Occurrence and transformations of iron and manganese in a colluvial terra rossa toposequence of northern Italy

Abstract

A soil toposequence in NE Italy was studied, which consists of a terra rossa on Cretaceous limestone on the upper slope grading downwards into a colluvial fan with terra rossa material and finally into alluvial river sediments. It is postulated that the red colluviated terra rossa material has come under a moister hydroregime which provided reducing conditions. Because hematite of the terra rossa dissolved preferentially over goethite, as shown by quantitative Fe oxide mineralogy, soil color changed from 2.5YR to 7.5YR. The soils contained two types of concretions, red ones with a low Fe o/Fe d ratio and a high content of hematite and low content of Mn-oxides and black ones with a high Fe o/Fe d ratio, a small amount of hematite and abundant Mn oxides. The red concretions are therefore considered as inherited from an earlier period of pedogenesis whereas the black ones are neoformed in the present pedoenvironment. This is further supported by the lower Al-for-Fe substitution of goethite in the black concretions as compared to a higher Al substitution in the goethite inherited from the terra rossa.