Short Communication: Increasing vertical attenuation length of cosmogenic nuclide production on steep slopes negates topographic shielding corrections for catchment erosion rates
Short Communication: Increasing vertical attenuation length of
cosmogenic nuclide production on steep slopes negates topographic shielding corrections for catchment erosion rates
复制标题
短通讯：陡坡上宇宙成因核素产生的垂直衰减长度的增加否定了流域侵蚀率的地形屏蔽校正
DOI：
10.5194/esurf-2018-48
复制
发表时间：
2018
影响因子：
4
通讯作者：
R. DiBiase
中科院分区：
文献类型：
--
作者：
R. DiBiase
抽象的。解释流域平均侵蚀率从原位产生的宇宙成因10 Be浓度流砂需要计算流域平均10 Be表面生产率和有效质量衰减长度，这两者都可以局部变化，由于地形屏蔽和坡度的影响。计算地形屏蔽的最常用方法仅考虑了地表的屏蔽效应，在陡峭的地形中，流域平均校正高达20%，并简化假设，即给定核素产生机制的有效质量衰减长度在空间上是均匀的。在这里，我评估了这一假设的有效性，使用一个简化的集水区几何计算的空间变化，在表面天际线屏蔽，有效质量衰减长度，和总的有效屏蔽因子的集水区的平均坡度为0°至80°。对于平坦集水区（即，边界山脊线的统一海拔），有效衰减长度的增加作为山坡角和天际线屏蔽的函数，导致流域平均总有效屏蔽因子为1，这意味着在计算流域平均垂直侵蚀率时不需要地形屏蔽因子。对于倾斜集水区（其特征在于与边界山脊线的平面拟合），集水区平均总有效屏蔽因子也为1，但极陡的范围前沿集水区除外，在这种情况下，屏蔽校正值违反直觉地大于1。这些结果表明，在大多数情况下，地形屏蔽校正是不合适的计算流域平均侵蚀速率，只需要陡峭的集水区与非均匀分布的石英和/或侵蚀速率。通过只考虑表面生产的屏蔽，现有的屏蔽方法引入了与坡度相关的系统误差，可能导致对地形和侵蚀率之间关系的错误解释。
Abstract. Interpreting catchment-mean erosion rate from in situ produced cosmogenic 10Be concentration in stream sands requires calculating the catchment-mean 10Be surface production rate and effective mass attenuation length, both of which can vary locally due to topographic shielding and slope effects. The most common method for calculating topographic shielding accounts only for the effect of shielding at the surface, leading to catchment-mean corrections of up to 20 % in steep landscapes, and makes the simplifying assumption that the effective mass attenuation length for a given nuclide production mechanism is spatially uniform. Here I evaluate the validity of this assumption using a simplified catchment geometry to calculate the spatial variation in surface skyline shielding, effective mass attenuation length, and the total effective shielding factor for catchments with mean slopes ranging from 0° to 80°. For flat catchments (i.e., uniform elevation of bounding ridgelines), the increase in effective attenuation length as a function of hillslope angle and skyline shielding leads to a catchment-mean total effective shielding factor of one, implying that no topographic shielding factor is needed when calculating catchment-mean vertical erosion rates. For dipping catchments (as characterized by a plane fit to the bounding ridgelines), the catchment-mean total effective shielding factor is also one, except for cases of extremely steep range-front catchments, where the shielding correction is counterintuitively greater than one. These results indicate that in most cases, topographic shielding corrections are inappropriate for calculating catchment-mean erosion rates, and only needed for steep catchments with non-uniform distribution of quartz and/or erosion rate. By accounting only for shielding of surface production, existing shielding approaches introduce a slope-dependent systematic error that could lead to spurious interpretations of relationships between topography and erosion rate.