Supplementary material to "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"

2021 ◽  
Author(s):  
Clément Outrequin ◽  
Anne Alexandre ◽  
Christine Vallet-Coulomb ◽  
Clément Piel ◽  
Sébastien Devidal ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Oxygen Isotope ◽  
Soil Water ◽  
Isotope Composition ◽  
Oxygen Isotope Composition ◽  
Supplementary Material ◽  
Atmospheric Relative Humidity ◽  
Water Isotope

scholarly journals 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

2021 ◽  
Author(s):  
Clément Outrequin ◽  
Anne Alexandre ◽  
Christine Vallet-Coulomb ◽  
Clément Piel ◽  
Sébastien Devidal ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Oxygen Isotope ◽  
Soil Water ◽  
Isotope Composition ◽  
Co2 Concentration ◽  
Oxygen Isotope Composition ◽  
Growth Chamber ◽  
Starting Point ◽  
Atmospheric Relative Humidity ◽  
Water Isotope

Abstract. Continental atmospheric relative humidity is a major climate parameter whose variability is poorly understood by global climate models. Models’improvement relies on model-data comparisons for past periods. However, there are no truly quantitative indicators of relative humidity for the pre-instrumental period. Previous studies highlighted a quantitative relationship between the triple oxygen isotope composition of phytoliths, and particularly the 17O-excess of phytoliths, and atmospheric relative humidity. Here, as part of a series of calibrations, we examine the respective controls of soil water isotope composition, temperature, CO2 concentration and relative humidity on phytolith 17O-excess. For that purpose, the grass species Festuca arundinacea was grown in growth chambers where these parameters were varying. The setup was designed to control the evolution of the triple oxygen isotope composition of phytoliths and all the water compartments of the soil-plant-atmosphere continuum. Different analytical techniques (cavity ring-down spectroscopy and isotope ratio mass spectrometry) were used to analyse water and silica. An inter-laboratory comparison allowed to strengthen the isotope data matching. Water and phytolith isotope compositions were compared to previous datasets obtained from growth chamber and natural tropical sites. The results show that the δ'18O value of the source water governs the starting point from which the triple oxygen isotope composition of leaf water, phytolith-forming water and phytoliths evolve. However, since the 17O-excess varies little in the growth chamber and natural source waters, this has no impact on the strong relative humidity-dependency of the 17O-excess of phytoliths, demonstrated for the 40–80 % relative humidity range. This relative humidity-dependency is not impacted by changes in air temperature or CO2 concentration either. A relative humidity proxy equation is proposed. Each per meg of change in phytolith 17O-excess reflects a change in atmospheric relative humidity of ca. 0.2 %. The ±15 per meg reproducibility on the measurement of phytolith 17O-excess corresponds to a ± 3.6 % precision on the reconstructed relative humidity. The low sensitivity of phytolith 17O-excess to climate parameters other than relative humidity makes it particularly suitable for quantitative reconstructions of continental relative humidity changes in the past.

scholarly journals 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

2021 ◽  
Vol 17 (5) ◽  
pp. 1881-1902
Author(s):  
Clément Outrequin ◽  
Anne Alexandre ◽  
Christine Vallet-Coulomb ◽  
Clément Piel ◽  
Sébastien Devidal ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Oxygen Isotope ◽  
Soil Water ◽  
Isotope Composition ◽  
Co2 Concentration ◽  
Oxygen Isotope Composition ◽  
Growth Chamber ◽  
Starting Point ◽  
Atmospheric Relative Humidity ◽  
Water Isotope

Abstract. Continental atmospheric relative humidity is a major climate parameter whose variability is poorly understood by global climate models. Models' improvement relies on model–data comparisons for past periods. However, there are no truly quantitative indicators of relative humidity for the pre-instrumental period. Previous studies highlighted a quantitative relationship between the triple oxygen isotope composition of phytoliths, particularly the 17O excess of phytoliths, and atmospheric relative humidity. Here, as part of a series of calibrations, we examine the respective controls of soil water isotope composition, temperature, CO2 concentration and relative humidity on phytolith 17O excess. For that purpose, the grass species Festuca arundinacea was grown in growth chambers where these parameters were varying. The setup was designed to control the evolution of the triple oxygen isotope composition of phytoliths and all the water compartments of the soil–plant–atmosphere continuum. Different analytical techniques (cavity ring-down spectroscopy and isotope ratio mass spectrometry) were used to analyze water and silica. An inter-laboratory comparison allowed to strengthen the isotope data matching. Water and phytolith isotope compositions were compared to previous datasets obtained from growth chamber and natural tropical sites. The results show that the δ′18O value of the source water governs the starting point from which the triple oxygen isotope composition of leaf water, phytolith-forming water and phytoliths evolves. However, since the 17O excess varies little in the growth chamber and natural source waters, this has no impact on the strong relative humidity dependency of the 17O excess of phytoliths, demonstrated for the 40 %–80% relative humidity range. This relative humidity dependency is not impacted by changes in air temperature or CO2 concentration either. A relative humidity proxy equation is proposed. Each per meg of change in phytolith 17O excess reflects a change in atmospheric relative humidity of ca. 0.2 %. The ±15 per meg reproducibility on the measurement of phytolith 17O excess corresponds to a ±3.6 % precision on the reconstructed relative humidity. The low sensitivity of phytolith 17O excess to climate parameters other than relative humidity makes it particularly suitable for quantitative reconstructions of continental relative humidity changes in the past.

The impact of an inverse climate–isotope relationship in soil water on the oxygen‐isotope composition of Larix gmelinii in Siberia

2015 ◽  
Vol 209 (3) ◽  
pp. 955-964 ◽  
Author(s):  
Matthias Saurer ◽  
Alexander V. Kirdyanov ◽  
Anatoly S. Prokushkin ◽  
Katja T. Rinne ◽  
Rolf T. W. Siegwolf
Keyword(s):  
Oxygen Isotope ◽  
Soil Water ◽  
Isotope Composition ◽  
Oxygen Isotope Composition ◽  
Larix Gmelinii ◽  
The Impact

scholarly journals Non-destructive estimates of soil carbonic anhydrase activity and soil water oxygen isotope composition

2017 ◽  
Author(s):  
Sam P. Jones ◽  
Jérôme Ogée ◽  
Joana Sauze ◽  
Steven Wohl ◽  
Noelia Saavedra ◽  
...  
Keyword(s):  
Oxygen Isotope ◽  
Soil Water ◽  
Isotope Composition ◽  
Soil Surface ◽  
Bulk Water ◽  
Oxygen Isotope Composition ◽  
Spatial And Temporal Variability ◽  
Co2 Fluxes ◽  
Carbonic Anhydrases ◽  
Water Pool

Abstract. The contribution of photosynthesis and soil respiration to net land-atmosphere carbon dioxide (CO2) exchange can be estimated based on the differential influence of leaves and soils on budgets of the oxygen isotope composition (δ18O) of atmospheric CO2. To do so, the activity of carbonic anhydrases (CA), a group of enzymes that catalyse the hydration of CO2, in soils and plants needs to be understood. Measurements of soil CA activity typically involve the inversion of models describing the δ18O of CO2 fluxes to solve for the apparent, potentially catalysed, rate of CO2 hydration. This requires information about the δ18O of CO2 in isotopic equilibrium with soil water, typically obtained from destructive, depth-resolved sampling and extraction of soil water. In doing so, an assumption is made about the soil water pool that CO2 interacts with, that may bias estimates of CA activity if incorrect. Furthermore, this can represent a significant challenge in data collection given the potential for spatial and temporal variability in the δ18O of soil water and limited a priori information with respect to the appropriate sampling resolution and depth. We investigated whether we could circumvent this requirement by inferring the rate of CO2 hydration and the δ18O of soil water from the relationship between the δ18O of CO2 fluxes and the δ18O of CO2 at the soil surface measured at different ambient CO2 conditions. This approach was tested through laboratory incubations of air-dried soils that were re-wetted with three waters of different δ18O. Gas exchange measurements were made on these soils to estimate the rate of hydration and the δ18O of soil water, followed by soil water extraction to allow for comparison. Estimated rates of CO2 hydration were 6.8 to 14.6 times greater than the theoretical un-catalysed rate of hydration, indicating that CA were active in these soils. Importantly, these estimates were not significantly different among water treatments suggesting that this represents a robust approach to assay the activity of CA in soil. As expected, estimates of the δ18O of the soil water that equilibrates with CO2 varied in response to alteration to the δ18O of soil water. However, these estimates were consistently more negative than the composition of the soil water extracted by cryogenic vacuum distillation at the end of the gas measurements with differences of up to −3.94 ‰ VSMOW. These offsets suggest that CO2 may be principally interacting with water pools associated with particle surfaces rather than the bulk water pool under the incubation conditions of this study.

scholarly journals Non-destructive estimates of soil carbonic anhydrase activity and associated soil water oxygen isotope composition

2017 ◽  
Vol 21 (12) ◽  
pp. 6363-6377 ◽  
Author(s):  
Sam P. Jones ◽  
Jérôme Ogée ◽  
Joana Sauze ◽  
Steven Wohl ◽  
Noelia Saavedra ◽  
...  
Keyword(s):  
Oxygen Isotope ◽  
Soil Water ◽  
Isotope Composition ◽  
Soil Surface ◽  
Carbonic Anhydrase Activity ◽  
Co2 Exchange ◽  
Oxygen Isotope Composition ◽  
Spatial And Temporal Variability ◽  
Co2 Fluxes ◽  
Carbonic Anhydrases

Abstract. The contribution of photosynthesis and soil respiration to net land–atmosphere carbon dioxide (CO2) exchange can be estimated based on the differential influence of leaves and soils on budgets of the oxygen isotope composition (δ18O) of atmospheric CO2. To do so, the activity of carbonic anhydrases (CAs), a group of enzymes that catalyse the hydration of CO2 in soils and plants, needs to be understood. Measurements of soil CA activity typically involve the inversion of models describing the δ18O of CO2 fluxes to solve for the apparent, potentially catalysed, rate of CO2 hydration. This requires information about the δ18O of CO2 in isotopic equilibrium with soil water, typically obtained from destructive, depth-resolved sampling and extraction of soil water. In doing so, an assumption is made about the soil water pool that CO2 interacts with, which may bias estimates of CA activity if incorrect. Furthermore, this can represent a significant challenge in data collection given the potential for spatial and temporal variability in the δ18O of soil water and limited a priori information with respect to the appropriate sampling resolution and depth. We investigated whether we could circumvent this requirement by inferring the rate of CO2 hydration and the δ18O of soil water from the relationship between the δ18O of CO2 fluxes and the δ18O of CO2 at the soil surface measured at different ambient CO2 conditions. This approach was tested through laboratory incubations of air-dried soils that were re-wetted with three waters of different δ18O. Gas exchange measurements were made on these soils to estimate the rate of hydration and the δ18O of soil water, followed by soil water extraction to allow for comparison. Estimated rates of CO2 hydration were 6.8–14.6 times greater than the theoretical uncatalysed rate of hydration, indicating that CA were active in these soils. Importantly, these estimates were not significantly different among water treatments, suggesting that this represents a robust approach to assay the activity of CA in soil. As expected, estimates of the δ18O of the soil water that equilibrates with CO2 varied in response to alteration to the δ18O of soil water. However, these estimates were consistently more negative than the composition of the soil water extracted by cryogenic vacuum distillation at the end of the gas measurements with differences of up to −3.94 ‰ VSMOW–SLAP. These offsets suggest that, at least at lower water contents, CO2–H2O isotope equilibration primarily occurs with water pools that are bound to particle surfaces and are depleted in 18O compared to bulk soil water.

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

2017 ◽  
Author(s):  
Anne Alexandre ◽  
Amarelle Landais ◽  
Christine Vallet-Coulomb ◽  
Clément Piel ◽  
Sébastien Devidal ◽  
...  
Keyword(s):  
Water Vapor ◽  
Relative Humidity ◽  
Oxygen Isotope ◽  
Soil Water ◽  
Isotope Composition ◽  
Leaf Water ◽  
Growth Chamber ◽  
Water Evaporation ◽  
Atmospheric Humidity ◽  
Oxygen Isotope Fractionation

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 to make 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 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 leaf tissue 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.

Factors controlling the triple oxygen isotope composition of grass leaf water and phytoliths: insights for paleo-environmental reconstructions

2020 ◽  
Author(s):  
Anne Alexandre ◽  
Clément Outrequin ◽  
Christine Vallet-Coulomb ◽  
Amaelle Landais ◽  
Clément Piel ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Vapor Pressure ◽  
Oxygen Isotope ◽  
Meteoric Water ◽  
Isotope Composition ◽  
Gross Primary Production ◽  
Leaf Water ◽  
Oxygen Isotope Composition ◽  
Phase Changes ◽  
Isotope Signature

&lt;p&gt;The oxygen isotope signature of leaf water is used to trace several processes at the soil-plant-atmosphere interface. During photosynthesis, it is transferred to the oxygen isotope signature of atmospheric CO&lt;sub&gt;2&lt;/sub&gt; and O&lt;sub&gt;2&lt;/sub&gt;, which can be used for reconstructing past changes in gross primary production. The oxygen isotope signature of leaf water additionally imprints leaf organic and mineral compounds, such as phytoliths, used as paleoclimate and paleovegetation proxies when extracted from sedimentary materials.&lt;/p&gt;&lt;p&gt;Numerous experimental and modelling studies were dedicated to constrain the main parameters responsible for changes in the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of leaf water. Although these models usually correctly depict the main trends of &lt;sup&gt;18&lt;/sup&gt;O-enrichment of the leaf water when relative humidity decreases, the calculated absolute values often depart from the observed ones by several &amp;#8240;. Moreover, the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of leaf water absorbed by plants is dependent on the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O value of meteoric and soil waters that can vary by several &amp;#8240; at different space and time scales. These added uncertainties make our knowledge of the parameters responsible for changes in the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of leaf water and phytoliths flawed.&lt;/p&gt;&lt;p&gt;Changes in the triple oxygen isotope composition of leaf water, expressed by the &lt;sup&gt;17&lt;/sup&gt;O-excess, are controlled by fewer variables than changes in &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O. In meteoric water the &lt;sup&gt;17&lt;/sup&gt;O-excess varies slightly as it is weakly affected by temperature or phase changes during air mass transport. This makes the soil water fed by meteoric water and the atmospheric vapour in equilibrium with meteoric water changing little from a place to another. Hence the &lt;sup&gt;17&lt;/sup&gt;O-excess of leaf water is essentially controlled by the evaporative fractionation. The latest depends on the ratio of vapor pressure in the air to vapor pressure in the stomata intercellular space, close to relative humidity. Leaf water evaporative fractionation can lead to &lt;sup&gt;17&lt;/sup&gt;O-excess negative values that can exceed most of surficial water ones.&lt;/p&gt;&lt;p&gt;Here we present the outcomes of several recent growth chamber and field studies, for the purpose of i) refining the grass leaf water and phytoliths &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O and &lt;sup&gt;17&lt;/sup&gt;O-excess modelling, ii) assessing whether the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O and &lt;sup&gt;17&lt;/sup&gt;O-excess of grass leaf water can be reconstructed from phytoliths, and iii) examining the precision of the &lt;sup&gt;17&lt;/sup&gt;O-excess of phytoliths as a new proxy for past changes in continental atmospheric relative humidity. Atmospheric continental relative humidity is an important climate parameter poorly constrained in global climate models. A model-data comparison approach, applicable beyond the instrumental period, is essential to progress on this issue. However, there is currently a lack of proxies allowing quantitative reconstruction of past continental relative humidity. The &lt;sup&gt;17&lt;/sup&gt;O-excess signature of phytoliths could fill this gap.&lt;/p&gt;

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