Stable isotopic composition of rice grain organic matter marking an abrupt shift of hydroclimatic condition during the cultural transformation of Harappan civilization

摘要

Several hypotheses have been proposed to solve the conundrum of the cause of transition of Harappan civilization to a de-urbanized form in its Late Phase. In view of this, high-resolution off-site palaeoclimatic records along with archaeological findings provide strong evidence of an abrupt climate change similar to 4000 yr BP (before present) that coincides with the civilization's cultural transition. The present study investigates whether this climatic shift recorded in palaeoclimate archives (speleothem, lake and marine sediments) at remote locations can be traced to the human settlements of Harappan civilization. This was accomplished by analyzing the remains of rice cereal-a highly water sensitive crop. The analysis involved measuring carbon and nitrogen isotopic compositions (delta C-13, delta N-15) of well-preserved rice grains recovered from archaeological sites belonging to the Harappan civilization and other contemporary regional cultures, representing the time windows between 4520 and 3400 yr BP. The parameter of intrinsic water use efficiency (WUEi) was used as an index for water availability in the rice crop's growth environment and was ascertained based on delta C-13 values measured in the bulk grain organic matter (OM). The observed WUEi values ranged between 49 and 69 mu mol mol(-1) and captured the variation in water availability. The delta N-15 values ranged from 5 parts per thousand to 6.5 parts per thousand, thereby allowing us to confirm the role of climate in controlling delta C-13 variation in the archaeological grains. The present findings will extend the usage of archaeobotanical cereal remains to deduce environmental changes at human settlements and will further our understanding of the process of Harappan cultural adaptation in response to decline in southwest monsoon at the Middle-Late Holocene boundary.