Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing, China
Corresponding author: J. Ji, Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China. (email@example.com)
All Supporting Information may be found in the online version of this article.
 High-resolution variations in carbonate minerals from the Jiaxian Red Clay section, located at the northern limit of the present East Asian summer monsoon (EASM) on Chinese Loess Plateau were quantified using Fourier transform infrared spectroscopy. We analyzed a large quantity of sediments dated from the late Miocene to Pliocene (8.2–2.6 Ma). The carbonates in this interval show high-frequency variations alternating between leached and calcareous horizons. The low carbonate contents and high values of magnetic susceptibility and high Rb/Sr ratios were found in the leached zones, a pattern that is consistent with that observed in the overlying Quaternary loess-paleosol sequences. This pattern suggests that East Asian Monsoon (EAM) rainwater enhanced leaching and accumulation processes of carbonate minerals in the Red Clay Formation in a way similar to the loess-paleosol sequence. Seven alternating leached and calcareous zones are identified, suggesting oscillations of the EASM and East Asian winter monsoon intervals. The calcareous zones were also found to have high Zr/Rb ratio. These indications of shifts from a strong EASM to East Asian winter monsoon dominance correlate well with the cooling transition indicated by deep sea δ18O isotopes. This evidence suggests that the EAM was active during the late Miocene and Pliocene and was similar to the Quaternary monsoon. The presence of a strong EAM during the Pliocene Warm Period also raises questions about the hypothesis that past and future warm climate conditions could produce a permanent El Niño-like state.
 The Pliocene epoch is considered the most recent analog of the current global warming, a time when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for the near-term as a result of climate change [Mix et al., 1995]. However, the fundamental climate characteristic of the Pliocene still remains controversial, especially a permanent El Niño-like or a La Niña dominant state [Wara et al., 2005; Watanabe et al., 2011]. As a result, an important research topic for the Red Clay Formation is to examine the nature of the East Asian Monsoon (EAM) climate forcing during the Pliocene [Suarez et al., 2011]. The EAM, an intrinsic component of the global climate system, responds to variations in Northern Hemisphere ice volume and solar insolation [Wang et al., 2008]. Evidence of changes for the past EAM system is found in the eolian deposits on the Chinese Loess Plateau (CLP). These deposits contain the most continuous and complete terrestrial record of the EAM signal, providing the ideal material to study the evolution of atmospheric movement across the boundary between the ocean and land masses [An et al., 2001]. However, the state of EAM prior to the Quaternary is highly controversial; the debate has focused on the existence of the EAM, and, if it did exist, what climate conditions the EAM produced, wetter or drier during the late Miocene and Pliocene in the northern CLP [Ding et al., 2001; Miao et al., 2004; Passey et al., 2009].
 The Red Clay Formation, which underlies the well-known Quaternary loess sequence, extends these eolian deposits from 2.6 Ma through the late Miocene making it a good archive to reconstruct Pliocene climate [Ding et al., 2001]. Paleoecological analysis of terrestrial snail communities from the Red Clay [Li et al., 2008] and pollen assemblages [Jiang and Ding, 2008] indicates the existence of monsoonal precipitation. More recently, clumped-isotope studies of carbonate [Suarez et al., 2011] provides evidence of an active EAM during the warm climates of the late Miocene and Pliocene. However, the temporal sampling resolution in these studies is low and detailed information of EAM variability remains poorly understood. Here we present a high-resolution, quantitative record of Red Clay carbonate variations from 8.2 to 2.6 Ma B.P. in addition to other climatic proxy data including magnetic susceptibility, Rb/Sr and Zr/Rb ratios. The goal of this study is to examine the phase relationship between the East Asian summer monsoon (EASM) and these parameters and to discuss the evolution of EASM during the Pliocene. To accomplish this goal, the Jiaxian (JX) section of the Red Clay Formation, located in the northern margin of the CLP, was chosen because this site provides an excellent archive for monitoring EASM near its northern limit.
2 Materials and Methods
 The Jiaxian Red Clay section (38.3°N, 110.1°E) is located at the northern margin of CLP (see Figure S1 in the Supporting Information); the mean annual precipitation (MAP) is presently 380 mm [Qiang et al., 2001]. The section is 63.6 m thick and, in order to obtain a high temporal resolution, was sampled at a 10 cm interval (approximately 9 kyr between each sample). A total of 607 samples were obtained for magnetic susceptibility (MS), elemental concentrations, and weight percent carbonate determination. The magneto-stratigraphy and lithology were recorded by Qiang et al.  (Table S1 and Figure S2). MS was tested using a Bartington MS2 susceptibility meter. Elemental concentrations of Rb, Sr, and Zr were obtained with XRF using an ARL9800 XP + XRF. Red Clay carbonate content was quantified using Nicolet 6700 Fourier transform infrared spectroscopy following methods of Ji et al.  and calibrated by the gasometric methods of Jones et al. .
 The bulk carbonate concentration of the JX Red Clay section shows high frequency variations (Figure 1) and ranges from 0 to 50.7%. Low and high carbonate values are found in zones we later interpret to be leached and calcareous horizons (nodule zones), respectively. The average carbonate concentration in the leached zone is 3.1% (0–9.1%) whereas in calcareous nodule zone it is 15.6% (5.0–50.7%). A total of 86 leached zones are present between 2.6 and 8.2 Ma with an average frequency of 64 kyr. The summed duration of all leached zones is 2.6 Myr, about 46% of the entire time interval studied.
 The 86 leached zones commonly show high Rb/Sr ratios, high MS values, and low Zr/Rb ratios (Figure 1). These geochemical proxies originated from different provenances [An et al., 1991; Chen et al., 2000, 2006] and are independent of soil carbonate dynamics, but they show strong correlations to low- carbonate content layers. Among 86 leached zones, seven clearly exhibit high MS values and Rb/Sr ratios with the carbonate concentrations near zero (Figure 1). These seven leached zones range from 39–175 kyr with a mean duration of 83 kyr (Table 1). In total, the seven intervals lasted 0.58 million years, about 10% of the entire time.
Table 1. Statistical Analysis for Timing and Duration of Seven Alternating Leached and Calcareous Horizons at JX Red Clay Sectiona
Red Clay Lchi
Red Clay Cai
Full Cycle Lchi + Cai
Lch refers to fully leached horizons; Ca refers to adjacent calcareous horizons.
 The Zr/Rb ratio markedly increases in the horizons immediately above fully leached zones, that is, in the carbonate nodule horizons where the MS values and Rb/Sr ratios are low and carbonate content is high (Figure 1). These calcareous zones lasted from 39 to 107 kyr with a mean duration of 72 kyr and a sum of 0.51 million years, about 9% of the entire time.
4.1 Carbonate Leaching and Monsoon Activity
 The JX Red Clay Formation is believed to be a succession of polygenic pedocomplexes. The striking characteristic of these complexes is that carbonate-leached horizons are intercalated with dense carbonate nodule horizons. Such polygenic pedogenesis appears to have resulted from long-term climate oscillations. A similar stratigraphic pattern was observed in the Lingtai Red Clay Section [Ding et al., 1999], which is located about 400 km south of the JX section. Hence, the Quaternary loess-paleosol sequence could provide a good analogy for understanding the mechanism of the soil carbonate dynamics in the Red Clay Formation. In the southern CLP, the carbonate content in paleosol horizons is close to zero, whereas in loess layers it averaged 11.6% [Liu, 1985]. In general, the degree of development of a soil involves factors of climate, parent material, landscape, time, and bioturbation. However, paleo-precipitation, a major component of the past climate, has been considered the main factor for Quaternary loess-paleosol alternations [Zhao, 1995]. Strong seasonally biased precipitation enhances the leaching process and produces thick leached horizons [Zhao, 1998]. For the Red Clay Formation, greater rainfall could amplify this mechanism causing a significant translocation of carbonate minerals from leached zones to deeper accumulation zones where carbonates are re-precipitated as nodules, concretions, and caliche beds.
 In Quaternary paleosols magnetic susceptibility, carbonate content, and elemental ratios in Quaternary paleosols indicate the strength of the EASM [An et al., 1991; Chen et al., 2000]; similarly, these proxy records in the Red Clay pedocomplex should reflect the strength of the EASM. High MS and Rb/Sr ratios typify leached horizons of the Red Clay (Figure 1) as in the Quaternary paleosols [Ji et al., 2001]. It has been demonstrated that high MS values in Quaternary paleosols depend on pedogenically enriched ultralfine magnetic minerals which are indicators of enhanced warm monsoonal rains [An et al., 1991]. Hence, high MS values in the leached zone of the Red Clay indicate intervals with intense summer monsoon activity. The MS values of the JX Red Clay section vary from 22 to 141 10–8m3kg–1 with an average for all samples at 67 10–8m3kg–1. Using these MS values and the regression equation from worldwide MS values distribution by Balsam et al.  the estimated MAP during the deposition of the JX Red Clay section ranged from 256 to 774 mm/yr with an average of 543 mm/yr. Based on the CLP equation of Porter et al.  and MS values from JX section, MAP estimates ranged from 218 to 660 mm/yr with an average of 483 mm/yr. Both estimates show that the average MAP in Pliocene Red Clay were clearly greater than the present 380 mm/yr at JX, suggesting that the Pliocene JX on the northern CLP was much wetter than present. These changes in MS values could not have been caused by dissolving the carbonate alone because the MS values vary in a similar pattern between the original measurement and the carbonate content-adjusted measurement (Figure 1).
 The Rb/Sr ratios also correlate well with MS values in both Quaternary loess [Ji et al., 2001] and Red Clay (Figure 2 A). The increase in Rb/Sr ratios results from the loss of Sr (mainly from carbonate leaching) and the stability of Rb during rainwater leaching [Chen et al., 2000]. Thus, the intensified EASM brought more rainfall into the northern CLP, facilitating leaching and pedogenesis and producing higher Rb/Sr ratios, higher MS values, and lower carbonate contents. On the other hand, the weakened EASM reduced the precipitation, causing lower Rb/Sr ratios, lower MS values, and higher carbonate contents (Figure 2A). It appears that the leached and calcareous zones in the Red Clay Formation reflect climate intervals dominated either by EASM or East Asian winter monsoon (EAWM) systems.
 Due to the immobility of both Zr and Rb, their ratio is independent from the leaching process. The Zr is enriched in coarser grained heavy minerals and the Rb is enriched in finer grained light minerals such as K-feldspar. Thus, Zr/Rb ratio primarily reflects the eolian grain size that is associated with dust transporting wind power, that is, the strength of EAWM [Chen et al., 2006]. As seen in the Quaternary paleosol, the Zr/Rb ratio was low in the leached zones of the Red Clay, which indicates either a consistently weaker EAWM or stronger EASM. The negative correlation between MS values and Zr/Rb ratios in these leached zones (Figure 2B) suggest that stronger pedogenesis corresponds to more finely grained deposits, more summer rainfall, and weak winter wind.
4.2 Correlations of Strong EAM Events With the Deep Sea δ18O Isotopes
 East Asian summer monsoon proxies such as magnetic minerals, carbonate concentrations, and Rb/Sr ratios consistently show seven major peaks, with high amplitude fluctuations (Lch 1 to Lch 7, Figure 1). The independent EAWM proxy, the Zr/Rb ratio, when adjacent to high amplitude EASM variations exhibits similar amplitude decreases, suggesting a stronger EASM and a synchronous weakening of the EAWM, associated with a warmer and wetter climate.
 As indicated by varying magnetic mineral concentrations, Rb/Sr ratios, carbonate concentrations, and Zr/Rb ratios (Figure 1), the climate was alternately controlled by the coupled strong EASM and weak EAWM which was rapidly replaced by a weak EASM and strong EAWM. The seven strong EASM and EAWM events in length averaged about 83 kyr and 72 kyr respectively (Table 1). These late Miocene-Pliocene events are temporally comparable to the last five intense Quaternary interglacial and glacial cycles since the mid-Brunhes in which the length of glacial-interglacial cycles averaged about 70 kyr and 52 kyr [Ji et al., 2006], respectively. These Quaternary interglacial-glacial cycles coincided with climate fluctuations of greater amplitude [Wang et al., 2003] characterized by larger fluctuations in temperature and longer durations of both EASM and EAWM as a result of a 100 kyr eccentricity succeeding the previous 41 kyr obliquity cycle [Augustin et al., 2004]. The comparable durations of both EASM and EAWM of late Miocene - Pliocene suggest an involvement of astronomical eccentricity cyclicity in EASM and EAWM.
 As indicated by our Red Clay geochemical proxies, the seven intense EAM events exhibit strong correlations with deep sea δ18O isotope fluctuations (Figure 1) suggesting a close dependence of EAM on global temperature changes. The leached zones correspond to lighter δ18O values associated with warmer global temperature whereas the calcareous zones correspond to heavier δ18O values associated with cooler global temperature.
 The shift of dominant EASM to EAWM intervals, indicated by the alternation of leached and calcareous zones in Red Clay Formation, coincided with the rapid global cooling, indicated by the enrichment of 18O/16O ratio and thus heavier δ18O values. It appears that the Asian Monsoon was operating during the late Miocene and Pliocene in a manner similar to the Quaternary.
4.3 Implications for Climate of the Pliocene Warm Period
 During the Pliocene Warm Period (PWP) (~5–3 Ma B.P.) the Western Pacific Warm Pool greatly expanded toward the Eastern Pacific and, some have suggested that a permanent El Niño-like state prevailed [Brierley et al., 2009; Fedorov et al., 2010]. This permanent El Niño is thought to be the result of a shift in atmospheric Hadley cells toward the Eastern Pacific causing a decrease in precipitation intensity over the western Pacific [Brierley et al., 2009] and a southward movement of terrestrial precipitation fronts on the Chinese mainland. These changes produce a reduction in precipitation over CLP, similar to the present El Niño influence on China [Wu et al., 2004].
 However, the JX Red Clay record of the Pliocene Epoch indicates that at least two strong EASM intervals became wetter than in the Quaternary loess-paleosol sequence. The evidence is recorded in the leached zone (Lch) 3 (5.1–4.9 Ma) and Lch 2 (3.1–3.0 Ma) (Figure 1 and Table 1). The mid-Pliocene warm period is likely a good analog for EASM precipitation as global surface temperature is about 2.5 °C higher than that during preindustrial conditions in the future. The alternations from Lch 2 to Ca 2 and Lch 3 to Ca 3 suggest that the impact of the long-term monsoonal system during the Pliocene resembles that during the Quaternary interglacial/glacial cycles (Figure S3). Our results do not appear to support the hypothesis that past and future warming leads to a permanent El Niño-like state [Wara et al., 2005; Brierley et al., 2009; Fedorov et al., 2010].
 More recently, high resolution Porites coral records from Equatorial Western Pacific, dated at approximately 3.8–3.5 Ma, indicate a pattern of El Niño/Southern Oscillation interannual variability [Watanabe et al., 2011] similar to modern conditions and suggest that permanent El Niño conditions did not exist during the PWP. Our paleo-monsoon records imprinted in the Red Clay Formation further confirm that climate intervals dominated by El Niño-like and La Niña-like conditions oscillated during the PWP. Our records also suggest that both the Pliocene La Niña-like conditions [Rickaby and Halloran, 2005; Watanabe et al., 2011] and El Niño-like states [Wara et al., 2005; Brierley et al., 2009; Fedorov et al., 2010] were intermittent, not permanent.
The JX Red Clay Formation contains 86 leached and calcareous horizons, and their carbonate content varies with high frequency and large amplitude. High MS values and Rb/Sr ratios and low Zr/Rb ratios are found in the leached horizons, whereas low MS values and Rb/Sr ratios and high Zr/Rb ratios are found in the calcareous horizons. These variations are comparable with those observed in the overlying Quaternary paleosol-loess sequence, suggesting the EAM system impacted the northern CLP during late Miocene and the Pliocene.
At the JX section, seven highly leached horizons, whose carbonate content is at or near zero, reflect climate intervals dominated by intense EASM. These summer monsoon dominated intervals were terminated rapidly by a climate interval with winter monsoon enhancement, indicated by a dramatic increase of the Zr/Rb ratios in the calcareous horizons. The cyclicity of EAM strength during the late Miocene to Pliocene is in phase with the deep sea δ18O isotope fluctuations, indicating a close dependence of EAM and global temperatures.
At least two leaching/carbonate accumulation cycles of the EAM occurred during the Pliocene Warm Period. The dramatic difference in the precipitation suggested by these cycles on the northern margin of the CLP do not appear to support the hypothesis that past and future warmer climate would lead to a permanent El Niño-like climate mode.
 This study was funded by the National Basic Research Program of China (Grant 2010CB83340) and the National Natural Science Foundation of China (through Grants 490973062, 41021002, and 41273111). We thank Hong Wang of the Illinois State Geological Survey for thoughtful discussion and comments.