Effects of leaf length and development stage on the triple oxygen isotope signature of grass leaf water and phytoliths: insights for a proxy of continental atmospheric humidity

Continental relative humidity (RH) is a key climate parameter, but there is a lack of quantitative RH proxies suitable for climate model-data comparisons. Recently, a combination of climate chamber and natural transect calibrations have laid the groundwork for examining the robustness of the triple oxygen isotope composition (delta O-'18 and O-17-excess) of phytoliths, that can preserve in sediments, as a new proxy for past changes in RH. However, it was recommended that besides RH, additional factors that may impact delta'O-18 and O-17-excess of plant water and phytoliths be examined. Here, the effects of grass leaf length, leaf development stage and day-night alternations are addressed from growth chamber experiments. The triple oxygen isotope compositions of leaf water and phytoliths of the grass species F. arundinacea are analysed. Evolution of the leaf water delta'O-18 and O-17-excess along the leaf length can be modelled using a string-of-lakes approach to which an unevaporated-evaporated mixing equation must be added. We show that for phytoliths to record this evolution, a kinetic fractionation between leaf water and silica, increasing from the base to the apex, must be assumed. Despite the isotope heterogeneity of leaf water along the leaf length, the bulk leaf phytolith delta'O-18 and O-17-excess values can be estimated from the Craig and Gordon model and a mean leaf water-phytolith fractionation exponent (lambda(Phyto-LW)) of 0.521. In addition to not being leaf length dependent, delta'O-18 and O-17-excess of grass phytoliths are expected to be impacted only very slightly by the stem vs. leaf biomass ratio. Our experiment additionally shows that because a lot of silica polymerises in grasses when the leaf reaches senescence (58 % of leaf phytoliths in mass), RH prevailing during the start of senescence should be considered in addition to RH prevailing during leaf growth when interpreting the O-17-excess of grass bulk phytoliths. Although under the study conditions O-17-excess(Phyto) do not vary significantly from constant day to day-night conditions, additional monitoring at low RH conditions should be done before drawing any generalisable conclusions. Overall, this study strengthens the reliability of the O-17-excess of phytoliths to be used as a proxy of RH. If future studies show that the mean value of 0.521 used for the grass leaf water-phytolith fractionation exponent lambda(Phyto-LW) is not climate dependent, then grassland leaf water O-17-excess obtained from grassland phytolith O-17-excess would inform on isotope signals of several soil-plant-atmosphere processes.