Accepted Manuscript Precipitation variability over Northwest Himalaya from ∼4.0 to 1.9 ka BP with likely impact on civilization in the foreland areas Bahadur S. Kotlia, Anoop K. Singh, Lalit M. Joshi, Kamini Bisht PII: DOI: Reference:
S1367-9120(17)30650-8 https://doi.org/10.1016/j.jseaes.2017.11.025 JAES 3318
To appear in:
Journal of Asian Earth Sciences
Received Date: Revised Date: Accepted Date:
31 March 2017 14 November 2017 14 November 2017
Please cite this article as: Kotlia, B.S., Singh, A.K., Joshi, L.M., Bisht, K., Precipitation variability over Northwest Himalaya from ∼4.0 to 1.9 ka BP with likely impact on civilization in the foreland areas, Journal of Asian Earth Sciences (2017), doi: https://doi.org/10.1016/j.jseaes.2017.11.025
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Precipitation variability over Northwest Himalaya from ~4.0 to 1.9 ka BP with likely impact on civilization in the foreland areas Bahadur S. Kotlia*, Anoop K. Singh, Lalit M. Joshi and Kamini Bisht Centre of Advanced Study in Geology, Kumaun University, Nainital, 263002, India *Corresponding Author: Bahadur S. Kotlia (
[email protected]) Abstract
We present a stalagmite based high resolution climatic record between ~4.0 and 1.9 ka BP from Dharamjali Cave in Pithoragarh (Kumaun Himalaya), an area which is influenced primarily by the Indian Summer Monsoon (ISM) and supplemented by the Indian Winter Monsoon (IWM). The chronology of the 41.5 cm long DH-1 stalagmite was constructed using a StalAge model on six have higher
232
230
Th/U dates. However, some samples
Th concentrations, leading to an increase in the final age uncertainties,
hence the chronology supports only the possible climatic boundaries. However, the significance of this work lies in its being one of the rare studies of speleothems to reconstruct high resolution climatic changes in the NW India for the Upper Holocene. The δ18O values, ranging between -5.3‰ to -10‰, show a large variation, compared to the areas dominated by a single monsoon, and this can be ascribed to the two sources of moisture (e.g., ISM and IWM) in the study area during the Upper Holocene. The sample consists of aragonite except for two sections of calcite growth from 0-7.3 cm and 8.513.5 cm from the top. The climatic reconstruction, within the age uncertainties, indicates strengthened precipitation from 4.0 ka BP with a sharp drop (>-2‰) in δ18O values, peaking at ~3.7 ka BP. A gradual decline in precipitation is observed from ~3.7- 3.0 ka BP with possible droughts, centered at ~3.4, ~3.2 and ~3.0 ka BP. Subsequently, climatic amelioration took place between ~3.0-2.9 ka BP, showing fluctuating trend in δ18O values with comparatively more rainfall, possibly generated by the IWM in the form of thunderstorms and hailstorms from ~2.9-2.7 ka BP. Precipitation declined from ~2.7 to 2.4 ka BP with a decadal scale major drought event, strongest in the present data set, at ~ 2.5-2.4 ka BP, whereas, an abrupt drop in stalagmite δ18O values from ~2.4-2.3 ka BP
1
points to increased precipitation intensity. Thereafter, the precipitation gradually decreased until ca. 2.1 ka BP with one of the driest events at ~2.1 ka BP. A century scale increasing trend in the precipitation intensity is observed from ~2.1-2.0 ka BP, following which the precipitation again declined. Accordingly, five sudden drought events are documented, centering at ~3.4, ~3.2, ~3.0, ~2.5-2.4 and ~2.1 ka BP. A gradual reduction in precipitation from ~3.7-3.0 ka BP coincides with reduction and deurbanisation and step-wise disintegration of the Harappan civilization along the Indus-Ghaggar-Sarasvati valleys in the foreland areas of northwestern India.
Keywords: Indian Summer Monsoon (ISM); Indian Winter Monsoon (IWM); Northwestern Himalaya; Speleothem isotopes; Upper Holocene; Harappan civilization
1. Introduction Two-thirds of the Earth’s population lives in areas dominated by the Asian Summer Monsoon (ASM) and its economic status depends on the monsoon intensity. For instance, the core locations of the Indian Summer Monsoon (ISM), Ocean and the South and East of Indian subcontinent, receive majority of annual rainfall through the summer months. During spring, the pressure gradient increases between the fast warming Asian continent and Indian Ocean, leading to an overturning of prevailing winter atmospheric circulation (Yang et al., 1992; Gupta and Anderson, 2005). The emerging and strengthening southwesterly winds transport large amounts of moisture from the Indian Ocean to the Indian subcontinent as far north as north-eastern China and Japan (Sampe and Xie, 2010; Cheng et al., 2012). Thus, the monsoon is linked to specific heat of the land and water and to the north south movement of the Intertropical Convergence Zone (ITCZ). Through the boreal summer, the ITCZ attains its northernmost position and moves southwards in the following months. Nevertheless, the ISM core sites are mainly controlled by changes in the precipitation; further the areas at present located near the northernmost segments of the ITCZ are also influenced by the seasonality of precipitation (Fleitmann et al., 2003). The millennial-scale variations in ISM strength and the past position of the ITCZ are
2
already well-understood (e.g., Sinha et al., 2005). However, for the regions, situated at the northwestern margin of the ISM and the northernmost extent of the boreal summer ITCZ, information about decadal to centennial shifts in the ITCZ is limited. Hence, stalagmite δ18O values are highly sensitive to changes in the position of the ITCZ and to precipitation strength. Therefore, the seasonal migration of the ITCZ controls the arrival, extent and withdrawal of the rainy season in the subtropics and its configuration affects the tropical and also extra tropical climate on short to long term time scales. Despite the major sensitivity of the ISM and accompanying contribution of the IWM (Kotlia et al., 2012, 2016, 2017; Liang et al., 2015), little information is available in terms of decadal to centennial-scale past climate variability and associated forcing mechanisms in the Indian Himalaya.
Further, the winter monsoon is an important, if not major, component of the Himalayan climate system and its input to the Himalaya has been well documented by Dimri et al. (2016). Western disturbances (WDs) originate in the Mediterranean Sea as extra-tropical cyclones and gradually travel across the Middle East from Iran, Afghanistan and Pakistan and finally enter northwestern India where these show reduction in frequency and intensity southwards and eastwards (Yadav et al., 2012). It is clearly understood that on millennial as well as decadal timescales, past fluctuations in the Himalayan glaciers have been controlled almost equally by the variations in ISM and WDs (Benn and Owen, 1998). Further, the climate influenced by the WDs has been considered as a significant driver of past and future changes at least in High Asia (Mölg et al., 2014). The first comprehensive tree-ring-based precipitation record indicating an increase in winter and spring precipitation in the NW India during the recent centuries (Yadav et al., 2017) supports this hypothesis. Interestingly, modelers have also noticed an increase in precipitation over NW India during winters of the twentieth century (Dash et al., 2007). This observation is further supported by Syed et al. (2009) revealing that the WDs have been amplified in southwest Asia from 1951–2000, owing to a strengthening of eastward moving synoptic disturbances from the Mediterranean. Based on modeling as well as palaeoclimatic studies, it has been demonstrated that the winter monsoon plays a critical
3
role in the Himalayan climate, hence the previously used term as Western Disturbances (WDs) was replaced by the Indian Winter Monsoon (IWM) (see Laat and Lelieveld, 2002; Yadav et al., 2013; Dimri, 2013; Dimri and Chevuturi, 2016; Dimri et al., 2016 and references therein). We may further state that a number of earlier researches on past climatic changes have not given attention to the role of the IWM compared to the ISM in the Indian Himalaya. However, recent high resolution speleothem researches have demonstrated a reasonable contribution of the IWM in the palaeo records of the Indian Himalaya and have shown that the pattern of the mid-upper Holocene climate of the Himalaya is inversely correlated with that of Peninsular India (Kotlia et al., 2012, 2015, 2016, 2017; Sanwal et al., 2013; Liang et al., 2015; Dimri et al., 2016).
Another key problem is the migration and collapse of Indus-Ghaggar-Sarasvati civilizations in the foreland regions that have been correlated with a gradual reduction in precipitation resulting in water scarcity (Tripathi et al., 2004; Berkelhammer et al., 2012; Kotlia et al., 2015). For instance, the Harappan civilization flourished all along the Indus, Ghaggar and Sarasvati River valleys (e.g., Mohenjo-daro, Harappa, Kalibangan etc.), older Harappan sites were concentrated in the lower reaches of the Sarasvati River (e.g., Banawali), while later Harappan settlements thrived in its upper reaches in the Siwalik domain. The River Sarasvati originates in the glaciers of the western Garhwal (Uttarakhand) in the same province as our Dharamjali Cave site and the sediments of the NW Himalayan foreland basin have been considered as a possible sediment source for the Ghaggar-Sarasvati River system (Najman et al., 2000; Robinson et al., 2001; Tripathi et al., 2004). Notably, the IWM is the chief rain producing system during non-monsoonal months over NW India including Uttarakhand and is also responsible for bringing rains to low-lying areas and snowfall to the higher reaches. It is the major and most important source for production of the Rabi crop, particularly wheat, which is the staple diet of inhabitants of northwestern India. Therefore, this study will also examine the possible relation of reduced precipitation resulting in IWM generated food insufficiency with breakdown of the above mentioned civilization using stalagmite δ18O data as the stalagmites have been consistently used to understand the high resolution past monsoon
4
intensity (e.g., Yadava et al., 2004; Fleitmann et al., 2003, 2007; Sinha et al., 2007, 2011; Berkelhammer et al., 2010; Kotlia, et al., 2012, 2015, 2016, 2017; Duan et al., 2013; Sanwal et al., 2013; Joshi et al., 2017). This is because carbonate cave deposits in the monsoonal regions are believed to record the intensity of monsoon precipitation as the δ18O of the carbonate tracks the isotopic signatures of precipitation, with lighter δ18O swings indicating a stronger monsoon and increased rainfall (Li et al., 1998, 2011; Wang et al., 2001, 2005; Yin et al., 2014).
2. Study Area and Climate Perspectives Dharamjali Cave (29031’27.8”N: 80012’40.3”E; altitude, 2,200m) is located 15 km south west of Pithoragarh town in Kumaun Himalaya, India (Fig. 1). The 35m long sub-vertical cave with the main entrance of 4.5 x 2 m, becomes narrower in the last chamber (ca 0.8 x 0.9 m) and has been described in detail by Sanwal et al. (2013) and Rajendran et al. (2016). The cave falls within the area of subtropical climate and is formed in Thalkedar limestone (e.g., Nautiyal, 1990) of Riphean-Vendian age (Valdiya, 1980). The thickness of the host rock around the cave is about 50m with an additional 3-5m thick brownish black soil. However, the rains directly fall on the soil above the cave ceiling. Stalagmite DH-1 was collected from deep in the cave, about 33.9m from the main entrance and was inactive when sampled (October, 2013). During sampling, the temperature and humidity inside the cave were 30C and 96% respectively. The average annual precipitation (19012000) at Pithoragarh town is 1,245mm (http://www.indiawaterportal.org/ met_ data). Long term mean for δ18O values (1960-2012) of monsoon rains at New Delhi (about 300 km SW of the cave) indicate that precipitation during the ISM months (i.e., JuneSeptember) has generally lower δ18O values compared to IWM months (i.e., OctoberMay) (see Fig. 2a). Also, the higher rainfall leads to lower temperature and minimum evaporation under reduced kinetic fractionation, thus maintaining the lower δ18O values (Kotlia et al., 2015). Therefore, δ18O values can be used as a primary proxy with lower values indicating higher precipitation and vice-versa even though the amount effect also
5
adds to the total precipitation. In contrast to the data set from New Delhi, present day rainwater δ18O values at Uttarkashi (GNIP, http://www.iaea.org/water) in the vicinity of cave show considerably higher rainfall during IWM months (see Figs. 2a, b). Further, average δ18O values (1960-2006) during ISM months at New Delhi are -6.8‰ (GNIP, http://www.iaea.org/water). However, New Delhi is about 200 amsl and Dharamjali Cave is 2,200 amsl. In general, the altitude difference is responsible for lower δ18O values at higher elevations with a gradient of about -1.5 to -2.4‰/km (Gonfiantini et al., 2001). Preliminary work on the isotopic composition of ground waters across the Tibetan Plateau (Yu and Zhang, 1981) presents a clear evidence for a strong “altitude effect” on the δ18O of precipitation of ~0.3‰/100 m elevation. More recently, Garzione et al. (2000a, b) and Rowley et al. (2001) have further verified this “altitude effect” in the Himalayan front. In general, the δ18O decreases by ~0.29‰ per 100m hypsometric basin elevation change, a value comparable to empirical data for the Himalayan front (Hren et al., 2009). If we assume a mean value of -2‰/km for the Himalaya, the mean δ18O value of precipitation at the cave site should be about 4‰ lower compared to New Delhi due to the altitude effect. Therefore, -10.8‰ is advocated to be the most probable δ18O value of ISM rain in the cave vicinity. Present day average ISM δ18O values at Uttarkashi are -5.16‰ (Fig. 2b), much higher than expected and this may indicate the contribution of two moisture sources at the cave site, argument for which is elaborated in section 4.2.
3. Methodology
The DH-1 stalagmite was halved along the growth axis using a diamond coated band saw. For
230
Th/U dating, six samples were drilled with layer thickness
