GRAIN SIZE DISTRIBUTIONS OF MEDITERRANEAN SOILS AND ASSOCIATED IMPLICATIONS FOR SILT GENERATION

Loess, or eolian silt, covers a significant proportion of land today (~6%) and is found in both glaciated and desert regions, but generation of the silt that forms loess deposits remains debated. Whereas glacial grinding is a known means to produce silt in large volumes, many have suggested that other processes, including pedogenesis, can also produce significant silt. Here we present new granulometric distributions from soils forming in temperate climates (Köppen-Geiger classifications Csa and Cfb) testing the previously published hypothesis that soil formation in Mediterranean climate zones results in significant silt production, even from coarse-grained (granitoid) precursors.

Weathering profiles and associated granitoid-bedrock were collected to assess in situ weathering potential. One-meter profiles were collected at 10 cm intervals, together with weathered and fresh bedrock samples for all localities. Preliminary data from two profiles reveal minimal mud contents (<62.5µm; 5 – 8%); both soils are predominantly sand (62.5µm – 2mm; 40 – 85%) and gravel (>2mm; 10 – 50%). The soil forming in the Csa climate (temperate, dry and hot summer) consists predominantly of sand (60 – 85%), while the soil forming in the Cfb (temperate, no dry season and warm summer) was largely gravel (up to 50%). Modes of the mud fraction for each soil fall within the medium (16 – 31μm) and coarse (31 – 62.5μm) silt fractions and range from 23 – 52µm; however, the Csa soil contains slightly finer modes (23 – 44µm) that vary throughout the profile, and the Cfb soil contains slightly coarser modes (32 – 52µm) that increase then decrease in size down-section. These preliminary results indicate that these soils do contain some silt in the typical loess grain-size (modes between 20 – 40µm); albeit in very small volumes compared to the amount of silt needed for a geologically significant loess deposit. To further explore these results, future work will include particle-size analyses on additional soils, as well as geochemical analyses of the parent-rock, soils, and dust trap sediment to aid in determining whether the silt in the soil profiles formed autochthonously (from bedrock weathering) or allochthonously (from eolian additions).