Formation of calcareous nodules in loess–paleosol sequences: Reviews of existing models with a proposed new “per evapotranspiration model”

Abstract

Loess is a product of aeolian deposition during Quaternary glaciation cycles. Loess–paleosol sequences are rich in calcareous nodules (CNs). In the literature, two models are widely cited for the formation of CNs, namely “per descendum” and “per ascendum”. However, there has been no direct testing or monitoring to support either of these contradictory models. This paper reviews a large number of multidisciplinary literature to evaluate the consistency, reliability and rationality of these two models. Three main conclusions are drawn: (1) the causative factors (variation of pH value along loess–paleosol sequence, decrease of CO2 partial pressure, and reduction of solvent water) that are used to support the per descendum model do not completely explain the supersaturation of infiltration solution with CaCO3, thereby making this model questionable; (2) the per ascendum model explains the formation of CNs along narrow horizons through upward evaporation; however, it fails to produce sporadic distributions and irregular shapes of nodules on loess slope faces and the frequent appearance of nodules around plant roots. In order to reconcile these deficiencies, we conducted an extensive field survey in various parts of Shanxi province. Based on this new set of observations, it was concluded that the “per ascendum” model can be extended to explain all occurrences of CNs. This extended model is called “per evapotranspiration”.