Assessing Silt Generation and Origins in Granitoid-Hosted Soils: Implications for Loess Formation

The origin and production of silt are key factors in the formation of loess deposits. Although many processes can potentially lead to silt generation, few are known to produce silt in the volumes and particle-size modes required to form geologically significant loess deposits. Here we investigate the hypothesis that pedogenic weathering in tropical and Mediterranean climates can generate abundant in situ silt, and therefore contribute significantly to loess formation throughout geologic time. We utilize granulometric and geochemical data from soils formed in Puerto Rico (hot-humid) and Southern California (hot-arid) to discern whether the mud fraction (<62.5 μm) is generated from bedrock (autochthonous) or sourced from eolian contributions (allochthonous). Our study demonstrates that the Puerto Rico soil contains abundant (up to 72%) silt- and clay-sized grains compared to the Anza Borrego soil (<6%). However, the silt fraction of the Puerto Rico soil is at least partially derived from eolian inputs, and the silt fraction of the Anza Borrego soil is geochemically indistinguishable from allochthonous dust sources. Furthermore, while intense chemical weathering in a tropical climate can produce abundant fines, the majority are significantly finer (average mode ~15 µm) than the modes of most “typical” loess deposits (modes more than 20 – 30 µm). In contrast, weathering in Mediterranean climates produces volumetrically sparse silt. Hence, pedogenic weathering in hot climates appears to be ineffectual for producing the volume and size distributions of silt-sized material needed to generate significant loess deposits.