Red soils in loess deposits of the Western Ebro Valley

Most of the loess in the southeast of the Ebro Valley encompass mostly the Marine Isotope Stage 2 (MIS2) but some exceptions date back to more than 115 ka. Soils formed on these older loess deposits are normally categorised as red soils. We studied the pedogenesis of two of these red soils. Macro-micromorphological, particle size, color-redness rating, and iron form analyses were performed in combination with luminescence and radiocarbon dating. Mas de l'Alerany is a primary loess deposit, 3.15 m thick. Recarbonation is shown as carbonate coatings covering previous clay coatings in a red matrix, besides carbonate nodules and rhizocretions. Its deepest horizon dates back to ca. 140 ka. Chiprana is a 3.2 m loess-like sequence. It also reveal carbonate nodules and rhizocretions throughout the profile, together with a strong bioturbation and frequent queras, which are channel infillings of biosparite surrounded by decarbonated reddish hypocoatings. The bottom of the sequence has an age between 100 and 130 ka and shows residual clay coatings. The age of the upper part is about 30 ka, while the biosparite grains have an age of 26 ka. The genesis of the Mas de l'Alerany red soil, after its deposition, comprises the subsequent processes of decarbonation - rubefaction - clay illuviation - recarbonation. Chiprana presents a similar succession, but with a more recent decarbonation due to the biological activity that generated the queras, which were responsible for revealing the previously rubefacted material. Although Mas de l'Alerany and Chiprana red soils were probably formed from loess of the penultimate glaciation and both underwent a similar cycle of processes, the origin of the red colours in Chiprana is due to micro-site, biologically driven recent decarbonation. Our results suggest that the biological activity may play an important role in the reddening process in similar Mediterranean soils.

Automated summary

The study focused on the pedogenesis of two red soils in the southeast of the Ebro Valley, revealing that one soil formed around 140 ka ago, while the other soil formed between 100 and 130 ka ago and underwent a new cycle of pedogenesis around 30 ka ago. The results suggest that biological activity may play a significant role in the reddening process of similar Mediterranean soils.