Abstract. 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 (δ′18O and 17O-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
δ′18O and 17O-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 δ′18O and
17O-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 δ′18O and
17O-excess values can be estimated from the Craig and Gordon model and
a mean leaf water–phytolith fractionation exponent (λPhyto-LW)
of 0.521. In addition to not being leaf length dependent, δ′18O
and 17O-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 17O-excess of grass bulk
phytoliths. Although under the study conditions 17O-excessPhyto 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 17O-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 λPhyto-LW is not
climate dependent, then grassland leaf water 17O-excess obtained from
grassland phytolith 17O-excess would inform on isotope signals of
several soil–plant-atmosphere processes.
Leaf water oxygen isotope measurement by direct equilibration