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Abstract

Understanding Indian Summer Monsoon (ISM) variability in the context of global climate is critical for predicting and forecasting its impacts in an evolving future climate change scenario, thereby achieving sustainable economic growth. This study presents speleothem oxygen isotope-based inferences regarding changes in ISM conditions during the marine isotope stage 2 (MIS-2). The study suggests a strong summer monsoon during global warm phases and a weak monsoon in cold intervals. The ISM was weak during 26–19 kyr BP, while strengthened between 19 and 12.8 kyr BP. The western Himalaya suffered a rapid reduction in ISM precipitation at around 12.8 kyr BP linked to the global cooling and weakened oceanic circulation at the onset of the Younger Dryas (YD) interval. Sea-ice expansion in the North Atlantic led to cooling in the western Himalaya through westerlies and caused a reduction in ISM precipitation in the Indian subcontinent. After the YD, the climate became warm with the onset of the Holocene, and ISM conditions significantly improved.

Keywords

Bhiar Dhar cave Himalaya speleothem climate change South Asia

HIGHLIGHTS

Indian Summer Monsoon variability in the western Himalaya during the MIS-2

Rapid weakening of ISM at the onset of the Younger Dryas

Strong ISM conditions during the Bølling–Allerød interval

INTRODUCTION

Monsoon winds are vital for heat and water transport across the atmosphere, hydrosphere, and lithosphere. The monsoon controls the hydrological budget directly for a large region of the Earth, supports agriculture and industries, and ensures potable freshwater supply to the human population. The importance of monsoon-driven moisture increases manifold for densely populated developing and agrarian economies in South Asia, Africa, and South America.

India, with the second-largest population in the world, is also largely dependent on summer monsoon rainfall for its economic well-being via agricultural production and growth of allied sectors (Singariya & Sinha, 2015). Additionally, precipitation is critically important for the hydrological budget (glaciers’ expansion/retreat, rivers’ discharge, water storage in lakes and aquifers) and the vast ecology of the Indian subcontinent (Kumar et al., 2021). About 80% of India’s annual rainfall is received between June and September, through the Indian Summer Monsoon (ISM) (Parthasarathy, 1960), except for a few southern areas. Untimely or excess/deficit ISM rainfall had caused socio-political turmoil, infrastructure collapse, and cultural disturbances in the past (Dobhal et al., 2013; Dutt et al., 2018; Kotlia et al., 2018; Sengupta et al., 2025). A below-average summer rainfall in past decades caused drying of springs and lowering of the static water level in the mountains and plains of the Indian subcontinent (Thakur et al., 2022). Recently, the frequency of extreme rainfall events and erratic onset/retreat has increased, which could be attributed mainly to recent global warming. However, an unprecedented rise in such events raised concerns about our understanding of monsoon dynamics, forcing mechanisms, and future predictions.

Meteorological studies have developed a significant understanding of the forcing factors of ISM conditions, enabling the prediction and forecast of rainfall in India (Gadgil, 2003). However, the meteorological data in India is available for the last 175 years and may not be sufficient to record impacts of millennium-scale global climate events on ISM conditions in Earth’s recent past, such as during the Bølling–Allerød interstadials, Younger Dryas (YD) cold event, and 8.2 kyr cold event, etc. Therefore, proxy records are developed to supplement the understanding of past meteorological studies. In the present study, we have developed a speleothem oxygen isotopes proxy record from the Bhiar Dhar cave, Uttarakhand, to comprehend ISM changes in the western Himalaya between 26 and 11 kyr BP, covering mainly the marine isotope stage 2 (MIS-2), and significant forcing driving these changes.

STUDY AREA

The study site, Bhiar Dhar cave, is located near Gorchha village in the state of Uttarakhand, India (30.79°N, 77.78°E, 2290 m a.s.l., Figure 1). The cave has been formed in dolomitic Deoban Limestone of Meso to Neoproterozoic age (Tewari, 1997) and previously studied for geotourism potential and past climate variability (Dutt et al., 2025; Sengupta et al., 2022, 2023). The regional climate ranges from subtropical humid to a moderate type, with distinct seasonality. At present, the moisture supply is dominated by the ISM, contributing around 1300 mm (75%–80%) of the annual precipitation. Mid-latitude westerlies (MLWs) in boreal winters provide some moisture in the form of snowfall mainly between November and February.

Figure 1.

Location map showing the site of the Bhiar Dhar cave. The dashed line indicates the ITCZ during the ISM season. Curved arrows show winds related to the ISM, and straight arrows indicate westerlies.

MATERIALS AND METHODS

A 115 mm long stalagmite sample, BH-4, was collected from the Bhiar Dhar cave. The sample’s growth had paused, and the water drip that fed it was dormant. The stalagmite was taken from the deepest part of the cave to reduce the chances of kinetic fractionation in isotopes during speleothem growth. The temperature inside the cave was at ~14.1°C, which is close to the average regional temperature, and the relative humidity was above 90% which indicates fair chances of speleothem deposition in isotopic equilibrium.

Chronology

The chronology of the stalagmite BH-4 was established using three U-Th dates measured at the University of Minnesota. ~200 mg powder was recovered for every subsample from the central growth axis, which was then dissolved in 0.1N hydrochloric acid (HCl). This was followed by spiking of the solution with ²³³U, ²³⁶U, and ²²⁹Th spikes to measure concentrations of various isotopes precisely. Subsequently, U and Th fractions were extracted using column chromatography from the spiked solution. The measurements for various U and Th isotopes were then carried out. The complete procedure, instrumentation, and calculations used to obtain the U-Th ages in carbonate speleothems are explained by Cheng et al. (2013), and the same procedure was followed for the present analysis.

Oxygen isotopes

For the studied part of the stalagmite BH-4, 47 samples have been analysed for stable oxygen isotopes at the Wadia Institute of Himalayan Geology, Dehradun. The powdered subsamples were extracted close to the central axis to avoid possibilities of kinetic fractionation. The measurement procedure is adopted from Gupta et al. (2019). Each subsample (~80 µg) was taken in a precleaned glass vial, which was then closed and flushed internally using Helium gas (99.999% pure) to remove ambient gases. This was followed by the addition of approximately 80 µl of 100% orthophosphoric acid (H₃PO₄) into the vial to dissolve carbonate subsamples kept at 72°C for an hour. After the complete reaction, CO₂ is sent to the Isotope Ratio Mass Spectrometer for measurements. Repeated analysis of random samples and standards, NBS-18, Carrara marble, and Merck Carbonate were also performed to observe the accuracy of data. The findings are presented using standard delta (δ) notation by the Vienna Pee Dee Belemnite (VPDB) standard. For oxygen isotopes, the analytical uncertainty (2 sigma) is close to 0.1‰.

RESULTS

The U-Th dating results are given in Table 1. Except for the top age, the error for the other two ages is less than 1%. The age-depth model for the stalagmite BH-4 was established using three U-Th ages in Constructive Proxy Records from Age Model (COPRA) (Breitenbach et al., 2012) algorithm in MATLAB (Figure 2). The chronology indicates a continuous growth of the sample during 26–11 kyr BP.

Table 1.

²³⁰Th dating results of sample BH-4 from the Bhiar Dhar cave, Uttarakhand, northwest Himalaya.

Sample Number	Depth (mm)	²³⁸U (ppb)	²³²Th (ppt)	²³⁰Th/²³² Th Atomic × 10⁻⁶	δ²³⁴U* (measured)	²³⁰Th/²³⁸U (activity)	²³⁰Th Age (yr) (uncorrected)	²³⁰Th Age (yr) (corrected)	δ²³⁴U_initial** (corrected)	²³⁰Th Age (yr BP)*** (corrected)
BH4_T	6	148.1 ± 0.1	2,0961 ± 420	20 ± 0	211.7 ± 1.6	0.1728 ± 0.0019	16,703 ± 201	13,277 ± 2,435	220 ± 2	13,213 ± 2,435
BH4-M	32	336.4 ± 0.5	765 ± 16	1,327 ± 28	200.3 ± 1.7	0.1830 ± 0.0010	17,951 ± 110	17,896 ± 116	211 ± 2	17,832 ± 116
BH4-a	57	370.2 ± 0.5	110 ± 4	1,3885 ± 547	197.2 ± 1.8	0.2501 ± 0.0009	25,383 ± 108	25,376 ± 108	212 ± 2	25,314 ± 108

Notes: The error is 2σ.The method is based on Cheng et al. (2013). Years before the present (ad1950). Bold characters highlight the ²³⁰Th age (year; corrected) and ²³⁰Th age (year B.P.; corrected) used for constraining the chronology in the study.

Figure 2.

Age-depth model for the stalagmite BH-4 reconstructed using three U-Th ages shown in Table 1. The horizontal bars indicate 2σ uncertainty in ²³⁰Th ages.

The δ¹⁸O values for BH-4 fluctuate between −10.2‰ and −2.9‰ (Figure 3). A wide range in δ¹⁸O time series suggests large-scale climatic changes in the region during the MIS-2. Between 26 and 11 kyr BP, three different phases of δ¹⁸O changes have been observed (Figure 3). In the first phase, the δ¹⁸O values remained less negative between 26 and 19 kyr BP with no major excursion and devoid of any sudden fluctuation. In the second phase, from 19 kyr BP, δ¹⁸O values became more negative, which continued until around 12.9 kyr BP. The δ¹⁸O ratio registers a secular trend in this phase too, except for a small excursion at ~17.3 kyr BP. During the third phase, at around 12.9 kyr BP, the δ¹⁸O ratio again shifted towards less negative values abruptly and reached the heaviest δ¹⁸O values of the record at 11.6 kyr BP. After 11.6 kyr BP, δ¹⁸O values again decreased abruptly.

Figure 3.

δ¹⁸O proxy record of Indian Summer Monsoon variability from (a) Bhiar Dhar cave, Uttarakhand (present study), (b) Timta cave, Uttarakhand (Sinha et al., 2005), (c) Mawmluh cave, Meghalaya (Dutt et al., 2015), and (d) Bittoo cave, Uttarakhand (Kathayat et al., 2016) concerning GISP two ice core record (Stuiver & Grootes, 2000).

Cave studies and moisture monitoring in the Indian subcontinent indicate that δ¹⁸O fluctuations in speleothems track the oxygen isotope ratio of regional precipitation, which in itself is controlled by changes in moisture source, amount of rainfall, distance of moisture travelled, etc. (Breitenbach et al., 2010; Sinha et al., 2015). The δ¹⁸O ratio in speleothems and regional precipitation in India is linked, as a more negative δ¹⁸O ratio suggests higher regional rainfall and a less harmful δ¹⁸O ratio indicates lower precipitation (Dutt et al., 2015; Kaushal et al., 2018). In the Bhiar Dhar cave region, ISM is the primary source of moisture, and therefore δ¹⁸O variations in the carbonate speleothems are linked mainly with changes in ISM conditions. The stronger the ISM and enhanced moisture input from the Bay of Bengal (BoB) branch, the more negative oxygen isotope ratio is recorded in summer precipitation over the Bhiar Dhar cave region (Dutt et al., 2025; Sinha et al., 2015). The contribution by the westerlies is less significant at present, but in the past, these might have played an important role in determining the δ¹⁸O variations and regional precipitation changes (Dutt et al., 2025).

DISCUSSION

Based on the δ¹⁸O time series, changes in the ISM in the Bhiar Dhar cave region can be categorised into three time slices:

26–19 kyr BP

The less negative δ¹⁸O values between 26 and 19 kyr BP suggest weak ISM conditions in the western Himalaya. ISM rainfall remained below average throughout this interval, as marked by a high δ¹⁸O ratio. Several cave records indicate reduced rainfall in the ISM and East Asian summer monsoon regimes in this time interval (Cheng et al., 2009; Dutt et al., 2015; Kathayat et al., 2016). Marine records from the BoB and the Arabian Sea also indicate significantly decreased discharge by Himalayan rivers (Govil & Naidu, 2010; Kumar, Dutt, et al., 2020; Saraswat et al., 2014), as a result of decreased rainfall. The sea level lowered and sea surface temperature decreased by around 3°C on average from the pre-industrial era in this interval (Govil & Naidu, 2010; Saraswat et al., 2013). This phase of reduced ISM rainfall corresponds to the Last Glacial Maximum (LGM). During the LGM, solar insolation decreased, which led to the expansion of sea ice in the North Atlantic Ocean (Berger & Loutre, 1991; Stuiver & Grootes, 2000). The cold signal is transported through westerlies, resulting in more snow deposition in the Himalaya (Dutt et al., 2015). This led to reduced convective activities in the Indian Ocean and decreased ISM precipitation in the Himalayan region. Several studies related to the chronology of glacial landforms indicate glacier advancement in the western Himalaya during and post-LGM interval. This cold phase led to decreased ISM and increased westerlies contribution to precipitation in the western Himalaya (Kumar, Shukla, et al., 2020; Mehta et al., 2012). The extent and time of the LGM is, however, highly debated in the glacial records of the Himalaya.

19–12.9 kyr BP

The δ¹⁸O values show a long-term gradual trend towards more negative values from 19 to 12.9 kyr BP, suggesting strengthened ISM conditions (Figure 3). Several continental paleoclimatic records as well as marine archives suggest higher rainfall and enhanced river discharge to the Indian Ocean because of stronger monsoon conditions in this interval (Cheng et al., 2009; Dutt et al., 2015; Govil & Naidu, 2011; Gupta et al., 2021). This is because of higher solar insolation associated with convective activities and more evaporation from the ocean, which led to increased rainfall in the Indian subcontinent, including the western Himalaya. These intervals encompass the Bølling–Allerød interstadials, showing marked enhancement in summer monsoon rainfall across South Asia and China (Sinha et al., 2005). However, there is asynchronity in the timing of monsoon strengthening in different regions of the ISM realm, which needs to be addressed, either because of differential ISM behaviour in different regions (Dutt et al., 2021) or due to age uncertainties. Marine records suggest the beginning of deglaciation after the LGM and increasing temperature at ~19 kyr BP (Saraswat et al., 2013). However, cave studies from India and China indicate continuous weakening of the summer monsoon following the LGM, with the weakest conditions at ~16–15 kyr BP, which was coupled with Heinrich event 1 (Dutt et al., 2015; Gupta et al., 2003). Another cave record from the same region also indicates weak ISM conditions during the Heinrich one event (Kathayat et al., 2016). Glacier records from the western Himalaya indicate post-LGM advancement, which is suggested as a delayed response of glaciers to climate change (Kumar, Shukla, et al., 2020). Our data has high age uncertainty, and further studies are required to decipher ISM changes between 19 and 12.9 kyr BP.

12.9–11.5 kyr BP (YD interval)

The δ¹⁸O values show a marked increase at ~12.9 kyr BP. This increase in δ¹⁸O values was very rapid and continued up to 11.5 kyr BP. The sudden increase of the δ¹⁸O values suggests abrupt weakening of the ISM, which is an expression of one of the most rapid climate events in Earth’s history, known as the YD. The YD, a cold event, continued for about 1300 yrs (12.9 to 11.6 kyr BP), which is believed to have initiated due to massive freshwater flooding in the North Atlantic from Lake Agassiz or Mackenzie valley which weakened the Atlantic Meridonial Overturing Circulation (AMOC) by 15%–30% and resulted into rapid sea-ice expansion in the North Atlantic (Broecker et al., 1989; Condron & Winsor, 2012). The signal of YD cooling was transported rapidly in a few decades to Antarctica, through both atmospheric and oceanic processes (Cheng et al., 2020). For the Indian subcontinent and Himalayan region, westerlies transport the cold signal from the North Atlantic, coupled with meridional southward positioning of the Intertropical Convergence Zone (ITCZ) (Cheng et al., 2020; Dutt et al., 2015; Gupta et al., 2003; Jaglan et al., 2021). The cooling in the Himalayas led to less evaporation in the oceans and less convective activity, ultimately reducing ISM rainfall in the region.

The evidence of ISM weakening during the YD has earlier been reported from the northeast Himalaya (Dutt et al., 2015; Gupta et al., 2021). The weakening of the reduced summer monsoon rainfall in Asia is well established in a few records from other regions also (Cheng et al., 2009, 2013). However, its evidence is sparse and less discussed in the western Himalaya (Kathayat et al., 2016; Rawat et al., 2012) except for the glacial records, where age uncertainty is high (Kumar, Shukla, et al., 2020). The present record expressed the YD in the western Himalaya in terms of reduced ISM rainfall. Very high δ¹⁸O values further indicate the simultaneous increased contribution of westerlies moisture to the precipitation in the western Himalaya, as suggested by Dutt et al. (2025). This record implies high-latitude northern hemisphere forcing of the precipitation in the western Himalaya on the centennial to millennial scale. However, we understand that age uncertainty in the present record is high, especially concerning the YD event. More precisely, dated high-resolution records are needed to further decipher the timing and characteristics of the YD in the western Himalaya.

CONCLUSIONS

Our record of ISM variability suggests enhanced/reduced summer monsoon conditions linked with the global warm/cold conditions. The ISM was weak from 26 to 19 kyr BP and strengthened between 19 and 12.9 kyr BP. The western Himalaya experienced a rapid decrease in ISM precipitation at ~12.9 kyr BP, which was linked to global cooling and hindered oceanic circulation during the YD interval. Cooling in the North Atlantic led to decreased temperature in the western Himalaya due to strong westerlies, leading to decreased ISM precipitation. Stronger ISM conditions prevailed in the region at the onset of the early Holocene warming.

Footnotes

Acknowledgements

SD and SG thank the Director,Wadia Institute of Himalayan Geology,for the necessary laboratory and infrastructure facilities required for this work. SD and JJ collected the sample,AKG and HC were involved in U-Th dating,and SG performed the oxygen isotopes analysis. SD wrote the first draft of the manuscript. AKG and HC have contributed to the preparation and finalisation of the manuscript. All the authors have participated in the preparation of the manuscript and have agreed to publish it in the present form. The contribution number from the Wadia Institute of Himalayan Geology for this article is WIHG/405.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: Wadia Institute of Himalayan Geology has provided infrastructure and funding during field work,oxygen isotopes analysis,and writing of the manuscript. This work was partially funded by a core research grant (CRG/2022/008935) to SD,and J C Bose Fellowship (JBR/2021/000019) to AKG from the Anusandhan National Research Foundation,Govt. of India.

ORCID iD

Som Dutt

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Sudden weakening of the Indian Summer Monsoon during the Younger Dryas

Abstract

Keywords

HIGHLIGHTS

INTRODUCTION

STUDY AREA

Location map showing the site of the Bhiar Dhar cave. The dashed line indicates the ITCZ during the ISM season. Curved arrows show winds related to the ISM, and straight arrows indicate westerlies.

MATERIALS AND METHODS

Chronology

Oxygen isotopes

RESULTS

230Th dating results of sample BH-4 from the Bhiar Dhar cave, Uttarakhand, northwest Himalaya.

Age-depth model for the stalagmite BH-4 reconstructed using three U-Th ages shown in Table 1. The horizontal bars indicate 2σ uncertainty in 230Th ages.

DISCUSSION

26–19 kyr BP

19–12.9 kyr BP

12.9–11.5 kyr BP (YD interval)

CONCLUSIONS

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

ORCID iD

References

²³⁰Th dating results of sample BH-4 from the Bhiar Dhar cave, Uttarakhand, northwest Himalaya.

Age-depth model for the stalagmite BH-4 reconstructed using three U-Th ages shown in Table 1. The horizontal bars indicate 2σ uncertainty in ²³⁰Th ages.