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Oxygen Isotope in Archaeological Bioapatites From India: Implications to Climate Change and Decline of Bronze Age Harappan Civilization

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Oxygen Isotope in Archaeological Bioapatites From India: Implications to Climate Change and Decline of Bronze Age Harappan Civilization

Anindya Sarkar et al. Sci Rep.

Abstract

The antiquity and decline of the Bronze Age Harappan civilization in the Indus-Ghaggar-Hakra river valleys is an enigma in archaeology. Weakening of the monsoon after ~5 ka BP (and droughts throughout the Asia) is a strong contender for the Harappan collapse, although controversy exists about the synchroneity of climate change and collapse of civilization. One reason for this controversy is lack of a continuous record of cultural levels and palaeomonsoon change in close proximity. We report a high resolution oxygen isotope (δ(18)O) record of animal teeth-bone phosphates from an archaeological trench itself at Bhirrana, NW India, preserving all cultural levels of this civilization. Bhirrana was part of a high concentration of settlements along the dried up mythical Vedic river valley 'Saraswati', an extension of Ghaggar river in the Thar desert. Isotope and archaeological data suggest that the pre-Harappans started inhabiting this area along the mighty Ghaggar-Hakra rivers fed by intensified monsoon from 9 to 7 ka BP. The monsoon monotonically declined after 7 ka yet the settlements continued to survive from early to mature Harappan time. Our study suggests that other cause like change in subsistence strategy by shifting crop patterns rather than climate change was responsible for Harappan collapse.

Figures

Figure 1
Figure 1
(A) Map of Northwest India and Pakistan (created by Coreldraw x7; http://www.coreldraw.com) showing the locations of main Harappan settlements including phosphate sampling site of Bhirrana, Haryana, IWIN precipitation sampling station at Hisar and two paleo-lakes Riwasa and Kotla Dahar studied earlier (see Fig. 3 and text for details). Black dotted lines represent 100 mm rainfall isohyets. Approximate trace of dried paleo-channel of ‘Saraswati’ (dashed white lines in Fig. 1A) is also shown. Black arrow indicates the direction of monsoon moisture transport from Bay of Bengal. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). Figure created by CorelDRAW Graphics Suite X7 (http://www.coreldraw.com) (B) Panoramic view of the excavation of mature Harappan stage at Bhirrana view from North-east (photograph reproduced with the permission of Archeological Survey of India).
Figure 2
Figure 2
(A) Settlement pattern of period 1A (pre-Harappan Hakra) along with locations of trenches at Bhirrana mound. Figure created by CorelDRAW Graphics Suite X7 (http://www.coreldraw.com) (B) Tentative lateral time correlation of different cultural levels between the trenches based on radiocarbon and OSL dates. Contours are in cm. above msl. Only the trench YF-2 yielded continuous bioapatite samples (see text).
Figure 3
Figure 3
(A) Arabian Sea upwelling intensity as monsoon index. (B) Carbonate δ18O and lake level records from paleo-lakes Riwasa and Kotla Dahar, Haryana (refs and 6). (C) Bioapatite based paleo-meteoric water δ18O (monsoon proxy) record at Bhirrana along with characteristic archaeological and faunal elements from different cultural levels. Note monsoon intensification from ~9 ka to 7 ka BP (blue shaded region and arrows) and monotonous decline from ~7 ka to 2.8 ka BP (brown shaded region, red arrows); dotted pink lines denote approximate time correlation of these two phases across the sites. (D) Bhirrana chronology based on archaeological evidences, 14C and new OSL dates. OSL dates are from trench YF-2; the oldest 14C date is from correlatable level of trench ZE-10 (E) Conventional chronology; note new dates, archaeological evidences and climate pattern are all suggestive of a much older age for the beginning of Harappan civilization at Bhirrana.
Figure 4
Figure 4
(A) Seasonal variation in temperature and rainfall and (B) Time series of precipitation δ18O at IWIN station Hisar, close to Bhirrana archaeological site.

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