Evidences of first organized life-style and agrarian human subsistence are found since the beginning of the Indus Valley Civilization that spread along the Indus and the Ghaggar-Hakra river systems (present day India-Pakistan international border) (Giosan et al., 2012; Levey and Burkey 1959; Possehl 2002; Weber et al., 2010). While human subsistence during the early phase was mainly dependent on agricultural and pastoral activities, however the mature phase witnessed variety of artisan activities and trade of goods with other contemporary civilizations (Park and Shinde, 2014). It has been argued that the Indus Civilization experienced a significant shifts in the environmental conditions i.e. arrival of monsoonal dryness during the mature phase which possibly led migrations and decline of this civilization (Dixit et al., 2014; Enzel et al., 1999; Giosan et al., 2012; Kathayat et al., 2017; Mac Donald, 2011; Petrie et al., 2017; Pokharia et al., 2017; Possehl, 2002; Sarkar et al., 2016; Sharma et al., 2020a; Singh et al., 1971; Staubwasser et al., 2003; Wright, 2010). Did these environmental changes affect agricultural strategies and production especially from early to mature phase of Indus era? While palaeo-climatologists broadly in agreement about monsoonal dryness prevailing during the latter part of the mature phase (Dixit et al., 2014; Giosan et al., 2012; 2018; Gupta et al., 2003; Kathayat et al., 2017; Mac Donald, 2011; Sarkar et al., 2016; Staubwasser et al., 2003 Wasson et al., 1984), archaeo-botanists continue to debate its impact(s) on agriculture (Petrie and Bates, 2017; Petrie et al., 2017; Pokharia et al., 2011, 2014, 2017; Sharma et al., 2020a; Singh et al., 1971, 1974). These studies yield two schools of thoughts: (i) monsoonal climate played a major role in shaping up the Harappan life-style, especially its subsistence (Bates et al., 2017; Kaushal et al, 2019; Petrie and Bates, 2017; Petrie et al., 2016; Pokharia et al. 2011, 2014, 2017; Sharma et al., 2020a; Sarkar et al., 2016; Weber 2003) and (ii) Harappan farming ironically arrived during deteriorating monsoonal conditions and was adaptive in nature, since its inception. Geological records largely support the latter view (Berkelhammer et al., 2012; Dixit et al., 2014; Enzel et al., 1999; Giosan et al., 2012, 2018; Gupta et al. 2003; Kathayat et al., 2017; Mac Donald, 2011; Prasad and Enzel 2006; Staubwasser et al., 2003).
In this communication, we present stable isotopic C and N isotopic data of archaeological grains/seeds recovered from an archaeological site 4MSR of western Rajasthan (India) which tends to reinforce the latter hypothesis. In addition to these, we also measured C and N isotopes of soils derived from host habitational sediments. As the study site is a rural settlement and it can be well expected that archaeological/ habitational soil-sediment recovered from different cultural layers would have come from nearby arable land. Combined usage of C and N isotopes of crops vis-à-vis host soil-sediments of the studied archaeological sites was aimed to provide integrated information of cropping pattern and agro-hydrological status in arable Harappan agri-fields (Agnihotri et al., 2021; Wang et al., 2008). Where macro-botanical grains are expected to provide information about past crop-hydrological status, C and N isotopic data (along with their contents viz. TOC% and TN%) of soils recovered from different cultural periods may provide information about contemporary ecological status i.e. vegetation type (C-3 versus C-4) and in turn dryness of soils (Agnihotri et al., 2021; Araus et al., 1997; Ma et al., 2012; Parker et al., 2011; Peukert et al., 2012; Pokharia et al., 2017; Rosen et al., 1999; Simpson et al., 1999; Styring et al., 2016; Wang et al., 2008). This rationale is predominantly rely on the principle that C isotopic data is a good indicator of palaeo-hydrology and vegetation type (Ferrio et al., 2005; Ma et al., 2012; Parker et al., 2011; Wang et al., 2008)d isotopic data of plants/crops and soils can be governed by both environmental factors as well as agricultural amendments employed in agricultural fields (such as manuring, irrigation etc.) (Araus et al., 1997; Aguilera et al., 2008, 2017; Bogaard et al., 2013; Ferrio et al., 2005; Lee et al., 2005; Riehl et al., 2014; Styring et al., 2016, 2017; Wang et al., 2008; Wallace et al., 2013, 2015).
Different geological locations may have different sensitivities of these isotopic proxies (Jones et al., 2021 and reference therein), semi-arid conditions of the studied site 4MSR (Binjor) situated on the bank of dried Ghaggar river channel (~ 160 km away from major urban centre of Indus Civilization i.e. Harappa town) could be an ideal locale to investigate agricultural manifestations versus hydrological status of agri-fields in the past. For the 4MSR site, twelve (12) cultural layers were identified by archaeologists based on the material culture (such as pottery type, tools, seals etc.) and these cultural layers cover a total time span from ~ 2900 BCE to ~ 1800 BCE. Scientific chronologies of cultural layers were established by a combination of AMS and conventional radiocarbon dating of macro-botanical remains and soil organic matter recovered from nine strata (Sharma et al. 2020a and b).
It has to be noted that majority of the archaeological researches carried out so for on Indus valley sites provide wealthy information about their material-culture, advent of metal technologies, architectural prowess, trade, and overall socio-economic status of ancient settlers (Agarwal, 1971; Asthana, 1993; Bhan et al., 2002; Giosan et al., 2012; Kenoyer and Miller, 1999; Lal et al., 2003; Marshall, 1931; Possehl, 2002; Sana Ullah, 1931, 1940; Sharma et al., 2020b; Shinde, 2016; Vats, 1940). To the best of our knowledge, studies on Indus agronomy, crop-diversity, strategies and intensification are relatively sparse (García-Granero et al., 2016; Miller et al. 2006, 2015; Petrie and Bates, 2017; Petrie et al., 2016; Weber et al. 2010). This study provides first set of C and N isotopic data of Indus crops from a well dated archaeological site spanning the beginning of early phase to the end of mature phase. We also made an attempt to contextualize the derived palaeo-agricultural information with available knowledge gleaned from contemporary European and Chinese archaeological sites.
Chronology, archaeological background and macro-botanical details
The site 4MSR (29°12'87.2"N; 73°9'421"E), is situated at dry alluvial bed of Ghaggar river in the western Rajasthan (Fig. 1A). This area along the India-Pakistan border of western Rajasthan (District Anupgarh) is known to be a semi-arid region. Based on 14C dating of charcoal, archaeological grains (barley, wheat and rice) and habitational soil-sediments, chronology of all the twelve cultural layers were ascertained (please see supplementary Table 1S). The total time-span covered by the obtained chronology has been sub-divided into
Figure 1 (A) Map showing location of archaeological study site 4MSR, Rajasthan along with other Indus sites (created by using mapping software ArcGIS 10.3). (B) Contour map of the study site shows excavated trenches and upper right shows the aerial view of the excavated site taken by dronne. (C) Section shows the location of sample collection from the mature Harappan and transitional phase stratigraphic layers. (D) Trench representing the sample collection area from the early Harappan phase deposit.
three time-windows following traditional Indus chronology (Kenoyer 1991). According to this convention, three periods are: the early phase ca. ~2900 − 2600 BCE; transitional phase ~ 2600 − 2500 BCE and the mature phase ~ 2500 − 1800 BCE. Figure 1B, contour map of the site shows excavated trenches with stratigraphic details from natural bed sediment to modern humus i.e. decreasing depth in mean sea level unit. Section in the Fig. 1C and D show the location of soil-sediment sample collection from the early, transitional and mature phase stratigraphic cultural layers. As stated earlier, there are twelve distinct identified cultural layers. The upper layers (one to four) stored remnants of mature phase, while middle layers (five to seven) represented a transitional phase followed by deeper cultural layers belonging to the early phase of the Indus era.
Several studies have been conducted along the palaeo-channel of Ghaggar (erstwhile Saraswati) river, originating from northwest Himalayas and supposed to have flown southwest towards the Gujarat Kachchh region (Chatterjee et al., 2019; Ghose et al., 1979; Giosan et al., 2012; Gupta et al., 2004; Joshi et al., 1984; Kar et el., 2004; Lal, 2002; Marshall, 1931; Oldham, 1886, 1893; Possehl, 1999; Singh et al., 2017; Yashpal et al., 1980). This relict channel is thought to have catered several pre-historic phases of human civilizations of Indus culture in the north-western India (Giosan et al., 2012; Lal et al., 2003; Marshall, 1931; Mughal, 1997; Possehl, 2002, Sharma et al., 2020a; Vats, 1940 and references therein). The site 4MSR (locally known as Binjor) evolved from a typical agricultural settlement to a major rural industrial centre that manufactured copper artefacts, beads from semi-precious stones and a wide variety of terracotta products and may have exported them to other urban Harappan sites nearby and far-flung areas. A large series of different shapes of hearths, furnaces and kilns with a cluster of multi-purpose workshops for industrial activities indicate a rural metal-factory settlement at the site 4MSR (Sharma et al., 2020b). A range of recovered artifacts of gold and copper, seashells and terracotta such as pendant frames, earrings, beads, spacers, chisels, bangles, needles, fish hooks, big storage pots, twin pots, broken perforated jars, terracotta beads and broken terracotta bangles, weights, seals, terracotta toys (humped bulls) confirmed the industrial nature of the site (Sharma et al., 2020b).
The macro-botanical assemblage recovered from the site is shown in supplementary Fig. 1S (data adopted from Sharma et al., 2020a). The assemblage was comprising of variety of cereals and leguminous crops viz., Hordeum vulgare, Triticum aestivum/durum, Oryza sativa, Setaria sp., Pisum arvense, Lens culinaris, Cicer arietinum, Lathyrus sp. and Vigna sp. etc. Besides these, oleiferous and fibrous crops viz. Sesamum indicum, Linum usitatissimum and Gossypium sp. were also been recorded (Sharma et al., 2020a). Figure 2 (data adopted from Sharma et al., 2020a) presents pie charts depicting relative proportions of Indus summer (Oryza sativa, Setaria sp., Vigna sp., Gossypium Sp. and Sesamum indicum) and winter (Hordeum vulgare, Triticum sp., Pisum arvense, Lens culinaris, Cicer arietinum, Lathyrus sp. and Linum usitatissimum) crop species during early, transitional and the mature phase. It is noteworthy here that summer crops appear to have introduced mainly during the transitional phase (Fig. 2B). Intensification and diversification of various crops appear to dominate Indus croplands belonging to the mature phase (Fig. 2C). The 4MSR archaeological site has yielded various metal artifacts along with variety of domestic hearths from the cultural layers belonging to the mature phase of Indus era (Sharma et al., 2020a and b).