Coupled 100 kyr cycles between 3 and 1 Ma in terrestrial and marine paleoclimatic records

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1 Article Volume 12, Number October 2011 Q10Z32, doi: /2011gc ISSN: Coupled 100 kyr cycles between 3 and 1 Ma in terrestrial and marine paleoclimatic records Junsheng Nie Key Laboratory of Western China s Environment System, Ministry of Education, Lanzhou University, 222 Tianshui South Road, Lanzhou, Gansu , China (jnie@lzu.edu.cn) Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing , China [1] Earth s climate over the last one million years experienced several 100 kyr glacial cycles, but no simple forcing mechanism has been identified. Numerous studies have tried to explain strong 100 kyr glacial cycles without recognizable forcing, which has come to be known as the 100 kyr problem. Few studies have examined 100 kyr band paleoclimatic signals before 1 Ma. A recent study has demonstrated that benthic oxygen and carbon isotope records are phase locked and amplitude coupled at the 100 kyr band, but that neither is phase locked to and amplitude coupled with the 100 kyr eccentricity signal between3and1ma.thisphasingandamplitudemismatchofthe100 kyrbandbetween3and1ma between marine records and the eccentricity forcing signal has been called the late Pliocene early Pleistocene 100 kyr problem. However, it remains unknown whether terrestrial paleoclimate records are consistent with marine records at the 100 kyr band. Here I show that loess monsoon records from China are amplitude coupled with benthic oxygen and carbon isotope records at the 100 kyr band, but not with the 100 kyr eccentricity forcing between 3 and 1 Ma. This observation provides further evidence in support of a free 100 kyr oscillation as the cause of the 100 kyr band amplitude variability in paleoclimatic records between 3 and 1 Ma. In contrast, benthic oxygen isotope records and loess monsoon records at the 100 kyr band are not amplitude coupled with 100 kyr benthic carbon isotope records over the last 0.4 million years, indicating that the late Pleistocene 100 kyr climatic cycles may not result exclusively from a free oscillation. Components: 6400 words, 4 figures. Keywords: East Asian monsoons; Pleistocene; Pliocene; benthic isotopes; eccentricity; loess. Index Terms: 1620 Global Change: Climate dynamics (0429, 3309); 4910 Paleoceanography: Astronomical forcing; 4914 Paleoceanography: Continental climate records. Received 1 July 2011; Revised 16 September 2011; Accepted 26 September 2011; Published 26 October Nie, J. (2011), Coupled 100 kyr cycles between 3 and 1 Ma in terrestrial and marine paleoclimatic records, Geochem. Geophys. Geosyst., 12, Q10Z32, doi: /2011gc Theme: Magnetism From Atomic to Planetary Scales: Physical Principles and Interdisciplinary Applications in Geoscience Guest Editors: B. Moskowitz, J. Feinberg, F. Florindo, and A. Roberts Copyright 2011 by the American Geophysical Union 1 of 10

2 1. Introduction [2] Glacial cycles are a fundamental aspect of the climate system. Since Milankovitch suggested that glaciation occurs when summer insolation is weak at high northern latitudes [Milankovitch, 1941], his hypothesis has been supported by numerous pieces of geological evidence [Hays et al., 1976]. Milankovitch theory deepens our understanding of the dynamic mechanisms of orbital timescale climatic change compared to longer (tectonic ) and shorter (sub orbital) timescale climate change. Three assumptions were made in Milankovitch s hypothesis: (1) insolation (i.e., intensity), instead of integrated insolation (i.e., amount), is important; (2) summer insolation, instead of winter insolation, is important; and (3) northern hemisphere insolation, instead of southern hemisphere insolation, is important. Recent studies have challenged assumptions 1 and 3 and suggest that integrated summer insolation and southern hemisphere insolation might be important in understanding the dynamic mechanisms of glacial cycles [Huybers, 2006; Raymo et al., 2006]. Furthermore, paleoclimate cycles recorded by sediments reveal that 100 kyr and 41 kyr cycles have larger variance than 23 kyr cycles although 23 kyr cycles have largest variance in northern hemisphere summer insolation time series [Hays et al., 1976]. The relative power mismatch at the 100 kyr and 41 kyr bands between insolation forcing and climatic response has been called the 100 kyr problem [Imbrie et al., 1993] and the 41 kyr problem [Raymo, 1998; Raymo and Nisancioglu, 2003], respectively. The 100 kyr problem refers to the last 1 million years (Myr) and the 41 kyr problem refers to the time interval between 3 and 1 Ma. Recently, we observed that benthic d 13 C and d 18 O records are phase locked and highly correlated for amplitude envelopes at the 100 kyr band, but neither is phase locked to and amplitude coupled with the 100 kyr eccentricity orbital forcing signal between 3 and 1 Ma. This phasing and amplitude mismatch at the 100 kyr band between 3 and 1 Ma for benthic d 13 C and d 18 O records and the eccentricity forcing signal has been called the late Pliocene early Pleistocene 100 kyr problem [Nie et al., 2008a]. A more precise understanding of the dynamic mechanisms that give rise to orbital timescale climatic changes requires better answers to these problems. [3] Free oscillation inherent in the Earth s climate system has long been proposed as a mechanism for 100 kyr glacial cycles in paleoclimate records [Saltzman and Maasch, 1988, 1990, 1991; DeBlonde and Peltier, 1993; Tarasov and Peltier, 1997; Berger et al., 1998; Shackleton, 2000; Saltzman, 2002; Paillard and Parrenin, 2004; Herbert et al., 2010]. If the late Pliocene early Pleistocene 100 kyr cycles result from free oscillation inherent in the Earth s climate system, the 100 kyr cycles in different components of the climate system should be highly correlated with each other. However, this coupling has only been reported in ice volume and ocean circulation change/global carbon mass balance systems recorded by oxygen and carbon isotopes of benthic microorganisms, respectively [Nie et al., 2008a]. No work has examined 100 kyr cycles for proxy records that represent different components of the climate system as encoded in terrestrial sediments during the Pliocene Pleistocene and compared them with those of the benthic records. Such studies might be able to cast light on complicated internal climate feedbacks that generated the inferred 100 kyr period free oscillation. [4] Here I examine the 100 kyr amplitude relationship between loess magnetic susceptibility (MS) records from the Chinese Loess Plateau, the orbital eccentricity forcing signal, and benthic oxygen and carbon isotope records during the Plio Pleistocene period (Figure 1), in order to test if terrestrial and marine paleoclimate records are coupled at the 100 kyr band. 2. Data and Methods [5] I use a synthesized MS stack from the Chinese Loess Plateau as a proxy for the intensity of the East Asian summer monsoon (EASM) [Sun et al., 2006]. Recent studies [Liu et al., 2003; Nie et al., 2007] have shown that pedogenic enhancement mechanisms for the red clay sediments deposited between 8 and 2.7 Ma are similar to those of the overlying loess paleosol sediments deposited between 2.7 and 0 Ma and, thus, that red clay MS should reflect EASM intensity as can loess paleosol MS [Zhou et al., 1990; Maher et al., 1994]. Benthic oxygen isotope records used to compare with the 100 kyr MS record are the synthesized benthic d 18 O records [Lisiecki and Raymo, 2005] and benthic d 18 O records from Site 659, North Atlantic Ocean [Tiedemann et al., 1994], which are used as a proxy for global ice volume. I use the benthic d 13 C records at ODP Site 659 [Tiedemann et al., 1994], the same site as one of our benthic d 18 O records as a proxy for ocean circulation change/ global carbon mass balance, although this proxy might be affected by local air sea equilibration and 2of10

3 Figure 1. Paleoclimate time series and the 100 kyr band variability associated with each record. (a) Magnetic susceptibility (MS) stack record of Sun et al. [2006] from the Chinese Loess Plateau (MS of Chinese loess sediments is a widely used EASM intensity proxy [An et al., 2001]). (b) Benthic d 18 O record from ODP Site 659 (blue) and benthic d 18 O stack record synthesized by Lisiecki and Raymo [2005] (red). Benthic d 18 O is generally used as paleo ice volume proxy. (c) Benthic d 13 C time series from ODP Site 659. Benthic d 13 C is generally used as a proxy for paleo ocean circulation change/global carbon mass balance [Tiedemann et al., 1994; Piotrowski et al., 2005]. (d) The filtered 100 kyr band variability of Earth s orbital eccentricity [Laskar, 1990]. (e, f, and g) The filtered 100 kyr band variability of Figures 1a, 1b, and 1c, respectively. In Figure 1e the filtered 100 kyr band variability of the Chaona MS record is also shown for comparison (brown). I note that there is weak forcing but strong response from 3 to 2.3 Ma (the shaded bar), and strong forcing but weak response from 1.8 to 1.3 Ma (the shaded bar). The trend lines in Figures 1e, 1f, and 1g indicate coupled amplitude variation in these three data sets. Revised from Nie et al. [2008a]. productivity changes [Shackleton et al., 1983; Boyle and Keigwin, 1987; Piotrowski et al., 2005]. The age models for the benthic d 18 O and d 13 C records for the last 5 Myr, and of the MS records for the last 3.6 Myr are all based on astronomical tuning [Tiedemann et al., 1994; Lisiecki and Raymo, 2005; Sun et al., 2006]. For the monsoon stack record, Sun et al. [2006] used lagged orbital obliquity (69 lag) and precession (78 lag) as the tuning target. For the ODP Site 659 data between 2.6 and 0 Ma (marine isotope stages 104 1), Tiedemann et al. [1994] correlated visually the oxygen isotope record from Site 659 to that from ODP Site 677 [Shackleton et al., 1990], which was generated using the Imbrie and Imbrie [1980] nonlinear ice volume model as the tuning target with constant time lag. For the interval from 2.85 to 2.6 Ma, Tiedemann et al. [1994] correlated visually the oxygen isotope record to that from ODP Site 607 [Ruddiman et al., 1989]. For the ODP Site 659 data between 5 and 2.85 Ma, Tiedemann et al. [1994] used precession and 400 kyr eccentricity as the tuning target. For the Lisiecki and Raymo [2005] stack data, the tuning target is still the nonlinear ice volume model of Imbrie and Imbrie [1980]. However, they allowed the nonlinearity coefficient and mean time constant of the ice volume model to increase with time due to the longterm increase in global ice volume [Lisiecki and Raymo, 2005]. The Imbrie and Imbrie [1980] ice volume model uses the same lagged precession and obliquity that Sun et al. [2006] used for tuning. The only difference is that the ice volume model also incorporated a nonlinearity coefficient that allowed the resultant record to have the saw tooth nature observed in the benthic oxygen isotope record (S. Clemens, personal communication, 2011). Therefore, it is reasonable to compare the phasing relationship between the MS stack established by Sun et al. [2006] and the Site 659 benthic d 18 O and d 13 C records. [6] 100 kyr band signals of eccentricity [Laskar, 1990] and benthic oxygen and carbon isotopes [Tiedemann et al., 1994; Lisiecki and Raymo, 2005] have been isolated by Nie et al. [2008a]. To isolate the 100 kyr band signal from the MS stack record for comparison [Sun et al., 2006], I filtered the original data using the same bandpass filter (bandwidth kyr) as in the study by Nie 3of10

4 Figure 2. A comparison of variance at the 100 kyr band between eccentricity [Laskar, 1990], benthic d 18 O ratios [Lisiecki and Raymo, 2005], a benthic d 13 C record from ODP Site 659 [Tiedemann et al., 1994], and the MS stacked record from the Chinese Loess Plateau [Sun et al., 2006]. The variance was calculated in a 500 kyr moving window using the STATNARY software designed by Dr. Michael Schulz. et al. [2008a]. For comparison, I also isolated the 100 kyr band signals between 2.6 and 0 Ma from a MS record from the Chaona section ( E, 35 6 N) [Song et al., 2001], central Chinese Loess Plateau, using the same bandpass filter. Filtering was performed using AnalySeries 1.2 [Paillard et al., 1996]. I also perform the wavelet analysis [Torrence and Compo, 1998] on the MS record from the Chaona section, instead of the MS stack generated by Sun et al. [2006], because the MS stack is only 3.6 Myr long and all other time series data that I use are 5.3 Ma long [Laskar, 1990; Tiedemann et al., 1994; Lisiecki and Raymo, 2005]. The age model of the Chaona section between 2.6 and 0 Ma is based on orbital tuning (tuned to the benthic d 18 O record from Site 677, as is the case for the Site 659 benthic d 18 O record) and the age model between 5.3 and 2.6 Ma is based on magnetostratigraphy and linear interpolation [Lü et al., 2001; Song et al., 2001]. The MS curve from Chaona has been shown many times [Lü et al., 2001; Song et al., 2001; Chen et al., 2007] and is not shown here. 3. Results [7] The 100 kyr band amplitude variabilities are similar between the MS records and the benthic oxygen and carbon isotope records over the last 3 Myr (Figures 1, 2, and 3). The Pearson correlation R between the filtered MS stack and the filtered Lisiecki and Raymo [2005] oxygen isotope stack records, between the filtered MS stack and the filtered Site 659 oxygen isotope records, and between the filtered MS stack and the filtered Site 659 carbon isotope records, during the last 3 Myr, is 0.86, 0.83, and 0.66, respectively. The Pearson correlation R between the filtered MS stack and the filtered eccentricity records over the last 3 Myr is only 0.47 (Figures 1, 2, and 3). [8] Before 3 Ma, I observe a statistically significant (p = 0.95) 100 kyr signal in the benthic d 13 C record (Figure 4), but the 100 kyr signal is much weaker in the benthic d 18 O, MS stack, and the Chaona MS records in the early Pliocene (Figures 1, 2, and 3). In addition, the 100 kyr signals in the Site 659 benthic d 13 C record, in the Lisiecki and Raymo [2005] d 18 O stack record, and in the MS stack record between 3 and 2.3 Ma are significant at the 99%, 90%, and 99% levels, respectively (auxiliary material). 1 These analyses were performed using a software package [Schulz and Stattegger, 1997] based on a Lomb Scargle Fourier transform combined with the Welch Overlapped Segment Averaging procedure, which does not require data points that are equally spaced in time. [9] The 100 kyr band amplitude variation of the MS and the benthic d 18 O records is also correlated with that of the benthic d 13 C records during the interval Ma. During the last 0.4 Ma, the 100 kyr band amplitude variation of the MS and the benthic d 18 O records is correlated but not with that of the benthic d 13 C records for unknown reasons: the 100 kyr band amplitude of the d 13 C record decreased at 0.4 Ma, whereas the 100 kyrband amplitude of the MS record and the d 18 O record did not. Benthic d 13 C records from Site 659 have clear 400 kyr signals before 1.8 Ma but gradually change to 500 kyr signals after 1.8 Ma. However, this 400 kyr to 500 kyr shift was not observed from the monsoon and ice volume proxy records examined in this paper (Figures 1 and 3). [10] In terms of phasing relationship, the filtered MS record is generally phase locked to the filtered benthic d 13 C and negative d 18 O records (Figure 1). However, the filtered MS record is not phaselocked to the filtered eccentricity record during the Ma and Ma intervals (Figure 1). For the other time intervals over the last 3 Myr, MS is 1 Auxiliary materials are available in the HTML. doi: / 2011GC of10

5 Figure 3. Wavelet transforms [Torrence and Compo, 1998] of (a) the summer insolation at 65 N on June 21 [Laskar, 1990], (b) the MS of the Chaona section, (c) synthesized benthic d 18 O[Lisiecki and Raymo, 2005], and (d) benthic d 13 C at ODP Site 659. The paleoclimate time series were detrended before performing the wavelet transform. Gray scale indicates power, which is scaled to percent total power. Hatched areas illustrate the cone of influence of edge effects of the transform. The 100 kyr period is labeled as a white line. That there is weak forcing but strong response from 3 to 2.3 Ma (the shaded bar), and strong forcing but weak response from 1.8 to 1.3 Ma (the shaded bar). Revised from Nie et al. [2008a]. phase locked to both the filtered eccentricity record and filtered benthic d 13 C and d 18 O records. 4. Discussion [11] A better understanding of forcing mechanisms of glacial cycles requires better answers to the 41 kyr problem and the 100 kyr problem. The 41 kyr problem is perplexing but less attention has been paid to it by paleoclimatologists [Raymo and Nisancioglu, 2003] compared to the 100 kyr problem. Several hypotheses have been proposed to explain the 41 kyr problem. Inspired by previous studies [Berger, 1976; Young and Bradley, 1984; Johnson, 1991], Raymo and Nisancioglu 5of10

6 Figure 4. Spectral analysis [Schulz and Mudelsee, 2002] of benthic d 13 C from ODP Site 659 between 5 and 3 Ma [Tiedemann et al., 1994]. 400 kyr and 100 kyr signals are labeled. The dashed line represents the 95% confidence level with respect to first order autoregressive [AR(1)] red noise background [Mann and Lees, 1996]. [2003] proposed that the insolation gradient between high and low latitudes, instead of higher latitude insolation intensity, may be responsible for the dominant 41 kyr period glacial cycles between 3 and 1 Ma. They argued that the insolation gradient can control glacial cycles via its influence on the poleward flux of moisture which determines ice sheet growth. More recently, Raymo et al. [2006] proposed that the lack of 23 kyr precession signals and the dominance of 41 kyr obliquity signals in glacial cycles between 3 and 1 Ma might be because 23 kyr period ice volume changes in the northern hemisphere are canceled by those in the southern hemisphere. On the other hand, 41 kyrperiod ice volume changes in the northern hemisphere are intensified by in phase 41 kyr period ice volume changes in the southern hemisphere [Raymo et al., 2006]. This work, therefore, requires relaxation of assumption 3 made by Milankovitch [1941]. In contrast, Huybers [2006] challenged assumption 1 made by Milankovitch [1941] and proposed that integrated summer insolation, which has a dominant 41 kyr signal, instead of summer insolation intensity, is the forcing mechanism for glacial cycles. Although we do not know which model is correct, the answer to the Milankovitch 41 kyr problem might lie with one of these three hypotheses. [12] Compared to the 41 kyr problem, the 100 kyr problem has been studied extensively. Numerous models have been proposed to solve this problem. These models can be classified in two categories [see Imbrie et al., 1993]: (1) with free oscillation, and (2) without free oscillation. After reviewing these models, Imbrie et al. [1993] concluded that the most likely explanation for the late Pleistocene 100 kyr problem lies somewhere between a group of models with free oscillation [Oerlemans, 1982; Saltzman et al., 1984; Maasch and Saltzman, 1990] and a group of models without free oscillation [Birchfield and Weertman, 1978; Oerlemans, 1980; Pollard, 1983; Birchfield and Grumbine, 1985; DeBlonde and Peltier, 1991; Gallée et al., 1991]. The group of models with free oscillation [Oerlemans, 1982; Saltzman et al., 1984; Maasch and Saltzman, 1990] explains the 100 kyr glaciation cycles as a self sustaining internal oscillation paced by ice sheets. The group of models without free oscillation [Birchfield and Weertman, 1978; Oerlemans, 1980; Pollard, 1983; Birchfield and Grumbine, 1985; DeBlonde and Peltier, 1991; Gallée et al., 1991] explains the 100 kyr glaciation cycles as a nonlinear response to orbital forcing which can channel energy into the 100 kyr band through feedbacks paced by the large time constant of ice sheets. Within the framework set by Imbrie et al. [1993], much progress has been made concerning the 100 kyr problem. For example, several studies [Mix et al., 1995; Ravelo et al., 2004; Lisiecki and Raymo, 2007; Liu et al., 2008] suggest that, in contrast to increased 100 kyr variability, ice volume and sea surface temperature have a diminished response to the 41 kyr obliquity forcing from the early Pleistocene to the late Pleistocene. If one assumes that the climate system had a steady response to 41 kyr obliquity forcing through the Pleistocene, the diminished response to obliquity forcing and increased 100 kyr variability means that some of the 41 kyr cycles have been channeled into a circa 100 kyr expression in the late Pleistocene via nonlinear mechanisms [Huybers, 2007; Liu et al., 2008]. Several recent studies have also examined amplitudes and phases of the 100 kyr cycles in marine Plio Pleistocene paleoclimate records [Nie et al., 2008a; Lisiecki, 2010]. For example, Nie et al. [2008a] named the phasing and amplitude mismatch at the 100 kyr band during the 3 1 Ma interval between eccentricity forcing and paleo ice volume records as the late Plioceneearly Pleistocene 100 kyr problem. Lisiecki [2010] found that the power of the 100 kyr eccentricity signals is anticorrelated with 100 kyr period ice volume signals over the last 5 Myr. Furthermore, Lisiecki [2010] proposed that the anticorrelation might come from disruption of strong precession forcing associated with strong eccentricity forcing to 100 kyr free oscillation generated from internal climate feedbacks. These two studies suggest that, at least for the late Pliocene early Pleistocene period, the 100 kyr glacial cycles might have resulted from free oscillation inherent in the Earth s climate system. 6of10

7 [13] As stated above, if the 100 kyr cycles during the late Pliocene early Pleistocene period resulted from free oscillation inherent in the Earth s climate system, the 100 kyr cycles in different components of the climate system should be highly correlated with each other. In this paper, I have shown that 100 kyr cycles in the EASM records from the Chinese Loess Plateau, the ice volume records and the circulation change/global carbon mass balance records are amplitude coupled and phase locked between 3 and 1 Ma. In addition, none of the examined 100 kyr band paleoclimate records is amplitude coupled with and phase locked to the 100 kyr eccentricity signals between 3 and 1 Ma. These observations do not support direct 100 kyr eccentricity forcing in the climate system even before 1 Ma when there is less apparent 100 kyr power in paleoclimate records. The phasing relationships also do not support an indirect role of eccentricity, through modulating precession forcing, in pacing 100 kyr paleoclimate cycles before 1 Ma. On the other hand, the analysis presented here suggests that a free 100 kyr oscillation internal to the climate system operating on an Earth characterized by a high inertia deep thermohaline ocean that can store carbon and heat [Saltzman and Maasch, 1988, 1990, 1991; Saltzman, 2002; Paillard and Parrenin, 2004; Nie et al., 2008a] could be the key for explaining the late Plioceneearly Pleistocene 100 kyr problem. The results presented here call for further studies aimed at deconvolving the carbon budget and oceancirculation signals encoded in benthic d 13 C records, similar to those done for the last 100 kyr [Boyle and Keigwin, 1987]. Although looking at filtered signals is potentially dangerous when the amplitude of the filtered signal is small compared to the original series, especially at around 1.5 Ma, where the 100 kyr band amplitude of the filtered MS, d 13 C, and d 18 O is at a minimum, the different records have a clear common and robust pattern. This common pattern suggests that the conclusion about the lack of an amplitude and phase relationship with eccentricity [Nie et al., 2008a] is valid. [14] Compared with the last 3 Myr, the benthic d 13 C record has a strong 100 kyr signal between 5 and 3 Ma (Figure 2) and the 100 kyr spectral peak is statistically significant (Figure 4), but the EASM and ice volume did not have a comparably strong 100 kyr signal before 3 Ma (Figures 1 and 2). I speculate that intensification of Northern Hemisphere glaciation at 2.75 Ma [Raymo, 1994; Shackleton et al., 1984] and resulting polar ocean stratification [Sigman et al., 2004] increased coupling between different components of the climate system and introduced the 100 kyr signal from the ocean circulation change/global carbon mass balance into the ice volume and EASM system and thus caused the coupled 100 kyr signal in these three paleoclimate records over the last 3 Ma. A stratified ocean would be able to store more carbon and cause lower atmospheric CO 2 levels [Sigman et al., 2004]. When atmospheric CO 2 levels are lower, the size of ice sheets is more sensitive to CO 2 variations and thus these two climate components are better coupled. Size variations of ice sheets and resulting sea level variations can consequently determine how far the EASM can reach into inland China. For larger ice volume and lower sea levels, the EASM will retreat southeastward and thus the Chinese Loess Plateau will receive less rainfall [Liu and Ding, 1998]. For smaller ice volume and higher sea levels, the EASM will advance northwestward and thus the Chinese Loess Plateau will receive more rainfall [Liu and Ding, 1998]. This proposed scenario could account for the coupling of different components of the climate system after 3 Ma in association with onset of intensive Northern Hemisphere glaciation and polar ocean stratification. [15] Although the 100 kyr cycles in paleoclimate records between 3 and 1 Ma likely result from a free oscillation internal to Earth s climate system, I argue that the late Pleistocene 100 kyr problem may not be exclusively caused by a free oscillation internal to Earth s climate system. This is because the benthic d 13 C record is not correlated with the benthic d 18 O record and the MS record from the Chinese Loess Plateau for the 100 kyr band variation during the last 0.4 Myr. The late Pleistocene 100 kyr problem might result from both a free oscillation and grouping of precession and/or obliquity cycles [Imbrie et al., 1993; Raymo, 1998; Ruddiman, 2003; Liu and Herbert, 2004; Huybers and Wunsch, 2005; Huybers, 2006; Liu et al., 2008]. [16] Benthic d 13 C records from Site 659 and many other sites contain a clear 400 kyr signal before 4 Ma but this gradually changes to a 500 kyr signal after 1.8 Ma for unknown reasons [Wang et al., 2004]. However, such a shift from 400 kyr to 500 kyr power at around 1.8 Ma is not observed in either the MS records or the benthic d 18 O records. I argue that, although the 100 kyr ice volume and EASM signals between 3 and 1 Ma might result from a free 100 kyr oscillation internal to the climate system, the 400 kyr cycles in ice volume and EASM proxy records might result directly from 7of10

8 eccentricity forcing. The 400 kyr cycles intensified in the benthic d 18 O record and in the MS record at around 4 Ma (Figure 3), which might be related to increased tectonic activity of the Tibetan Plateau during the Pliocene [An et al., 2001; Nie et al., 2008b]. Increased tectonic activity of the Tibetan Plateau during the Pliocene could have changed mean and variance of the EASM and global ice volume, making these two systems have a clipped response to insolation and thus increasing the power of the 400 kyr signal [Clemens and Tiedemann, 1997; Nie et al., 2008b]. The reason for 400 kyr power decline after 2 Ma in proxy records for the EASM and global ice volume is unknown. 5. Conclusions [17] EASM records from the Chinese Loess Plateau are phase locked to, and amplitude coupled with, benthic d 13 C and d 18 O records for amplitude envelopes at the 100 kyr band, but are not phaselocked to or amplitude coupled with the 100 kyr eccentricity orbital forcing between 3 and 1 Ma. [18] Although variances between benthic d 13 C and d 18 O records and MS records from the Chinese Loess Plateau are comparable between 3 and 1 Ma, variance of the benthic d 13 C is much larger than that of the benthic d 18 O records and the MS records from the Chinese Loess Plateau before 3 Ma. [19] These observations provide further evidence to support the hypothesis that a free 100 kyr oscillation internal to the climate system operating on an Earth with a high inertia deep thermohaline ocean that can store carbon and heat, might be the cause of the 100 kyr band amplitude variability in paleoclimatic records between 3 and 1 Ma. [20] 100 kyr benthic d 18 O records and loess MS records are not amplitude coupled with 100 kyr benthic d 13 C records for the last 0.4 Myr. This mismatch indicates that the late Pleistocene 100 kyr problem might not exclusively result from a free oscillation inherent to Earth s climate system, but probably result partially from grouping of precession and/or obliquity cycles. Acknowledgments [21] J. Nie acknowledges R. Tiedemann for supplying the data from ODP Site 659. This paper benefited significantly from formal reviews from D. Paillard and A. Hong, and detailed suggestions and edits from the Associate Editor A. Roberts. J. Nie is co supported by the National Science Foundation of China ( ; ; ), the (973) National Basic Research Program of China (2010CB833401), the New Century Talent project of the Ministry of Education of China (NCET ), the Fundamental Research Funds for the Central Universities (Lanzhou University: ), and Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences (LCPU201000). References An, Z. S., J. E. Kutzbach, W. L. Prell, and S. C. Porter (2001), Evolution of Asian monsoons and phased uplift of the Himalaya Tibetan plateau since Late Miocene times, Nature, 411, 62 66, doi: / Berger, A. (1976), Long term variations of daily and monthly insolation during the last Ice Age, Eos Trans. AGU, 57, 254. Berger, A., et al. (1998), Sensitivity of the LLN climate model to the astronomical and CO 2 forcings over the last 200 ky, Clim. Dyn., 14, , doi: /s Birchfield, G. E., and R. W. Grumbine (1985), Slow physics of large continental ice sheets and underlying bedrock and its relation to the Pleistocene ice ages, J. Geophys. Res., 90, 11,294 11,302, doi: /jb090ib13p Birchfield, G. E., and J. Weertman (1978), A note on the spectral response of a model continental ice sheet, J. Geophys. Res., 83, , doi: /jc083ic08p Boyle, E. A., and L. D. Keigwin (1987), North Atlantic thermohaline circulation during the past 20,000 years linked to high latitude surface temperature, Nature, 330, 35 40, doi: /330035a0. Chen, X., X. Fang, Z. An, W. Han, X. Wang, Y. Bai, and Y. Hong (2007), An 8.1 Ma calcite record of Asian summer monsoon evolution on the Chinese central Loess Plateau, Sci. China, Ser. D, 50, , doi: /s x. Clemens, S. C., and R. Tiedemann (1997), Eccentricity forcing of Pliocene Early Pleistocene climate revealed in a marine oxygen isotope record, Nature, 385, , doi: / a0. DeBlonde, G., and W. R. Peltier (1991), A one dimensional model of continental ice volume fluctuations through the Pleistocene: Implications for the origin of the mid Pleistocene climate transition, J. Clim., 4, , doi: / (1991)004<0318:aodmoc>2.0.co;2. DeBlonde, G., and W. R. Peltier (1993), Late Pleistocene ice age scenarios based on observational evidence, J. Clim., 6, , doi: / (1993)006<0709:lpiasb> 2.0.CO;2. Gallée, H., J. P. van Ypersele, T. Fichefet, C. Tricot, and A. Berger (1991), Simulation of the last glacial cycle by a coupled, sectorially averaged climate ice sheet model 1. The climate model, J. Geophys. Res., 96, 13,139 13,161, doi: /91jd Hays, J. D., J. Imbrie, and N. J. Shackleton (1976), Variations in the earth s orbit: Pacemaker of the ice ages, Science, 194, , doi: /science Herbert, T. D., L. C. Peterson, K. T. Lawrence, and Z. Liu (2010), Tropical ocean temperatures over the past 3.5 million years, Science, 328, , doi: /science Huybers, P. (2006), Early Pleistocene glacial cycles and the integrated summer insolation forcing, Science, 313, , doi: /science of10

9 Huybers, P. (2007), Glacial variability over the last two million years: An extended depth derived age model, continuous obliquity pacing, and the Pleistocene progression, Quat. Sci. Rev., 26, 37 55, doi: /j.quascirev Huybers, P., and C. Wunsch (2005), Obliquity pacing of the late Pleistocene glacial terminations, Nature, 434, , doi: /nature Imbrie,J.,andJ.Z.Imbrie(1980), Modeling the climatic response to orbital variations, Science, 207, , doi: /science Imbrie, J., et al. (1993), On the structure and origin of major glaciation cycles: 2. The 100,000 year cycle, Paleoceanography, 8, , doi: /93pa Johnson, R. G. (1991), Major Northern Hemisphere deglaciation caused by a moisture deficit 140 ka, Geology, 19, , doi: / (1991)019<0686:mnhdcb> 2.3.CO;2. Laskar, J. (1990), The chaotic motion of the solar system: A numerical estimate of the size of the chaotic zones, Icarus, 88, , doi: / (90)90084-m. Lisiecki, L. E. (2010), Links between eccentricity forcing and the 100,000 year glacial cycle, Nat. Geosci., 3, , doi: /ngeo828. Lisiecki, L. E., and M. E. Raymo (2005), A Pliocene Pleistocene stack of 57 globally distributed benthic d 18 O records, Paleoceanography, 20, PA1003, doi: / 2004PA Lisiecki, L., and M. E. Raymo (2007), Plio Pleistocene climate evolution: Trends and transitions in glacial cycle dynamics, Quat.Sci.Rev., 26, 56 69, doi: /j.quascirev Liu, T., and Z. Ding (1998), Chinese loess and the paleomonsoon, Annu. Rev. Earth Planet. Sci., 26, , doi: / annurev.earth Liu, X., T. C. Rolph, Z. S. An, and P. Hesse (2003), Paleoclimatic significance of magnetic proporties on the Red Clay underlying the loess and paleosols in China, Palaeogeogr. Palaeoclimatol. Palaeoecol., 199, , doi: / S (03) Liu, Z., and T. Herbert (2004), High latitude influence on the eastern equatorial Pacific climate in the early Pleistocene epoch, Nature, 427, , doi: /nature Liu, Z., L. Cleaveland, and T. Herbert (2008), Early onset and origin of 100 kyr cycles in Pleistocene tropical SST records, Earth Planet. Sci. Lett., 265, , doi: /j.epsl Lü, L., X. Fang, J. Mason, J. J. Li, and Z. S. An (2001), The evolution of coupling of Asian winter monsoon and high latitude climate of Northern Hemisphere, Sci. China, Ser. D, 44, Suppl., doi: /bf Maasch, K. A., and B. Saltzman (1990), A low order dynamical model of global climatic variability over the full Pleistocene, J. Geophys. Res., 95, , doi: / JD095iD02p Maher, B. A., R. Thompson, and L. P. Zhou (1994), Spatial and temporal reconstructions of changes in the Asian palaeomonsoon a new mineral magnetic approach, Earth Planet. Sci. Lett., 125, , doi: / x (94) Mann, M. E., and J. M. Lees (1996), Robust estimation of background noise and signal detection in climatic time series, Clim. Change, 33, , doi: /bf Milankovitch, M. (1941), Canon of Insolation and the Ice Age Problem [in German], Spec. Publ., vol. 132, R. Serb. Acad., Belgrade. [English translation, Israel Program for Sci. Transl., Jerusalem, 1969.] Mix, A. C., J. Le, and N. J. Shackleton (1995), Benthic foraminiferal stable isotope stratigraphy of Site 846: Ma, Proc. Ocean Drill. Program Sci. Results., 138, Nie, J., J. King, and X. Fang (2007), Enhancement mechanisms of magnetic susceptibility in the Chinese Red Clay sequence, Geophys. Res. Lett., 34, L19705, doi: / 2007GL Nie, J., J. King, and X. Fang (2008a), Late Pliocene early Pleistocene 100 ka problem, Geophys. Res. Lett., 35, L21606, doi: /2008gl Nie, J., J. King, and X. Fang (2008b), Tibetan uplift intensified the 400 k.y. signal in paleoclimate records at 4 Ma, Geol. Soc. Am. Bull., 120, , doi: /b Oerlemans, J. (1980), Model experiments on the 100,000 yr glacial cycle, Nature, 287, , doi: /287430a0. Oerlemans, J. (1982), Glacial cycles and ice sheet modelling, Clim. Change, 4, Paillard, D., and F. Parrenin (2004), The Antarctic ice sheet and the triggering of deglaciations, Earth Planet. Sci. Lett., 227, , doi: /j.epsl Paillard, D., L. D. Labeyrie, and P. Yiou (1996), Macintosh program performs time series analysis, Eos Trans. AGU, 77(39), 379, doi: /96eo Piotrowski, A., S. Goldstein, S. Hemming, and R. Fairbanks (2005), Temporal relationships of carbon cycling and ocean circulation at glacial boundaries, Science, 307, , doi: /science Pollard, D. (1983), A coupled climate ice sheet model applied to the Quaternary ice ages, J. Clim., 1, Ravelo, A. C., D. H. Andreasen, M. Lyle, A. Olivarez Lyle, and M. W. Wara (2004), Regional climate shifts caused by gradual global cooling in the Pliocene epoch, Nature, 429, , doi: /nature Raymo, M. E. (1994), The initiation of Northern Hemisphere glaciation, Annu.Rev.EarthPlanet.Sci., 22, , doi: /annurev.ea Raymo, M. E. (1998), Glacial puzzles, Science, 281, , doi: /science Raymo, M. E., and K. Nisancioglu (2003), The 41 kyr world: Milankovitch s other unsolved mystery, Paleoceanography, 18(1), 1011, doi: /2002pa Raymo, M. E., L. Lisiecki, and K. Nisancioglu (2006), Plio Pleistocene ice volume, Antarctic climate, and the global d 18 O record, Science, 313, , doi: /science Ruddiman, W. F. (2003), Orbital insolation, ice volume, and greenhouse gases, Quat.Sci.Rev., 22, , doi: /s (03) Ruddiman, W. F., et al. (1989), Late Miocene to Pleistocene evolution of climate in Africa and the low latitude Atlantic: Overview of Leg 108 results, Proc. Ocean Drill. Program Sci. Results, 108, Saltzman, B. (2002), Dynamical Paleoclimatology: Generalized Theory of Global Climate Change, Academic, San Diego, Calif. Saltzman, B., and K. A. Maasch (1988), Carbon cycle instability as a cause of the late Pleistocene ice age oscillation: Modelling theasymmetricresponse,global Biogeochem. Cycles, 2, , doi: /gb002i002p Saltzman, B., and K. A. Maasch (1990), A first order global model of late Cenozoic climate change, Trans. R. Soc. Edinburgh Earth Sci., 81, of10

10 Saltzman, B., and K. A. Maasch (1991), A first order global model of late Cenozoic climate change, Clim. Dyn., 5, , doi: /bf Saltzman, B., A. Sutera, and A. R. Hansen (1984), Earth orbital eccentricity variations and climatic change, in Milankovitch and Climate, Part 2, edited by A. Berger, pp , D. Reidel, Norwell, Mass. Schulz, M., and M. Mudelsee (2002), REDFIT: Estimating red noise spectra directly from unevenly spaced paleoclimatic time series, Comput. Geosci., 28, , doi: / S (01) Schulz, M., and K. Stattegger (1997), SPECTRUM: Spectral analysis of unevenly spaced paleoclimatic time series, Comput. Geosci., 23, , doi: /s (97) Shackleton, N. J. (2000), The 100,000 year ice age cycle identified and found to lag temperature, carbon dioxide, and orbital eccentricity, Science, 289, , doi: / science Shackleton, N. J., J. Imbrie, and M. A. Hall (1983), Oxygen and carbon isotope record of East Pacific core V19 30: Implications for the formation of deep water in the late Pleistocene North Atlantic, Earth Planet. Sci. Lett., 65, , doi: / x(83) Shackleton, N. J., et al. (1984), Oxygen isotope calibration of the onset of ice rafting and the history of glaciation in the North Atlantic region, Nature, 307, , doi: / a0. Shackleton, N. J., A. Berger, and W. R. Peltier (1990), An alternative astronomical calibration of the lower Pleistocene timescale based on ODP Site 677, Trans. R. Soc. Edinburgh Earth Sci., 81, Sigman, D., S. L. Jaccard, and G. H. Haug (2004), Polar ocean stratification in a cold climate, Nature, 428, 59 63, doi: /nature Song, Y., X. Fang, J. J. Li, Z. S. An, and X. Miao (2001), The Late Cenozoic uplift of the Liupan Shan, China, Sci. Chin., D, 44, suppl., Sun, Y., S. C. Clemens, Z. S. An, and Z. Yu (2006), Astronomical timescale and palaeoclimatic implication of stacked 3.6 Myr monsoon records from the Chinese Loess Plateau, Quat.Sci.Rev., 25, 33 48, doi: /j.quascirev Tarasov, L., and W. R. Peltier (1997), A high resolution model of the 100 ky ice age cycles, Ann. Glaciol., 25, Tiedemann, R., M. Sarnthein, and N. J. Shackleton (1994), Astronomic timescale for the Pliocene Atlantic d 18 Oand dust flux records of ODP Site 659, Paleoceanography, 9, , doi: /94pa Torrence, C., and G. P. Compo (1998), A practical guide to wavelet analysis, Bull. Am. Meteorol. Soc., 79, 61 78, doi: / (1998)079<0061:apgtwa>2.0. CO;2. Wang, P. X., J. Tian, X. Cheng, C. Liu, and J. Xu (2004), Major Pleistocene stages in a carbon perspective: The South China Sea record and its global comparison, Paleoceanography, 19, PA4005, doi: /2003pa Young, M. A., and R. S. Bradley (1984), Insolation gradients and the paleoclimatic record, in Milankovitch and Climate, Part 2, editedbya.l.berger,pp , D. Reidel, Norwell, Mass. Zhou, L., F. Oldfield, A. Wintle, S. Robinson, and J. T. Wang (1990), Partly pedogenic origin of magnetic variations in Chinese loess, Nature, 346, , doi: / a0. 10 of 10