17O-excess of grass leaf phytoliths across the North American prairies records late-growing season daytime relative humidity Guo, M. E. Alexandre, A. Longstaffe, F. J. /Peugeot, Christophe Webb, E. A. Phytoliths Relative Humidity Transpiration North American prairies O-17-excess Because it is an important climate parameter, an accurate proxy is required for the reconstruction of past Relative Humidity (RH). Triple oxygen isotopes of grass-leaf phytoliths have a high potential for this purpose. A strong correlation between RH and O-17-excess of grass leaf phytoliths has been previously demonstrated from calibration experiments in growth chambers and at natural Sub-Saharan African and Mediterranean sites. Here we focus on the potential of 17 O-excess of grass leaf phytoliths as a proxy of RH across the North American Great Plains. We tested the relationship between the O-17-excess of naturally grown Calamovilfa longifolia phytoliths and RH for different periods of the grass growing season. The results show that O-17-excess of leaf phytoliths has a strong correlation with late-growing season (July-August) daytime RH, which varies from 29 to 72 % across North America. The correlation can be expressed as: O-17-excess leaf phytoliths = 4.14(+0.47) x RH (%) - 497(+26) (R-2 = 0.85, p-value<0.0001). To use the O-17-excess of bulk phytoliths buried in soils and sediments as a quantitative proxy of past RH, future calibration studies on soil phytoliths are needed to evaluate the contribution of non-transpiring phytoliths. The(17)O-excess of stem water, which was determined by O-17-excess of stem phytoliths, shows similar values to the O-17-excess of precipitation in North America without evaporation, which therefore does not affect the O-17-excess of leaf phytoliths in North America. These results demonstrate that the O-17-excess of phytoliths is a promising proxy for regional growing season RH. 2025 text https://www.documentation.ird.fr/hor/fdi:010092600 fdi:010092600 Guo M. E., Alexandre A., Longstaffe F. J., Peugeot Christophe, Webb E. A.. 17O-excess of grass leaf phytoliths across the North American prairies records late-growing season daytime relative humidity. 2025, 671, 122484 [11 p.] EN AMERIQUE DU NORD