EXPLORING SILT ORIGINS THROUGH GRAIN SIZE ANALYSIS OF MEDITERRANEAN SOILS AND DUST

Chemical weathering via pedogenesis has been suggested as a means to generate silt to form loess, especially for equatorial, and Mediterranean climates, but has not been investigated robustly. Here we present data from three soil profiles hosted on granitoid bedrock from varying climatic regions of southern France and northeastern Spain, including temperate dry and humid continental climates, to assess the efficacy of pedogenic weathering in silt production.

At each locality, weathering profiles were sampled at 10-cm intervals from the surface to 1 m depth. Additionally, we collected both weathered and fresh granitoid bedrock samples at each locality, and dust traps were deployed for 10 months at two sites near the study areas in Spain. Weathering profile samples were sieved to assess gravel (>2 mm), sand (2 mm – 62.5 µm), and mud (<62.5 µm) fractions. The <62.5 µm fraction, and the dust trap samples were then processed with hydrogen peroxide and acetic acid to remove organics and carbonates, respectively, and analyzed using a Laser Particle Size Analyzer (LPSA). Granulometric results indicate that most sediment falls within the gravel (up to 68%) and sand (up to 86%) fractions and contains minor mud (2 – 13%). LPSA analysis of the mud fractions from each profile yield modes of medium- to coarse silt (23 – 44 µm), consistent with typical size modes of loess deposits. Overall, soil profiles display a vertical pattern with finer modes towards the surface, and coarser modes at depth. Additionally, LPSA results from the dust-trap sediment indicate modes between 29 – 34 µm, which overlap with modes in the mud fractions of the soils.

These preliminary data suggest that, while size modes of the muds mimic those of loess, the quantity of mud (on average <10% of the total profile) is minimal, and potentially insufficient to produce significant loess. Furthermore, geochemical data are needed to determine whether the mud originated by in situ weathering, or records allochthonous (eolian) contributions to the soils.