Wildfire thermochronology and the fate and transport of apatite in hillslope and fluvial environments
Wildfire thermochronology and the fate and transport of apatite in hillslope and fluvial environments
复制标题
DOI：
10.1029/2007jf000759
复制
发表时间：
2007-12
影响因子：
--
通讯作者：
P. Reiners;S. Thomson;D. McPhillips;R. Donelick;J. Roering
中科院分区：
文献类型：
--
作者：
P. Reiners;S. Thomson;D. McPhillips;R. Donelick;J. Roering
[1]野火加热暴露在外的岩石或土壤的外部几厘米产生短时间，高温热事件，产生矿物的特征热年代学签名。对比裂变径迹退火和He在磷灰石中扩散的活化能，导致动力学交叉，野火加热重置裂变径迹（FT）年龄比（U-Th）/He年龄快得多，导致单个晶粒中的“倒置"FT-He年龄。这可以用来追踪地球表面受野火影响的碎屑。我们发现，在暴露的基岩中，反向磷灰石FT-He年龄随深度的变化而系统地变化到-3厘米，山坡上的碎屑碎屑也显示出强烈但异质的野火重置特征。在土壤，崩积层，和低阶通道沉积物，强烈野火复位磷灰石颗粒是丰富的，在某些情况下，占主导地位的人口碎屑磷灰石，深度至少高达10厘米。野火重置磷灰石是罕见的，但是，在河流沉积物中采样较大的盆地，表明从山坡到河流的磷灰石种群的强烈分馏。山坡磷灰石和其他常见矿物的相对溶解速率较慢的溶解特性表明，野火重置磷灰石颗粒在河流中是罕见或不存在的，因为它们在土壤剖面中溶解相对较快。对河流沉积物有贡献的磷灰石可能主要来自陡峭地区的基岩滑坡或含有受野火加热和溶解保护的颗粒的大碎屑。这意味着河流沉积物中的磷灰石在空间上是分馏相对于其来源的集水区，即使集水区的侵蚀率是空间上均匀的。
[1] Wildfire heating of the outer few centimeters of exposed rock or soil generates shortduration, high-temperature thermal events that produce characteristic thermochronologic signatures in minerals. Contrasting activation energies of fission track annealing and He diffusion in apatite lead to a kinetic crossover whereby wildfire heating resets fission track (FT) ages much faster than (U-Th)/He ages, resulting in ‘‘inverted’’ FT-He ages in single grains. This can be used to trace wildfire-affected detritus at the Earth’s surface. We show that in exposed bedrock, inverted apatite FT-He ages vary systematically with depth to � 3 cm, and detrital clasts on hillslopes also show strong but heterogeneous wildfire-resetting signatures. In soils, colluvium, and low-order channel sediments, strongly wildfire-reset apatite grains are abundant, and in some cases dominate the population of detrital apatite, to depths at least as great as 10 cm. Wildfire-reset apatite is rare, however, in fluvial sediments sampled from larger basins, indicating a strong fractionation of apatite populations from hillslopes to rivers. Characteristic dissolution features in hillslope apatite and slower relative dissolution rates of other common minerals suggest that wildfire-reset apatite grains are rare or absent in rivers because they dissolve relatively rapidly in soil profiles. Apatite that does contribute to fluvial sediments is likely to be dominantly derived from bedrock landslides in steep regions or from large clasts containing grains protected from both wildfire heating and dissolution. This means that apatite in fluvial sediment is spatially fractionated with respect to its sources in the catchment, even if catchment erosion rates are spatially uniform.