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.

中文翻译：

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硅藻土的三重氧同位素组成，大气相对湿度的新代表：控制土壤水同位素组成，温度，CO ₂浓度和相对湿度

摘要。大陆大气相对湿度是一个主要的气候参数，其可变性在全球气候模型中了解得很少。模型的改进依赖于过去时期的模型数据比较。但是，在仪器使用前，没有真正的相对湿度定量指标。先前的研究强调了硅藻土的三重氧同位素组成，尤其是¹⁷ O过量的硅藻土与大气相对湿度之间存在定量关系。在这里，作为一系列校准的一部分，我们研究了植物水体¹⁷ O过量时土壤水同位素组成，温度，CO ₂浓度和相对湿度的相应控制。为此，草种Festuca arundinacea在这些参数变化的生长室中生长。设计该装置的目的是控制植硅体的三重氧同位素组成以及土壤-植物-大气连续体的所有水室的演化。使用不同的分析技术（腔衰荡光谱法和同位素比质谱法）分析水和二氧化硅。实验室间的比较可以加强同位素数据的匹配。将水和植物硅酸盐的同位素组成与从生长室和自然热带地区获得的先前数据集进行了比较。结果表明，δ” ¹⁸源水的O值控制着叶片水，形成植物石的水和植物石的三重氧同位素组成的起始点。但是，由于¹⁷ O过量在生长室和天然水源中变化不大，因此对40 O-80％相对湿度范围内的¹⁷ O过量的裸露石的强烈相对湿度依赖性没有影响。该相对湿度依赖性也不受空气温度或CO ₂浓度变化的影响。提出了一个相对湿度代理方程。每毫克藻酸石¹⁷ O-过量的每变化反映了大气相对湿度的变化。0.2％。植硅体测量值的±15 per meg再现性¹⁷ O过量对应于重构的相对湿度的±3.6％精度。phytolith ¹⁷ O过量对气候参数（相对湿度除外）的低敏感性使其特别适用于过去大陆相对湿度变化的定量重建。