RESEARCH LETTER A High‐Resolution Speleothem Record of Marine Isotope 10.1029/2019GL083836 Stage 11 as a Natural Analog to Asian Key Points: • A new high‐resolution record of the Summer Monsoon Variations Asian summer monsoon is Xinnan Zhao1 , Hai Cheng1,2 , Ashish Sinha3, Haiwei Zhang1, Jonathan L. Baker1 , reconstructed over Marine Isotope 4 4 4 2 1 Stage 11 as a natural analog to the Shitao Chen , Xinggong Kong , Yongjin Wang , R. Lawrence Edwards , Youfeng Ning , Holocene and Jingyao Zhao1 • The Marine Isotope Stage 11 record with a chronology constrained by 1Institute of Global Environmental Change, Xi'an Jiaotong University, Xi'an, China, 2Department of Earth Sciences, annual band counting shows climate University of Minnesota, Minneapolis, MN, USA, 3Department of Earth Sciences, California State University, Dominguez variability similar to the Holocene 4 • The projection of the Marine Isotope Hills, Carson, CA, USA, College of Geography Science, Nanjing Normal University, Nanjing, China Stage 11‐Holocene comparison into the future suggests a weakening trend for the Asian summer Abstract A full‐spectrum characterization of past climate is a necessary prerequisite for the monsoon detection and attribution of climate changes during the current interglacial. Here we present a speleothem record of Asian summer monsoon (ASM) during Marine Isotope Stage (MIS) 11 interglacial Supporting Information: • Supporting Information S1 (MIS 11c), from Yongxing cave, China. The record's unprecedented chronologic constraints and • Table S1 decadal‐scale temporal resolution allow a precise and direct comparison of ASM between the MIS 11c and the Holocene. Our data suggest that orbital‐centennial patterns of ASM were remarkably similar during both interglacial, including their pacing and structure. Notably, a multi‐millennial stronger monsoon late in Correspondence to: MIS 11c, the “Late‐MIS 11c shift,” is similar to the Late Holocene strengthening of the ASM, the “2‐Kyr H. Cheng, ” ‐ [email protected] shift. Thus, the multicentennial ASM weakening at the end of the Late MIS 11c shift could imply that the current century‐long ASM waning trend may persist into the future, if only natural forcings are considered.

Citation: Plain Language Summary The interglacial of Marine Isotope Stage 11 (MIS 11c) is generally Zhao, X., Cheng, H., Sinha, A., considered to be an appropriate natural climate analog to the current interglacial, the Holocene. A new Zhang, H., Baker, J. L., Chen, S., et al. ‐ δ18 (2019). A high‐resolution speleothem speleothem oxygen isotope ( O) record is reconstructed to characterize Asian summer monsoon (ASM) record of Marine Isotope Stage 11 as a variability across MIS 11c on a wide range of timescales. This new record has an unprecedented high natural analog to Holocene Asian resolution (~11 years) and precise chronology constrained by 230Th dating and annual band counting. It thus summer monsoon variations. fi Geophysical Research Letters, 46, allows for the rst time a precise comparison of ASM variability between MIS 11c and the Holocene. We 9949–9957. https://doi.org/10.1029/ find that the observed pattern of decreasing ASM strength during the late portion of MIS 11c was interrupted 2019GL083836 by a multi‐millennial period of stronger monsoon (the “Late‐MIS 11c shift”), which is similar to the Late Holocene increase in ASM strength (the “2‐Kyr shift”), which also occurred against a backdrop of Received 29 MAY 2019 ‐ ‐ Accepted 14 AUG 2019 long term decline in Northern Hemisphere summer insolation. Notably, the Late MIS 11c shift was Accepted article online 20 AUG 2019 terminated by a multicentennial ASM weakening trend, thus suggesting that the current century‐long ASM Published online 28 AUG 2019 waning trend could persist into the future, provided that the anthropogenic forcing does not supersede the natural variability.

1. Introduction A precise characterization of climate variability during the past Quaternary interglacial episodes can have an important bearing on our understanding of the past, present, and future evolution of the current interglacial period, the Holocene (Candy et al., 2014; PAGES, 2016; Tzedakis et al., 2009). For example, it remains unclear whether the observed Asian summer monsoon (ASM) weakening trend during the last century, which is clearly evident in instrumental (Ding et al., 2008) and cave oxygen isotope (δ18O) records from the ASM region (Li et al., 2017; Zhao et al., 2018), is a manifestation of anthropogenic forcings such as aero- sols (Li et al., 2016) or greenhouse gases or whether it results from natural variability. In this context, the challenge lies in finding appropriate past interglacial analogs for the Holocene and archives that can be sampled with sufficient temporal resolutions and be placed on a precise chronological framework to allow for an analytically meaningful comparison between previous interglacial periods and the Holocene. In this regard, evaluation of ASM variability (Cheng et al., 2016) between the current interglacial

©2019. American Geophysical Union. period MIS 1 (or Holocene) and interglacial of MIS 11, the longest interglacial period of the last 500,000 years All Rights Reserved. and among the best candidates for a Holocene analog (Berger & Loutre, 2002; Candy et al., 2014; EPICA

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Community Members, 2004; Herold et al., 2012; Koutsodendris et al., 2012; Loutre & Berger, 2003; Pol et al., 2011; Ruddiman et al., 2016), constitutes a frontier of scientific research. Previous studies have demonstrated that δ18O in Chinese speleothems closely tracks Northern Hemisphere summer insolation (NHSI) on a precessional rhythm while recording quasiperiodic centennial‐ to millennial‐scale climate fluctuations during both glacial and interglacial periods (Cheng et al., 2006; Cheng, Fleitmann, et al., 2009; Cheng, Edwards, et al., 2009; Cheng et al., 2016; Dykoski et al., 2005; Wang, 2005; Wang et al., 2001, 2008). We propose, therefore, that Chinese speleothem records of past inter- glacial periods may potentially serve as a suitable paleoanalog to natural Holocene climate variability. Prior to our data set, the Sanbao Cave speleothem record had been the only speleothem‐based reconstruction of ASM variability in China that covered MIS 11 (Cheng et al., 2016). However, the quality of this time series is limited by a relatively low temporal resolution and large age uncertainties during MIS 11, which preclude a precise characterization of ASM variability on suborbital time scales for comparison to the much higher‐ resolution Holocene cave records. Herein we present a new δ18O record from Yongxing cave, which overcomes these limitations. Importantly, our record has an exceptionally high temporal resolution (decadal‐scale) and precise chronology that is unprecedented in the literature to date. The latter is constrained both by 230Th dates and annual‐band counting via Confocal Laser Fluorescent Microscopy (CLFM), thereby allowing for a direct comparison between MIS‐11 and Holocene climate variability without the need for downsampling either data sets.

2. Materials and Methods 2.1. Materials Yongxing Cave (31°35′N, 111°14′E, elevation 700 metres above sea level) is located in central China, within the ASM domain (supporting information Figure S1). Precipitation occurs mainly during boreal summer, when the ASM advances over the Asian continent. Mean annual precipitation and temperature in the study area are ~900 mm and 14.6 °C, respectively. Yongxing Cave is composed of two levels connected by a narrow chimney so that the upper level of the cave is poorly ventilated with constant relative humidity close to 100%. Stalagmite sample YX50 was collected from the upper level, ~110 m from the cave entrance (Figure S2).

2.2. 230Th dating Subsamples for dating were obtained by drilling the polished stalagmite section along the growth axis with a carbide dental burr (Figure S3). Dating work was performed at the University of Minnesota (USA) and Xi'an Jiaotong University (China). We used identical standard chemistry procedures between the two laboratories to separate U and Th for geochronological analysis (Edwards et al., 1987). U and Th isotopes were measured using Thermo‐Finnigan Neptune/Neptune plus multicollector inductively coupled plasma mass spectro- meters, equipped with Faraday cups (for some of U measurements) or a MasCom multiplier behind the retarding potential quadrupole in peak‐jumping mode. A more detailed summary of the methodology employed here, as well as Th and U decay constants, is available from earlier studies by Cheng et al. (2000, 2013). A total of 12 230Th ages was obtained, and all ages are in stratigraphic order with 2–σ uncertain- ties of ~1.66% to 0.46% (Table S1 and Figure S4).

2.3. CLFM YX50 was cut into smaller sections, suitable for the microscope bracket (30–35 mm long each). Images of fluorescent bands were obtained by Nikon A1+ CLFM at Xi'an Jiaotong University. A total of 47 images were obtained, and each image was integrated by overlapping photos collected via a setting of 10X objective lens and 10X eyepiece. The focal‐plane depth, laser power, and signal gain were held constant (Figure S5).

2.4. Annual Band Counting and Uncertainties Annual bands are identified as paired dark‐bright fluorescence laminae via CLFM described in earlier stu- dies (Zhao & Cheng, 2017). We followed a band‐counting approach similar to that used for a ice core (Rasmussen et al., 2006) to establish a relative age model. The defined annual bands were counted and then used for estimates of the total number of seasonal couplets (or years) per millimeter. Major uncer- tainties, however, are fourfold: (a) Sample growth is geometrically far from ideal and results in a complex stalagmite stratigraphy. For instance, the deposition center could change if the drip water position shifts

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slightly, which will potentially affect band counting along different lines (Figure S5). Pores can also result in large uncertainties, particularly at the bottom part of the sample (Figure S3). (b) There are “spatial gaps” at a number of locations along the growth axis, where sample sections became unsuitable for CLFM, due to prior subsampling and sample breaks. These gaps are ~2–5 mm wide each, with a cumulative gap length of ~70 mm. To account for missing bands in these gaps, we first attempted to shift the counting areas along courses parallel to the growth axis. Where this approach was not feasible, we estimated the number of missing bands by linear interpolation based on the number of bands in the intervals 1 mm prior and subsequent to each gap. (c) Insufficient spatial resolution sometimes resulted from the fact that CLFM images were taken at 100X magnification so that annual bands narrower than ~5 μm were difficult to identify. However, annual bands with widths of ~5 μm or less are rare in this sample. (d) Misinterpretation of annual bands can occur, despite that annual bands are ostensibly identifiable throughout the sample (Tan et al., 2006), which may introduce additional uncertainties, espe- cially when the bright fluorescence laminae—our counting mark—were too ambiguous to be definitively resolved. Overall, it is challenging to quantify accurately the aforementioned uncertainties. However, the counting results appear to agree well with the 230Th dating results within age‐model uncertainty (Figures S4 and S6). Our calculation for the initial age of the band‐counting datum follows a recent method (Domínguez‐ δ18 Figure 1. Chinese cave O records covering MIS 11. (a) The Sanbao cave Villar et al., 2012), which can anchor the floating age sequences obtained δ18 O record (Cheng et al., 2016) and 21 July insolation at 65°N (Laskar et al., 230 ‐ 2004). (b) The Yongxing cave δ18O record (YX50, this study) reconstructed by our band counting method to Th dates via a least squares regression by MOD‐age (Hercman & Pawlak, 2012) based on 12 230Th ages and method. The band‐counting chronology record is essentially consistent 21 July insolation at 65°N (Laskar et al., 2004). (c) Twelve 230Th dating within uncertainty with the record reconstructed by the MOD‐age results of YX50 with age error bars (2σ; Table S1). The strong monsoon (Hercman & Pawlak, 2012) based on 230Th ages (Figure S7). interval (III) during MIS 11 is synchronous with 11c Interglacial, but the 18 pattern of Chinese cave δ O records, which is parallel with insolation, but 2.5. Stable‐Isotope Analysis different with the marine records (Figure S10). MIS = Marine Isotope Stage. A total of 3,675 stable isotope (δ18O) subsamples was analyzed at Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, China, and Xi'an Jiaotong University, China (Table S2). The measurements in two laboratories were made on Thermo‐Finnigan MAT‐253 mass spectrometer fitted with a Kiel Carbonate Device III and IV, respec- tively. The δ18O values are reported in per mil (‰) deviations, relative to the Vienna Pee Dee Belemnite stan- dard. All subsamples were calibrated against the TTB1 standard, and the long‐term reproducibility for δ18O measurements over the course of this study was typically ~0.1‰ (1σ) or better.

2.6. Analyses of Weak Monsoon Events Singular Spectral Analysis was used to reconstruct a skeleton of the underlying dynamical system's structure (Ghil, 2002). For this analysis, we interpolated the Yongxing and Dongge records onto an evenly spaced time series (equivalent to average resolution) and then used singular spectral analysis to normalize the records (https://dept.atmos.ucla.edu/tcd/singular‐spectrum‐analysis‐ssa). In this approach, heavier δ18O excursions exceeding 0.5 (Figure S8) offset from the mean value in normalized cave δ18O records were marked as weak monsoon events (WMEs; Figure S9).

3. Results 3.1. Yongxing δ18O Record Over MIS 11 The Yongxing δ18O record covers a major part of MIS 11, including most of the MIS 11 interglacial (MIS11c), stadial MIS11b, and early portions of interstadial MIS11a (Figure S10). Although differing in resolution and amplitude, the Sanbao and Yongxing δ18O records are virtually identical over contemporaneous growth per- iods, providing a robust replication test (Figure 1) that suggests insignificant oxygen‐isotope fractionation during calcite formation (Figure S11). Furthermore, a recently published δ18O record from Yongxing cave

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(Figure S2) convincingly demonstrated that Yongxing cave δ18O records reflect primarily ASM variations on a wide range of timescales, which unambiguously correlate with global climate changes (Chen et al., 2016; Figure S12). Additionally, many previous cave δ18O studies from the same area (Chen et al., 2016; Cheng et al., 2016; Wang et al., 2001) have also demonstrated that lighter speleothem‐δ18O values generally indicate stronger ASM intensity and vice versa. Following the same reasoning, we interpret low and high δ18O values to refer to strong and weak ASM, respectively, consistent with the results from theoretical (Liu et al., 2014) and empirical studies (Cheng et al., 2016). Remarkably, our new Yongxing record has an unprecedented average temporal resolution of ~11 years (range from ~100 to 2 years) and precise age control, which for the first time, allow us to characterize the multidecadal‐millennial cli- mate variability over a large and critical portion of MIS 11 (Cheng et al., 2016). The Yongxing and Sanbao records exhibit four orbital‐scale peaks (labeled I to IV in Figures 1 and 2), which correspond to NHSI peaks at ~424, 407, 387 and 372 Kyr BP, respectively (Figure 2). Five prominent millennial events are also documented by the records in MIS 11a and 11b (labeled i to v in Figure 2). These observed orbital pattern and millen- nial events in Yongxing and Sanbao records are coherent with Antarctic 18 ice core δ Oatm (Severinghaus et al., 2009), CH4 (Loulergue et al., 2008), and δD (Jouzel et al., 2007), as well as a lacustrine record from North Asia (Prokopenko et al., 2006; Figures 2 and S9). Additionally, the 18 MIS 11 portion of Antarctic ice core δ Oatm record (Severinghaus et al., 2009) clearly manifests a four‐peak pattern that can be correlated with the cave records on an orbital scale (Figure 2) in accordance with their close link, established by previous studies (Cheng et al., 2006, 2016; Cheng, Edwards, et al., 2009). 3.2. MIS 11c as an Analog of the Holocene A global stack of marine benthic δ18O records (Dutton et al., 2015; Spratt & Lisiecki, 2016) suggests a sea level high stand during the MIS 11c inter- Figure 2. Comparison of Chinese cave δ18O records with other MIS 11 glacial (Figures 2 and S10). Ice core (EPICA Community Members, 2004, climate records. (a) The 21 July insolation at 65°N (Laskar et al., 2004). ‐ (b) The Sanbao record (Cheng et al., 2016). (c) The Yongxing record (green 2010; Loulergue et al., 2008; Lüthi et al., 2008; Masson Delmotte et al., line) in comparison with atmosphere δ18O data (pink line) (Severinghaus 2010; Petit et al., 1999; Pol et al., 2011) and terrestrial (lacustrine/loess/ et al., 2009). (d) to (i) are Atmospheric CH4 (Loulergue et al., 2008), Lake cave) records (An et al., 2011; Cheng et al., 2016; Hao et al., 2012; Baikal (Prokopenko et al., 2006), North Atlantic SST (Stein et al., 2009), Koutsodendris et al., 2010, 2012; Meckler et al., 2012; Prokopenko et al., Antarctic ice core δD (Jouzel et al., 2007), Atmospheric CO (Lüthi et al., 2 2006, 2010) further indicate that MIS 11c was a warm, high‐biomass 2008), and sea level (Spratt & Lisiecki, 2016), respectively. We use Termination V event to correlate all other records to our cave records via the interglacial period with high atmospheric CO2 and CH4, largely similar same strategies described in earlier studies (Cheng, Edwards, et al., 2009; to the Holocene (Figure 3). Considering its duration of ~30 Kyr, identify- Cheng et al., 2016) while keeping the relative ages of other records on their ing which portion of MIS 11c is the most appropriate analog to the original chronologies. I to IV show orbital peaks, and i to v show millennial Holocene remains a significant issue (Candy et al., 2014). The two events. Green and gray vertical bars indicate Termination V and MIS 11c, respectively. VPDB = Vienna Pee Dee Belemnite; SMOW = Standard Mean prevalent comparison schemes are based on aligning precession insola- Ocean Water; SST = sea surface temperature; MIS = Marine Isotope Stage. tion maximum and obliquity signal (or terminations; Berger & Loutre, 2002; Candy et al., 2014; Ruddiman et al., 2011, 2016). The former scheme mismatches ice age Termination I (T‐I) with T‐V, while the latter correlates the Holocene to a portion of MIS 11c that has a dissimilar NHSI pattern and thus an incongruent ASM pattern (Figure S13). In light of the view that the ASM is largely driven by insolation forcing on orbital timescale (Cheng et al., 2016), we suggest that the MIS 11‐Holocene matching based on NHSI would be more appropriate for comparing the Holocene with the late portion (ca. 11 Kyr) of MIS 11c (Figure S13). Our reasoning for using these comparison criteria is based on a number of previous observations that are briefly summarized as follows: (a) Changes in NHSI during the Holocene and the late portion of MIS 11c are almost equivalent, with both periods characterized by an initial high NHSI for ~3 Kyr, followed by a gradual decreasing trend (~40 W/m2 as referred from 21 July insolation at 65°N) over the subsequent ~6 Kyr (Figure 3); and (b) the initial climate conditions

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during the early portion of the late MIS 11c and the early Holocene are

comparable with typical interglacial CH4 and CO2 values and high sea‐ level stand. In addition to broad similarities between the MIS 11c and Holocene as noted above, there are also marked differences between the two periods. Our cave data show that the ASM gradually weakened during the late MIS 11c (~6 Kyr from ~408 to 402 Kyr BP) in tandem with decreasing

NHSI (Laskar et al., 2004), CH4 (Loulergue et al., 2008), Antarctic temperature (δD; Jouzel et al., 2007), and possibly CO2 (Lüthi et al., 2008) and sea level (Spratt & Lisiecki, 2016) and North Atlantic SST (Stein et al., 2009; Figure 2). In contrast, the climate trends since the mid‐Holocene are notably different. While the ASM intensity declines

concomitantly with NHSI until ~2 Kyr ago, atmospheric CO2 and CH4 show increasing trends starting at ~8 and ~5 Kyr ago, respectively, possibly due to anthropogenic forcing (Ruddiman et al., 2016). These observations, on the other hand, suggest that NHSI remains the dominant driver of the ASM, notwithstanding the gradual increase of atmospheric

CO2 and CH4 since the mid‐Holocene.

4. Paleoclimate Implications 4.1. ASM Events During the Holocene in the Context of MIS 11c Two salient aspects of our new record are its high resolution (~11 years) and a precise 230Th dated chronologic framework, which is additionally constrained by annual‐band counting that allows a precise comparison of ASM variability between the Holocene and MIS 11c. Previous studies of European pollen records have revealed an abrupt vegetation change during the MIS 11 interglacial (MIS 11c; Koutsodendris et al., 2012), which represents a North Atlantic climate event in MIS 11c analogous to the 8.2‐Kyr event in the Holocene (Cheng, Fleitmann, et al., 2009; Koutsodendris et al., 2012). Two additional abrupt cold events have also been recognized in MIS 11 by a high‐resolution lacustrine record from North Asia (Prokopenko et al., 2010). Furthermore, a temperature record from the North Atlantic (Kandiano et al., 2017) exhibits a transi- ent cold event at ~411 Kyr BP. Similarly, between ~418 and 396 Kyr BP during MIS 11c, our cave record is marked by 16 WMEs, namely, the Yongxing Events (YXEs; Figures 4 and S9). Notably, the YXEs show average pacing of ~1.4 Kyr and amplitude of ~0.8‰ (Table S3 and Figure 3. Holocene climate variations in the context of MIS 11. (a) The 21 Figure S9), which broadly resembles the Holocene WMEs inferred by July insolation at 65°N (Laskar et al., 2004). (b) Dongge record (Dykoski the counterpart cave records (Dykoski et al., 2005; Wang, 2005). et al., 2005; Wang, 2005). (c) Yongxing record (this study). Red dash lines in Durations of YXEs vary considerably from ~1,200 to 100 years (Table (b) and (c) indicate the trends modeled by Ramp regression program S3) with an average of ~400 years. Of note is the longest YXE estimated (Mudelsee, 2000). Light blue bar in (b) and (c) indicate the temporal dura- based on annual layers during MIS 11c (the YXE 5: ~1,200 years around tions of the “2‐Kyr shift” and “Late‐MIS 11c shift,” respectively. (d) to (g) 406 Kyr BP), which appears to coincide with one of the most significant Atmospheric CH4 (Loulergue et al., 2008), CO2 (Lüthi et al., 2008), Antarctic relative temperature (Jouzel et al., 2007), and sea level (inferred from climate events documented recently by a pollen‐based climate recon- benthic δ18O data; Spratt & Lisiecki, 2016), respectively. Gray and green struction from SE Europe (Kousis et al., 2018). The Yongxing millennial curves depict the Holocene and MIS 11 records with their timescales at event centered on 390 Kyr BP during MIS 11b similarly coincides with a bottom and top, respectively. The climate conditions of the early Holocene significant event in the pollen‐based record during the same interval and corresponding portion of MIS 11c are approximately similar (yellow bar). VPDB = Vienna Pee Dee Belemnite; MIS = Marine Isotope Stage. (Kousis et al., 2018). However, it remains challenging to correlate directly and precisely the MIS 11 events between various records from different climate domains, which requires comparable resolutions and age controls. In short, the presence of a set of distinct YXEs during MIS 11c suggests that centennial‐ millennial scale climate events are a persistent feature of ASM variability during both Holocene and

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MIS 11 interglacial periods, albeit with smaller magnitude relative to those in glacial periods. We note that this places new constraints on relevant climate models.

4.2. The Late‐MIS 11c Shift Over the past ~2 Kyr, the ASM has increased anomalously compared to the downward trend in NHSI (Figures 3 and 4). One previous study referred to this Late Holocene anomaly as the “2‐Kyr shift” and inter- preted it as a manifestation of a recurring pattern of millennial events that occurred throughout much of the past several hundred kiloyears (Cheng et al., 2016; Figure 4). A similar multi‐millennial shift to stronger ASM against a backdrop of a downward trend in NHSI during the late portion of MIS 11c is also evident (Figure S14). Close examination of the Yongxing δ18O profile indicates that a long‐term trend toward increasing δ18O values during the late MIS 11c was interrupted by an interval of slightly decreasing δ18O value from ~402 to 397 Kyr BP (Figures 3 and 4). We refer to this interval of anomalous decrease in δ18O as the “Late‐MIS 11c shift,” which in the Yongxing MIS 11c record culminated abruptly with a remarkably weak ASM state for several centuries, as determined by Figure 4. Comparison between ASM records during the Holocene and MIS annual band counting (Yongxing event S; Figure 4). The ASM waning 11c. (a) The Yongxing record (this study, green line) and 21 July insolation at trend is defined by a ~2‰ change in δ18O, which is nearly equivalent to 65°N (orange line; Laskar et al., 2004). Vertical yellow bars depict the Yongxing events (labeled by S and 0–7) during MIS 11c (Figure S9). Vertical the magnitudes of MIS 11c or Holocene events (Figure 4). The distinct arrows depict amplitudes (~2‰)of“Yongxing event S.” The “Late‐MIS 11c Late‐MIS 11c shift and its abrupt end are further evident in the Sanbao shift” in the late MIS 11c is shown by blue bar with a duration of ~4 Kyr. record, although its amplitude is muted, likely due to an extremely slow (b) Dongge record (Dykoski et al., 2005; Wang, 2005; black line) with 21 July speleothem growth rate (~1 Kyr/mm; Cheng et al., 2016). insolation at 65°N (orange line; Laskar et al., 2004). Vertical yellow bars depict weak monsoon events possibly linked to Bond events (Wang, 2005). Intriguingly, a number of proxy records, as well as instrumental data Vertical arrows depict amplitudes (~1‰) of last ~100‐year trend of the ASM from the wider ASM domain, reveal a declining trend in ASM over (Li et al., 2017; blue line). The 2‐Kyr shift (Cheng et al., 2016) in the Late the last ~100 years (Ding et al., 2008; Li et al., 2017; Sinha et al., Holocene is shown by blue bar with a duration of ~2 Kyr. Blue curves in 2015; Zhang et al., 2008). This observed decline in the ASM intensity inset box show the last ~100‐year trend of the ASM (Li et al., 2017) in comparison with an interval marking the abrupt culmination of the over represents a marked departure from the strong coupling between Late‐MIS 11c shift (gray line, this study) at the end of MIS 11c. Vertical the ASM and NH temperature that had persisted over the last 2 Kyr arrows (insert box) depict amplitudes (~2‰) of the Yongxing event S. Red (Kathayat et al., 2017; Sinha et al., 2015; Zhang et al., 2008). It remains dashed lines in (a) and (b) indicate the MIS 11c and Holocene trends unclear whether this centennial‐scale waning trend of the ASM can be revealed by the Ramp regressions (Mudelsee, 2000), respectively. The MIS solely attributed to anthropogenic forcing. Our MIS 11c data suggest 11c (Yongxing) and Holocene (Dongge) records are aligned via their inso- lation maxima at 408 and 9 Kyr BP, respectively. VPDB = Vienna Pee Dee that the natural forcing may also play an important role, given the fact Belemnite; ASM = Asian summer monsoon; MIS = Marine Isotope Stage. that a much longer and larger decline in ASM was observed under a similar orbital forcing condition around the late stage of MIS 11c (Figures 3 and 4). Provided that anthropogenic forcing does not supersede natural variability, the waning trend in ASM over the last ~100 years could, therefore, be viewed as a manifestation of the early stage of a larger and longer declining trend, perhaps comparable in temporal duration to the one observed during the late portion of MIS 11c (Figure 4). If this interpretation is correct, then our observations may have significant socioeconomic implications. Although a number of hypotheses have been proposed to explain the underlying mechanisms of these WMEs, including the 2‐Kyr shift, such as changes in the solar varia- tions (Bond et al., 2001), Antarctic Ice Sheet discharge (Bakker et al., 2017), volcanic forcing (Cooper et al., 2018), and Atlantic Meridional Overturning Circulation (Cheng et al., 2016; Thornalley et al., 2018), our understanding of the recurrence of these events remains limited and controversial (Wanner et al., 2011). In this context, our new record assumes significance, because it provides a high‐resolution and precisely dated record of ASM variations during MIS 11, which opens the door for placing the ASM changes during Holocene into a wider context of long‐term natural variability.

5. Conclusions New oxygen‐isotope data from Yongxing Cave show similar orbital patterns of ASM variability between MIS 11c and the Holocene. Superimposed on the orbital trend during MIS 11c (~408 to 397 Kyr BP) are 16

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millennial‐centennial WMEs (or YXEs), which broadly resemble the Holocene WMEs inferred from coun- terpart cave records in terms of pacing, amplitude, and structure. In particular, the “Late MIS 11c shift” toward stronger ASM conditions is comparable to the Late Holocene 2‐Kyr shift. Intriguingly, the end of the Late MIS 11c shift is characterized by an abrupt, multicentennial ASM weakening, akin to the observed weakening trend in ASM over the past ~100 years but much longer in duration and larger in amplitude. This observation may suggest that unless superseded by anthropogenic forcing, weakening of the ASM over the past century may represent the early stage of a naturally forced, multicentennial WME of the Late Holocene, which would have significant socioeconomic implications for the ASM region.

Acknowledgments References This work was supported by China's ‐ NSFC grants 41888101, 41731174, An, Z., Clemens, S. C., Shen, J., Qiang, X., Jin, Z., Sun, Y., et al. (2011). Glacial interglacial Indian summer monsoon dynamics. Science, – 41230524, 4157020432 and 333(6043), 719 723. https://doi.org/10.1126/science.1203752 ‐ fi 41561144003 and U.S. NSF grant Bakker, P., Clark, P. U., Golledge, N. R., Schmittner, A., & Weber, M. E. (2017). Centennial scale Holocene climate variations ampli ed by – 1702816. H. C. designed the research Antarctic Ice Sheet discharge. Nature, 541(7635), 72 76. https://doi.org/10.1038/nature20582 – and experiments; S. T. C. and X. G. K. Berger, A., & Loutre, M. F. (2002). An exceptionally long interglacial ahead? Science, 297(5585), 1287 1288. https://doi.org/10.1126/ collected the samples. X. N. Z., H. C., science.1076120 fl and Y. F. N. performed all stable isotope Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M. N., Showers, W., et al. (2001). Persistent solar in uence on North Atlantic climate 230 – measurements and Th dating work. during the Holocene. Science, 294(5549), 2130 2136. https://doi.org/10.1126/science.1065680 X. N. Z. counted the annual bands by Candy, I., Schreve, D. C., Sherriff, J., & Tye, G. J. (2014). Marine Isotope Stage 11: Palaeoclimates, palaeoenvironments and its role as an ‐ – using Confocal Laser Fluorescent analogue for the current interglacial. Earth Science Reviews, 128,18 51. https://doi.org/10.1016/j.earscirev.2013.09.006 Microscopy. X. N. Z., A. S., and H. C. did Chen, S., Wang, Y., Cheng, H., Edwards, L. R., Wang, X., Kong, X., & Liu, D. (2016). Strong coupling of Asian Monsoon and Antarctic ‐ fi the data analyses. H. C., X. N. Z., and A. climates on sub orbital timescales. Scienti c Reports, 6(1), 32995. https://doi.org/10.1038/srep32995 ‐ ‐ S. wrote the manuscript, which was Cheng, H., Adkins, J., Edwards, R. L., & Boyle, E. A. (2000). U Th dating of deep sea corals. Geochimica Et Cosmochimica Acta, 64(14), – ‐ ‐ edited by all of the coauthors. The 2401 2416. https://doi.org/10.1016/S0016 7037(99)00422 6 authors declare no competing financial Cheng, H., Edwards, R. L., Broecker, W. S., Denton, G. H., Kong, X., Wang, Y., et al. (2009). Ice Age terminations. Science, 326(5950), – interests. We thank X. L. Q from State 248 252. https://doi.org/10.1126/science.1177840 key laboratory for manufacturing Cheng, H., Edwards, R. L., Sinha, A., Spötl, C., Yi, L., Chen, S., et al. (2016). The Asian monsoon over the past 640,000 years and ice age – system engineering, Xi'an Jiaotong terminations. Nature, 534(7609), 640 646. https://doi.org/10.1038/nature18591 University for technical support. Cheng, H., Edwards, R. L., Wang, Y., Kong, X., Ming, Y., Kelly, M. J., et al. (2006). A penultimate glacial monsoon record from Hulu Cave ‐ – Speleothem data in this research are and two phase glacial terminations. Geology, 34(3), 217 220. https://doi.org/10.1130/G22289.1 available in Table S2 and online Cheng, H., Fleitmann, D., Edwards, R. L., Wang, X., Cruz, F. W., Auler, A. S., et al. (2009). Timing and structure of the 8.2 event inferred δ18 – reference (http://doi.org/10.5281/ from O records from China, Oman and Brezil. Geology, 37(11), 1007 1010. https://doi.org/10.1130/G30126A.1 230 230 zenodo.3365932). Cheng, H., Lawrence Edwards, R., Shen, C. C., Polyak, V. J., Asmerom, Y., Woodhead, J., et al. (2013). Improvements in Th dating, Th and 234U half‐life values, and U–Th isotopic measurements by multi‐collector inductively coupled plasma mass spectrometry. Earth and Planetary Science Letters, 371‐372,82–91. https://doi.org/10.1016/j.epsl.2013.04.006 Cooper, C. L., Swindles, G. T., Savov, I. P., Schmidt, A., & Bacon, K. L. (2018). Evaluating the relationship between climate change and volcanism. Earth‐Science Reviews, 177, 238–247. https://doi.org/10.1016/j.earscirev.2017.11.009 Ding, Y., Wang, Z., & Sun, Y. (2008). Inter‐decadal variation of the summer precipitation in East China and its association with decreasing Asian summer monsoon. Part I: Observed evidences. International Journal of Climatology, 28(9), 1139–1161. https://doi.org/10.1002/ joc.1615 Domínguez‐Villar, D., Baker, A., Fairchild, I. J., & Edwards, R. L. (2012). A method to anchor floating chronologies in annually laminated speleothems with U‐Th dates. Quaternary Geochronology, 14,57–66. https://doi.org/10.1016/j.quageo.2012.04.019 Dutton, A., Carlson, A. E., Long, A. J., Milne, G. A., Clark, P. U., DeConto, R., et al. (2015). Sea‐level rise due to polar ice‐sheet mass loss during past warm periods. Science, 349(6244), aaa4019–aaa4164. https://doi.org/10.1126/science.aaa4019 Dykoski, C., Edwards, R., Cheng, H., Yuan, D., Cai, Y., Zhang, M., et al. (2005). A high‐resolution, absolute‐dated Holocene and deglacial Asian monsoon record from Dongge Cave, China. Earth and Planetary Science Letters, 233(1‐2), 71–86. https://doi.org/10.1016/j. epsl.2005.01.036 Edwards, R. L., Chen, J. H., & Wasserburg, G. J. (1987). 238U‐234U‐230Th‐232Th systematics and the precise measurement of time over the past 500,000 years. Earth and Planetary Science Letters, 81(2‐3), 175–192. https://doi.org/10.1016/0012‐821X(87)90154‐3 EPICA Community Members (2004). Eight glacial cycles from an Antarctic ice core. Nature, 429(6992), 623–628. https://doi.org/10.1038/ nature02599 EPICA Community Members (2010). Stable oxygen isotopes of ice core EDML. 2010. https://doi.org/10.1594/PANGAEA.552270 Ghil, M. (2002). Advanced spectral methods for climatic time series. Reviews of Geophysics, 40(1), 1003. https://doi.org/10.1029/2000RG000092 Hao, Q., Wang, L., Oldfield, F., Peng, S., Qin, L., Song, Y., et al. (2012). Delayed build‐up of Arctic ice sheets during 400,000‐year minima in insolation variability. Nature, 490(7420), 393–396. https://doi.org/10.1038/nature11493 Hercman, H., & Pawlak, J. (2012). MOD‐AGE: An age‐depth model construction algorithm. Quaternary Geochronology, 12,1–10. https://doi.org/10.1016/j.quageo.2012.05.003 Herold, N., Yin, Q. Z., Karami, M. P., & Berger, A. (2012). Modelling the climatic diversity of the warm . Quaternary Science Reviews, 56,126–141. https://doi.org/10.1016/j.quascirev.2012.08.020 Jouzel, J., Masson‐Delmotte, V., Cattani, O., Dreyfus, G., Falourd, S., Hoffmann, G., et al. (2007). Orbital and millennial Antarctic climate variability over the past 800,000 years. Science, 317(5839), 793–796. https://doi.org/10.1126/science.1141038 Kandiano, E. S., Mt, V. D. M., Schouten, S., Fahl, K., Sinninghe Damsté, J. S., & Bauch, H. A. (2017). Response of the North Atlantic surface and intermediate ocean structure to climate warming of MIS 11. Scientific Reports, 7(1), 46192. https://doi.org/10.1038/srep46192 Kathayat, G., Cheng, H., Sinha, A., Yi, L., Li, X., Zhang, H., et al. (2017). The Indian monsoon variability and civilization changes in the Indian subcontinent. Science Advances, 3(12). https://doi.org/10.1126/sciadv.1701296 Kousis, I., Koutsodendris, A., Peyron, O., Leicher, N., Francke, A., Wagner, B., et al. (2018). Centennial‐scale vegetation dynamics and climate variability in SE Europe during Marine Isotope Stage 11 based on a pollen record from Lake Ohrid. Quaternary Science Reviews, 190,20–38. https://doi.org/10.1016/j.quascirev.2018.04.014

ZHAO ET AL. 9955 Geophysical Research Letters 10.1029/2019GL083836

Koutsodendris, A., Müller, U. C., Pross, J., Brauer, A., Kotthoff, U., & Lotter, A. F. (2010). Vegetation dynamics and climate variability during the Holsteinian interglacial based on a pollen record from Dethlingen (northern Germany). Quaternary Science Reviews, 29(23‐24), 3298–3307. https://doi.org/10.1016/j.quascirev.2010.07.024 Koutsodendris, A., Pross, J., Müller, U. C., Brauer, A., Fletcher, W. J., Kühl, N., et al. (2012). A short‐term climate oscillation during the Holsteinian interglacial (MIS 11c): An analogy to the 8.2 ka climatic event? Global and Planetary Change, 92‐93, 224–235. https://doi. org/10.1016/j.gloplacha.2012.05.011 Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A. C. M., & Levrard, B. (2004). A long‐term numerical solution for the insolation quantities of the Earth. Photographic Science and Photochemistry, 428(1), 261–285. https://doi.org/10.1051/0004‐6361:20041335 Li, X., Cheng, H., Tan, L., Ban, F., Sinha, A., Duan, W., et al. (2017). The East Asian summer monsoon variability over the last 145 years inferred from the Shihua Cave record, North China. Scientific Reports, 7(1), 7078. https://doi.org/10.1038/s41598‐017‐07251‐3 Li, Z., Lau, W. K. M., Ramanathan, V., Wu, G., Ding, Y., Manoj, M. G., et al. (2016). Aerosol and monsoon climate interactions over Asia. Reviews of Geophysics, 54, 866–929. https://doi.org/10.1002/2015RG000500 Liu, Z., Wen, X., Brady, E. C., Otto‐Bliesner, B., Yu, G., Lu, H., et al. (2014). Chinese cave records and the East Asia Summer Monsoon. Quaternary Science Reviews, 83, 115–128. https://doi.org/10.1016/j.quascirev.2013.10.021 Loulergue, L., Schilt, A., Spahni, R., Masson‐Delmotte, V., Blunier, T., Lemieux, B., et al. (2008). Orbital and millennial‐scale features of atmospheric CH4 over the past 800,000 years. Nature, 453(7193), 383–386. https://doi.org/10.1038/nature06950 Loutre, M. F., & Berger, A. (2003). Marine Isotope Stage 11 as an analogue for the present interglacial. Global and Planetary Change, 36(3), 209–217. https://doi.org/10.1016/S0921‐8181(02)00186‐8 Lüthi, D., le Floch, M., Bereiter, B., Blunier, T., Barnola, J. M., Siegenthaler, U., et al. (2008). High‐resolution carbon dioxide concentration record 650,000‐800,000 years before present. Nature, 453(7193), 379–382. https://doi.org/10.1038/nature06949 Masson‐Delmotte, V., Stenni, B., Pol, K., Braconnot, P., Cattani, O., Falourd, S., et al. (2010). EPICA Dome C record of glacial and inter- glacial intensities. Quaternary Science Reviews, 29(1‐2), 113–128. https://doi.org/10.1016/j.quascirev.2009.09.030 Meckler, A. N., Clarkson, M. O., Cobb, K. M., Sodemann, H., & Adkins, J. F. (2012). Interglacial hydroclimate in the Tropical West Pacific through the Late . Science, 336(6086), 1301–1304. https://doi.org/10.1126/science.1218340 Mudelsee, M. (2000). Ramp function regression: A tool for quantifying climate transitions. Computers & Geosciences, 26(3), 293–307. https://doi.org/10.1016/S0098‐3004(99)00141‐7 Past Interglacials Working Group of PAGES (2016). Interglacials of the last 800,000 years. Reviews of Geophysics, 54, 162–219. https://doi. org/10.1002/2015RG000482 Petit, J. R., Jouzel, J., Raynaud, D., Barkov, N. I., Barnola, J. M., Basile, I., et al. (1999). Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature, 7(1), 413–429. https://doi.org/10.1038/s41598‐017‐07251‐3 Pol, K., Debret, M., Masson‐Delmotte, V., Capron, E., Cattani, O., Dreyfus, G., et al. (2011). Links between MIS 11 millennial to sub‐ millennial climate variability and long term trends as revealed by new high resolution EPICA Dome C deuterium data—A comparison with the Holocene. Climate of the Past, 7(2), 437–450. https://doi.org/10.5194/cp‐7‐437‐2011 Prokopenko, A. A., Bezrukova, E. V., Khursevich, G. K., Solotchina, E. P., Kuzmin, M. I., & Tarasov, P. E. (2010). Climate in continental interior Asia during the longest interglacial of the past 500 000 years: The new MIS 11 records from Lake Baikal. Climate of the Past, 6(1), 31–48. https://doi.org/10.5194/cp‐6‐31‐2010 Prokopenko, A. A., Hinnov, L. A., Williams, D. F., & Kuzmin, M. I. (2006). Orbital forcing of continental climate during the Pleistocene: A complete astronomically tuned climatic record from Lake Baikal, SE Siberia. Quaternary Science Reviews, 25(23‐24), 3431–3457. https://doi.org/10.1016/j.quascirev.2006.10.002 Rasmussen, S. O., Andersen, K. K., Svensson, A. M., Steffensen, J. P., Vinther, B. M., Clausen, H. B., et al. (2006). A new Greenland ice core chronology for the last glacial termination. Journal of Geophysical Research, 111, 907, D06102–923. https://doi.org/10.1029/ 2005JD006079 Ruddiman, W. F., Fuller, D. Q., Kutzbach, J. E., Tzedakis, P. C., Kaplan, J. O., Ellis, E. C., et al. (2016). Late Holocene climate: Natural or anthropogenic? Reviews of Geophysics, 54,93–118. https://doi.org/10.1002/2015RG000503 Ruddiman, W. F., Kutzbach, J. E., & Vavrus, S. J. (2011). Can natural or anthropogenic explanations of late‐Holocene CO2 and CH4 increases be falsified? Holocene, 21(5), 865–8879. https://doi.org/10.1177/0959683610387172 Severinghaus, J. P., Beaudette, R., Headly, M. A., Taylor, K., & Brook, E. J. (2009). Oxygen‐18 of O2 records the impact of on the terrestrial biosphere. Science, 324(5933), 1431–1434. https://doi.org/10.1126/science.1169473 Sinha, A., Kathayat, G., Cheng, H., Breitenbach, S. F. M., Berkelhammer, M., Mudelsee, M., et al. (2015). Trends and oscillations in the Indian summer monsoon rainfall over the last two millennia. Nature Communications, 6(1), 6309. https://doi.org/10.1038/ncomms7309 Spratt, R. M., & Lisiecki, L. E. (2016). A late Pleistocene sea level stack. Climate of the Past, 12(4), 1079–1092. https://doi.org/10.5194/cp‐12‐ 1079‐2016 Stein, R., Hefter, J., Grützner, J., Voelker, A., & Naafs, B. D. A. (2009). Variability of surface water characteristics and Heinrich‐like events in the Pleistocene midlatitude North Atlantic Ocean: Biomarker and XRD records from IODP Site U1313 (MIS 16‐9). Paleoceanography, 24, PA2203. https://doi.org/10.1029/2008PA001639 Tan, M., Baker, A., Genty, D., Smith, C., Esper, J., & Cai, B. (2006). Applications of stalagmite laminae to paleoclimate reconstructions: Comparison with dendrochronology/climatology. Quaternary Science Reviews, 25(17‐18), 2103–2117. https://doi.org/10.1016/j. quascirev.2006.01.034 Thornalley, D. J. R., Oppo, D. W., Ortega, P., Robson, J. I., Brierley, C. M., Davis, R., et al. (2018). Anomalously weak Labrador Sea con- vection and Atlantic overturning during the past 150 years. Nature, 556(7700), 227–230. https://doi.org/10.1038/s41586‐018‐0007‐4 Tzedakis, P. C., Raynaud, D., McManus, J. F., Berger, A., Brovkin, V., & Kiefer, T. (2009). Interglacial diversity. Nature Geoscience, 2(11), 751–755. https://doi.org/10.1038/ngeo660 Wang, Y. (2005). The Holocene Asian monsoon: Links to solar changes and North Atlantic climate. Science, 308(5723), 854–857. https:// doi.org/10.1126/science.1106296 Wang, Y., Cheng, H., Edwards, R. L., An, Z. S., Wu, J. Y., Shen, C. C., & Dorale, J. A. (2001). A high‐resolution absolute‐dated late Pleistocene Monsoon record from Hulu Cave, China. Science, 294(5550), 2345–2348. https://doi.org/10.1126/science.1064618 Wang, Y., Cheng, H., Edwards, R. L., Kong, X., Shao, X., Chen, S., et al. (2008). Millennial‐ and orbital‐scale changes in the East Asian monsoon over the past 224,000 years. Nature, 451(7182), 1090–1093. https://doi.org/10.1038/nature06692 Wanner, H., Solomina, O., Grosjean, M., Ritz, S. P., & Jetel, M. (2011). Structure and origin of Holocene cold events. Quaternary Science Reviews, 30(21‐22), 3109–3123. https://doi.org/10.1016/j.quascirev.2011.07.010 Zhang, P., Cheng, H., Edwards, R. L., Chen, F., Wang, Y., Yang, X., et al. (2008). A test of climate, sun, and culture relationships from an 1810‐year Chinese cave record. Science, 322(5903), 940–942. https://doi.org/10.1126/science.1163965

ZHAO ET AL. 9956 Geophysical Research Letters 10.1029/2019GL083836

Zhao, J., & Cheng, H. (2017). Applications of laser scanning confocal microscope to paleoclimate research: Characterizing and counting laminae. Quaternary Science, 37, 1472–1474. https://doi.org/10.11928/j.issn.1001‐7410.2017.06.28 Zhao, J., Cheng, H., Yang, Y., Tan, L., Spötl, C., Ning, Y., et al. (2018). Reconstructing the western boundary variability of the Western Pacific Subtropical High over the past 200 years via Chinese cave oxygen isotope records. Climate Dynamics, 52, 3741–3757. https://doi. org/10.1007/s00382‐018‐4456‐0

References From the Supporting Information Bond, G. (1997). A pervasive millennialscale cycle in North Atlantic Holocene and glacial climates. Science, 278(5341), 1257–1266. https://doi.org/10.1126/science.278.5341.1257 Hendy, C. H. (1971). The isotopic geochemistry of speleothems—I. The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeoclimatic indicators. Geochimica Et Cosmochimica Acta, 35(8), 801–824. https://doi.org/10.1016/0016‐7037(71)90127‐X Imbrie, J., Hays, J. D., Martinson, D. G., McIntyre, A., Mix, A. C., Morley, J. J., et al. (1984). The orbital theory of Pleistocene climate: Support from a revised chronology of the Marine d18O record. Milankovitch and Climate Part, 126, 269–305. https://doi.org/10013/ epic.48655 Lisiecki, L. E., & Raymo, M. E. (2005). A Pliocene‐Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanography, 20, PA1003. https://doi.org/10.1029/2004PA001071 Zhang, H., Griffiths, M. L., Chiang, J. C. H., Kong, W., Wu, S., Atwood, A., et al. (2018). East Asian hydroclimate modulated by the position of the westerlies during Termination I. Science, 362(6414), 580–583. https://doi.org/10.1126/science.aat9393

ZHAO ET AL. 9957