Reconstructing Past Humidity Conditions Using Rice (Oryza sativa L.) archive

Variation of the Indian Summer Monsoon (ISM) in the perspective of climate change is a matter of concern given its pivotal role in sustaining the food and financial security of the Indian subcontinent. In this context, reconstruction of variation in moisture conditions (relative humidity (RH) and rainfall) associated with ISM has been carried out using tree ring archives primarily located over the Tibetan plateau [1,2]. However, limited studies have documented past variation of monsoonal RH over the main Indian landmass [3]. Here we present the potential of rice, the staple food of many ancient civilisations in Asia, to infer atmospheric moisture conditions prevalent over north-western and eastern India during 4,600 - 3500 BP. Since rainfall is a crucial factor which governs rice productivity over Indian subcontinent, it is cultivated primarily during the ISM season. The sensitivity of oxygen and carbon isotope composition in organic matter of rice and other plants (d18OOM, d13COM) to RH has been demonstrated [4,5]. Thus, the present study uses stable isotope technique to decipher paleo-RH conditions using rice grains retrieved from archaeological sites belonging to the Mature Harappan phase and Chalcolithic period. The archaeo- samples were subjected to SEM/EDAX analysis to identify morphology and elemental composition followed by analysis for both d18OOM and d13COM, after treatment for carbonate removal. The results obtained were compared with the observations of d18OOM and d13COM measured on modern day rice grains sampled across twenty four sites across India which experienced RH ranging from 68% to 89% (Kaushal and Ghosh (under preparation)). The study showed that decrease in RH was manifested as significant increase in d18OOM values together with increase in the crop's intrinsic water use efficiency, as calculated from d13COM. Based on this comparison, we inferred that the regional climate during the cultivation of the archaeo-rice samples was relatively moist, as compared to modern day. 1.Wernicke et al., Clim. Past 11, (2015). 2.Grießinger et al., Quat. Int. 1-8 (2016) 3.Managave 37,(2010). 4.Kaushal et al., Ecol. Indic. 61, (2016). 5.Winter et al., J. Exp. Bot. 33, 88-91 (1982)