The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations
Authors: Alexandre Anne , Landais Amarelle , Vallet-Coulomb Christine , Piel Clément , Devidal Sébastien , Pauchet Sandrine , Sonzogni Corinne , Couapel Martine , Pasturel Marine , Cornuault Pauline , Xin Jingming , Mazur Jean-Charles , Prié Frédéric , Bentaleb Ilhem , Webb Elizabeth , Chalié Françoise , Roy Jacques 
Published: May 2018

Abstract. Continental atmospheric relative humidity (RH) is a key climate
parameter. Combined with atmospheric temperature, it allows us to estimate
the concentration of atmospheric water vapor, which is one of the main
components of the global water cycle and the most important gas contributing
to the natural greenhouse effect. However, there is a lack of proxies
suitable for reconstructing, in a quantitative way, past changes of
continental atmospheric humidity. This reduces the possibility of making
model–data comparisons necessary for the implementation of climate models.
Over the past 10 years, analytical developments have enabled a few
laboratories to reach sufficient precision for measuring the triple oxygen
isotopes, expressed by the 17O-excess (17O-excess = ln
(δ17O + 1) – 0.528 × ln
(δ18O + 1)), in water, water vapor and minerals. The
17O-excess represents an alternative to deuterium-excess for
investigating relative humidity conditions that prevail during water
evaporation. Phytoliths are micrometric amorphous silica particles that form
continuously in living plants. Phytolith morphological assemblages from soils
and sediments are commonly used as past vegetation and hydrous stress
indicators. In the present study, we examine whether changes in atmospheric
RH imprint the 17O-excess of phytoliths in a measurable way and
whether this imprint offers a potential for reconstructing past RH. For that
purpose, we first monitored the 17O-excess evolution of soil water,
grass leaf water and grass phytoliths in response to changes in RH (from 40
to 100 %) in a growth chamber experiment where transpiration reached a
steady state. Decreasing RH from 80 to 40 % decreases the
17O-excess of phytoliths by 4.1 per meg/% as a result of kinetic
fractionation of the leaf water subject to evaporation. In order to model
with accuracy the triple oxygen isotope fractionation in play in plant water
and in phytoliths we recommend direct and continuous measurements of the
triple isotope composition of water vapor. Then, we measured the
17O-excess of 57 phytolith assemblages collected from top soils
along a RH and vegetation transect in inter-tropical West and Central Africa.
Although scattered, the 17O-excess of phytoliths decreases with RH
by 3.4 per meg/%. The similarity of the trends observed in the growth
chamber and nature supports that RH is an important control of
17O-excess of phytoliths in the natural environment. However, other
parameters such as changes in the triple isotope composition of the soil
water or phytolith origin in the plant may come into play. Assessment of
these parameters through additional growth chambers experiments and field
campaigns will bring us closer to an accurate proxy of changes in relative
humidity.