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

Abstract

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.

Triple oxygen isotope systematics of evaporation and mixing processes in a dynamic desert lake system

“…Also, the effect of high salinity on the evaporation trajectory of the C-G model is less significant (Surma et al, 2018). Hence, the combined analysis of δ 17 O, δ 18 O, and δ 2 H provides information on fundamental processes of the hydrological cycle such as humidity, moisture sources, evaporation conditions, and mixing, which would be masked by temperature variability if δ 18 O and δ 2 H were analysed alone (Angert et al, 2004; Luz, 2005, 2007;Landais et al, 2006Landais et al, , 2008Landais et al, , 2010Surma et al, 2015Surma et al, , 2018Herwartz et al, 2017). As such, the general potential of the 17 O-excess parameter as a tool to quantitatively reconstruct paleo-humidity from plant silica particles (Alexandre et al, 2018(Alexandre et al, , 2019 and lake sediments (Evans et al, 2018;Gázquez et al, 2018) could be demonstrated.…”

The triple oxygen isotope composition of phytoliths, a new proxy of atmospheric relative humidity: controls of soil water isotope composition, temperature, CO<sub>2</sub> concentration and relative humidity

“…2), is influenced by fewer processes than δ 18 O alone (Aron et al, 2020;Luz and Barkan, 2010;Sharp et al, 2018). Indeed, the 17 O excess of rainwater that feeds the soil water absorbed by the roots is less sensitive to temperature (Barkan and Luz, 2005;Uemura et al, 2010) or phase changes that occur during the air masses trajectories (Angert et al, 2003;Barkan and Luz, 2007;Landais et al, 2008;Uemura et al, 2010). As a result, the 17 O excess of rainwater varies spatially much less than its δ 18 O (from −30 to 60 per meg when δ 18 O varies between −25 ‰ and −5 ‰; Aron et al, 2020).…”

17O‐excess and d‐excess of atmospheric water vapor measured by cavity ring‐down spectrometry: Evidence of a matrix effect and implication for the calibration procedure

“…leaves. 13,14 The 17 O-excess of precipitation and cryogenically trapped atmospheric water vapor can be used to reconstruct evaporative conditions at ocean sources, identify raindrop re-evaporation, and trace continental moisture recycling. [15][16][17][18][19][20][21][22][23][24] In particular, Uemura et al 24 showed that the 17 O-excess of atmospheric water vapor above the ocean correlates negatively with relative humidity normalized to seasurface temperature.…”