An updated astronomical timescale for the Plio-Pleistocene deposits from South China Sea and new insights into Asian monsoon evolution Hong Ao a, b, * , Mark J. Dekkers c , Li Qin d , Guoqiao Xiao e a State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xian 710075, China b Paleomagnetism and Geochronology Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China c Paleomagnetic Laboratory ‘Fort Hoofddijk’, Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands d Chongqing China Three Gorges Museum, Chongqing 400015, China e State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China article info Article history: Received 21 June 2010 Received in revised form 5 April 2011 Accepted 8 April 2011 Available online 30 April 2011 Keywords: Astronomical chronology South China Sea ODP Site 1143 Asian monsoon Pliocene Pleistocene abstract Here we present an improved astronomical timescale since 5 Ma as recorded in the ODP Site 1143 in the southern South China Sea, using a recently published Asian summer monsoon record (hematite to goethite content ratio, Hm/Gt) and a parallel benthic d 18 O record. Correlation of the benthic d 18 O record to the stack of 57 globally distributed benthic d 18 O records (LR04 stack) and the Hm/Gt curve to the 65 N summer insolation curve is a particularly useful approach to obtain refined timescales. Hence, it constitutes the basis for our effort. Our proposed modifications result in a more accurate and robust chronology than the existing astronomical timescale for the ODP Site 1143. This updated timescale further enables a detailed study of the orbital variability of low-latitude Asian summer monsoon throughout the Plio-Pleistocene. Comparison of the Hm/Gt record with the d 18 O record from the same core reveals that the oscillations of low-latitude Asian summer monsoon over orbital scales differed considerably from the glacialeinterglacial climate cycles. The popular view that summer monsoon intensifies during interglacial stages and weakens during glacial stages appears to be too simplistic for low-latitude Asia. In low-latitude Asia, some strong summer monsoon intervals appear to have also occurred during glacial stages in addition to their increased occurrence during interglacial stages. Vice versa, some notably weak summer monsoon intervals have also occurred during interglacial stages next to their anticipated occurrence during glacial stages. The well-known mid-Pleistocene transition (MPT) is only identified in the benthic d 18 O record but not in the Hm/Gt record from the same core. This suggests that the MPT may be a feature of high- and middle-latitude climates, possibly determined by high- latitude ice sheet dynamics. For low-latitude monsoonal climate, its orbital-scale variations respond more directly to insolation and are little influenced by high-latitude processes, thus the MPT is likely not recorded. In addition, the Hm/Gt record suggests that low-latitude Asian summer monsoon intensity has a long-term decreasing trend since 2.8 Ma with increased oscillation amplitude. This long-term vari- ability is presumably linked to the Northern Hemisphere glaciation since then. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The Asian monsoon system is an important component of the global climatic system (Webster, 1994), which controls much of the Asian climate changes (Sun and Wang, 2005; Wang et al., 2005a; Clift and Plumb, 2008). Knowledge of the long-term spatial and temporal evolution of the Asian monsoon would aid in understanding of its underlying forcing mechanisms. This would in turn enable a better prediction of future climate change scenarios with regard to the Asian monsoon. The Asian monsoon is characterized by seasonal reversal of winter and summer monsoons, which results in cold/dry winters and warm/wet summers over the Asian mainland (Wang et al., 2005a; Clift and Plumb, 2008). The Asian summer monsoon evolution in South China during the Holocene and the late Pleistocene (i.e. from 387 ka to present) can be reflected in detail (from orbital down to millennial variations) from the d 18 O record of stalagmites from South China, which are dated by high-resolution U-series analyses (Wang et al., 2001, 2005b, 2008; Yuan et al., 2004; Cheng et al., 2006, 2009; Zhang et al., 2008), although measuring stalagmite d 18 O is * Corresponding author. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xian 710075, China. Tel.: þ86 29 88321470; fax: þ86 29 88320456. E-mail address: [email protected] (H. Ao). Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.04.009 Quaternary Science Reviews 30 (2011) 1560e1575
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Quaternary Science Reviews 30 (2011) 1560e1575
Contents lists avai
Quaternary Science Reviews
journal homepage: www.elsevier .com/locate/quascirev
An updated astronomical timescale for the Plio-Pleistocene deposits from SouthChina Sea and new insights into Asian monsoon evolution
Hong Ao a,b,*, Mark J. Dekkers c, Li Qin d, Guoqiao Xiao e
a State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xian 710075, Chinab Paleomagnetism and Geochronology Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, Chinac Paleomagnetic Laboratory ‘Fort Hoofddijk’, Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The NetherlandsdChongqing China Three Gorges Museum, Chongqing 400015, Chinae State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China
a r t i c l e i n f o
Article history:Received 21 June 2010Received in revised form5 April 2011Accepted 8 April 2011Available online 30 April 2011
Keywords:Astronomical chronologySouth China SeaODP Site 1143Asian monsoonPliocenePleistocene
* Corresponding author. State Key Laboratory of LoInstitute of Earth Environment, Chinese Academy of STel.: þ86 29 88321470; fax: þ86 29 88320456.
E-mail address: [email protected] (H. Ao).
0277-3791/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.quascirev.2011.04.009
a b s t r a c t
Here we present an improved astronomical timescale since 5 Ma as recorded in the ODP Site 1143 in thesouthern South China Sea, using a recently published Asian summer monsoon record (hematite togoethite content ratio, Hm/Gt) and a parallel benthic d18O record. Correlation of the benthic d18O recordto the stack of 57 globally distributed benthic d18O records (LR04 stack) and the Hm/Gt curve to the 65�Nsummer insolation curve is a particularly useful approach to obtain refined timescales. Hence, itconstitutes the basis for our effort. Our proposed modifications result in a more accurate and robustchronology than the existing astronomical timescale for the ODP Site 1143. This updated timescalefurther enables a detailed study of the orbital variability of low-latitude Asian summer monsoonthroughout the Plio-Pleistocene. Comparison of the Hm/Gt record with the d18O record from the samecore reveals that the oscillations of low-latitude Asian summer monsoon over orbital scales differedconsiderably from the glacialeinterglacial climate cycles. The popular view that summer monsoonintensifies during interglacial stages and weakens during glacial stages appears to be too simplistic forlow-latitude Asia. In low-latitude Asia, some strong summer monsoon intervals appear to have alsooccurred during glacial stages in addition to their increased occurrence during interglacial stages. Viceversa, some notably weak summer monsoon intervals have also occurred during interglacial stages nextto their anticipated occurrence during glacial stages. The well-known mid-Pleistocene transition (MPT) isonly identified in the benthic d18O record but not in the Hm/Gt record from the same core. This suggeststhat the MPT may be a feature of high- and middle-latitude climates, possibly determined by high-latitude ice sheet dynamics. For low-latitude monsoonal climate, its orbital-scale variations respondmore directly to insolation and are little influenced by high-latitude processes, thus the MPT is likely notrecorded. In addition, the Hm/Gt record suggests that low-latitude Asian summer monsoon intensity hasa long-term decreasing trend since 2.8 Ma with increased oscillation amplitude. This long-term vari-ability is presumably linked to the Northern Hemisphere glaciation since then.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
The Asian monsoon system is an important component of theglobal climatic system (Webster, 1994), which controls much of theAsian climate changes (SunandWang, 2005;Wanget al., 2005a; Cliftand Plumb, 2008). Knowledge of the long-term spatial and temporalevolution of the Asian monsoon would aid in understanding of its
ess and Quaternary Geology,ciences, Xian 710075, China.
All rights reserved.
underlying forcing mechanisms. This would in turn enable a betterpredictionof future climatechangescenarioswith regard totheAsianmonsoon. TheAsianmonsoon is characterizedbyseasonal reversal ofwinterandsummermonsoons,which results in cold/drywinters andwarm/wet summers over the Asian mainland (Wang et al., 2005a;Clift and Plumb, 2008). The Asian summer monsoon evolution inSouth China during the Holocene and the late Pleistocene (i.e. from387 ka to present) can be reflected in detail (from orbital down tomillennial variations) from the d18O record of stalagmites fromSouthChina, which are dated by high-resolution U-series analyses (Wanget al., 2001, 2005b, 2008; Yuan et al., 2004; Cheng et al., 2006,2009; Zhang et al., 2008), although measuring stalagmite d18O is
H. Ao et al. / Quaternary Science Reviews 30 (2011) 1560e1575 1561
not the same as (directly) measuring summer monsoon intensity.Note that the summer monsoon intensity contains not only themonsoon precipitation, but also other issues, such as temperatureand wind speed. The Chinese loess-paleosol sequences offer a goodopportunity to trace the Asian monsoon evolution in north Chinaduring the Pleistocene (e.g. An et al., 1990, 2001; Liu and Ding,1998;An, 2000; Ding et al., 2002, 2005; Sun et al., 2006). Up to now,however, published high-resolution records that reveal orbitalchanges in the Asianmonsoon extending back to the pre-Pleistoceneperiods are rare.
Recently, Zhang et al. (2007, 2009) established a high-resolutionAsian summermonsoon record throughout the last 5 Myr using theratio of hematite and goethite contents (Hm/Gt) from the OceanDrilling Program (ODP) Site 1143 in the southern South China Sea(Fig. 1). Generally, a weaker summer monsoon would result indecreased humidity and subsequently higher Hm/Gt ratios,whereas a stronger summer monsoon would lead to increasedhumidity with related lower Hm/Gt ratios, because dry and humidconditions are more favorable for the formation of hematite andgoethite, respectively (Harris and Mix, 1999; Ji et al., 2004; Zhanget al., 2007, 2009). The Plio-Pleistocene part of the ODP Site 1143is 190 m long. As suggested by presently accepted chronology ofthis ODP site, which was generated by tuning the benthic d18Orecord to the orbital obliquity (41-kyr) and precession (23-kyr)(Tian et al., 2002) (from now on referred to as the T2002 timescale),the 190-m deposits span the last 5 Myr. Inferred from the T2002timescale, the resolution of Hm/Gt record is as high as 2 kyr, whichenables a detailed study of the orbital and millennial as well aslong-term changes of Asian monsoon during the last 5 Myr.
Although the T2002 timescale was established based on orbitaltuning, the Hm/Gt record plotted on the T2002 timescale showspoor orbital periodicities, which cannot be matched with the ETP
South China
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Fig. 1. Location map of the ODP Site 1143 (modified from Zhang et al., 2007). Graydashed line represents the 100 m isobaths (approximate position of the coastlineduring glacial sea level low stands) in the South China Sea. Back dashed line representsthe Molengraaff paleo-river. The arrow represents the Asian summer monsoondirection.
curve (Fig. 2). Here the ETP curve is the sum of normalized eccen-tricity (E, w100-kyr), obliquity (T, 41-kyr) and reversed precession(P, 19- and 23-kyr), which reflects the characteristics of the Earth’sorbital elements. A good tuning should result in high coherenciesbetween climate and ETP curves, generally above the 95% confi-dence (Brüggemann, 1992). Since the precession cycles are quiteweak in the Plio-Pleistocene benthic d18O record, the tuning ofprecession signal in the benthic d18O record to orbital precession asadopted by Tian et al. (2002) would result in considerable uncer-tainties of the assignment of short-term sedimentary precessioncycles. The stalagmite d18O record is the most accurately datedmonsoon record on the relevant 100-kyr timescale, with errors ofmere decades (Wang et al., 2001, 2005b, 2008; Yuan et al., 2004;Cheng et al., 2006, 2009; Zhang et al., 2008). When comparingthe Hm/Gt record in the T2002 timescale with the compositeChinese stalagmite d18O record (from the Sanbao, Linzhu, Donggeand Hulu caves, see their locations in Fig. 1) during the pastw390 kyr (Wang et al., 2001, 2008; Cheng et al., 2009), the corre-lation between the two timescales appears to be less consistent aswell (Fig. 3). A rearrangement of these tie points (dashed lines)could result in a more plausible correlation between the Hm/Gt andstalagmite d18O records that with the current timescale showsa rather variable sedimentation rate (Fig. 3). The shifts in sedimentaccumulation rate when compared to the speleothem timescale (cf.Fig. 3) and the rather poor assignment of precession cycles in theT2002 timescale suggest that it is appropriate to re-assess thistimescale for the ODP Site 1143. Especially the summer monsoonrecord inferred from the Hm/Gt ratio is now available for the last5 Myr for that site. The Hm/Gt record has strong short-term oscil-lations related to precession cycles, which in principle wouldenable a better assignment of the sedimentary precession cyclesallowing amore precise timescale. These observationsmotivated usto construct an improved timescale for the Plio-Pleistocene part ofthe ODP Site 1143 by considering the structures of both Hm/Gt andbenthic d18O records. A series of major events, such as the finalclosure of the Panama Isthmus and the Indonesian seaway, the finaluplift stage and expansion of the northern edge of the Tibetan
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Fig. 2. Cross-spectral comparisons of the ETP curve with the Hm/Gt record (Zhanget al., 2007, 2009) expressed on the T2002 timescale (Tian et al., 2002). The shadedareas represent the coherency between Hm/Gt and ETP, and the horizontal dashed lineindicates 95% confidence limit for coherence peaks. Here the ETP curve is the sum ofnormalized eccentricity (E), obliquity (T), and reversed precession (P). The cross-spectral analyses were performed with Arand software developed by Brown Univer-sity. Bandwidth we used is 0.007 kyr�1.
Fig. 3. Comparison of the Hm/Gt monsoon proxy (Zhang et al., 2007, 2009) expressed on the original depth and on the T2002 timescale (Tian et al., 2002) with the compositeChinese stalagmite d18O record (from the Sanbao, Linzhu, Dongge and Hulu caves) (Wang et al., 2001, 2008; Cheng et al., 2009), another accurately dated Asian summer monsoonrecord. Note that the time resolution and chronology are different in the two monsoon records. The chronology of the stalagmites was derived directly from high-resolution 230Thdating. The stalagmite d18O record is the most accurately dated monsoon record on the 100-kyr timescale that is relevant here, with errors of mere decades (Wang et al., 2001,2005b, 2008; Yuan et al., 2004; Cheng et al., 2006, 2009; Zhang et al., 2008). The T2002 timescale was formulated through simultaneously tuning the precession and obliquityin the benthic d18O record to the orbital precession and obliquity, respectively. Dashed lines show tie-lines of equal age (or depth) between the Hm/Gt and stalagmite d18O monsoonrecords.
H. Ao et al. / Quaternary Science Reviews 30 (2011) 1560e15751562
Plateau and the onset of Northern Hemisphere glaciations, whichare closely linked to the appearance of the current patterns in theclimate system, are within the Plio-Pleistocene period (Raymo,1994; Zheng et al., 2000; An et al., 2001; Cane and Molnar, 2001;Schmittner et al., 2004; Clift and Plumb, 2008). In addition, theSouth China Sea area was also under the influence of Asianmonsoon during the Plio-Pleistocene and may thus containimportant monsoonal information. Therefore, a refined timescalefor the ODP Site 1143 will enable a better understanding of theorbital variations of low-latitude Asian and global climate as well asof the paleoceanographic history of the South China Sea throughoutthe Plio-Pleistocene.
Using the recently published Hm/Gt (Zhang et al., 2007, 2009)and benthic d18O records (Tian et al., 2002), we present here animproved timescale for the ODP Site 1143 throughout the last5 Myr. We calibrate firstly the benthic d18O record to the stack of 57globally distributed benthic d18O records (LR04 stack) (Lisiecki andRaymo, 2005) and secondly the Hm/Gt record to the 65�N summerinsolation curve (Laskar et al., 2004). Because of more robusttuning, our new timescale (hereinafter referred to as A2011 time-scale) appears to be more consistent than the T2002 timescale. Asmoother sediment accumulation is the result in line witha continuous monsoon-derived riverine input to the depositionalenvironment. Particularly within the time interval between 5 and3.2 Ma, the new timescale shows considerable improvement. Basedon the A2011 timescale, we further employ the Hm/Gt record to
discuss the variations of low-latitude Asian monsoon throughoutthe last 5 Myr over short-term orbital scales and long-term trendsas well as the possible relationship with variations of global icevolume.
2. Geological setting and materials
ODP Site 1143 (9�21.720N, 113�17.110E; 2777 mwater depth) wasdrilled in a depression on the carbonate platform that forms thesouthern continental shelf of the southern South China Sea (Fig. 1).As a semi-enclosed deep-sea basin, the South China Sea is thelargest marginal sea of the western Pacific, covering an area ofw3.5 � 106 km2. The seasonally reversed circulations of the Asianwinter and summer monsoon result in cold/dry winters and warm/wet summers over the South China Sea. Due to strong monsoonprecipitation and intrusion of low-salinity water from along shoreof Borneo, the sea surface salinity in the southern South China Sea isquite low, ranging from w31e34.3& (Tian et al., 2004). The seasurface salinity in the openwestern Pacific is as much as 35e35.5&throughout the upper 560m of thewater column (Tian et al., 2004).The deposits at the ODP Site 1143 mainly consist of terrigenousquartz, feldspar and clay minerals, with only a minor biogeniccomponent (<2%) (Wan et al., 2006). Perhaps rather surprising inan overall detrital setting, only the Brunhes/Matuyama (B/M)paleomagnetic reversal was identified at a depth ofw42.5 m; otherpaleomagnetic reversals were not found (Fig. 4) (Tian et al., 2002).
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B/M boundary (0.78 Ma)LO Globigerinoides fistulosus (1.77 Ma)
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LO Sphaeroidinellopsis seminulina (3.12 Ma)FO Globorotalia tosaensis (3.35 Ma)
LO Globorotalia plesiotumida (3.77 Ma)
LCO Globorotalia margaritae (3.96 Ma)
LO Dentoglobigerina altispira (3.09 Ma)
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Fig. 4. First-order correlations of the benthic d18O record (Tian et al., 2002) of the ODP Site 1143 to the LR04 stack of benthic d18O (Lisiecki and Raymo, 2005). B, Brunhes;M,Matuyama;FO, first occurrence; LO, last occurrence; LCO, last common occurrence. The ages of panktonoic foraminiferal events refer to themagnetostratigraphic dating of these events from otherODP sites (Berggren et al., 1995). The age of B/M boundary refers to the astronomically tuned Neogene timescale (Lourens et al., 2004). Numbers arranged on the benthic d18O curvesrepresent the marine isotope stages. Dashed lines show the typical correlations between the benthic d18O records of the ODP Site 1143 and LR04 stack.
H. Ao et al. / Quaternary Science Reviews 30 (2011) 1560e15751564
This makes it impossible to establish a chronology for the ODP Site1143 based on magnetostratigraphy. The reason for the poorpaleomagnetic record is presumably related to poor demagnetiza-tion of the characteristic remanent magnetization (ChRM) ordiagenetic removal of magnetite which dissolvesmore quickly thanhematite and goethite (Shipboard Scientific Party, 2000; Larrasoañaet al., 2003a, b; Liu et al., 2004; Ao et al., 2010a), but a full account isbeyond the scope of this paper.
Hematite (Hm) and goethite (Gt) contents over the last 5 Myr(from 0 to 190 m) were assembled by Zhang et al. (2007, 2009) for2122 samples from the ODP Site 1143 using a Perkin Elmer Lambda900 diffuse reflectance spectrophotometer in the SurficialGeochemistry Institute of Nanjing University, China (see Zhanget al., 2007 for a detailed description of the measuring method).Subsequently, the Hm/Gt ratio is calculated and interpreted asa proxy for changes in Asian summer monsoon intensity in SouthChina (Zhang et al., 2007, 2009). This published Hm/Gt record is animportant ingredient for our tuning. Another key component is thepublished benthic d18O record from the same core (Tian et al.,2002). Within the interval of our tuning, a total of 1992 samplesof benthic foraminifers were measured for stable oxygen isotopesusing a FinniganMAT252mass spectrometer in theMarine GeologyLaboratory of Tongji University, China (Tian et al., 2002). Thebenthic d18O values are from the benthic foraminifers Cibicidoideswuellerstorfi and Uvigerina peregrina. The values of U. peregrina areadjusted by subtracting 0.64& (Tian et al., 2002).
The sensitivity of the Hm/Gt ratio to precipitation has beendemonstrated by its correlation with topography: generally soilswith relatively higher hematite concentrations occur on the drierslopes in the landscape while higher goethite concentrations arenoted in the wetter depressions (Curi and Franzmeier, 1984;Santana, 1984; da Motta and Kampf, 1992). In an overall wetterclimate the proportion of comparatively dry slopes is thereforelower while the expression of wetter depressions is restricted indrier climate. The Hm/Gt ratio has been used in a number ofoccasions for studying the precipitation variations in the sourceareas of sedimentary basins (e.g. Harris and Mix, 1999; Ji et al.,2004; Clift, 2006). For example, the Hm/Gt ratio record retrievedfrom Ceara Rise sediments in the western tropical Atlantic Oceanwas used to estimate the Pleistocene variability of the precipitationin the Amazon Basin (Harris and Mix, 1999). The Hm/Gt recordretrieved from the Chinese loess deposits was used to estimate theAsian summer monsoon evolution, which has high values at loesslayers (comparatively dry climate, weak summer monsooninterval) and low values at paleosol layers (comparatively humidclimate, strong summer monsoon interval) (Ji et al., 2004). Clift(2006) used the Hm/Gt ratios of the ODP Site 1148 from thenorthern South China Sea to infer the Asian summer monsoonvariations for the past 25 Myr. As already adopted by Zhang et al.(2007, 2009), the Hm/Gt ratios of the ODP Site 1143 can be usedas an indicator of summer monsoon intensity because of thefollowing four reasons. (1) The terrigenous deposits, including thehematite and goethite, at the ODP Site 1143 were mostly derivedfrom the Mekong Basin (Fig. 1) through fluvial and marine trans-portation (Wan et al., 2006), with a discharge of w160 � 106 t ofsediment per year (Milliman andMeade,1983). Other rivers such asthe Baram River from northwest Borneo and the Chao Phraya Riverfrom western Indochina have a combined annual sedimentdischarge of w23 � 106 t to the southwest South China Sea. Thesetherefore were not an important source of the ODP Site 1143sediments compared with the Mekong River (Milliman andSyvitski, 1992; Hiscott, 2001). The dominant surface currentduring interglacial periods from the southwest makes terrigenousinput from the Red River and Pearl River insignificant as well.During glacial periods with lower sea levels, however, the
Molengraaff River may have transported some additional sedi-ments from the Paleo-Sunda shelf to the ODP Site 1143(Molengraaff andWeber, 1920) (Fig. 1). (2) Hematite and goethite inthe ODP Site 1143 are not significantly affected by pore-waterreduction after burial (Zhang et al., 2007, 2009) as suggested byfollowing evidence. A commonly used sign for strongly reductivealteration is the presence of pyrite in sediments (Berner, 1981,1984). However, pyrite was not found in the ODP Site 1143 (Wanget al., 2001). This implies that anoxic diagenesis was not signifi-cant here. Furthermore, unlike the low-coercivity magnetite, thehigh-coercivity hematite and goethite are rather resistant to pore-water reduction processes (Snowball, 1993; Nowaczyk et al., 2002;Demory et al., 2005). Prolonged diagenesis generally requiresabundant (reactive) organic matter (Berner, 1984; Robinson et al.,2000; Demory et al., 2005), however, the so-called green layers inthe ODP site 1143 sediments have a low total organic carbon (TOC)content (Tamburini et al., 2003). (3) The relative abundance ofgoethite to hematite on the paleo-Sunda shelf and in the MekongBasin varies with climatic conditions: dry and humid conditions aremore favorable for formation of hematite and goethite, respectively(Curi and Franzmeier, 1984; Santana, 1984; da Motta and Kampf,1992; Zhang et al., 2007, 2009). (4) The dry and humid conditionsover the South China Sea are mainly modulated by Asian summermonsoon (Tian et al., 2004, 2005, 2006; Wang et al., 2005a; Zhanget al., 2007, 2009), although it is debatable whether the climate inBorneo is significantly affected by the monsoon circulations (Cobbet al., 2007). Therefore, the strong summer monsoon periodswould result in more goethite deposition in the South China Sea,whereas the weak summer monsoon periods would result in morehematite deposition. So, for this region low and high Hm/Gt ratioswould imply strong and weak summer monsoons, respectively.
3. Calibration of the astronomical timescale
Astronomical calibration is a powerful approach to constructtimescales which, in principle, have a higher resolution and betteraccuracy than conventional timescales based on linear interpola-tion between geomagnetic reversals and/or radiometrically datedcalibration points. This approach has beenwidely used to constructage models for deep-sea sediments (e.g. Raymo et al., 1989;Ruddiman et al., 1989; Shackleton et al., 1990; Hilgen, 1991a,b;Lisiecki and Raymo, 2005; Hüsing et al., 2010), Chinese loess (e.g.Ding et al., 1994; Lu et al., 1999; Heslop et al., 2000; Sun et al., 2006),and other continental deposits (van Vugt et al., 1998; Aziz et al.,2003, 2004; Ao et al., 2010b).
For the construction of an astronomical timescale, the selectionof suitable target curves is crucial. In this study, we selected the65�N summer insolation (Laskar et al., 2004) for the tuning of theODP Site 1143, because it is a major forcing factor of the orbital-scale changes in the Asian summer monsoon in South China(Wang et al., 2008; Cheng et al., 2009). The orbital solution of La2004(Laskar et al., 2004) computed with present-day input values forthe dynamical ellipticity of the Earth and tidal dissipation in theevolution of the Earth-Moon system, is demonstrated to have anaccurate solution with respect to the geological records (Pälikeet al., 2006a,b; Tian et al., 2008; Ao et al., 2010b; Hüsing et al.,2010). Before starting the tuning procedure, the phase relation-ship between orbital forcing and Asianmonsoon responses must beknown. As proposed by Ruddiman (2006a), the Asian monsoonshould respond to the Northern Hemisphere summer insolationwith a near-zero phase lag. This is recently confirmed by thecomparison of the d18O record retrieved from the well-datedChinese stalagmites with the 65�N summer insolation (Wanget al., 2008). Further, the in-phase correlation between theprecession (23-kyr) signal in the stalagmite d18O (Wang et al., 2001,
H. Ao et al. / Quaternary Science Reviews 30 (2011) 1560e1575 1565
2008; Cheng et al., 2009) and the orbital precession signal (Laskaret al., 2004) supports a zero phase lag as well (SupplementaryFig. 1). Thus during the present tuning procedure, we utilizeda zero time lag between the insolation forcing and the monsoonresponse. This differs from Tian et al. (2002) who used the thengenerally accepted 8-kyr lag for the obliquity curve and 5-kyr lagfor the precession curve in their tuning.
Unlike the T2002 timescale that uses a single isotope curve fortuning, the A2011 timescale was formulated by visual correlation ofthe benthic d18O record to the LR04 benthic stack (Lisiecki andRaymo, 2005) and the Hm/Gt monsoon record to the 65�Nsummer insolation (Laskar et al., 2004). The LR04 stack wasselected over other marine benthic d18O records because it is con-structed by averaging 57 globally distributed sites. It thus has anincreased signal-to-noise ratio, which would better removeregional variability and better capture the global ice volume signal
Fig. 5. Second-order correlations of the Hm/Gt cycles (Zhang et al., 2007, 2009) of the ODP(weak summer monsoon) and minima (strong summer monsoon) were correlated with refiltered from the Hm/Gt record expressed on the A2011 timescale is also used to promote adetailed correlations between the Hm/Gt and the insolation records. The red points on the HThese tie-points are selected from feature correlated specifies maxima (minima) in Hm/Gt rcolour in this figure legend, the reader is referred to the web version of this article.)
(Lisiecki and Raymo, 2005). This cross-correlation with severalproxies (here two) is generally more robust than tuning with onlya single climate proxy. The visual tuning carried out here wassimilar to the procedure adopted by Hilgen (1991a, b) and Hilgenet al. (1995, 2006), Ruddiman and Raymo (2003) and Heslopet al. (2000). For example, Hilgen et al. (1995) extended theastronomical (polarity) timescale into the Miocene based on thecorrelation of characterisitic sedimentary cycle patterns in marinesections in the Mediterranean to the 65�N summer insolation. Bycorrelation of CH4 to insolation, Ruddiman and Raymo (2003)constructed a new timescale for the Vostok ice core. Heslop et al.(2000) formulated a refined timescale for the Chinese loessdeposits by correlation of the unfiltered grain-size (a proxy of Asianwinter monsoon in north China) and magnetic susceptibility (aproxy of Asian summer monsoon in north China) records to theinsolation and marine d18O curves.
Site 1143 to the 65�N summer insolation curve (Laskar et al., 2004). Hm/Gt maximagional minima and maxima in insolation, respectively. The precession (21-kyr) cyclebetter correlation between the Hm/Gt and insolation records. Dashed lines show the
m/Gt curves represent the final tie-points we adopted to establish the A2011 timescale.ecord and minima (maxima) in 65�N insolation. (For interpretation of the references to
Fig. 5. (continued).
Table 1Ages of planktonic foraminiferal events estimated by the A2011 and T2002 time-scales (Tian et al., 2002) from the ODP Site 1143 and bymagnetostratigraphic datumlevels of other ODP sites (Berggren et al., 1995).
Planktonic foraminiferal events Depth (m) Age (Ma)
A2011 T2002 Berggren
FO pink Globigerinoides ruber 25.0 0.41 0.41 0.40LO Globigerinoides fistulosus 83.4 1.73 1.73 1.77FO Globorotalia truncatulinoides 96.1 1.99 2.03 2.00LO Dentoglobigerina altispira 134.8 3.03 3.04 3.09LO Sphaeroidinellopsis seminulina 138.0 3.12 3.14 3.12FO Globorotalia tosaensis 144.4 3.32 3.35 3.35LO Globorotalia plesiotumida 161.1 3.82 3.82 3.77LCO Globorotalia margaritae 166.5 4.06 4.05 3.96
FO, first occurrence; LO, last occurrence; LCO, last common occurrence.
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As a first step in our tuning, the magnetostratigraphic (i.e. the B/M boundary) and biostratigraphic data provided 9 initial tie pointsbetween the benthic d18O records of the ODP Site 1143 and the LR04stack (Fig. 4).Next,weattempt to correlate thebenthic d18O recordofthe ODP Site 1143 to the LR04 benthic stack, because benthic d18O isglobally correlative during the Plio-Pleistocene (Lisiecki and Raymo,2005). The characteristics of the benthic d18O records retrieved fromdifferent sites are quite similar, therefore, the correlation of thelarge-scale d18O cycles is relatively straightforward. The outcome ofthis first-order tuning is presented in Fig. 4. The validity of thistuning is supported by the cycle-by-cycle correlation between thetwo benthic d18O records and their similar amplitude modulation.Thisfirst-order large-scale tuning is crucial to avoid over-correlationof the data during the following second-order tuning. Unlike us,however, Tianet al. (2002)didnot use suchavisual correlationof thebenthic d18O record of the ODP Site 1143 to otherwell-dated benthicd18O record to constrain their orbital tuning. Keep in mind that theages of the B/Mboundary and biostratigraphic eventswe selected toestablish our initial timescale aswell as the ages from the first-order
tuning, are not fixed (generally with a relaxed age less than 40 ka)during our following second-order tuning that focuses on fine-scalefeatures. So, after the first-order tuning, we then further correlatethe small-scaleHm/Gt cycles to the 65�Nsummer insolation. During
Fig. 6. Comparisons of the Hm/Gt (Zhang et al., 2007, 2009) and benthic d18O (Tian et al., 2002) records expressed on the A2011 timescale and the T2002 timescale (Tian et al.,2002). Hm/Gt (Zhang et al., 2007, 2009) and benthic d18O records from the ODP Site 1143 plotted on the (A) original depth, (B) A2011 timescale and (C) T2002 timescale. (D)LR04 benthic d18O stack (Lisiecki and Raymo, 2005). Numbers arranged on the benthic d18O curves represent the marine isotope stages. Dashed lines show the sedimentarycorrelations among the original depth, A2011 timescale and T2002 timescale. The arrows show a long-term varying trend of the climate proxies.
Fig. 6. (continued).
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H. Ao et al. / Quaternary Science Reviews 30 (2011) 1560e1575 1569
this second-order tuning, Hm/Gtmaxima (weak summermonsoon)and minima (strong summer monsoon) were correlated cycle-by-cycle with regional minima and maxima in insolation, respectively.This relationship between theAsianmonsoon and the insolationhasbeendocumentedby the d18Odata of theChinese stalagmites (Wanget al., 2001, 2008; Yuan et al., 2004; Cheng et al., 2009). This astro-nomical calibration of the small-scale Hm/Gt cycles is unfortunatelyless straightforward at some intervals, such as at ca 0.6, 0.75, 1.1, 2,2.95, 3.2, 3.86 and 4.66 Ma. In addition to the correlation betweeninsolation and Hm/Gt curves, therefore, meanwhile the correlationbetween the insolation cycles and the precession (23-kyr) cyclesfiltered from the Hm/Gt record was also used to assist the tuning(Fig. 5). The strong precession signals in the Hm/Gt record enableda detailed correlation betweenHm/Gt and insolation records,whichconstitutes the basis for our refined tuning. An important extraconstraint in this second-order tuning, is that the resulting agesshould not implychanges in sedimentation rate that are geologicallyimplausible (e.g. too sharp shifts in sedimentation rate), or differ toomuch from the ages constrained by the magnetostratigraphy,biostratigraphy and first-order tuning. The most likely final corre-lation is presented in Fig. 5 (see Supplementary Table 1 for the finaltie-pointsweadoptedandSupplementaryTable 2 for theHm/Gt andbenthic d18O data expressed on the A2011 timescale). Both the Hm/Gt curveand theprecession curvefiltered fromtheHm/Gt record arewell correlated with the insolation curve, with only occasionalpresence of minor discrepancies (Fig. 5).
During our tuning, we tried various alternative tuning options,which involved different tie-points. However, they did not result inconsistent correlations between the benthic d18O records from theODPSite1143and theLR04stackon theonehand, orbetweentheHm/Gt record and the insolation record on the other. These tuning optionsresulted in either a good correlation between the Hm/Gt record andthe insolation record with a rather bad correlation between the ODPSite 1143 and the LR04 stacked d18O records, or vice versa. Also toosharp shifts in sedimentation rate were the outcome. All these alter-natives resulted in correlations which are not convincing and lessconsistent than the correlations presented in Figs. 4 and 5. Therefore,these alternative tuning options were discarded.
4. Evaluation of the astronomical timescale
After our tuning, the M/B boundary in the ODP Site 1143 has anage of 0.77 Ma, which is consistent with its recent astronomical ageof 0.78 Ma (Lourens et al., 2004; Lisiecki and Raymo, 2005). Theages of the planktonic foraminiferal events from the A2011 time-scale broadly concur with those of the T2002 timescale (Tian et al.,2002) and with magnetostratigraphic datum levels of other ODPsites (Berggren et al., 1995) (Table 1).
Compared to the T2002 timescale, the present A2011 timescalehas several features that indicate improvement. Firstly, visualcomparison shows that the tuning of the A2011 timescale is moreconsistent than of the T2002 timescale. For ages younger than3.2 Ma, the differences are marginal, no more than 40 kyr (Fig. 6).Before 3.2 Ma, however, differences are more prominent, withdifferences as much as 160 kyr (Fig. 6). These major differences areinterpreted to be mainly due to the ‘over-tuning’ in the T2002timescale. As suggested by the Hm/Gt and benthic d18O records(Fig. 6), the 3.2e3.7 Ma interval of the T2002 timescale is over-stretched. Following this overstretched interval, there is an ‘over-compressed’ interval at ca 3.8 Ma, with a sedimentation rate closeto an order of magnitude higher than that of its neighboringintervals (Fig. 6). Further back in time, another overly stretched andcompressed ‘couplet’ occurs at ca 4.2e4.5 and 4.5e4.7 Ma in theT2002 timescale (Fig. 6). In contrast, both the benthic d18O recordand the Hm/Gt record indicate that the A2011 timescale shows
a more consistent tuning and is more plausible between 3.2 and5 Ma than the T2002 timescale, without showing any distinct shiftsin sedimentation rate for which there is no reason from lithologicalobservation. The sedimentation rate in the A2011 age model variessmoothly between 1 and 6 cm/kyr.
Secondly, the benthic d18O record ismore consistently expressedin the A2011 timescale than in the T2002 timescale before 3.2 Ma(Fig. 6). For example, the LR04 stack suggests that the marineoxygen isotopic stage (MIS) M2 is the coldest glacial period before3.2 Ma (Lisiecki and Raymo, 2005) (Fig. 6). This is also the case forthe A2011 timescale. In the T2002 timescale, however, MIS MG2becomes the coldest glacial period before 3.2 Ma (Fig. 6). Appar-ently, MIS M2 was mistakenly tuned to MIS MG2 in the T2002timescale. The T2002 timescale reveals several sub-glacial and sub-interglacial stages (as suggested by several regional d18O maximaand minima) within MIS Gi11. However, these sub-glacial and sub-interglacial stages are not revealed by the LR04 stack and the A2011timescale (Fig. 6). As suggested by the LR04 stack and A2011timescale, glacial MIS Gi26 and NS6 should be colder than theirneighboring glacial MIS Gi28 and Si2, respectively (Lisiecki andRaymo, 2005) (Fig. 6). In the T2002 timescale, however, MIS Gi28and Si2 are colder than MIS Gi26 and NS6, respectively (Fig. 6).Unlike the A2011 timescale, the T2002 timescale also results insome other visible inconsistencies of benthic d18O (e.g. the subtlecharacteristics of the marine oxygen isotopic stages) comparedwith the LR04 stack for ages between 3.2 and 5 Ma, in addition tothese inconsistencies we pointed out above (Fig. 6).
Thirdly, the A2011 timescale results in a better (almost cycle-by-cycle) correlation between the Hm/Gt record and the compositeChinese stalagmite d18O record during the past 400 kyr than theT2002 timescale (Fig. 7). The good correlation between the Hm/Gtrecord and the composite Chinese stalagmite d18O record alsosuggests that Hm/Gt is a good proxy parameter of the Asian summermonsoon intensity. Otherwise, a good correlation between the Hm/Gt record and the composite Chinese stalagmite d18O record aftertuning would be non-existent. Summarizing, we are convinced thatthe proposed revisions to theODP Site 1143 timescale are essentiallycorrect. It is based on consistent tuning of continuous sedimentarysuccessions. This tuning has resulted in a more accurate timescalefor the Plio-Pleistocene part of the ODP Site 1143.
5. Asian monsoon variability during the Plio-Pleistocene
5.1. Orbital-scale variability of the Asian monsoon
It is well-known that the Asianmonsoon circulations result fromthe reversal of the temperature gradient between the Asiancontinent and the adjacent oceans (Ding and Liu, 1998; Websteret al., 1998; Clift and Plumb, 2008). However, the underlyingforcingmechanisms of the changes in this temperature gradient arestill an open question. Both ice volume and solar-insolation changeshave been proposed to be possible driving forces of this tempera-ture gradient and thus the Asian monsoon circulations (Kutzbach,1981; Prell and Kutzbach, 1987, 1992; An et al., 1990; Liu andDing, 1993; Ding et al., 1995; Ding and Liu, 1998; Liu et al., 1999;Clemens and Prell, 2003; Tian et al., 2004; Yuan et al., 2004;Clemens et al., 2008; Kutzbach et al., 2008; Wang et al., 2008;Cheng et al., 2009). According to the ice volume forcing mecha-nism (Liu and Ding, 1993; Ding et al., 1995; Liu et al., 1999; Tianet al., 2004), insolation is just an initial factor in driving gla-cialeinterglacial cycles, but not a major forcing factor in driving theorbital cycles of monsoon. This mechanism links the monsooncycles mainly to the ice-volume cycles. In response to an increasedice volume during glacial stages, the winter monsoon wouldstrengthen, whereas the summer monsoon would weaken. In
Fig. 7. (A) 65�N summer insolation (Laskar et al., 2004), (B) Hm/Gt monsoon record from the ODP Site 1143 (Zhang et al., 2007, 2009) expressed on the T2002 timescale (Tian et al.,2002), (C) Hm/Gt monsoon record from the ODP Site 1143 (Zhang et al., 2007, 2009) expressed on the A2011 timescale, (D) Composite Chinese stalagmite d18O record (from theSanbao, Linzhu, Dongge and Hulu caves) (Wang et al., 2001, 2008; Cheng et al., 2009), (E) Benthic d18O record from the ODP Site 1143 (Tian et al., 2002) expressed on the A2011timescale. Numbers arranged on the benthic d18O curves represent the marine isotope stages.
H. Ao et al. / Quaternary Science Reviews 30 (2011) 1560e15751570
response to a decreased ice volume during interglacial stages, thewinter and summer monsoon would weaken and strengthen,respectively. In contrast, the solar-insolation mechanism(Kutzbach, 1981; Prell and Kutzbach, 1987; An et al., 1990; Yuanet al., 2004; Ruddiman, 2006a; Kutzbach et al., 2008; Wang et al.,2008; Cheng et al., 2009) directly links the changes in thetemperature gradient between land and ocean and in turn theAsian monsoon to changes in the Northern Hemisphere summerinsolation. According to this view, the summer monsoon wouldstrengthen in pace with increases in insolation rather than onlyduring each interglacial stage. For example, the Chinese stalagmited18O data reveal an intensified monsoon for the late parts of MIS 6,8 and 10, which correspond to insolation maxima within glacialstages (Fig. 7). Our tuned Hm/Gt and d18O records from the ODP Site1143 reveal significant differences between the Asian summermonsoon cycles and the glacialeinterglacial climate rhythms(Fig. 6). Some strong summer monsoon intervals appear to havealso occurred during glacial stages in addition to their increasedoccurrence during interglacial stages. Vice versa, some notablyweak summer monsoon intervals also occurred during interglacialstages next to their anticipated occurrence during glacial stages(Fig. 6). This indicates that the ice-volume mechanism to explainsummer monsoon intensification during interglacial stages andweakening during glacial stages may be overly simplistic for South
China. As indicated by our study, the orbital cycles in low-latitudehydrological/atmospheric circulation are reasonably independentfrom thewaxing and waning rhythms of the global ice volume. Thisfinding may have profound ramifications for understanding of theforcing mechanism of Asian monsoon changes over orbital scales,as well as the monsoon regions’ ecology and hydrology.
With the T2002 timescale, it is difficult to investigate the orbitalchanges of the Hm/Gt record during the last 5 Myr (Fig. 2).However, our refined timescale enables an analysis of this issue.The benthic d18O records of the ODP Site 1143 and LR04 stack(Lisiecki and Raymo, 2005) show consistent orbital evolutionduring the last 5 Myr (Fig. 8A and B). Before ca 2.8 Ma the recordshave a weak obliquity signal (compared to the interval from 2.8 Mato present), which intensifies significantly after ca 2.8 Ma. Till ca1 Ma, obliquity is the dominant periodicity, with a notably strongexpression of the eccentricity periodicity between 2 and 2.8 Ma.After ca 1 Ma, eccentricity becomes the dominant periodicityinstead. Precession signal is very poor throughout the entire last5-Myr record. Unlike the benthic d18O records, the Hm/Gt recordfrom the ODP Site 1143 is characterized by precession cyclicityacross the last 5 Myr, with a less important signal of obliquity(Fig. 8c). Both precession and obliquity have slightly stronger powerafter about 3 Ma, whereas strong eccentricity signals only occur atabout 0.7 Ma and between 2.3 and 2.8 Ma (Fig. 8C).
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The shift of dominant periodicity from w41-kyr tow100-kyr atabout 1 Ma revealed by the benthic d18O data (Fig. 8A and B) isknown as the mid-Pleistocene transition (MPT), which has beenwidely detected in the marine and continental climate records (seeClark et al., 2006 and references therein). The occurrence of MPTchallenges a fundamental tenet of traditional Milankovitch theory,which states that the 65�N summer insolation at the periods ofprecession and obliquity forces the global climate changes(Milankovitch, 1941; Hays et al., 1976). The enigma of the MPT isthat it involved the emergence of w100-kyr cycles when theinsolation does not show a shift of periodicity and the forcing at theeccentricity periodicity is distinctly weaker than the power atprecession and obliquity. Therefore, it is difficult to explain theMPTby direct insolation forcing, which is an unresolved issue ofMilankovitch theory. It is important that our tuned Hm/Gt recorddoes not show the MPT, with a rather strong precession anda comparatively weak eccentricity during the late Pleistoceneinstead (Fig. 8C). Consistent with this, low-latitude Asian summermonsoon inferred from the composite Chinese stalagmite d18Orecord (Wang et al., 2001, 2008; Cheng et al., 2009) (Fig. 7D), thepollen record from Lake Biwa in Japan (Nakagawa et al., 2008) andthe clay mineral record from the South China Sea (Boulay et al.,2005) was also dominated by precession during the late Pleisto-cene, with a rather weak eccentricity periodicity. This implies thatthe MPT may be not recorded in low-latitude Asian monsoon thatresponds directly to insolation, consistent with the Milankovitchtheory. Although the benthic d18O record from the same core showsthe MPT, it is interpreted as being a signal of high-latitude climate,i.e. the high-latitude ice volume. Our spectral analyses on theterrigenous dust flux in marine sediments from the Arabian Sea(ODP Sites 721/722) (deMenocal et al., 1991; deMenocal, 1995), thesubtropical West Africa (ODP Site 659) (Tiedemann et al., 1994) andthe eastern Mediterranean Sea (ODP Site 967) (Trauth et al., 2009),which reflect climatic changes in low-latitude Africa during thePlio-Pleistocene, do not reveal a clear MPT as well (Fig. 8DeF). Assuggested by the previous studies, the MPT was mostly found inhigh- and middle-latitude climate records and in (sub-) tropicalmarine climate records (Ruddiman et al., 1989; Mudelsee andSchulz, 1997; Schmieder et al., 2000; Heslop et al., 2002; Medina-Elizalde and Lea, 2005; Clark et al., 2006; Liu et al., 2008).However, most of the (sub-)tropical marine climate records, such asthe records of benthic d18O, sea surface temperature and sea level,were actually determined by the high-latitude ice volumedynamics (Hays et al., 1976; Imbrie et al., 1984; Ruddiman, 2003,2006a,b; Liu and Herbert, 2004). This combined evidence seemsto imply that the MPT may reflect processes in high-latitudeclimate. Its occurrence is therefore possibly restricted to high-and middle-latitude climate records and some climate recordsretrieved from low-latitudes but which are significantly influencedby high-latitude ice sheet dynamics. In low-latitude monsoonalclimate, which varies dominantly and directly in response tochanges in insolation with little influence from high-latitude icedynamics, the MPT likely did not occur. This finding has profoundimplications for solving the MPT enigma. It supports the proposi-tion that the MPT may be a consequence of long-term cooling of
Fig. 8. Orbital evolution of the Plio-Pleistocene Asian summer monsoon, African climate andstack and the climate records from the ODP Sites 1143, 721/722, 659 and 967. (A) LR04 stackHm/Gt (Zhang et al., 2007, 2009) records from the ODP Site 1143 plotted on the A2011 timdeMenocal, 1995), 659 (Tiedemann et al., 1994) and 967 (Trauth et al., 2009). The evolutiGrinsted et al. (2004). The stronger periodicities have darker red colors (higher intensity),enclosed by thick contours are calculated confidence levels with a significance of >95% usingby edge effects of the time series are plotted with lighter shades at the beginning and end olegend, the reader is referred to the web version of this article.)
high-latitude climate, especially the increasing global ice volume(Clark et al., 2006; Raymo et al., 2006; Lisiecki and Raymo, 2007).
5.2. Long-term evolution of the Asian monsoon
The ODP Site 1143 Hm/Gt record shows a long-term increasingtrend from 2.8 Ma to the present (Fig. 6), which indicates a long-term decreasing trend in low-latitude Asian summer monsoonintensity. This long-term trend in the Asian summer monsoon hasalso been revealed bymineral magnetic and geochemical studies ofthe Chinese loess and the Nihewan fluvio-lacustrine deposits fromNorth China (Chen et al., 2001; Deng et al., 2005, 2006; Ao et al.,2009, 2010c; Ao, 2010). When compared with the benthic d18Odata from the ODP Site 1143 and the LR04 stack (Fig. 6), the declinedtrend in the summer monsoon intensity during the past 2.8 Myr isconsistent with the onset of Northern Hemisphere glaciation atabout 2.8 Ma (Shackleton et al., 1984; Raymo, 1994). Before 2.8 Ma,the global ice volume was dominated by the Southern Hemisphereice sheets. However, with the occurrence of sustained majorNorthern Hemisphere glaciation since 2.8 Ma, the NorthernHemisphere ice sheets became dominant instead (Shackleton et al.,1984). In response to the increasing importance of the sustainedNorthern Hemisphere ice sheets, a long-tem decreasing trend inthe Asian summer monsoon intensity is expected (Prell andKutzbach, 1992; An et al., 2001; Clift and Plumb, 2008).
After 2.8 Ma, the Hm/Gt record also has increased amplitudes inits variation upon its long-term increasing trend (Fig. 6), whichimplies that a larger difference between peaks (stronger monsoon)and troughs (weaker monsoon) is superimposed on the overallweakening trend in summer monsoon intensity. This increasedamplitude variability of the summer monsoon may be linked to theoccurrence of sustained major ice sheets in the Northern Hemi-sphere after 2.8 Ma as well, because general-circulation-modelsimulations suggest that the increased ice volume would increasethe sensitivity of the Asian monsoon system, which would in turnincrease the oscillation amplitude of the monsoon (Prell andKutzbach, 1992; deMenocal and Rind, 1993). In addition, the Hm/Gt ratios show an abrupt decrease at ca 2.9e2.8 Ma (Figs. 5 and 6),which presumably suggests an abrupt intensification of the Asiansummer monsoon at this time interval.
6. Conclusions
We have formulated a refined 5-Myr astronomical timescale forthe ODP Site 1143 by calibration of the benthic d18O record to theLR04 stack, and the hematite/goethite ratio, a good estimate of theAsian summer monsoon intensity, to the 65�N summer insolation.It is the strong contribution from the precession signal in the Hm/Gtmonsoon proxy that helps to refine the timescale of the ODP Site1143. In comparison with the T2002 timescale, which was gener-ated by tuning the single benthic d18O record to the orbital obliquityand precession (Tian et al., 2002), our combined tuning of monsoonand isotope curves for the development of the present A2011timescale results in a more accurate and robust chronology.
global ice volume suggested by evolutionary spectral analysis of the LR04 benthic d18Oof benthic d18O (Lisiecki and Raymo, 2005). (B) benthic d18O (Tian et al., 2002) and (C)escale. (E) Terrigenous dust records from ODP Sites 721/722 (deMenocal et al., 1991;
onary spectral analysis was performed by the Matlab software package developed bywhile the weaker periodicities have lighter blue colors (lower intensity). The regionsa red-noise process with a lag-1 autocorrelation coefficient. Regions that are influencedf the frequency-time plane. (For interpretation of the references to colour in this figure
H. Ao et al. / Quaternary Science Reviews 30 (2011) 1560e1575 1573
Comparison of the parallel Hm/Gt and d18O data from the ODPSite 1143 suggests considerable differences between fluctuations inAsian summer monsoon intensity and the glacialeinterglacialclimate cyclicity. This indicates that the popular view of summermonsoon intensification during interglacial stages and weakeningduring glacial stagesmay be overly simplistic for low-latitude areas.The well-knownMPT is only identified in the d18O record but not inthe Hm/Gt record from the same core. This indicates that the MPTmay be a feature of high- and middle-latitude climates that is‘inherited’ in this core. The expression of the MPT is less likely tooccur in low-latitude monsoonal climates, in which the orbitalvariations are more directly forced by insolation. In addition, low-latitude Asian summer monsoon intensity shows a long-termdecreasing trend since 2.8 Ma with increased oscillation ampli-tude, which is presumably linked to the development of sustainedmajor ice sheets in the Northern Hemisphere after 2.8 Ma. Thepresent study has ignored other possible influences on the Hm/Gtrecord such as the changes in provenance. Future more detailedstudies such as reconstruction of long-term high-resolution climaterecords from South China Sea and other continental areas of low-latitude Asia and climate-model simulations are crucial fortesting our conclusions.
Acknowledgments
We thank Profs. M. Raymo, Y.B. Sun and Q.S. Liu for reading anearlier version of this paper; their useful suggestions were grate-fully implemented. We also thank the editor and three anonymousreviewers for their insightful comments, which significantlyimproved this paper. We are grateful to Y.G. Zhang, J. Tian, P.B.deMenocal, J.C. Larrasoaña and R. Tiedemann for providing accessto their published climate records, which were used in the presentstudy. This study was supported by the Key Projects of NationalBasic Research Program of China (Grant 2010CB833400), Innova-tion Program of Chinese Academy of Sciences (KZCX2-YW-Q09-04),National Natural Science Foundation of China (Grant 40921120406)and West Light Foundation of Chinese Academy of Sciences.
Appendix. Supplementary data
Supplementary data related to this article can be found online atdoi:10.1016/j.quascirev.2011.04.009.
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