of Continental Collision and Plateau Uplift, Institute of
547 Tibetan Plateau Research (LCP201602).
548
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Figures and tables
(a) (b) 80°E 100°E 120°E 140°E GTS 2012 Xiaoshuizi Section Gradstein et al.
Polarity Lithology
E Incline
3 E 90 0 -90
E C2An.2n 0 50°N
C2An.3n
E E
E
E E
E 4 * Xiaoshuizi 10
C3n.1n E
(Ma) 30°N
E
C3n.2n E
Age E E
E C3n.3n E
E E 20 5
E C3n.4n
E E
E
E E E
E E
10°N E
30 E E Thicknesss(m)
E E
6
E E
C3An.1n
E
E E E
E E C3An.2n
E
40 10°S E
E
E E E E Red clay E 48
E Carbonate nodule layer
E
E E E Red clay with gravel
30°S E E
E E E E
Precambrian migmatite E Mammal fossil (c) 102°E 104°E 106°E 108°E 110°E
WJ Yellow river
40°N
EASM
Tengger Dersert Jiaxian ! 38°N
Ordos platform ● Shilou !Xining
Lanzhou ! 36°N ● Xiaoshuizi ! !Xifeng Chaona ! Dongwan! ● Lingtai
0 100 34°N Miles
Fig. 1. The location of the study area and atmospheric circulation patterns. (a) 850 mb vector windaveraged from
June to August for 1982-2012 based on NOAA Earth System Research Laboratory reanalysis data (Compo et al.,
2013). (b) Lithology and magnetostratigraphy of the XSZ drill core (Li et al., 2017). (c) The Chinese Loess Plateau
with locations of the studied Xiaoshuizi site and other sections mentioned in the text.
35 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
(a) (b)
(c) (d)
Fig. 2. Photos of the XSZ planation surface and the red clay. (a) XSZ planation surface.
(b) Red clay outcrop, XSZ. (c) Position of the XSZ drilling hole. (d) The XSZ drill core.
36 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
Carbonate content Al O Rb of χ of XSZ χ of XSZ 2 3 Na2O of hf fd of XSZ(%) of XSZ(%) XSZ(%) XSZ(ppm) (10-8m3/kg) (10-8m3/kg) >40μm(%) 7 14 21 28 35 10 15 0.5 1.0 1.5 90 120 20 40 60 0 2 4 6 0 7 14 3.5 3.5
4.0 4.0
4.5 4.5
5.0 5.0
Age(Ma) 5.5 5.5 Age(Ma)
6.0 6.0
6.5 6.5
7.0 7.0 0 15 30 2 3 4 150 200 250 400 500 600 700 20 40 60 5 10 -8 3 CaO of K2 O Sr of XSZ(ppm) Ba of XSZ(ppm) (10 m /kg) <2μm(%) XSZ(%) of XSZ(%) χ lf of XSZ Fig. 3. Variations in carbonate content, major element concentration, minor element concentration, magnetic
susceptibility and grain size from the XSZ red clay section, spanning 6.7-3.6 Ma.
37 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
(b) (a) 6 3.5 χpedo 2 χlf = 0.11*χfd-1.4 (R =0.9) 5 χ0
4 /kg) 3 m -8 3 4.0 (10 fd χ 2
1 4.5 0 10 20 30 40 50 60 χ (10-8m3/kg) (c) 60 lf
50 (4.8-3.6Ma) 5.0
/kg) 40 3 m -8 30 (10
Age(Ma) pedo
χ 20
5.5 10
0 10 20 30 40 50 60 -8 3 60 χlf(10 m /kg) (d) 6.0 50 (6.7-4.8Ma) /kg) 3 40 m -8
(10 30 6.5 pedo χ 20
10
0 7.0 10 20 30 40 50 60 -8 3 χlf(10 m /kg) 0 20 40 60 (10-8m3/kg) (e) 300 R2=0.12 (f) 300 R2=0.45 280 280 260 260 240 240 220 220 200 200
Sr(ppm)
Sr(ppm) 180 180 160 160 140 140 0 5 10 15 20 25 30 35 40 0.0 0.1 0.2 0.3 CaO*(%) CaCO3(%)
Fig. 4. (a) Scatter plots of χlf versus χfd. (b) Separation of χpedo and χ0. (c) Scatter plot of χlf versus χpedo during
4.8-3.6 Ma. （d) Scatter plot of χlf versus χpedo during 6.7-4.8 Ma. (e) Scatterplot of Sr versus CaCO3. (f) Scatter
plot of Sr versus CaO*. Solid squares and triangles are the average values during 4.8-3.6 Ma and 6.7-4.8 Ma,
respectively. χpedo is the magnetic susceptibility of pedogenic origin and χ0 is the magnetic susceptibility of the
detrital material. 38 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
(a) (b) 12000 10000 Carbonate content χpedo
8000
9000 rewoP murtceps
murtceps rewoP murtceps 6000
6000 100k 4000 71k 56k 30k 3000 41k 2000 27k 41k 21k 21k
0 0 0.00 0.01 0.02 0.03 0.04 0.05 0.00 0.01 0.02 0.03 0.04 0.05 (c) Frequency Frequency
3500
4000
4500
)ak(egA 5000
5500
6000
6500
7000 -0.04 0.00 0.04 22 23 24 0.00 0.03 0.063.3 3.4 3.5 3.2 3.4 3.6 3.3 3.4 3.5 12 16 20 6 12 18 24 12 16 20 4 8 12 6 9 12 6 9 12 Precession Obliquity Eccentricity 21-kyr filter 41-kyr filter 100-kyr filter 21-kyr filter 41-kyr filter 100-kyr filter 21-kyr filter 41-kyr filter 100-kyr filter 18 Earth Orbital parameters δ O Carbonate content χpedo (d) χ Carbonate content 1 pedo 1 12.5 10.5
2 2
3 3 2.46 0.52 4 4 21ka
5 5 log2 scale log2 scale 41ka 6 6 100ka 7 7
8 8
4000 4500 5000 5500 6000 6500 4000 4500 5000 5500 6000 6500 Age(ka) Age(ka)
Fig. 5. Spectrum analysis of the red clay. (a) χpedo and (b) carbonate content(blue) on original paleomagnetism
chronology. Dashed lines are 90% confidience limit lines. (c) Comparison of orbital parameters (i.e., eccentricity,
18 obliquity and precession,Laskar et al., 2004) with filtered components of the carbonate content, χpedo and δ O
records (Zachos et al.,2001) at the 21-kyr, 41-kyr, and 100-kyr bands. Yellow shading denote the largest amplitude
of filtered components of carbonate and χpedo at the three orbital bands. Dashed lines indicate a large shift in the
East Asian monsoon circulation occurred around 4.8 Ma. (d) Results of the wavelet transform of χpedo and carbonate
content time series.
39 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
Magnetic susceptibility <2 μm/>43 μm χ (10-8m3/kg) UK'37 Temp(℃ ) XSZ Rb/Sr of XSZ lf of EEP of site 882 0.3 0.6 0.9 0 3 6 20 40 60 24 26 28 40 30 20 10 0 3.5
4.0
4.5
5.0
Age(Ma) 5.5
6.0 Increased IRD input
cold warm 6.5
7.0 35 28 21 14 7 0 1 2 3 4 5 0 8 16 24 32 40 2.5 3.0 3.5 23 24 25 26 27 Carbonate content(%) K O/Na O -8 3 Benthic δ18O 2 2 χpedo (10 m /kg) SST (℃) (per mil) Mg/Ca
Fig. 6. Temporal evolution of the palaeo-ASM. The dark blue line represents changes in effective
precipitation at XSZ, the orange line represents changes in chemical weathering intensity, and the
brown lines represent changes in pedogenic intensity. The blue line is the stacked deep-sea benthic
foraminiferal oxygen isotope curve compiled from data from DSDP and ODP sites (Zachos et al.,
2001). The black line is a reconstruction of sea surface tempeature in the eastern equatorial Pacific
(EEP) from ODP Site 846 (Lawrence et al., 2006). Green line is a reconstruction temperature at the
edge of warm pool from southwest Pacific Ocean Site 590B (Karas et al., 2011). Purple line is
magnetic susceptibility from ODP Site 882 (Haug et al., 2005). Gray shading shows intervals of
strong palaeo-ASM and the light-yellow shading shows intervals of carbonate accumulation.
40 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
(10-8m3/kg) -8 3 -8 3 Hematite/goethite χpedo χlf (10 m /kg) χ (10 m /kg) CA(%) lf of SCS of XSZ of SL of LT of DW 0 20 40 0 100 200 50 100 0 20 40 60 80 1.0 0.5 0.0 3.5 (a) (b) (c) (d) (e) (f) (g) (h) (i) (j)
4.0
4.5 More seasonsity
5.0
Age(Ma) 5.5
6.0
6.5
7.0 10 20 30 40 50 60 40 80 120 25 50 75 100 0 20 40 60 80 0 3 6 9 -8 3 -8 3 -8 3 Smectite/Kaolinite χ (10 m /kg) χ (10 m /kg) χ (10 m /kg) TH (%) lf lf lf of SCS of XSZ of XF of CN of DW
Fig. 7. Comparison of late Miocene-Pliocene paleoclimatic records from Asia. (a-b) χpedo and χlf from the
XSZ section. (c-f) χlf record from Shilou (Ao et al., 2016), Xifeng (Guo et al., 2001), Lingtai (Sun et al.,
2010) and Chaona (Song et al., 2007). (g-h) Percentages of cold-aridiphilous (CA) mollusk group and
thermo-humidiphilous (TH) mollusk group from Donwan(Li et al., 2008), (i) Hematite/goethite ratio
from the South China Sea (Clift, 2006). (j) Smectite/Kaolinite ratio from the South China Sea (Wan et al.,
2010; Clift et al., 2014).
41 Clim. Past Discuss., https://doi.org/10.5194/cp-2018-73 Manuscript under review for journal Clim. Past Discussion started: 4 July 2018 c Author(s) 2018. CC BY 4.0 License.
Table 1. major element compositions of XSZ red clay.
Content SiO2(%) Al2O3(%) Fe2O3(%) CaO(%) MgO(%) Average 49.16 12.61 5.38 11.36 2.76 6.7-4.8Ma 48.85 12.22 5.18 11.20 3.06 4.8-3.6Ma 49.50 13.22 5.69 11.60 2.30 Content K2O(%) Na2O(%) Rb(ppm) Sr(ppm) Ba(ppm) Average 2.76 1.22 106.2 212.8 519.0 6.7-4.8Ma 2.59 1.20 103.9 211.7 494.3 4.8-3.6Ma 3.03 1.23 109.9 214.6 558.0
42