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