A Review on Weathering, Erosion and Denudation in the Critical Zone of Central Highlands of Sri Lanka - the State under Natural and Man-made Environments

抄録

The Critical Zone (CZ) is the most important layer of the Earth, in which chemical elements circulate through rocks, sediment, soil, pore water, groundwater and plants before they are exported to rivers and oceans as physical and dissolved fluxes. In addition, as a result of ever-increasing pressure by humans to the natural environment, the CZ has been transformed in many parts of the world. In this review, the state of the CZ of the mountainous and humid tropical island of Sri Lanka is discussed. A handful of locations in the upper region of central Sri Lanka, characterized with natural rain forests, are ideal sites to disclose the nature of a natural CZ. But, due to the intense deforestation over the last two centuries, rain forest covers in many areas of the country have drastically reduced, leading to a few segregated patches of forest on mountain tops. In this paper, weathering and erosion processes of this CZ operating under both natural and human-altered environments are elaborated. Landscapes prevailing under the natural forests are mantled with thick regolith profiles formed by spheroidal weathering. These profiles are intensely weathered and primary minerals are completely altered at the top of the profile, enriching weathering resistant quartz and secondary minerals, making the landscape a supply-limited weathering regime. Under this regime, pathways of the weathering reactions that occur in the coherent bedrock are extensively discussed in this review. Physical erosion at the surface and silicate weathering through the regolith under natural conditions are surprisingly slow, illuminating that climatic control on denudation has become insignificant due to the lack of tectonic rejuvenation of the landscape. Tectonic stability of the terrain, protection of the soil surface by thick vegetation, hardness of the underlying bedrock and impermeability of the thick regolith profile were identified as the main reasons for slow physical erosion and chemical weathering in this landscape. In view of human perturbations, comparison of soil production rates with soil denudation and soil erosion rates in agriculturally-utilized catchments suggests that soil is being lost 10-100 times faster than it is formed under natural conditions. This assessment shows that CZ in many parts of Sri Lanka has been notably impacted by anthropogenic activities.