Assessing Weathering, Pedogenesis, and Silt Generation in Granitoid-Hosted Soils From Contrasting Hydroclimates

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 soil-associated 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 two granitoid-hosted soils formed in Puerto Rico (hot-humid climate) and Southern California (hot-arid climate) to discern whether the silt + clay fraction (<62.5 mu m) is generated from bedrock weathering (autochthonous) or sourced from eolian contributions (allochthonous). Our study demonstrates that the PR soil contains abundant (up to 72%) silt- and clay-sized particles compared to the Southern California soil (<6%). However, the silt fraction of the PR soil is at least partially derived from eolian inputs, and the silt fraction of the Southern California soil is geochemically indistinguishable from allochthonous dust sources. Furthermore, while intense chemical weathering in PR can produce abundant fines, the majority are significantly finer (average mode similar to 15 mu m) than the modes of most "typical" loess deposits (modes more than 20-30 mu m). In contrast, weathering in the Southern California soil produces volumetrically sparse silt. Hence, weathering processes within these two profiles, representing hot-humid and hot-dry climates, appear to be ineffectual for producing the volume and size distributions of silt needed to generate significant loess deposits. Plain Language Summary Global sediment movement is important to understand when reconstructing Earth's past climate. Terrestrial windblown silt, known as loess, is particularly useful for paleoclimate interpretation, but the origin of the silt that forms large loess accumulations remains controversial: glacial grinding produces silt, but does weathering in soils produce it? In this study, we aim to illuminate processes that generate silt (particles <62.5 mu m in diameter) in large quantities by investigating whether significant silt forms within soils as underlying bedrock weathers, or if the silt in soils was delivered from elsewhere. We approached this study as geologists interested in how Earth's near-time record can lend insight to deep-time deposits. We compare silt in two soils-from a hot-dry and a hot-wet climate. Our findings demonstrate that the soil in the hot-wet climate contains abundant silt, but of a size smaller than most loess deposits, whereas the soil in the hot-dry climate contains minimal silt; furthermore, both show evidence for silt inputs from external sources. Neither soil in our study contains significant silt in size modes typical of common loess deposits, indicating that soil formation is ineffectual for the production of silt that contributes to loess deposits.