Indian summer monsoon variability in southern India during the last deglaciation: Evidence from a high resolution stalagmite δ18O record

Accepted Manuscript Indian summer monsoon variability in southern India during the last deglaciation: Evidence from a high resolution stalagmite δ18O record

Waseem Raza, Syed Masood Ahmad, Mahjoor Ahmad Lone, Chuan-Chou Shen, Drona Srinivasa Sarma, Anil Kumar PII: DOI: Reference:

S0031-0182(16)30472-2 doi: 10.1016/j.palaeo.2017.07.003 PALAEO 8353

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

15 September 2016 30 June 2017 3 July 2017

Please cite this article as: Waseem Raza, Syed Masood Ahmad, Mahjoor Ahmad Lone, Chuan-Chou Shen, Drona Srinivasa Sarma, Anil Kumar , Indian summer monsoon variability in southern India during the last deglaciation: Evidence from a high resolution stalagmite δ18O record, Palaeogeography, Palaeoclimatology, Palaeoecology (2017), doi: 10.1016/j.palaeo.2017.07.003

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ACCEPTED MANUSCRIPT Indian summer monsoon variability in southern India during the last deglaciation: Evidence from a high resolution stalagmite δ18O record Waseem Raza1, Syed Masood Ahmad*1, Mahjoor Ahmad Lone2, Chuan-Chou Shen2, Drona Srinivasa Sarma1 and Anil Kumar1 CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad – 500 007, India

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High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC),

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*Corresponding author’s: [email protected]

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Abstract

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Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan, ROC

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We present a high resolution record of Indian summer monsoon (ISM) variability during the last deglacial period. The record is based on high precision measurements of oxygen isotopes

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(δ18O) in 263 sub-samples from a ~18 cm long stalagmite (VSPM1) sample collected from the

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Valmiki cave in the southern India. The chronological framework is based on high precision UTh dating in 8 sub-samples at regular intervals. The stalagmite exhibits a variable growth rate

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ranging from ~0.03 to 0.80 mm/yr with 9 years as an average sampling resolution for δ18O. X-

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ray diffraction analysis shows that the sample is composed of aragonite mineral. The δ 18O record provides a strong evidence of abrupt changes in ISM activity between approximately 15,610 and

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13,160 yr BP (before 1950 AD). Highly negative δ18O values at ~15,610 and 15,250 yr BP are suggestive of a wet climate followed by less negative δ18O at ~15,240 to 14,750 yr BP indicating relatively dry climate. These dry and arid conditions are punctuated by two wet events at 15,030 and 14,910 yr BP. A marked increase in δ18O between approximately 14,660 and 14,370 yr BP suggests weakening in ISM activity, followed by a gradual decrease in δ18O between 14,370 and 13,560 yr BP indicating strengthening of ISM. Spectral analysis of δ18O time series suggests

ACCEPTED MANUSCRIPT strong influence of solar forcing on ISM activity. The amplitude variance of δ18O reveal decadal to centennial scale variability in monsoonal activity during the last deglaciation. Keywords

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Speleothem; Oxygen isotope; Paleomonsoon; Valmiki cave; Wavelet analysis.

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1. Introduction

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Speleothems, particularly stalagmites, are considered to be one of the very useful archives to reconstruct past climate and paleomonsoon regimes (Richards and Dorale, 2003;

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Fairchild et al., 2006; Scholz and Hoffmann, 2008) as they provide high resolution climate data

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sets (Wang et al., 2001; Dayem et al., 2010; Pausata et al., 2011). The speleothem based climate records are precious in sub-humid and semi-arid regions, where several other terrestrial climate

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proxies are incomplete (Ünal-İmer et al., 2015). The most frequently used climate proxy in

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stalagmites are isotopic composition of oxygen (δ18O) (McDermott, 2004; Yuan et al., 2004; Lachniet, 2009; Midhun and Ramesh, 2015) and a variety of trace elements (Fairchild and

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Treble, 2009). The oxygen isotope records based on stalagmites have been extensively used to

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reveal regional climatic changes worldwide (Tan et al., 2015 and references therein). Reconstruction of climatic variations over time involves selecting a suitable archive,

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precise measurements of a proxy variable, dating and statistically analyzing the proxy data to deduce the properties of the climatic processes. Understanding the past ISM variability at high spatiotemporal resolution and its relationship to regional climate change is of paramount importance for accurate model simulations to predict future climate/monsoon changes (Shakun et al., 2007). High resolution data ranges from annual to centennial time scales provided by speleothem δ18O records makes it an ultimate proxy for understanding the past monsoon variability. A large number of studies have been carried out on speleothems from various parts of

ACCEPTED MANUSCRIPT India to infer past monsoon variation during different time intervals (Yadava and Ramesh, 2001, 2005, 2006; Yadava et al., 2004; Sinha et al., 2005, 2007, 2011a, 2011b; Laskar et al., 2011, 2013; Lone et al., 2014). However high-resolution records pertaining to last deglaciation are very limited (Sinha et al., 2005; Lone et al., 2014)

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Oxygen isotopic composition of speleothem is a reliable proxy for the past variability in

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δ18O of meteoric water and is linked to the amount effect in tropical regions like India

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(Dansgaard, 1964; Rozanski et al., 1992; Neff et al., 2001; Wang et al., 2001, 2008; Burns et al., 2002; Yadava et al., 2004). Annual to sub-decadal high resolution δ18O records using

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speleothems from Asia have assisted our understanding of the nature and cause of abrupt change

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in ancient monsoon systems (Sinha et al., 2005; Shakun et al., 2007; Shen et al., 2010; Laskar et al., 2013). Several records relevant to ISM intensity during the last deglaciation provides

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information on monsoon activity during a period manifested by main global climate transitions.

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However most of these studies are based on marine sediments (Overpeck et al., 1996; Rashid et al., 2007; Govil and Naidu, 2010). But these records are inadequate by their spatiotemporal

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resolution and chronological uncertainties and therefore there is a necessity to produce high

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resolution ISM records, mainly from continental archives to investigate our perceptions of past climatic changes in the Indian sub-continent. The monsoon variability in the Indian subcontinent

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is regulated by coupled heating-cooling between the Himalaya and southern Indian Ocean (Chauhan, 2003; Rehfeld et al., 2012; Gupta et al., 2013) that also control the strength of the ISM in the Indian subcontinent (Webster et al., 1998; Chauhan, 2003). The ISM provides essential precipitation to southern Asia and is distinguished by a narrow coastal region along the Western Ghats, with moisture from the Arabian Sea and a monsoon zone around 20°N in northeastern India, where storms originate from the Bay of Bengal (Cai et al., 2015). The monsoon patterns

ACCEPTED MANUSCRIPT associated with India are: Southwest (SW) monsoon which brings rain over India during June to September and Northeast (NE) monsoon winds which transport winter rains to the southern part of the country (Duplessy., 1982; Kotlia et al., 2012). The earlier observations suggest that the ISM weakened during the cold events (Sinha et al., 2005; Gupta et al., 2013). In present study,

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we have used stalagmite δ18O variations to generate high resolution ISM records. It is essential to

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study abrupt climatic fluctuations across varying latitudes in order to identify and understand the

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possible leads or lags between the low and high latitudes and to determine the possible epicenter of these changes (Shakun et al., 2007). Speleothem records from southern China (Dykoski et al.,

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2005; Yang et al., 2010) are coupled to ISM or East Asian Monsoon activity or whether they are

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associated to both have been a topic of discussion so far (Tan et al., 2015) . There is only one speleothem based last deglacial record from southern India (Lone et al., 2014), however, it only

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covers 1 kyr time interval between 15.7 and 14.7 kyr BP. Therefore, in order to generate a longer

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deglacial record and to add our understanding of forcing mechanisms of climate change, we used δ18O variation in an ~18 cm long stalagmite from Valmiki cave dated between ~13,160 and

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15,610 yr BP to reconstruct the ISM variability in southern India.

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2. Geological setting, climate and location of the cave

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Valmiki cave is located in Boylavadlapalle village in Dhone Taluk of Kurnool district in the southern India (15° 09′N: 77° 49′E) (Fig. 1). The cave is one of the deepest caves in India. It has a very narrow entrance (Fig. 2a) at an elevation of 420 m above sea level (asl). Other details of cave settings are discussed elsewhere by Lone et al. (2014). The stalagmite sample VSPM1 was collected from the inner chamber of the cave (Fig. 2b), where humidity is about 95% and exposure to outside air is confined to a small crawl-in channel to the outer chamber. The cave is located in the Paleoproterozoic Vempalle stromatolitic dolomite (Chakrabarti et al., 2011). The

ACCEPTED MANUSCRIPT climate is semi-arid with mean annual rainfall of 670 mm (mean annual precipitation δ18OVSMOW = -4‰; 95% CI=0.3‰; Bowen and Revenaugh, 2003) and mean annual temperature of 28°C. Local climate is characterized by warm/wet summers and cool/dry winters. Most of the annual precipitation (80-90%) occurs during the Indian summer monsoon season (June to September)

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with almost invariable annual temperature, indicating that the summer monsoon rainfall

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dominates the annual precipitation at the cave site and rest of the precipitation falls during winter

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season (October to May) (Fig. S1, supplementary material). The vegetation in and around the

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cave site is predominantly open scrub and irrigated crops (Dar et al., 2011). 3. Material and Analytical procedures

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A stalagmite sample (VSPM1) of ~18 cm in length was collected from the Valmiki cave.

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The sample was cut along the growth axis and polished prior to its isotopic analysis and XRD measurements (Fig. 3). Three distinct layers from the top, middle and bottom of the sample were

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chosen for Hendy test to ensure that whether or not the sample was deposited under equilibrium

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conditions (Fig. 4). For this rationale, six subsamples were drilled and analyzed from three individual layers to determine the relationship between δ13C and δ18O and to check whether these

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two are associated as per the criterion of Hendy test for equilibrium condition.

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Eight different layers were selected for U-Th dating on a Thermo Fisher NEPTUNE multi-collector inductively couples plasma mass spectrometer (MC-ICP-MS) at the HighPrecision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, following the methods developed by Shen et al. (2003, 2012). The detailed chemical procedure is described in Shen et al. (2003). The instrumental methods involved in dating procedure and off-line data reduction are described in Shen et al., (2002, 2012). The age model was constructed by using StalAge model and linear

ACCEPTED MANUSCRIPT model as well. All the dates (yr BP), are in correct stratigraphic order with a 2-sigma error from ±26 to ±76 years. The StalAge model uses

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Th ages along with their associated age

uncertainties and stratigraphic information to improve the age model. It provides modeled ages with 95% confidence levels using a Monte-Carlo simulation. Because of the large error obtained 230

Th dates (13,253 and 14,597 yr BP) in top portion, the

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by StalAge model between the two

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linear interpolation method was used to model the ages for the δ18O values between 13,253 and

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14,597 yr BP whereas the StalAge modeling was carried out to determine the ages for the δ18O values between 14,597 and 15,669 yr BP. The linear model was also constructed for all δ18O

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values to compare it with the StalAge model. Our record of 263 δ18O values is representing wide

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range of temporal resolutions.

The mineralogical study of the sample was carried out by chemical staining on thin slabs

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of VSPM1 using Feigl’s solution (Kato et al., 2003). Three samples, ~1 gm each, were drilled

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using micro-drilling machine (Manix-180) from top, middle and bottom portion of the sample for X-ray diffraction analysis on a Philips X-ray diffractometer (XRD) using nickel filtered Cu Kα

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radiation at the CSIR-National Institute of Oceanography (NIO), Goa, India (Kessarkar et al.,

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2010). X-ray diffraction patterns have recorded 2θ position ranges at 20-50°. Continuous microdrilling of VSPM1 was carried out along the growth profile using a micro-drilling machine

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(Manix-180) with a 0.8 mm drill bit. The drilled sub-samples of VSPM1 were reacted with saturated orthophosphoric acid at 70°C in a vacuum system according to the method described by Ahmad et al. (2012) and the evolved CO2 was analyzed for δ18O measurements by Isotope Ratio Mass Spectrometer (IRMS). The δ18O measurements were carried out at the CSIRNational Geophysical Research Institute, Hyderabad, India using a MAT-253 model of IRMS attached with a Kiel IV automated carbonate device. To ensure reproducibility of the

ACCEPTED MANUSCRIPT instruments, international carbonate standards (NBS-18 and NBS-19) were analyzed with every batch of 8 samples. The calibration to the VPDB standard was achieved by repeated measurements of international reference standards NBS-18 and NBS-19 (Ahmad et al., 2008). Isotopic results are expressed against Vienna Pee Dee Belemnite (VPDB) standard and analytical

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precision was found to be better than ±0.06‰ for δ18O.

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Spectral analysis was performed by wavelet transform analysis to investigate the hidden

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periodic components in our δ18O record. The generated δ18O time series is unevenly distributed (unequally spaced) in time like most of the paleoclimatic time series. The data properties of

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climate time series that are essential to take into account are data spacing and the persistence in

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the data (Ólafsdóttir et al., 2016). Here we used PAST software v. 3.10 (Hammer et al., 2001) to

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execute wavelet transform analysis. The code in PAST is based on Torrence and Compo (1998).

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4. Results and Discussion

U-Th isotopic compositions and determined

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Th ages are displayed in Table 1. A high

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content of U (~6,500 to 13,500 ppb) and low content of 232Th (~400 to 4,000 ppt) is an indicator

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of very low contribution of detrital thorium and establishes the robustness of 230Th chronology of VSPM1. The ages are in stratigraphic order with 2σ error from ±26 to ±76 years and covering

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the time span from ~13,250 to 15,670 yr BP. The combination of StalAge and linear interpolation models were used to deduce the ages for the δ18O values of all the drilled samples. The StalAge model based on Monte-Carlo simulation is described in Scholz and Hoffmann (2011). Linear interpolation model was used to calculate the ages for each δ18O value corresponding to the period between 13,250 and 14,600 yr BP due to the large error obtained from the StalAge model for the top age (13,253 yr BP), while the StalAge model was applied to determine the ages for all δ18O values between 14,600 and 15,670 yr BP. As a result, the age

ACCEPTED MANUSCRIPT model is a combination of linear and StalAge modeled ages. The age model suggests that the growth rate of VSPM1 varies from ~0.03 to 0.80 mm/yr with the average growth rate of 0.07 mm/yr. Initially, at the bottom of the VSPM1, the growth rate is 0.14 mm/yr from approximately 15,610 to 15,250 yr BP. With a sudden increase of 0.77 mm/year at ~15,250 yr BP, the growth

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rate of VSPM1 declines to 0.07 mm/yr from ~15,240 to 14,780 yr BP, might be due to the less

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availability of drip water and amplified significantly to 0.52mm/yr from ~14,770 to 14,710 yr BP

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with the better availability of drip water due to the intensification in ISM activity. Subsequently, it decreases to 0.26 mm/yr for almost a century, from ~14,700 to 14,610 yr BP. The growth rate

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is very low (~0.03 mm/yr) in the top portion of VSPM1 starting from approximately 14,600 until

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13,250 yr BP. Linear age model constructed for all δ18O values is in well association with StalAge model except for the values between two initial ages (Fig. 5). All

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Th/U ages, except

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the initial age (13,253 yr BP), are plotted with respect to their sampling depths in Figure 6. XRD

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analysis of three subsamples taken from the top, middle and bottom portions of VSPM1 sample confirms aragonitic nature of the stalagmite (Fig. S2, supplementary material). An aragonitic

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precipitation may also be driven by water evaporation (Bar-Matthews et al., 1991; Railsback et

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al., 1994). This evaporation amplifies drip-water δ18O values and the aragonite precipitated from the solution would not replicate the primary δ18O signature (Lachniet, M. S., 2015). Lone et al.

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(2014) established that the aragonitic mineralogy of speleothems in Valmiki cave is due to dolomitic nature of host rock and not because of evaporation. The host rock of Valmiki cave is Vempalle dolomite because the cave is located in Paleoproterozoic Vempalle dolomite (Chakrabarti et al., 2011) with humidity about 100% and almost invariable temperature. High amplitude δ18O variability in VSPM1 stalagmite shows significant variations in ISM activity during the last deglacial period. The δ18O values in VSPM1 ranges from -0.31 to -

ACCEPTED MANUSCRIPT 3.23‰, with a mean value of -1.56‰ vs. VPDB. The δ18O time series, based on 263 measurements is plotted with respect to their StalAge derived ages in Figure 7. The sudden and abrupt fluctuations in δ18O in the oldest section of the stalagmite, between ~15,610 and 15,250 yr BP, suggest the unstable climate with intermittently wet and dry events at approximately 15,480,

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15,390, 15,290 and 15,530, 15,440, 15,340 yr BP respectively (Fig. 7). Moreover, this was a

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relatively wet period inferred from the average amplitude (-1.51‰) of δ18O variations during this

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period. The growth rate during this phase is nearly two times higher (0.14 mm/yr) than the average growth rate of VSPM1 (0.07 mm/yr). This wet period is then followed by a relatively

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dry period between ~15,240 and 14,750 yr BP (avg. δ18O = -1.42‰) with lesser growth rate of

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0.07 mm/yr. The dry period at approximately 15,240 to 14,750 yr BP was punctuated by two wet events at ~15,030 and 14,910 yr BP. A significant increase in ISM precipitation occurred during

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Termination 1a, between ~14,750 and ~14,660 yr BP, represented by a decrease of ~2.21‰ in

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δ18O. This period demonstrates the higher growth rate of 0.52 mm/yr. Thereafter, the δ18O gradually increases on centennial scale between ~14,660 and 14,370 yr BP. This arid phase is

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marked by an increase of ~2‰ in δ18O, showing weaker ISM activity which driest phase at

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~14,370 yr BP. The growth rate also decreases to 0.26 mm/yr during this period. Subsequently, a gradual decrease of ~2.9‰ in δ18O between ~14,370 and ~13,500 yr BP indicates strengthening

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in ISM activity during this period. Based on the average δ18O variations throughout this phase (1.63‰), we conclude that, the period between ~14,370 and 13,500 yr BP was a relatively wet period interrupted by two dry events at ~14,370 and 14,030 yr BP and three major wet events at approximately 13,890, 13,800 and 13,560 yr BP with maximum precipitation occurring at ~13,560 yr BP. Finally, during the most recent period in our record, between ~13,500 and 13,160 yr BP, the climate seems to be almost steady and moderately wet with overall lower δ18O values

ACCEPTED MANUSCRIPT (avg. δ18O = -2.43‰), representing an enhanced and invariable monsoonal activity between ~13,500 and 13,160 yr BP (Fig. 7). Hendy (1971) recommended that the oxygen isotopic composition in a single growth layer should remain identical if the sample was formed under isotopic equilibrium. However recent

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studies have demonstrated that stalagmites need not be in equilibrium with cave drip water for

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the stable isotopic values to provide precise information for past environmental conditions

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(Kotlia et al., 2012; Lone et al., 2014). Regardless of the Hendy test failure, speleothems can still be used as precipitation proxy mainly because its δ18O reveal rainfall rather than the temperature

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(Fleitmann et al., 2004; Dorale and Liu, 2009; Mühlinghaus et al., 2009).

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The δ18O and δ13C values in VSPM1 are found to be nearly constant along each studied layer (Fig. 4). This establishes that the deposition of VSPM1 occurred in isotopic equilibrium.

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Lone et al. (2014) also reported that the precipitation of speleothems in Valmiki cave occurs

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under isotopic equilibrium. The low value of correlation coefficient between δ18O and δ13C for all three layers further provides evidence that the degassing of CO2 was sufficiently slow to

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sustain the isotopic equilibrium throughout the process of aragonite precipitation. The low R2

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values in Figure 4c also indicate that the kinetic fractionation was negligible during aragonite precipitation. These tests confirm that VSPM1 faithfully records ISM precipitation and preserved

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important climatic information. The δ18O record of VSPM1 is in good association with the speleothem records from Chinese caves (Dongge cave, Dykoski et al., 2005; Yamen cave, Yang et al., 2010) representing the regional implication of VSPM1 as a climate indicator. The variations in ISM activity during the last deglaciation can be seen by high amplitude δ18O fluctuations in VSPM1 (Fig. 7) reflecting significant temporal changes in monsoonal precipitation. Previous studies on Indian

ACCEPTED MANUSCRIPT speleothems are suggestive of a direct relationship between the ‘amount effect’ and δ18O (Yadava et al., 2004; Sinha et al., 2005, 2007; Yadava and Ramesh, 2005; Berkelhammer et al., 2010; Lone et al., 2014). Kumar et al. (2010) also demonstrated the ‘amount effect’ in southern India during the summer monsoon. Moreover, a negative correlation between δ18O of

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precipitation and monsoon activity over the Asian monsoon region has been established by

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Vuille et al. (2005). Data provided by ‘Global Network of Isotopes in Precipitation’ (GNIP)

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reveals that the amount effect is the root cause of δ18O variability during the monsoon period in the Indian region (Bhattacharya et al., 2003). Our interpretation of δ18O variations in VSPM1

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stalagmite as an indication of oscillation in the strength of the ISM precipitation is based on all

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these studies.

A number of evidences exist to suggest sudden changes in past ISM activity with

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multidecadal to centennial scale variability (Berkelhammer et al., 2010; Sinha et al., 2005;

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Shakun et al., 2007). The abrupt δ18O variations in VSPM1 are indicative of multi-decadal variability in ISM precipitation. Our high resolution δ18O record provides the signature of

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foundation and development of rapid climate change during the last deglaciation. The studied

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stalagmite, VSPM1 is anticipated to have enriched (higher) δ18O values than the speleothem δ18O values of caves from South China and Himalayan region (Fig. 7) as the Valmiki cave is closer to

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Indian Ocean than these regions. The yearly average δ18O values of precipitation at the studied cave are high at present due to the nearness of this cave from northern Indian Ocean (Bowen and Revenaugh, 2003) (Fig. 1). The ISM has a very strong control on δ18O values of Chinese speleothems (Pausata et al., 2011, Lone et al., 2014). The comparison of our VSPM1 stalagmite record with the caves from South China (Dongge cave; Dykoski et al., 2005, Yamen cave; Yang et al., 2010) reveals that the

ACCEPTED MANUSCRIPT isotopic studies on Chinese speleothems reflects the changes in ISM activity shown by synchronized intensification in precipitation strength during the last deglaciation. Various studies made by several researchers also substantiate the same (Pausata et al., 2011; Wang and Chen, 2012). The analogous tendency of δ18O values in Indian and Chinese speleothem records during

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the last deglacial period confirms the synchronous variability in monsoon systems (Fig. 7). Wang

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and Chen (2012) demonstrated that the atmospheric water vapor over southern China throughout

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the summer period is mainly carried from the Indian Ocean. The ISM and EAM varies on different timescales depending on the environment and degree of the climate forcing revealed by

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speleothem records from eastern China (Cai et al., 2015). An anti-phase association between

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δ18O records of speleothems from China and India is also demonstrated by Tan et al., (2015).

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5. Comparison of VSPM1 record with a stalagmite from the same cave In Figure 8, we have plotted our VSPM1 δ18O time series with another deglacial δ18O

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record from the same cave stalagmite VSPM4 (Lone et al., 2014). The δ18O variations in VSPM1

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between 15,590 and 15,530 yr BP representing moderately wet climatic conditions, with two wet events at ~15,580 and 15,540 yr BP followed by a dry event at ~15,530 yr BP, are broadly

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consistent with the δ18O record of VSPM4 between 15,650 and 15540 yr BP with two wet events

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at ~15,630 and 15,600 yr BP followed by a dry event at ~15,540 yr BP within age uncertainties of the two stalagmite samples. Two dry events at ~14,750 and 14,710 yr BP, interposed by a wet event with strengthened ISM activity at ~14,720 yr BP registered in the VSPM1 δ18O record are precisely correlated with the dry events at ~14,760 and 14,710 yr BP, punctuated by a wet event at ~14,730 yr BP in VSPM4 stalagmite record. A small decrease in ISM activity between approximately 15,030 and 14,920 yr BP, marked by an increase of ~1.35‰ in δ18O of VSPM1 with average variation of -1.52‰ in δ18O is also encountered between 15,100 and 14,980 (avg.

ACCEPTED MANUSCRIPT δ18O = -1.48‰) in VSPM4 record with an increase of ~1.10‰ in δ18O. The average amplitude of δ18O variations (-1.52‰) between 15,030 and 14,920 yr BP in VSPM1 and ~ -1.48‰, between 15,100 and 14,980 yr BP in VSPM4, revealing almost equal precipitation occurred in both the records. In addition, the high temporal resolution in VSPM1 during Termination 1a, between

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~14,760 to 14,700 yr BP is also evident in VSPM4 record, approximately in the similar time

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period, between ~14,830 and 14,730 yr BP. Thus, most of the periods of weakening and

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strengthening in ISM precipitation are coinciding in two stalagmites within their age

6. Wavelet Analysis of VSPM1 δ18O time series

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uncertainties.

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The wavelet analysis of VSPM1 δ18O time series reveals a range of significant

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periodicities (Fig. 9). Firstly, the wavelet analysis was carried out for the whole δ18O time series

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(Fig. 9a). The average resolution of entire δ18O time series is ~9.3 years. The shape of the mother wavelet is set to Morlet (6db bandwidth) which usually performs best. The y-axis in Figure (9) is

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a logarithmic size scale (base 2). One unit on this axis corresponds to a doubling of the size scale. Two bands can be seen in Figure (9a) at around 5.5 and 4.3 on y axis (25.5 = 45 samples

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and 24.3 = 20 samples) corresponds to periods of about 420 (45 x 9.3) and 180 (20 x 9.3) years. A

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weaker band around 3.1 (23.1 = 9 samples) corresponds to a period of ~84 (9 x 9.3) years. Stuiver and Braziunas (1989) analyzed a long high precision

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C chronology using the Maximum

Entropy method of spectral analysis and identified a basic long term period of about 420 years and they link this periodicity with cyclic changes in the solar constant. The existence of a period ~180 years cycle has been proposed (Cohen and Lintz, 1974: Okal and Anderson, 1975) which has been reported to have found within sunspot numbers (Tiwari et al., 2012). A solar cycle of the order of 80-90 years in variation of the peaks in successive cycle has been reported by

ACCEPTED MANUSCRIPT Gleissberg (1958). This ‘Gleissberg cycle’ is associated with the change in the periods between successive peaks of solar activity. These cycles shows multi-decadal to centennial scale variations in our δ18O record and possibly reveals the influence of solar mechanism on ISM activity. Several previous studies also reveals the direct linkage of solar activity with ISM

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variability (Kodera, 2004; Gupta et al., 2005; Lone et al., 2014) on decadal and centennial

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timescales (Neff et al., 2001; Agnihotri et al., 2002; Fleitmann et al., 2003).

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In addition, wavelet analysis of δ18O time series covering the time span of 170 years (14,770 to 14,600 yr BP) was also performed separately (Fig. 9b). The purpose is to perceive

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whether there is any short term variations between 14,770 and 14,600 yr BP because of a very

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high resolution record (~2.1 years) obtained during this period. Two bands in Figure (9b) at around 4.7 and 4.0 on y axis (24.7 = 26 samples and 24 = 16 samples) correspond to periods of

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about 55 (26 x 2.1) and 33 (16 x 2.1) years. A weaker band at around 2.4 (22.4 = 5 samples)

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corresponds to a period of ~11 (5 x 2.1) years. The 11 years cycle may be associated with the solar sunspot cycle. These 55 and 33 year cycles may also be the multiple amplification of the

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sun spot cycle. All three cycles can be attributed to solar forcing on ISM activity between 14,770

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and 14,600 yr BP. A 33 year cycle has also been reported by Lone et al., (2014), from a speleothem record of same Valmiki cave. In addition, several speleothem records from China

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have been shown to have a 33 year cycle (Ku and Li, 1998; Paulsen et al., 2003; Zhang et al., 2013). A 32 year cycle is also reported by Tiwari et al. (2012) from a tree ring record of the western Himalayan region. Hence, the sudden fluctuations in ISM activity probably reflect solar forcing mechanism/sun spot variability during the last deglacial period. 7. Conclusions

ACCEPTED MANUSCRIPT A ~2,500 yr δ18O record from a southern Indian stalagmite VSPM1 shows high amplitude variations in ISM precipitation during the last deglaciation. This is the first absolutely dated high resolution deglacial record from the southern India covering a time span of 2.5 ka. The data reveals changes in ISM activity on decadal to multi-decadal timescales during last

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deglacial period. The oxygen isotope record provides a strong evidence of abrupt changes in ISM

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activity between 15,610 and 13,160 yr BP. Wavelet analysis of δ18O time series suggest 420,

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180, 80 year cycles indicating solar control on Indian summer monsoon dynamics for the studied time interval. Also 55, 33 and 11 year cycles were obtained from a short high resolution δ18O

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record between 14,770 and 14,600 yr BP reveals sun spot variability during this period.

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Therefore the solar variability may have resulted in sudden changes in ISM activity during the last deglaciation with decadal to centennial phases.

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8. Acknowledgements

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The authors are thankful to Dr. V. M. Tiwari, Director, CSIR-NGRI, Hyderabad. The first author is thankful to Ms. K. Zainab, S. Singh and colleagues at paleoclimate group, CSIR-

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NGRI for their support. We are also grateful to the Council of Scientific and Industrial Research

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(CSIR) for financial support provided through 12th plan projects INDIAS IDEA, INDEX and SHORE. U-Th dating was supported by grants from Taiwan ROC MOST (104-2119-M-002-003,

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MOST-105-2119-M-002-001) and the National Taiwan University (105R7625). 9. References

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Figure Captions:

Fig. 1. Map showing locations of Valmiki cave and other caves discussed in this study. The map

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is displaying modern weighted mean annual precipitation δ18O over the Asian region (Lone et al., 2014).

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Fig. 2. (a) Image of the cave entrance, and (b) Sketch diagram of Valmiki cave interior showing

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sample locations of VSPM1 (this study) and VSPM4 (Lone et al., 2014). Fig. 3. VSPM1 stalagmite. Black line represents the drilling points for δ18O measurements. Red

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lines represent eight 230Th dating samples. Blue lines show layers of Hendy test and green boxes

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mark XRD analysis points.

Fig. 4. Hendy test results. Red, blue and green lines represent three dissimilar test layers. (a)

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δ18O values, (b) δ13C values, (c) shows correlation between δ18O and δ13C. R2 in (c) represents that the sample formed under equilibrium conditions. Fig. 5. Comparing Linear and StalAge models for the ages between 13,160 and 15,610 yr BP. Fig. 6. The StalAge model. Green line represents modeled ages and red lines showing the 95% confidence limits. The black dots with vertical error bars represent eight standard error. Text in blue color shows the average growth rate in mm/yr.

230

Th samples with

ACCEPTED MANUSCRIPT Fig. 7. Low VSPM1 δ18O values (green) reflects intense rainfall events at 15,480, 15,390, 15,290, 15,030, 14,910, 14,710, 14,660, 13,890, 13,800, 13,560 and 13,500 yr BP and high δ18O values at 15,530, 15,340, 15,300, 14,750, 14,370 and 14,030 yr BP, suggest extremely dry events. Comparison between the δ18O time series of VSPM1, with southern China cave records

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Yamen (red; Yang et al., 2010) and Dongge (blue; Dykoski et al., 2005). The vertical bars

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represent the coincident variability in δ18O of VSPM1 with cave records from southern China. 2σ

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age errors in the records are highlighted by horizontal error bars with the same color (before

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1950 AD).

Fig. 8. (a) Complete δ18O record of VSPM1 plotted against VSPM4 δ18O time series and (b)

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Comparison of δ18O variations in VSPM1 with VSPM4 (Lone et al, 2014) during the same time

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period.

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Fig. 9. Spectral analysis of VSPM1 δ18O time series by wavelet transform in PAST v. 3.10. Graph (a) represents the wavelet analysis of complete δ18O time series whereas (b) showing the

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wavelet analysis of δ18O time series from 14,770 to 14,600 yr BP respectively. Signal power is

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shown in color at the extreme right of the graphs (a) and (b).

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Uranium and Thorium isotopic compositions and 230 Th ages for Valmiki speleothem sample (India) by MC-ICPMS, Thermo Electron Neptune, at NTU.

g

238

U

Th

232

a

ppb

ppt

238

Th/

d U 234

U

[

]

a

measured

activity

c

± ± 0.0004 1.9 0.21198 3

Va1_0 2

± ± 9. 0.1170 10100 10 815.1 8

822. 8

± ± 0.0003 1.9 0.23148 1

Va1_0 3

± ± 6. 4. 0.0974 6594.5 1 786.5 9

909. 0

± ± 0.0003 1.9 0.24445 3

Va1_0 4

± ± 8. 2490. 6. 0.1031 7911.5 1 7 0

913. 8

± ± 0.0005 2.4 0.24702 0

Va1_0 5

± ± 10260. 9. 5. 0.0909 0 5 649.3 2

Va1_0 6

± ± 4. 0.1046 13236 13 671.0 6

894. 5

± ± 0.0003 2.0 0.25048 4

Va1_0 7

± ± 7. 3. 0.1569 8527.9 8 708.1 2

879. 5

Va1_0 8

± ± 3. 0.1441 12616 40 386.9 4

915. 2

Age (yr BP)

d

uncorrecte d

Age (yr BP)

d

Uinitia

l

corrected c,e

corrected

b

± 13,25 3 3 2

859.1 ± 2.0

47289 ± 572 14,598 ± 27

± 14,59 2 7 7

857.5 ± 1.9

33792 ± 213 14,720 ± 26

± 14,71 2 8 6

947.7 ± 1.9

12937 ± 39 14,845 ± 38

± 14,84 3 1 8

953.0 ± 2.5

66230 ± 538 15,121 ± 28

± 15,12 2 0 8

976.4 ± 1.9

81475 ± 561 15,232 ± 28

± 15,23 2 1 8

934.0 ± 2.0

± ± 0.0004 1.7 0.25002 0

49649 ± 234 15,333 ± 30

± 15,33 3 2 0

918.6 ± 1.7

± ± 0.0009 5.3 0.26006 4

13979 ± 9 1258 15,669 ± 76

± 15,66 7 9 6

956.8 ± 5.5

AN

M

ED

PT

234

Th

7948 ± 24 13,259 ± 32

US

827. 4

CE

232

ppm

± ± 7. 3672. 8. 0.0656 8352.1 9 6 9

AC

Th/ ]

Va1_0 1

935. 5

230

T

ID

230

[

IP

Weigh t

CR

Sampl e

± ± 0.0003 1.8 0.25421 7

Chemistry was performed on 3rd Dec., 2015 (Shen et al., 2003), and instrumental analysis on MC-ICPMS (Shen et al., 2012). Analytical errors are 2 sigma of the mean.

ACCEPTED MANUSCRIPT a 238

[ U] = [235U] x 137.818 (±0.65‰) (Hiess et al., 2012); 234U = ([234U/238U]activity - 1) x 1000.

b

δ234Uinitial corrected was calculated based on 230Th age (T), i.e., δ234Uinitial = δ234Umeasured X eλ234*T, and T

is corrected age. c 230

[ Th/238U]activity = 1 - e- λ230T + ( 234Umeasured/1000)[ λ230/( λ230 - λ234)](1 - e-( λ230 - λ234) T ), where T is the

T

age.

IP

Decay constants are 9.1705 x 10-6 yr -1 for 230Th, 2.8221 x 10-6 yr -1 for 234U (Cheng et al., 2013), and

d

CR

1.55125 x 10-10 yr -1 for 238U (Jaffey et al., 1971).

The degree of detrital 230Th contamination is indicated by the [230Th/232Th] atomic ratio instead of

e

US

the activity ratio.

Age (yr BP, before 1950 AD) corrections for samples were calculated using an estimated initial

AN

atomic 230Th/232Th ratio of 4 ± 2 ppm. Those are the values for a material at secular equilibrium,

AC

CE

PT

ED

M

with the crustal 232Th/238U value of 3.8. The errors are arbitrarily assumed to be 50%.

ACCEPTED MANUSCRIPT Highlights: 

An absolutely dated high resolution deglacial record from southern Indian region covering a time span of ~2.5 ka indicating high amplitude variability in Indian summer monsoon (ISM) precipitation. The oxygen isotope record reveals decadal to centennial variations in ISM activity.



Wavelet analysis of δ18O time series shows ~420, 180 and 84 years periodicities

CR

IP

T



AC

CE

PT

ED

M

AN

US

indicating solar control on Indian summer monsoon dynamics.