Morphology, distribution and origin of soil biogenic carbonates “queras” presents in Loess-palaeosols of Ebro Valley

Several types of secondary carbonate accumulations have been reported, but some of them are not completely well defined in the field due to unclear nomenclature. This is the case of the “queras”, reported in several Loess-palaeosol sequences of the Ebro Valley, which have often been described as pseudomycelia. Micromorphologically, they are complex pedofeatures (including calcified root cells, infillings and hypocoatings of carbonates and a decarbonated zone), resulting from calcification/decalcification processess at a microscale. They are composed of a central channel (1-2 mm wide and 2-3 cm long) filled with biosparite crystals (Herrero et al., 1992). The study of these secondary carbonate bioaccumulations are important archives for climatic reconstructions in terrestrial environments and can be used for paleoenvironmental reconstructions. The aims of this research are the characterization (morphological, optical and isotopically) of the biocalcifications present in Loess-palaeosols sequences, OSL-dated, to determine the main factors that originate them and their possible use as a palaeoenvironmental proxy. We collected soil samples from seven profile of Loess-palaeosols where the presence of these biocalcifications was recorded. We isolated and manually cleaned complete fragments of queras to describe them and to determine their isotopic composition. For that purpose, we used the queras fraction (sieved fraction of bulk soil between 100-250 µm) removing the residues of micrite with a buffer solution and manually separating the quera fragments with the help of a stereoscope. Thin sections were made to analyse the micromorphology in a petrographic microscope and cathodoluminescence techniques to determine the origin of the calcite. The micromorphology of these biocalcifications is similar in most cases: they present the same number of rows around the central channel (4 to 5), and a decarbonated hypocoating around it, supporting the hypothesis that their origin is derived from the calcification of cells of the root tips as a strategy to acidify the soil surrounding to absorb nutrients. Under cathodoluminescence biosparite has a different behaviour than non-biological calcite crystals. The age of the queras was similar in most horizons and their formation is independent of the age of the loess deposit. The isotopic composition of δ¹³C correspond mainly to CAM plants and the temperatures of precipitation calculated correspond to a Mediterranean template climate (Cerling and Quade, 1993), implying that the biocalcifications developed in warm environments. Finally, we hope to gain some more certainty of their origin and formation processes from the ongoing analyses of DNA sequencing and pollen recording.