Fingerprint of Seasonal Relative humidity in Rice (Oryza sativa L.): Potential for Paleoclimate Archive

Abstract:

Seasonal variability of relative humidity (RH) condition during southwest monsoon (SWM) is strongly related with availability of moisture for rainfall over the continental region. There are ongoing efforts to reconstruct the past monsoonal condition based on geochemical and isotopic records as proxies [1]. However, limited archives have been successfully retrieved for reconstructing moisture condition during SWM [2].

Potential of plants as climate archives and the mechanism involved in transfer of climate signature to the stable isotope composition of oxygen (δ¹⁸O) has been demonstarted successfully [3]. Here we are introducing δ¹⁸O of rice grain bulk organic matter (δ¹⁸O_OM) as a new proxy for precise determination of RH during SWM. Rice is a seasonal crop and widely cultivated over the Indian subcontinent during SWM. Here we present δ¹⁸O_OM of rice grains collected from 15 locations from different climatic zones over the Indian subcontinent, ranging from semi-arid to humid-perhumid. These samples were collected from the harvest of the crop grown at the time of SWM for the period 2012-2014. Each of these climatic zones are characterized by unique range of RH values, which is expected to leave distinct oxygen isotopic signature in the rice grain OM. We compared the δ¹⁸O_OM values with δ¹⁸O of precipitation water, RH and temperature during the period of cultivation. Precipitation δ¹⁸O values were obtained from were obtained from OPIC [4]. Our observations document a significant relationship between δ¹⁸O_OM and RH (R² =0.62, p<0.001). When the RH level fluctuated from 65% to 88%, δ¹⁸O_OM tended to vary between 31.5 ‰ to 15.4‰. However, the humid regions with rainfed crop showed significant relationship between δ¹⁸O_OM and precipitation δ¹⁸O. Remaining stations being fed by the both rain and irrigation from river/ groundwater, dampen the rainfall isotope signature in δ¹⁸O_OM.

This approach can be extended in deriving RH of regions of rice cultivation by measuring δ¹⁸O of grain OM. The study also demonstrated that δ¹⁸O of well-preserved rice grains can serve as a proxy for RH condition of ancient agrarian civilisations, with confidence.

[1] Ghosh and Brand, Int. J. Mass spectrom. 2003 [2] Yu et al., Geophys. Res. Lett. 2014; [3] Barbour et al. Funct. Plant Biol. 2007 [4] OIPC, http://waterisotopes.org