Thesis Format

Integrated Article

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

Author

Minger GuoFollow

Degree

Doctor of Philosophy

Program

Geology

Supervisor

Webb, Elizabeth

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.

Summary for Lay Audience

Relative Humidity (RH) is the percentage of water vapor in the air relative to the saturation state at the same temperature, which is required for the estimation of the atmospheric water concentration, and it is an important parameter in the natural greenhouse effect and the global water cycle. However, global climate models have trouble reconstructing past continental RH properly, so it is urgent to find a good proxy that can directly indicate changes in continental RH.

Water evaporation and transpiration is determined by RH. During plant transpiration in tissues like leaves, leaf water is progressively lost from the base to the tip of the leaf, and isotopes undergo equilibrium and kinetic fractionation that increases the δ¹⁷O, δ¹⁸O, and of δ²H, and decreases the d-excess and ¹⁷O-excess of remaining leaf water. Compared to other parameters, ¹⁷O-excess can record the most accurate RH because of its insensitivity to temperature. Phytoliths are amorphous silica microparticles formed during plant growth from partially evaporated plant water. Paleo-plant water can’t be directly measured but the ¹⁷O-excess of phytoliths records the ¹⁷O-excess of evaporated plant water, thus it has a high potential for being a good proxy for paleo-RH. Previous calibrations have shown a clear relationship between the ¹⁷O-excess of leaf phytoliths and RH in controlled growth chambers and in natural dry and Mediterranean climates. However, investigating the temporal and spatial relationships between ¹⁷O-excess of modern grass phytoliths and RH over various climates across North America and over one growing season is essential to determine whether the ¹⁷O-excess of phytoliths can be used as a worldwide paleo-RH indicator.

Here, we tested the spatial and temporal relationships between ¹⁷O-excess of phytoliths and RH in 16 grasslands across North American at the end of the growing season and monthly during the whole-growing season at one location. Our results show that the ¹⁷O-excess of grass phytolith is a good proxy for RH in North American grasslands, and it has a great potential to be used as a proxy to reconstruct paleoclimate.

Recommended Citation

Guo, Minger, "Triple oxygen isotopes of grass phytoliths as a proxy for relative humidity" (2024). Electronic Thesis and Dissertation Repository. 9986.
https://ir.lib.uwo.ca/etd/9986