Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 10
Biogeosciences, 15, 3223-3241, 2018
https://doi.org/10.5194/bg-15-3223-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Biogeosciences, 15, 3223-3241, 2018
https://doi.org/10.5194/bg-15-3223-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 31 May 2018

Research article | 31 May 2018

The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations

Anne Alexandre1, Amarelle Landais2, Christine Vallet-Coulomb1, Clément Piel3, Sébastien Devidal3, Sandrine Pauchet1, Corinne Sonzogni1, Martine Couapel1, Marine Pasturel1, Pauline Cornuault1, Jingming Xin2, Jean-Charles Mazur1, Frédéric Prié2, Ilhem Bentaleb4, Elizabeth Webb5, Françoise Chalié1, and Jacques Roy3 Anne Alexandre et al.
  • 1CEREGE UM34, Aix-Marseille Université, CNRS, IRD, INRA, Aix en Provence, France
  • 2Laboratoire des Sciences du Climat et de l'Environnement (LSCE/IPSL/CEA/CNRS/UVSQ), Gif-sur-Yvette, France
  • 3Ecotron Européen de Montpellier, UPS 3248, Centre National de la Recherche Scientifique (CNRS), Campus Baillarguet, Montferrier-sur-Lez, France
  • 4ISEM, Université de Montpellier, CNRS, IRD, EPHE, Montpellier, France
  • 5Department of Earth Sciences, The University of Western Ontario, London, Ontario, Canada

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.

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There is a lack of proxies suitable for reconstructing, in a quantitative way, past changes in continental atmospheric humidity, which is a key climate parameter. Here, we demonstrate through climate chamber and climate transect calibrations that the triple oxygen isotope composition of phytoliths offers a potential for reconstructing changes in relative humidity.
There is a lack of proxies suitable for reconstructing, in a quantitative way, past changes in...
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