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

Abstract

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.