The role of soil processes in δ18O terrestrial climate proxies
The role of soil processes in δ18O terrestrial climate proxies
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土壤过程在 δ18O 陆地气候代理中的作用
DOI：
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发表时间：
2014
期刊：
影响因子：
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通讯作者：
D. Noone
中科院分区：
文献类型：
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作者：
L. Kanner;N. Buenning;L. Stott;A. Timmermann;D. Noone
如果土壤水的同位素组成发生了同位素交换或分馏，则对陆地水文代用指标（如树木纤维素或洞穴沉积物方解石的δ18O）的古气候解释可能会有偏差或误解。在这项研究中，我们使用一个全球同位素启用的陆面模型来研究如何降水的δ 18 O可能会改变土壤柱由于蒸发和垂直水分输送。为了评估降水和蒸发如何影响土壤水同位素变异性，我们比较了模拟木质部水δ 18 O的季节和年际变化，并进行了一系列敏感性实验，其中降水δ 18 O、水蒸气δ 18 O和土壤水蒸发的影响被独立去除。1979年至2004年期间进行的模拟表明，在半干旱地区，如美国西南部，木质部水δ 18 O的季节性循环受到旱季蒸发损失的强烈影响，蒸发也可以构成高达50%的年际δ 18 O变化。额外的模拟，包括土壤水标记实验，表明土壤水向上通量发生在干燥时期。对于土壤水δ 18 O剖面，在深度处18 O同位素更贫乏，这在这样的干燥间隔期间赋予木质部水δ 18 O低同位素特征。因此，如果不考虑土壤中的垂直水分输送，低δ 18 O年份可能被误解为潮湿条件（由于蒸发富集减少），而干燥条件同样可能。
A paleoclimate interpretation of a terrestrial hydrologic proxy such as the δ18O of tree cellulose or speleothem calcite may be biased or misinterpreted if the isotopic composition of the soil water from which the proxy originated undergoes isotopic exchange or fractionation. In this study, we use a global isotope‐enabled land surface model to investigate how the δ18O of precipitation may be altered in a soil column due to evaporation and vertical moisture transport. In order to assess how precipitation and evaporation contribute to the soil water isotopic variability, we compare seasonal and interannual changes in simulated xylem water δ18O within a control simulation and in a suite of sensitivity experiments where the effects of precipitation δ18O, water vapor δ18O, and soil water evaporation are independently removed. The simulations, carried out for the period 1979 to 2004, reveal that in semiarid regions, such as the southwest United States, the seasonal cycle in xylem water δ18O is strongly affected by evaporative loss during the dry season and evaporation can also constitute as much as 50% of the interannual δ18O variance. Additional simulations, including soil water tagging experiments, indicate that upward fluxes of soil water occur during drier periods. For soil water δ18O profiles that are isotopically more depleted in 18O at depth, this imparts a low isotopic signature to xylem water δ18O during such dry intervals. Hence, without taking into account vertical moisture transport in the soils, low δ18O years could be misinterpreted as wet conditions (due to decreased evaporative enrichment) when instead drier conditions are equally as likely.