Triple oxygen isotopes of rice (Oryza sativa L.) phytoliths as a quantitative proxy for relative humidity

Relative humidity is a key climate parameter that varies with global temperature and the continentality of a location. It indicates the atmospheric moisture content, a significant component of the hydrological cycle. Notably, relative humidity has a strong potential to be recorded in the isotopic composition of silica precipitates within plant tissues, as the triple oxygen isotope compositions (δ¹⁷O, δ¹⁸O, and Δ¹⁷O) of evaporating plant waters vary systematically with relative humidity. Here, we investigated the triple oxygen isotope compositions of phytoliths, a biogenic silica mineral extracted from the grain husks of rice (Oryza sativa L.), to assess their utility as recorders of relative humidity during the growing season. We reconstructed triple oxygen isotopic values of equilibrium waters and compared them to isotopic compositions of environmental water at the sites and with the modeled plant waters. Results showed significant dependency of Δʹ¹⁷O of phytoliths and the Y intercept of the reconstructed meteoric waters on relative humidity, exhibiting lower and negative values for drier sites, which creates an application of the empirical relationship developed in the study as a quantitative recorder of relative humidity. Further, we measured the amount of structural water (3–5 wt%) in phytoliths and their hydrogen isotopic compositions (δ²H) (−165 to −119 ‰) to investigate their sensitivity to growing season ambient moisture levels and source water δ²H, and these lack correlations. While oxygen isotope composition (δ¹⁸O) of rice grain organic matter (expressed as ¹⁸O enrichment above source water) has already been established as a proxy for relative humidity, this new triple oxygen isotope approach on relatively environmentally resilient phytoliths will facilitate constraining hydroclimate estimates from the same rice grain specimen or from other environmental archives. As rice has been an important monsoonal cereal crop across tropical and subtropical Asia since the mid-Holocene, roughly 6000 years ago, while its wild progenitors date back to Cretaceous, further investigation is needed to establish the ability of phytoliths to preserve pristine signatures and be used as paleoenvironmental tool.