Isotopic composition of carbon dioxide lost by evasion from surface water to the atmosphere: Methodological comparison of a direct and indirect approach
Isotopic composition of carbon dioxide lost by evasion from surface water to the atmosphere: Methodological comparison of a direct and indirect approach
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从地表水逃逸到大气中损失的二氧化碳的同位素组成：直接和间接方法的方法比较
DOI：
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
M. Garnett
中科院分区：
文献类型：
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作者：
M. Billett;M. Garnett
从水面释放到大气的二氧化碳的年龄和来源，为控制生物圈关键区域，如泥炭地和热带河流系统中这一重要通量的过程提供了重要的洞察。目前有两种方法能够测量或估计通过逃逸(脱气)而损失的二氧化碳的同位素组成。~(13)C-CO_2和~(14)C-CO_2可通过安装在浮子室上的分子筛捕集系统直接测量，或通过DIC计算间接测量。我们从二氧化碳过饱和的低生产力英国泥炭地溪流中收集样本，对这两种方法进行了比较。六次采集海面上的逃逸二氧化碳样品，用于测定δ13C和14C。同时，采集溪水样品，测定DIC的同位素组成。我们的结果表明，在某些情况下，逃逸的二氧化碳和DIC的放射性碳年龄相似，而在另一些情况下，测量的14C-DIC年龄明显更老。还观察到直接测量和计算的δ13C-CO2值之间的差异。我们对这两种方法进行了严格的评估，强调了对逃逸二氧化碳进行适当的同位素校正的重要性，并得出结论，在应用间接方法时需要谨慎，因为与基础计算中使用的平衡条件相关的一些固有假设可能在某些水生系统中不成立。
The age and source of CO2 released from water surfaces to the atmosphere provides significant insight into the processes that control this important flux in key areas of the biosphere, such as peatlands and tropical river systems. Two approaches currently exist that enable the isotopic composition of CO2 lost by evasion (degassing) to be either measured or estimated. 13C‐CO2 and 14C‐CO2 can be measured directly using a molecular sieve trapping system attached to a floating chamber, or indirectly by calculation from DIC. We present a comparison of the two methods from samples collected from low‐productivity UK peatland streams supersaturated in CO2. On six occasions, samples of evaded CO2 were collected above the water surface for the determination of δ13C and 14C. Simultaneously, samples of streamwater were collected for the determination of the isotopic composition of DIC. Our results suggest that whereas on some occasions the radiocarbon ages of evaded CO2 and DIC were similar, on others the measured 14C‐DIC age was significantly older. Differences were also observed between the directly measured and calculated δ13C‐CO2 values. We critically evaluate both methods, highlight the importance of making the appropriate isotopic corrections for evasion CO2, and conclude that care needs to be taken in applying the indirect approach, as some of the inherent assumptions associated with equilibrium conditions used in the underlying calculation may not hold in certain aquatic systems.