利用反射光谱研究砂岩风化与排氮作用——以江西龙虎山为例

Reflectance Spectroscopy Applied in Sandstone Weathering and Nitrogen Excretion: a Case Study in Longhushan Mountain, Jiangxi Province

近期,《Nature》、《Science》等世界顶级期刊揭示了地表岩石与全球氮循环的相互作用,认为岩石是氮素的重要储库,其风化排氮是生态系统氮素的主要来源之一,对局部乃至全球氮循环具有重大影响。陆地地表岩石分布面积75%是沉积岩,丹霞地貌是受沉积岩风化侵蚀作用形成的重要地貌类型之一,研究丹霞地貌沉积岩的风化与排氮作用对深入了解全球氮循环过程具有重要意义。以江西龙虎山丹霞地貌出露的红色砂岩为研究对象,在系统的野外样品采集工作基础上,利用偏光显微镜、ASD光谱仪和碳氮分析仪对样品的矿物组成、光谱曲线和氮素含量进行鉴定和测量。结果表明:龙虎山砂岩主要为长石砂岩,颗粒矿物为长石和石英,胶结物以铁质、钙质为主,次为泥质;砂岩的化学风化类型主要有胶结物溶蚀和颗粒矿物水化两种,其中胶结物溶蚀以铁质溶蚀为主,溶蚀后岩石的光谱曲线在902 nm处(Fe^3+的特征吸收波段)的吸收指数下降,而颗粒矿物水化则主要体现为长石向粘土矿物转变,水化后岩石的光谱曲线在2210 nm处(粘土Al—OH的特征吸收波段)的吸收指数上升。针对同一件样品,已被化学风化的部分与未被风化的部分相比,其氮元素含量有所下降,且氮元素含量与2210 nm处的吸收指数存在一定的负相关系(R^2=0.8026),说明岩石风化有助于氮排放;针对不同样品,氮元素含量与2210 nm处的吸收指数相关性很差(R^2=0.0256),说明岩石矿物组成与结构差异对岩石初始氮含量和吸收指数产生影响,从而降低两者间的相关性。以上研究说明,反射光谱为砂岩风化程度检测与氮排放研究提供了一种重要的技术手段,但研究时必须注意岩性的一致性。

Nature and Science have revealed the interaction between surface rocks and the global nitrogen cycle,considering that rock is an important reservoir of nitrogen.Nitrogen excretion during rock weathering is one of the main sources of nitrogen in soil ecosystems,and has a major impact on the local and even global nitrogen cycle.Sedimentary rock,one of the important rock types,occupies 75%of the land surface.Danxia landform is one of the important geomorphologic types formed by the weathering and erosion of sedimentary rocks.Studies on Danxia landform sedimentary rock weathering and nitrogen excretion contribute to better understanding of the global nitrogen cycle.In this paper,we systematically collected red sandstone samples from Danxia landform in Longhushan Mountain of Jiangxi Province,and studied the mineral composition,spectral characteristics and nitrogen content of the rock samples using polarizing microscope,ASD spectrometer and carbon-nitrogen analyzer.The experimental results showed that the sandstones in the Longhushan Mountain area are dominated by feldspar sandstones.The granular minerals include feldspar and quartz,and the cement is mainly composed of iron and calcium with minor muddy.The chemical weathering of sandstone in Longhushan Mountain area includes cement dissolution and hydration.The dissolution of cement is mainly caused by iron dissolution,and the hydration of granular mineral is mainly represented by feldspar altering to clay mineral.The absorption index at 902 nm(the characteristic absorption band of Fe^3+)decreased when iron dissolved,while that at 2210 nm(the characteristic absorption band of clay Al-OH)increased after feldspar altered to clay minerals.For the same sample,the nitrogen content in the chemically weathered part is lower than that in the unweathered part,and the nitrogen content has a negative relationship with the absorption index at 2210 nm(R^2=0.8026),indicating that rock weathering contributes to nitrogen excretion.For different samples,the correlation between nitrogen content and the absorption index at 2210 nm is very poor(R^2=0.0256),indicating that the different mineral compositions and structures of rocks will decrease the correlation between absorption index and nitrogen content.The above studies showed that the reflectance spectroscopy provides a potential approach to study sandstone weathering and nitrogen excretion,but samples used in the study should have same lithology property.