ASSESSING THE ORIGIN OF SILT IN MEDITERRANEAN, GRANITOID-HOSTED SOILS

Abstract

The modes of production of silt in sufficient volumes to form loess deposits remains debated. In periglacial regions, glacial grinding typically prevails and is a non-controversial means to produce abundant silt. Nonglacial mechanisms proposed include eolian saltation, fluvial comminution, salt weathering, and derivation from silt-rich protoliths. Additionally, chemical weathering in soil profiles of tropical and mediterranean climates—a hypothesis first proposed in the 1980s—has garnered significant attention as a potentially effective means for silt production but has not been rigorously tested. Here we investigate weathering in Mediterranean, granitoid-hosted soils in northeastern Spain and southern France to test the efficacy of weathering for silt generation from coarse-grained hosts. Textural and geochemical data indicate that, despite significant evidence for chemical weathering, these profiles are dominated by sand and gravel, with only minimal (average across all profiles ~7.9%, average of individual profiles range 5-10%) mud (clay + silt). Provenance data indicate the influence of allochthonous additions of dust to several profiles, especially those in Spain and coastal France, with data from dust traps confirming contributions from sources in north Africa, as well as other potential sources in southern Europe. Although mud modes (17-52µm) from the profiles encompass common loess modes, the amount of mud produced in these weathering profiles is insufficient to create significant loess deposits, challenging the oft-cited hypothesis of the efficacy of chemical weathering for silt production.