Carbonatation in palaeosols  formed on terraces of the Tormes river basin (Salamanca, Spain)

Alonso, P.*; Dorronsoro, C.**; Egido, J. A.*

Abstract

A macro- and micromorphological study was made of the carbonate accumulations in a total of 21 relict (unburied) palaeosols (Haploxeralfs and Palexeralfs), of ages corresponding to the Late, Middle and Early Pleistocene, formed on fluvial terraces in the Tormes river basin (Salamanca, Spain) under a present-day subhumid continental Mediterranean climate.

The fluvial deposits were predominantly gravels and sands derived from the erosion of granite, slate, sandstone and quartzite, with a notable absence of calcareous materials.

Secondary carbonates commonly filled cracks, forming laminae in preferentially horizontal disposition (though also tipped and vertical), giving rise to distinctive patterns that have long attracted scientific attention. In the youngest soils of the lower terraces (Late Pleistocene, <50,000 years) the carbonatemorphologies are almost exclusively micrite, whereas sparry calcite was abundant in the oldest soils of the middle and upper terraces (Middle and Early Pleistocene, >300,000 years), especially in the upper levels within the calcic horizon. In rare cases, the pseudomycelia and the coatings were comprised of fine needle-fiber calcite. The carbonates in these soils are recrystallized, even in the some of the youngest soils of these terraces (≥40,000 years).

The older soils of the middle terraces, and particularly in the upper terraces (≥300,000 years) reflected an intense process of replacement of silicates by carbonates; in addition, the clays in the micromass and illuvial coatings, as well as the detrital grains, such as feldspars and quartz, were progressively replaced by carbonates. The carbonate accumulations, as well as the subsequent recrystallization and replacement, tended to increase with the age of the soils. In addition, the secondary carbonatation and dissolution of carbonate observed indicate the genetic complexity of these soils.

The form of the carbonates present (at the micro- as well as macroscopic level), their distribution (within the soil profiles and throughout the region), as well as the relationship with other processes (subsequent to clay illuviation and associated with hydromorphy) imply that vadose water, at certain geological stages, must have been responsible for the carbonate accumulation in these soils.

The carbonates were deposited during the coldest periods of the isotope substages 6.2, 7.4, 10.2, 12.2, 13.2 and 16.2. The presence of dissolution in the soils corresponding to the isotope substages 7.4 and 13.2 appears to indicate the development of two wetter periods towards 200.000 and 500.000 years B.P. The absence of carbonates in the soils of the youngest surfaces (plains flood and the lowest terraces; Holocene and Late Pleistocene, <30,000 years) indicates that this process is not currently occurring in these chronosequences. Therefore, simply the presence of carbonate accumulations in these soils suffices to characterize such paleosols. This criterion is applicable not only to the soils of the Tormes river basin, but also to a great number of soils throughout the region.