Triple oxygen isotopes of grass phytoliths as a proxy for relative humidity

Minger Guo

Abstract

The ¹⁷O-excess of grass phytoliths has high potential to be good proxy for past Relative Humidity (RH), which is an important climate parameter that is hard to accurately estimate. Strong correlations between ¹⁷O-excess of leaf phytoliths and RH have been previously demonstrated in modern plants in growth chamber experiments and in natural West African and Mediterranean climates. In this study, we focus on the potential of ¹⁷O-excess of grass leaf phytoliths as a proxy of RH across the North American Great Plains. A strong correlation between the ¹⁷O-excess of naturally grown grass-leaf phytoliths and late-growing season daytime RH (RH range: 29-72%) in sixteen grasslands across North America can be expressed as: ¹⁷O-excess_{leaf phytoliths} = 4.14(±0.47) x RH – 497(±26) (R² = 0.85, p-value17O-excess of estimated bulk soil phytolith assemblages that contain a mixture of phytoliths from leaves and stems, which can be expressed as: ¹⁷O-excess_{bulk phytoliths} = 2.1(±0.29) x RH – 322(±16) (R² = 0.82, p-value17O-excess of newly formed leaf phytoliths correlates with the seasonal RH, but the ¹⁷O-excess of phytoliths from leaves at the end of the growing season only records the late-growing season RH, and these relationships do not differ in phytolith mixtures that include different grass species. In addition, the similar triple oxygen isotope composition of precipitation and stem water estimated from stem phytoliths indicate that soil water taken up by plants is not influenced by evaporation in this humid continental climate and can be represented by the ¹⁷O-excess of monthly precipitation. The variation of ¹⁷O-excess of precipitation is negligible in interpreting ¹⁷O-excess of leaf phytoliths across North America. Overall, both the temporal and spatial relationships between ¹⁷O-excess of phytoliths and RH this study demonstrated that the ¹⁷O-excess of grass phytolith is a good proxy for RH across the North American grasslands, without complications from soil water evaporation, varied precipitation isotopic compositions, different and variable RH, and mixing of phytoliths from transpiring and non-transpiring tissues and different species.