Evolution of loess-paleosol sequence from the perspectives of thermodynamics and microstructure

Abstract

The process of entropy increase can promote the formation and transformation of natural structures, and also provide information on the evolution process of some inherently complex structures in nature. However, this process has rarely been discussed in the evolution of loess-paleosol sequences. In this study, taking the Xiushidu section of Jingyang as an example, we deciphered the evolution process of a loess-paleosol sequence from the perspective of thermodynamics and microstructure. The configuration entropy of microstructure was found to be sensitive to subtle differences and can be regarded as a new climate indicator. In other words, entropy values changed periodically in the loess strata, with paleosol layers showing lower values than the adjacent loess layers. In addition, the relationship between configuration entropy and external entropy flow induced by climatic change played an important role in the evolution of loess-paleosol sequence. During the paleosol development period, the soil structure underwent continuous change driven by strong weathering and pedogenic processes. The significant structural changes might have entered a new orderly state of the system. In contrast, during the loess accumulation period, the external conditions did not satisfy the threshold of qualitative change, and the soil structure continued to develop toward a disordered state. In the context of thermodynamic entropy, the evolution of the loess-paleosol sequence is a result of alternation between the “near disordered state” and “dissipative structure”. Macroscopically, the change process of the geological system was a dynamic cycle: an open system that constantly experienced a cycle of “external entropy input”-“self-adjustment”-“external entropy input”. We define the above entropy change process as a thermodynamic conceptual model of the loess-paleosol sequence evolution. These findings provide a new thermodynamics approach for deciphering loess-paleosol sequences.