Modelling the 14C bomb-pulse in young speleothems using a soil carbon continuum model
Modelling the 14C bomb-pulse in young speleothems using a soil carbon continuum model
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DOI：
10.1016/j.gca.2019.04.029
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发表时间：
2019-09
影响因子：
5
通讯作者：
M. Markowska;J. Fohlmeister;P. Treble;A. Baker;M. Andersen;Q. Hua
中科院分区：
文献类型：
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作者：
M. Markowska;J. Fohlmeister;P. Treble;A. Baker;M. Andersen;Q. Hua
“炸弹脉冲法”是一种按年代排列的方法，用来进一步限制1950年至今的洞穴沉积的年龄。建立可靠的年代学约束对于现代仪器气候记录的洞穴校正研究至关重要，它为古气候解释提供了基础。然而，由于有机质在洞穴系统上方的特定停留时间，14C是如何从大气转移到任何个体洞穴中的，这是一个很大的未知数。在这里，我们使用炸弹脉冲方法从14C测量数据建立年表，并结合一个新的非饱和区C模型，该模型将C分解视为一个连续体，以更好地理解非饱和区14C动力学。研究了澳大利亚南部三个不同气候区域的8个洞穴的炸弹脉冲曲线：半干旱的惠灵顿洞穴遗址、地中海的果戈塔洞穴遗址和山地雅兰戈比利洞穴遗址。总体而言，模拟的14C炸弹脉冲曲线与测量的14C喷发数据(R2= 0.82-0.99)有很好的拟合。C模型表明，C带在半干旱地区以年轻为主，加权平均停留时间为32 年，树根呼吸可能是包气二氧化碳的重要来源。在山地遗址，∼39%的C是年轻的(<；1岁)，但加权平均C年龄更老(145-220 )。地中海遗址对年轻C的贡献很小(<；12%：0-1 年)，加权平均年龄在157年至245年 年之间，可能是由于基质流动过程中上部包气带对有机质的更大吸附，以及年轻同生岩溶对C的再动员。在澳大利亚山地和半干旱带洞穴群落中，发现了新的低洞穴群死碳比例(DCP)端元(分别为2.19%和1.65%)，包气带现代CO2供应过剩导致DCP较低。这也证明了DCP在小的时间尺度上可能是非常可变的，过程可能很难解开，恒定的DCP假设可能是无效的。DCP随时间的变化主要受包气带CO2变化的控制，其中植被更新、野火和岩溶水文起着重要作用。
The ‘bomb-pulse’ method is a chronological approach to further constrain the age of speleothems that grew between 1950 CE – present. Establishing dependable chronological constraints is crucial for modern calibration studies of speleothems to instrumental climate records, which provides the basis for paleoclimate interpretations. However, a large unknown is how14C is transferred from the atmosphere to any individual speleothem owing to the site-specific residence times of organic matter above cave systems. Here, we employ the bomb-pulse method to build chronologies from14C measurements in combination with a new unsaturated zone C model which considers C decomposition as a continuum, to better understand unsaturated zone14C dynamics. The bomb-pulse curves of eight speleothems from southern Australia in three contrasting climatic regions; the semi-arid Wellington Caves site, the mediterranean Golgotha Cave site and the montane Yarrangobilly Caves site, are investigated. Overall, the modelled14C bomb-pulse curves produce excellent fits with measured14C speleothem data (r2= 0.82–0.99). The C modelling reveals that unsaturated zone C is predominately young at the semi-arid site, with a weighted-mean residence time of 32 years and that tree root respiration is likely an important source of vadose CO2. At the montane site, ∼39% of C is young (<1 years), but the weighted-mean C ages are older (145–220 years). The mediterranean site has very little contribution from young C (<12%: 0–1 years), with weighted-mean ages between 157 and 245 years, likely due to greater adsorption of organic matter in the upper vadose zone during matrix flow, and remobilisation of C from young syngenetic karst. New end members for low speleothem Dead Carbon Proportion (DCP) are identified (2.19% and 1.65%, respectively) for Australian montane and semi-arid zone speleothems, where oversupply of modern CO2in the vadose zone leads to lower DCP. It is also demonstrated that DCP can be quite variable over small time scales, that processes may be difficult to untangle and a constant DCP assumption is likely invalid. DCP variability over time is mainly controlled by the changes vadose zone CO2, where vegetation regeneration, wild-fires and karst hydrology play an important role.