GEOCHEMISTRY AND POTENTIAL ORIGINS OF SILT IN GRANITOID-HOST SOILS FROM HOT-HUMID AND HOT-ARID CLIMATES

The generation of silt to form loess deposits has been linked to a variety of geomorphological processes including chemical weathering during pedogenesis. Some authors suggest that this process can generate up to 50 – 75% in situ silt contents from granitoid bedrock in tropical climates. Here we present granulometric and geochemical data from soils formed on granitoids/first-cycle granitoid sediment from hot-arid (Anza Borrego, CA) and hot-humid (southeastern Puerto Rico) climates to assess the origins of pedogenic silt.

A 1.5-m soil profile from Anza Borrego consists predominantly of gravel (18 – 33%) and sand (62 – 79%) with minor mud (1 – 5.5%), while a 2.5-m soil profile from Puerto Rico consists of minor gravel (<1.5%), with predominant sand (28.5 – 56%) and mud (42.5 – 71.5%). Laser Particle Size Analysis indicates much coarser modes for the mud fraction from the Anza Borrego profile (36 – 44µm) compared to Puerto Rico (6 – 18μm) and show that the Anza Borrego mud is predominantly coarse- and medium-silt, with medium-silt making up 0.3 – 1.6% of total soil, whereas the Puerto Rico mud mostly consists of clay, very fine-, and fine-silt, with medium-silt making up 8 – 14% of the total soil. These data support the hypothesis that a granitoid-host soil from a tropical climate does contain more total silt (up to 50% of the total soil) than a soil forming on similar bedrock in an arid climate; however, Puerto Rico contains silt modes that are much finer than typical loess (25 - 30μm), and Anza Borrego modes are coarser. Additionally, the silt fraction of a Pleistocene carbonate beach rock from Puerto Rico (dust proxy) and the Anza Borrego soils are geochemically similar to far-travelled dust sources including African dust and Lesser Antilles volcanism for Puerto Rico and Mojave Desert Dust in Anza Borrego. These samples also show evidence for allochthonous contributions by enrichments in several immobile elements including Ti, Th, Nb, La, and Ta. However, geochemical analysis of the <20µm and 20 – 63µm fractions of the Puerto Rico soils do not show a clear geochemical signature of dust inputs possibly due to intense leaching and chemical weathering. These results warrant further investigation regarding the efficacy of pedogenic weathering for the generation of silt modes and quantities needed to form typical loess deposits.