What is left behind: Advancing interpretation of pastoral land-use in Harappan Gujarat using herbivore dung to examine biosphere strontium isotope (87Sr/86Sr) variation

The analysis of strontium isotopes in archaeologically preserved biological tissues is most productive when these can be compared to naturally occurring variation in strontium isotope ratios across the physical landscape. Such work is in its infancy in South Asia. Here, we report on the first attempt to monitor 87Sr/86Sr variation across the Indian state of Gujarat using herbivore dung. As it incorporates plant material from throughout an individual animal's grazing range, herbivore dung averages local isotopic variation in palatable vegetation and is therefore an ideal material for use in studies involving domestic livestock. In our analysis of 125 dung samples from 38 sampling locations across the study area, 87Sr/86Sr values and geographic variation are commensurate with expectations based on regional geology. The values that we report are significantly different from those reported for both ecosystem elements and archaeological humans and livestock that have been published for other regions of the Indus Civilization (2600-1900 BC). No individual humans or livestock in these studies appear to have their origins in Gujarat. The present study further allows for more detailed interpretations of our previously published study of strontium isotope ratios in faunal remains from the walled Indus manufacturing center of Bagasra in Gujarat (Chase et al., 2014b). Specifically, it is now clear that while most livestock show very little movement within the period of enamel formation, their places of origin were scattered throughout central Saurashtra, adjacent to the site, suggesting that a portion of the livestock consumed at Bagasra were initially raised in the many small unexcavated villages in the area. There is little evidence for the procurement of livestock from further afield within the region and none for livestock originating outside the region. These results demonstrate that monitoring geographic 87Sr/86Sr variation using herbivore dung has the potential to significantly advance archaeological interpretation of livestock mobility in the past and is applicable anywhere that modern livestock graze on natural vegetation.