Variability of the Asian Summer Monsoon Printer-friendly Version Interactive Discussion Climate of the past Variability of the Asian Summer Monsoon during the Penultimate Glacial/interglacial Period Inferred from Stalagmite Oxygen Isotope Records from Yangkou Cave, Chongqing, Southwestern China Cpd 

Discussions This discussion paper is/has been under review for the journal Climate of the Past (CP). Please refer to the corresponding final paper in CP if available. Abstract The orbital-timescale dynamics of the Quaternary Asian summer monsoons (ASM) are frequently attributed to precession-dominated Northern Hemisphere summer inso-lation. However, this ASM variability is inferred primarily from oxygen isotope records of stalagmites, mainly from Sanbao cave in mainland China, and may not provide a com-5 prehensive picture of ASM evolution. A new spliced stalagmite oxygen isotope record from Yangkou cave tracks summer monsoon precipitation variation from 124–206 thousand years ago in Chongqing, southwest China. When superimposed on the Sanbao record, the Yangkou-inferred precipitation time series is shown to support the strong ASM periods at marine isotope stages (MIS) 6.3, 6.5, and 7.1 and weak ASM inter-10 vals at MIS 6.2, 6.4, and 7.0. This consistency confirms that ASM events affected most of mainland China. We show that change in glacial/interglacial (G/IG) ASM intensity was also governed by the Walker Circulation by combining our results with published paleo-Pacific thermal and salinity records. One of the strongest ASM events over the past fiver G/IG cycles, at MIS 6.5, was enhanced by such zonal forcing associated with 15 prevailing trade winds in the Pacific.


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Full Our current understanding of ASM variation over past 500 kyr BP (before 1950 AD) has been reconstructed using oxygen isotope records of Chinese stalagmites (Wang et al., 2008;Cheng et al., 2012b) with the advantages of absolute and high-precision chronologies (e.g.Cheng et al., 2000Cheng et al., , 2013;;Shen et al., 2002Shen et al., , 2012)).Stalagmiteinferred orbital-scale ASM intensity closely follows the change in precession-dominated Northern Hemisphere (NH) summer insolation (NHSI) (Wang et al., 2008;Cheng et al., 2012b).However, these 100s-kyr records were mainly from a single cave, namely Sanbao cave, located in Hubei Province, China (Fig. 1; Wang et al., 2008;Cheng et al., 2012b).Utilizing only one site leads to uncertainties in the spatial extent of Quaternary ASM evolution.These uncertainties stem from differences in local or regional climatic and environmental conditions (Lachniet, 2009), hydrological variability of monsoonal sources (e.g.Dayem et al., 2010;Clemens et al., 2010;Pausata et al., 2011), and interactions between climatic subsystems (e.g.Maher and Thompson, 2012;Tan, 2013).Sanbao records, for example, show distinct ASM events at marine isotope stages (MIS) 6.3 and 6.5 during the penultimate glacial time and a weaker summer monsoon during the penultimate glacial maximum (PGM) at MIS 6.2 (Fig. 1 of Wang et al., 2008).To clarify whether this combination of weak PGM ASM intensities and strong ASM events during the penultimate glacial/interglacial (G/IG) period are local effects, we built an integrated stalagmite oxygen stable isotope record from Yangkou cave, Chongqing, China, covering 124-206 kyr BP (Fig. 1).Comparison with records from other Chinese caves would confirm the fidelity of Sanbao cave-inferred ASM intensities.Our results also demonstrate that a strong Walker circulation in the Pacific could enhance glacial ASM precipitation.Introduction

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Full 2 Material and methods

Regional settings and samples
Stalagmites were collected from Yangkou cave (29  (Wang et al., 2008).The cave air temperature is 7.5 • C and the average relative humidity is > 80 % (October 2011-October 2013).The regional climate is dominated by the AM and annual rainfall is 1400-1500 mm, 83 % from April to October (Zhang et al., 1998).Five stalagmites, YK05, YK12, YK23, YK47 and YK61, which formed within a time interval of 120-210 kyr BP were halved and polished for 230 Th dating and oxygen stable isotope analysis.

230 Th dating
Chemistry and instrumental analysis were conducted in the High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University.Fifty three powdered subsamples, 60-80 mg each, were drilled from the polished surface along the deposit lamina of the five stalagmites (Fig. 2, Table 1), on a class-100 bench in a class-10 000 subsampling room.Uranium-thorium (U-Th) chemistry (Shen et al., 2003) was performed in a class-10 000 clean room with independent class-100 benches and hoods (Shen et al., 2008).A multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Thermo Fisher Neptune, with secondary electron multiplier protocols, was used for the determination of U-Th isotopic contents and compositions (Shen et al., 2012).All errors of U-Th isotopic data and 230 Th dates are two standard deviations (2σ) unless otherwise noted.Introduction

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Stable isotopes
Five-to-seven coeval subsamples, 60-120 µg each, were drilled from one layer per stalagmite to measure the oxygen and carbon isotopic compositions as part of the socalled "Hendy Test" (Hendy, 1971).To obtain oxygen time series, 604 subsamples, 60-120 µg each, were drilled at 0.5-3.0mm intervals along the maximum growth axis.
Measurement of oxygen stable isotopes was performed by two isotope ratio mass spectrometers, including a Finnigan Delta V Plus in the Southwest University, China and a Micromass IsoPrime instrument at the National Taiwan Normal University.Oxygen isotope values were reported as δ 18 O (% ) with respect to the Vienna Pee Dee Belemnite standard (V-PDB).An international standard, NBS-19, was used in both laboratories to confirm that the 1-sigma standard deviation of δ 18 O was better than ±0.1 % .

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Yangkou oxygen isotope data
The well-known Hendy Test has been taken as an essential requirement when assessing the ability of stalagmites to serve as paleoclimate archives (Hendy, 1971) (Fig. 4).Despite relative large δ 13 C variations of 0.2-0.4% (1σ) for coeval subsamples on the five selected layers (Fig. 4a), only a small variations in δ 18 O of ±0.1-0.2 % (1σ) are observed on individual horizons of coeval subsamples (Fig. 4b).Also, there is no relationship between δ 18 O and δ 13 C values, which is an additional part of the Hendy Test (Fig. 4c).The replication of the δ 18 O records both within Yangkou cave (Fig. 5) and between Chinese caves (Fig. 6), as well as successful Hendy Tests, indicates that the stalagmites formed under an oxygen isotopic equilibrium condition.The Yangkou stalagmite δ 18 O data therefore represent rainfall oxygen isotopic change, which is a reflection of regional hydrological variability in the AM territory (e.g.Wang et al., 2001Wang et al., , 2008;;Cheng et al., 2009;Li et al., 2011).The oxygen isotope sequences for all of the Yangkou stalagmites are illustrated in Fig. 5a.The spliced record covers a time interval from 124-206 kyr BP, with three narrow hiatuses at 132. 1-133.5, 190.4-193.2, and 200.3-200.9

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Over the past 200 kyr BP, the weakest ASM interval has been suggested to be at MIS 6.2 in the Sanbao records (Wang et al., 2008).For example, the δ 18 O data are 1 % higher than those at weak ASM intervals of MIS 6.4, 7.0, and 7.2 (Fig. 5).Concurrence between ASM records and ice-rafted debris events in the North Atlantic supports the hypothesis of a forcing on the ASM from NH high-latitudes (Cheng et al., 2009).δ 18 O values at MIS 6.2 in Yangkou record are 1.5-2 % higher than those at MIS 6.4, 7.0, and 7.2 (Fig. 5).This large difference suggests that this event in Chongqing may have been relatively intensified through NH forcing as compared with the Hubei regions during the PGM.
The Sanbao record indicates that the strongest ASM condition over the past 500 kyr BP occurs in MIS 6.5 (Cheng et al., 2012b).This ASM event, lasting 13 kyr, is 3 kyr longer than a comparable event (in terms of intensity) at interglacial MIS 5.3, and was stronger than at any time during MIS 1, 5.5, 7.3, 9.5, and 11.3, which had higher sea-levels and thermal conditions (Fig. 1 of Cheng et al., 2012b).The lowest contemporaneous δ 18 O data in the Yangkou record (Fig. 5) show a similar ASM intensity at MIS 6.5 in southwest China.
Overall, consistency of the stalagmite δ 18 O sequences between Yangkou and other Chinese caves supports the idea that ASM intensity primarily follows NHSI on orbital timescales and is driven by precessional forcing and is punctuated by NH high-latitude climatic fluctuations (e.g.Wang et al., 2001Wang et al., , 2008;;Cheng et al., 2009).Agreement in the amplitude and the transition of δ 18 O dynamics during different MIS also confirms that the Sanbao stalagmite-inferred ASM events, including a very weak one at MIS 6.2 Introduction

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Full and the strongest one at MIS 6.5, are predominant over the entire mainland during the past five G/IG cycles (Cheng et al., 2012b) (Fig. 6).

Forcings for the abnormal strong ASM at MIS 6.5
The extraordinarily strong ASM condition at MIS 6.5 during the penultimate glacial period is one of the most striking features revealed by new Chinese cave records (Fig. 5).Wang et al. (2008) found a correlation between the stalagmite-inferred ASM intensity and the atmospheric δ 18 O records from Antarctic Vostok ice core O 2 bubbles (Sowers et al., 1991;Petit et al., 1999), and suggested that the Dole effect (Dole, 1936;Bender et al., 1994) (Kroopnick and Craig, 1972).
This inequality is caused by respiration, which prefers depleted 16 O 2 rather than 18 O 2 , and is balanced by photosynthesis, which emits oxygen with the δ 18 O value equal to the water used in the reaction (Guy et al., 1989), resulting in a net decrease of the δ 18 O atm value.Accordingly, the δ 18 O atm variation can depend on the activity of photosynthesis.Vostok ice core-inferred δ 18 O atm evolution most likely results from changes of summer insolation and precipitation in NH, where land provides space for the growth of vegetation and intense photosynthesis during glacial periods (Sun et al., 2000).The summer insolation at MIS 6.5 is less than the interglacial periods at MIS 5.5 and 7.3 (Fig. 5), suggesting that the strong ASM intensity should be associated with additional forcing(s).Climate conditions around Yangkou and Sanbao caves are influenced by the Indian summer monsoon (ISM) and East Asian summer monsoon (EASM) (Fig. 1).The ISM, a typical tropical monsoon system, is driven by a south-north land-sea thermal gradient; Introduction

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Full instead, the EASM is controlled by both south-north and east-west land-sea gradients (Wang and Lin, 2002).The EASM precipitation is influenced by the Northwestern Pacific Tropical High, developed by the mainland-Pacific thermal gradient (Wang et al., 2003).The Pacific climatic variability can, therefore, affect EASM precipitation (Tan, 2013).
Cai et al. ( 2010) and Jiang et al. (2011) argued for a significant impact of the western tropical Pacific sea surface temperature (SST) on the EASM precipitation.They proposed that the evolution and spatial asynchroneity of stalagmite-inferred Holocene precipitation histories at different AM regions could be attributed to SST changes in the western Pacific.Planktonic foraminiferal-inferred SST records of the marine sediment core ODP806B (0 • 19 N, 159 • 22 E) in the western Pacific warm pool (WPWP) and TR163-19 (2 • 16 N, 90 • 57 W) in the eastern equatorial Pacific (EEP) (Lea et al., 2000) are plotted in Fig. 6, along with the LR04 stacked benthic δ 18 O sequence (Lisiecki and Raymo, 2005) and Yangkou and Sanbao cave time series.A SST gradient between the WPWP and EEP during the glacial periods of MIS 6 and 8 is 2 • C, larger than 0.5-1.5 • C gradient during the warm interglacial windows of MIS 5.5 and 7 (Fig. 6).Combined with salinity gradient data, Lea et al. (2000) suggested that the transport of water vapor to the western Pacific was enhanced during glacial times.This large SST gradient could result in an enhanced Walker circulation in the Pacific, similar to the modern La Niña state, which moves the rainfall zone westward and intensifies EASM precipitation (Clement et al., 1999) (Fig. 1).Under a weak Walker circulation, analogous to present El Niño conditions, the rainfall zone in the Pacific migrated eastward and EASM precipitation was reduced (Clement et al., 1999).The extremely strong EASM precipitation at MIS 6.5 was not only governed by high NHSI, but also enhanced by the Pacific SST gradient.
Our arguments are also supported by modern meteorological observations (e.g.Xue et al., 2007;Tan, 2013) and decadally-resolved marine records (Oppo et al., 2009).La Niña years accompany above-normal precipitation probabilities above normal in mainland China (Tan, 2013 and references therein).Two thousand year-SST and Introduction

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Full -salinity records from the Makassar Strait (Oppo et al., 2009) also support a strong link between Pacific Ocean climate and the AM.

Conclusions
In this study, our new spliced δ 18 O record of five stalagmites from Yangkou cave, Chongqing, exhibits ASM variability over the time period during 124-206 kyr BP.The prominent consistency between the Yangkou and previous Chinese cave δ 18 O sequences confirms the duration and intensity of the enclosed ASM events in the entire mainland.Our data supports the hypothesis that the ASM change primarily follows NHSI on a precessional orbital timescale.The weakest ASM condition during low-insolation MIS 6.2 was influenced by meridional forcing originating from the North Atlantic.The strongest ASM intensity in the past 500 kyr BP, at MIS 6.5 (Cheng et al., 2012b), was partially due to zonal forcing associated with G/IG dynamics of Walker circulation in the Pacific.Introduction

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