17O-excess of grass leaf phytoliths across the North American prairies records late-growing season daytime relative humidity

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 ¹⁷O-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 ¹⁷O-excess of grass leaf phytoliths as a proxy of RH across the North American Great Plains. We tested the relationship between the ¹⁷O-excess of naturally grown Calamovilfa longifolia phytoliths and RH for different periods of the grass growing season. The results show that ¹⁷O-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: ¹⁷O-excess_{leaf phytoliths} = 4.14(±0.47) × RH (%) – 497(±26) (R² = 0.85, p-value<0.0001). To use the ¹⁷O-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 ¹⁷O-excess of stem water, which was determined by ¹⁷O-excess of stem phytoliths, shows similar values to the ¹⁷O-excess of precipitation in North America without evaporation, which therefore does not affect the ¹⁷O-excess of leaf phytoliths in North America. These results demonstrate that the ¹⁷O-excess of phytoliths is a promising proxy for regional growing season RH.