Initial generation of sand across climate zones of the Mojave, Sierra Nevada, and Klamath Batholiths in California, U.S.A.

Abstract

The generation of modern, plutoniclastic sediment across California varies with climate, resulting in significant mineralogical and geochemical differences between parent rock, weathering profile material, and low-order stream sand. With average precipitation of 15cm/yr in the arid zones and 120cm/yr in the temperate zones, feldspar is relatively decreased in stream sand (compared to parent rock) by <2% and ~20%, respectively. Potassium feldspar is more resistant to weathering than plagioclase but is less stable than quartz in temperate climates. Hornblende and biotite are more easily weathered in temperate than arid climates, although their occurrence in medium-grained stream sands is affected by hydraulic sorting and comminution during transport. Temperate-climate sands have lower concentrations of CaO and Na2O compared to K2O and Al2O3, reflective of the expected faster loss of plagioclase than potassium feldspar during weathering. With the exception of biotite, the mineralogy of medium-grained stream sand reflects that of medium-grained weathering profile material in both climates, confirming the findings from constant-climate case studies that the composition of proximal sand is largely achieved within weathering profiles and during initial erosion. We encourage continued attention to climate as a control on sand composition in provenance and paleoclimate studies, where a moderate precipitation shift is capable of producing a significant loss of feldspar. The differential response of common minerals to chemical weathering should be considered for studies and modeling related to bedrock erodibility, sediment supply, and landscape evolution.