High precise dating on the variation of the Asian summer monsoon since 37 ka BP

Comprehensive comparison of paleoclimate change based on records constrained by precise chronology and high-resolution is essential to explore the correlation and interaction within earth climate systems. Here, we propose a new stalagmite-based multidecadal resolved Asian summer monsoon (ASM) record spanning the past thirty-seven thousand years (ka BP, before ad 1950) from Furong Cave, southwestern China. This record is consistent with the published Chinese stalagmite sequences and shows that the dominant controls of the ASM dynamics include not only insolation and solar activity but also suborbital-scale hydroclimate events in the high latitudes of the northern hemisphere, such as the Heinrich events, Bølling-Allerød (BA), and Younger Dryas (YD). Benefit from the unprecedented accurate chronology, the timings of these events are precisely dated, with uncertainties of, at most, 40 years (2σ). The onset of the weak ASM during the YD began at 12.92 ka BP and lasted for 430 years. The occurrence of the 200-yr Older Dryas during the BA period was dated from 13.87 to 14.06 ka BP. The durations of the three Heinrich (H) events, H1, H2, and H3, are 14.33–18.29, 23.77–24.48, and 28.98–30.46 ka BP, respectively. Furong record shows surprisingly variable onset transitions of 980, 210, and 40 years for the corresponding weak ASM events. These discrepancies suggest different influences of the H events on ASM dynamics. During the periods of H 1–3, the obvious difference between our Furong record and NGRIP δ18O record indicated the decoupling correlation between the mid-low latitudes and high latitudes. On the other hand, synchronous climate change in high and low latitudes suggests another possibility which different to the dominant role of Northern high latitudes in triggering global climate change. Our high quality records also indicate a plausible different correlation between the high and mid-low latitudes under glacial and inter-glacial background, especially for the ASM regimes.

Over the past eight hundred thousand years, the orbital-scale glacial-interglacial cycles were punctuated with suborbital dynamics (EPICA, 2004). The internal hydroclimate fluctuations of the last glacial period (LGP) have remained the most studied such phenomena of the past decades 1,2 . One of the remarkable features of this period is a series of abrupt events in the northern hemisphere (NH) 3 .
During the last deglaciation, influxes of fresh water from Lake Agassiz 4 that cascaded into the North Atlantic decreased the salinity of surface water, weakened the Atlantic Meridional Overturning Circulation (AMOC), changed the thermohaline circulation (THC), and influenced the global climate 5 . These centurial-to-millennial scale climatic oscillations correspond to the Older Dryas (OD) 6 , Younger Dryas (YD) 7 , and 8.2 ka event 8 , all of which are associated with a decline of the intensity of the Asian summer monsoon (ASM) 9 through ocean-atmosphere coupling processes. Low-latitude monsoonal hydroclimatic responses are apparently slower www.nature.com/scientificreports/ than the transitions of the onsets and terminations of these high-latitude abrupt climate events 10 . Partin et al. 11 used stalagmite records in the Philippines and eastern and southern China to propose that changes in the AMOC may not produce the same effects across the entire AM territory. The robustness of this argument should be verified with additional regional ASM records. During the last glaciation, enormous icebergs intermittently calved from the ice shelves and traversed the North Atlantic. These so-called Heinrich (H) events were recorded at least six times through the ice-rafted debris (IRD) preserved in marine sediment 2,12 . During H events, the extensive amounts of fresh water entering the North Atlantic attenuated the density-driven THC and led to regional climate changes worldwide 2 . ASM intensity was also reduced 9,13,14 . Due to chronology limitations and proxy resolutions, the detailed transitions and dynamics of ASMs during H events have never been precisely dated. There are different mechanisms in the response of high and low latitudes climate variations to series H events [15][16][17][18][19] . Climate and environment changes at high and low latitudes can be identified from multiple climate proxy records (e.g. δ 18 O, 17 O-excess, d-excess) in the Greenland ice core (NGRIP 15,17,20,21 . During the H1 and H4, although the temperature in Greenland maintained a stable low temperature, large-scale climate reorganization related to warming and humidification in the low latitude areas [15][16][17] . These observations argued the traditional opinion that the dynamics of ASM were deeply dominated by climate changes in the north high latitudes 9 . It is still an open question whether this decoupling between high latitudes and mid-low latitudes existing during all the H events in the last glacial period (LGP).
Here, we present a 37-kyr stalagmite oxygen isotope record from southwestern China. For this stalagmite, the section of 6-16 ka BP had been reported by Li et al. 22 . This work reports the complete record of this stalagmite. The YD, OD, and H events are precisely dated, with only decadal-scale errors of just a few decades. The detailed structures of these events would provide a better understanding of low-latitude ASM responses through the internal forcings that originated from the NH high latitudes as well as allowing an evaluation of the fidelity of the Greenland ice core chronologies.

Regional settings and samples
Furong Cave (29°13′44″ N, 107°54′13″ E) is located in the Cambrian limestone and dolomite strata of the eastern bank of the Furongjiang River, which is a secondary branch of the Yangtze River, Chongqing, southwestern China (Fig. 1). The overlying vadose zone is 300-500 m thick, the elevation at the cave entrance is 480 m above sea level, the cave air temperature is 16.3 °C and the humidity is 95-100%. Furthermore, the regional climate is dominated by the Asian monsoon system, characterized by cold-dry winters and hot-wet summers. The current annual average precipitation is 1200 mm, with 70% of the annual rainfall occurring during the wet season from May to October. Details of the geographic and regional hydrological settings of the cave were described in Li et al. 22 .
FR5 is an active stalagmite with a total length of 50 cm and diameters vary from 6 cm at its top to 14 cm at its bottom ( Fig. 2A). The stalagmite was collected from a chamber 1 km from the natural cave entrance, halved along its growth axis and polished ( Fig. 2A). X-ray diffraction and electron backscatter diffraction techniques show that the stalagmite is 100% aragonite, demonstrating the total absence of aragonite-to-calcite transformation 22 .  www.nature.com/scientificreports/ at the High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), National Taiwan University 23 . The details of the chemistry, instrumental analysis, and off-line calculation are described in Shen et al. 23 . The half-lives of U-Th nuclides used are given in Cheng et al. 24 . The uncertainties in the U-Th isotopic data and 230 Th dates (ka BP, before 1950 ad) are calculated at the 2σ level or at two standard deviations of the mean (2σ m ) unless otherwise noted. For the stable oxygen isotope analysis, 956 subsamples of 50-100 µg each were drilled from along the growth axis using a 0.5-mm carbide dental bit ( Fig. 2A). The measurements were taken using an isotope ratio mass spectrometer, specifically the Finnegan DELTA V PLUS coupled with a Kiel IV automated carbonate device at the Laboratory of Geochemistry and Isotopes, Southwest University, China. Measurements of an international standard, NBS 19, and an in-house standard, NCKU 1, show a one-sigma reproducibility of ± 0.10‰ for δ 18 O. All the stalagmite oxygen stable isotopic data are reported in permil (‰) relative to the Vienna Pee Dee Belemnite (V-PDB) standard.

Methods
Identification of climate events. Here, we describe our method of defining the periods of climatic events ( Fig. 3). Taking event in Fig. 3 as an example, the onset transition is from the inflection point Oa to that of Ob; similarly, the termination transition is from Ta to Tb. The period between Oa and Tb, represents the duration of the event. A statistical regression approach, RAMPFIT 25 , was used to determine the end of the Holocene opti-  Table 1). This weighted least-squares method, which determines the ramp between states of a certain record, has been successfully applied to geochemical proxy time series [26][27][28] .

Results
Chronology. The uranium-thorium content, isotopic compositions and determined 230 Th ages are given

Discussion
Comparison within monsoon China. Previous multiyear in situ monitoring programs and Hendy test results demonstrated that both of the speleothems in Furong Cave and stalagmite FR5 are deposited at oxygen isotopic equilibrium condition and inherited the isotopic composition of precipitation 22 . Strong correlations among contemporaneous sequences are observed in the Furong, Hulu 9 , Sanbao 29 and Dongge 30 records from the last glacial period to the Holocene (Fig. 4B,C), supporting previous arguments for stalagmite δ 18 O records mainly capturing monsoonal hydroclimate variability, qualitatively representing ASM intensities (e.g., 31,32 , and changes in moisture sources which dominated by atmosphere circulations [33][34][35] . As the almost only source of energy for earth climate systems, solar activity dominates the energy input into earth surface. The tropical Indian Ocean and Pacific Ocean are the heat and vapor sources of ASM. Variations in energy input and distribution in earth surface influence on the thermodynamic gradient between continents and oceans, resulting the changes in ASM intensity, modulating the portion of summer precipitation (lighter δ 18 O values than that in winter precipitation), and/or changing the distance of moisture source via the re-organization of atmospheric circulations which may be regulated by ENSO 34,35 (see details in "Asynchronous regional Holocene ASM"). All these physical processes influenced on the precipitation δ 18 O and were finally reserved in speleothems. Multi-millennial-scale trends of FR5 δ 18 O-inferred ASM variability show a clear correlation with the precession-dominated NH solar insolation (NHSI) (Fig. 4). Series significant periodicities of 170, 210, 220, 510 and 1350 years (Fig. 5), produced by a spectral analysis method 36 , support that the solar activity plays an essential role in ASM variability on the centennial to millennial timescales 9,32 .
Asynchronous regional Holocene ASM. The degradation of regional ASM conditions during the Holocene was reconstructed 27 . The RAMPFIT-determined transitions from the Holocene optimum to the Middle-Holocene degradation occur at 7.31 ± 0.10, 7.01 ± 0.06, 6.64 ± 0.03, 5.74 ± 0.06 and 5.76 ± 0.06 ka BP, according to the stalagmite records from the Qunf cave 26 , Dongge cave 37 , Shigao cave 27 , Heshang cave 38 and Sanbao cave, respectively (Fig. 4 of ref. 27 ). This transition in the FR5 record is determined to be 7.05 ± 0.02 ka BP (Fig. 6A), which is consistent with the timing of the Dongge record, but differs from the others. Our record supports the previous observations of the HOP termination being regionally asynchronous over the AM area 27 . In southern and eastern China, the summer monsoon precipitation is influenced by both the ISM and the East Asian summer monsoon (EASM). Jiang et al. 27 proposed that the asynchronies could be attributed to sea surface temperature (SST) changes in the western tropical Pacific, the primary moisture source for the East Asian monsoon. www.nature.com/scientificreports/ www.nature.com/scientificreports/ A recent study by Zhao et al. 39 proposed that the millennial-scale change of the EASM intensity strongly correlated with the El Niño-Southern Oscillation (ENSO) during the late Holocene. Ta 40 argued that an SST gradient from east to west in the tropical Pacific modulated the ENSO-like state and affected climate change in the monsoon region of China (Fig. 7 of 40 ). The gradual weakening of the ASM from the middle to late Holocene observed in the FR5 records matches the enhancement of the El Niño activities (Fig. 6)     Younger Dryas, Older Dryas, and Bølling/Allerød. The YD, the most remarkable cold reversion of the climate after the termination of the LGP, was proposed to be caused by a slowdown or reduction of the AMOC and the associated abrupt increase of fresh melt water into the North Atlantic Ocean 45 . The YD was also characterized by a notable decrease of air temperature over central Greenland 28 . Due to the unprecedented high resolution and precise chronology of FR5 record, the details of a series of climate events can be refined accurately. Comprehensive comparison will be conducive to probe into the mechanism of interactions between climatic systems under different background. The onset of the YD as recorded in the FR5 δ 18 O record began at 12.92 ± 0.04 ka BP and ended at 12.50 ± 0.04 ka BP (Figs. 7D, 8C), followed by a weak millennial ASM interval reaching a δ 18 O maxima of -6.09‰ at 12.11 ± 0.04 ka BP. The beginning of the YD onset in the FR5 records closely matches that at 12.89 ± 0.14 ka BP in the NGRIP record and 13.0 ± 0.15 ka BP in the Timta Cave record, India (Fig. 7A, and a' in Fig. 8)    www.nature.com/scientificreports/ The onset of the transition into the YD lasted for 450 ± 140 years in central China (Fig. 7C) 48 , which is consistent with the 420 ± 60 years observed in Furong Cave. A shorter transition of 240 ± 60 years is recorded in northern China (Fig. 7B) 49 , and a more gradual transition of 570 ± 110 years is observed in Philippines in the western tropical Pacific 11 . This meridional difference suggests regional hydroclimatic responses to the southward-moving NH high-latitude forcing that originates in the North Atlantic 10,11 .
The short centennial OD event, triggered by the slowdown of the AMOC during the Bølling/Allerød (B/A) 6 , began from 13.87 ± 0.05 to 14.06 ± 0.04 ka BP based on three 230 Th dates in the FR5 record and lasted for 190 ± 60 years, which concurs with those dates noted in the Greenland δ 18 O and Indian stalagmite records which reflected the variation of Indian summer monsoon (ISM) (b' in Fig. 8) 46,47,50 . These agreements indicate an instantaneous influence of the NH high-latitude forcing on the ASM through a reorganization of atmospheric circulations 28,51 .
In the FR5 record, the transition to B/A, lasting from 14.81 ± 0.03 to 14.33 ± 0.04 ka BP, is centered at 14.57 ka BP and lasted 480 ± 50 years (Figs. 8, 9A). However, the gradual multicentennial enhancement of the ASM differs from the abrupt 40 year warming in Greenland (c′ in Fig. 8A) 46 , but consistent to the gradually strengthen of ISM (c′ in Fig. 8B) 47 . These slow low-latitude hydroclimatic responses could require a long adjustment time to the dominant hemispheric-scale thermal forcing from Greenland 11 . In addition, the consistency between the NGRIP and FR5 records during the period of YD and B/A, (a′, b′ and c′ in Fig. 8) validates the accuracy of GICC05 chronology (Table 1). www.nature.com/scientificreports/ During the B/A period, the NGRIP δ 18 O record shows a gradual millennial temperature decline, while the FR5 record expresses an increasing ASM intensity trend (Fig. 8). This discrepancy is also observed in other Chinese records from the caves of Hulu 9 and Dongge 30 , as well as in Japanese records from Maboroshi Cave 52 , and could be attributed to the rising NHSI (Fig. 4B) 49 or the enhanced cross-equatorial transport of moisture and heat from the southern oceans 52 .
Heinrich events. H events are identified by the massive surges and melts of icebergs in the North Atlantic, perturbed the global ocean circulation and triggered a slowdown of the AMOC 1,2,12 . Stalagmite records show that the ASM intensity could be weakened during H events through atmospheric teleconnections 9 . The 37-kyr FR5 δ 18 O record demonstrates weak ASMs during H1, H2, and H3, which are centered at 16.2, 24.0, and 29.8 ka BP, respectively (Figs. 4,9).
The onset of a weak ASM, accompanied by an increase of 2.5‰ in the δ 18 O during H1, spanned from 18.29 to 17.31 ka BP and lasted 0.98 kyrs. The recovery, spanning from 14.81 to 14.33 ka BP, lasted 0.48 kyrs (Fig. 9A). The maximum δ 18 O value of -3.65‰ is observed at 16.14 ka BP. This interval can be divided into two phases, 16.20-14.33 and 18.29-17.03 ka BP (Fig. 9A), corresponding to the substages of H1a and H1b in the North Atlantic sediment records (Fig. 9D). This double-peak event is also detected in the South American monsoon system, where it is called the "Maga-South Atlantic Convergence Zone (SACZ)" 53 . The evidence again indicates that this high-latitude forcing affected inter-hemispheric hydroclimates. The 1.26-kyr H1b is characterized by a slow 1-kyr onset and a fast 80-yr ending. While the onset of the 1.87-kyr H1a lasted only 60 years, the termination took 480 years (Fig. 9A). A 0.3-kyr weak ASM, registered by the FR5 record at 16.63 ka BP, has not yet been observed in any North Atlantic records (Fig. 9A), but could be a regional ASM event or limited by the intrinsic millennial resolutions of high-latitude marine records.
During H2, FR5 indicates the onset of a weak ASM, with an enrichment of 1.2‰ in δ 18 O from 24.48 to 24.27 ka BP; the recovery spanned from 23.83 to 23.77 ka BP (Fig. 9B). This single-peak event is consistent with evidence of IRD events and alkenone-inferred salinity changes recorded in the North Atlantic sediment core SU8118 1 . The corresponding weak ASM of H3 shows an onset transition of 1‰ in the FR5 δ 18 O record, lasting from 30.46 to 30.42 ka BP, and was only 40 years long. In contrast, the termination of H3, from 29.25 to 28.98 ka BP, lasted 270 years (Fig. 9C).
The abrupt onsets at H3 (40 years) and H1a (60 years) and the terminations at H2 (60 years) and H1b (80 years) in the FR5 record ( Fig. 9) are not unique in the monsoonal region. The stalagmite M1-2 δ 18 O record from Moomi Cave, Socotra Island, Yemen, shows a 25-yr recovery of the ISM intensity at the end of H5 54 . The observed abrupt hydrological change of the Indian monsoon could be attributed to the fast meridional migration of the ITCZ responding to the recovery of the AMOC via atmospheric reorganization 54 . www.nature.com/scientificreports/ The Furong record also clearly shows different transitional durations (TDs) of the onsets and terminations of the weak ASM periods of H1-3 (Fig. 9). The TDs of the onsets/terminations are 980/480, 210/60, and 40/270 years for the weak ASMs of H1, H2 and H3, respectively (Fig. 9). The relatively slow onsets and fast terminations of H1 and H2 are opposite those of H3. This discrepancy cannot be attributed to the global sea level change (Fig. 4), but the exact mechanism causing these inconsistent ASM responses between the different H events is not clear. We speculate that the composition of the orbitally controlled changes in insolation and the rapid changes in the oceanic and atmospheric circulation patterns during the NH cooling events control the different responses of the ASM. The strength of the ASM may be particularly sensitive to NH cooling when the continent is at its warmest due to a precessional insolation maximum (Fig. 4B), such that H3 is characterized by an abrupt weakening of the ASM (Fig. 9C). More comprehensive models and simulations are necessary to fully verify the physical mechanism of this hypothesis.
Decoupling between high and mid-low latitudes. There is obvious decoupling between the climate change in NH high latitudes and low latitude during H1, H2 and H3 (Fig. 10). The comparison of FR5 records with other paleoclimate records indicates that the Greenland ice core δ 18 O remains stable during H1, H2 and H3, which means that Greenland was in a state of relative stable low-temperature (Fig. 10A1-A3) Fig. 10A) 21,55 . While, the abrupt decline of ASM (increase of FR5 δ 18 O) at these times indicates that the low latitude climate systems related to ITCZ have undergone an intense change (Fig. 10D,E) 18,56,57 . On the other hand, the strengthening of ASM is several centuries prior to the warming of Greenland at the end of H 1-3 (Fig. 10A,D). The decoupling between the high and low latitude climate variations indicates that the climate change in other parts of the world during the period of H events cannot be completely attributed to the temperature change in the high latitude regions (e.g. Greenland) 15,17 .
Previous researches based on the comparison of ice cores and marine sediments indicate that the tropical atmospheric circulation in the period of H4 and last deglaciation is also decoupled from the temperature changes in Greenland 15,17 . Iceberg discharge from the Laurentide ice sheet occurred several centuries after the cooling of North Atlantic surface ocean and decrease of AMOC intensity 58,59 . During H4, southward shift of the ITCZ delayed by 550 ± 60 years after the beginning of the cold period in Greenland 15 . The abrupt increase of WAIS CH 4 concentration also occurred under the condition of stable temperature in Greenland, suggesting that the position of ITCZ was shifting further south at this time (Fig. 10F) 60 . Constrained by annually laminate chronology, three different phases were identified in the low latitude stalagmite records from Brazil, distinct to the relatively stable low-temperature state during the H4 in Greenland 18 . Our FR5 stalagmite record further support the inference that large-scale reorganization of atmospheric water cycle occurred in the low latitudes during the period of H events, while the temperature in Greenland remained stable [15][16][17] . However, recent researches on YD and Dansgaard-Oeschger events (D/O) since the last glacial indicate practically synchronous climate changes in high to low latitudes during the onsets of these abrupt interstadial events 61 . In addition, the onset of YD in the North Atlantic, Greenland, and the Asian Monsoon region are synchronous within a few decades (Fig. 8) 62 . All the above mentioned observations suggest possible different mechanisms for the correlation between high and mid-low latitudes climate change at the different background of H events, and YD, D/O events. This supposition argued the previous speculation which proposed that series centurial-millennial abrupt climatic events at low latitude during the LGP were dominated by the climate changes in high latitude. Our high quality records suggest possible different correlation between the high and mid-low latitudes under glacial and inter-glacial background. For example, in the late Holocene, at ~ 2 ka BP, the change of ASM could be modulated by the change in AMOC (Cheng et al. 63 ) or greenhouse gas forcing, which offsets summer insolation forcing 64 . More precise chronology and high-resolution constrained records and model simulations are essential to address this supposition.

Conclusions
We established an accurate chronology for the stalagmite-inferred evolution of the ASM over the past 37 ka from Furong Cave, southwestern China. The weak ASM events of the YD, OD, and H1-3, are precisely determined, with age uncertainties of few decades. Our record shows surprisingly different temporal responses of the monsoonal hydroclimate during the transitions of the onsets and recoveries of the weakened ASM of the first three H events. During the period of H1-3, the observed decoupling between the climate changes in the mid-low latitudes and Greenland indicates that the change of atmospheric water cycle in low latitudes was generated by the southward shift of ITCZ. On the other hand, under the deglacial boundary conditions, the onset of YD, OD and BA in the Northern high latitudes and the Asian Monsoon region are synchronous within a few decades (Fig. 8). There is possible different correlation between the high and mid-low latitudes under glacial and interglacial background. These observations argue the traditional opinion about the correlation between the climate change in high latitude and mid-low latitudes. The change of ASM could not be always triggered by the northern high latitudes. As the major energy and vapor source of ASM, the topical Indian Ocean and Pacific Ocean maybe play a crucial role in modulate the evolution of ASM, especially under the present background of global warming. www.nature.com/scientificreports/