Lithologic controls on regolith weathering and mass flux in forested ecosystems of the southwestern USA

Abstract

Parent material has a profound impact on chemical weathering, mineral transformation and chemical denudation. However, there remains a relative paucity of lithosequence studies that directly examine parent material control on pedogenic processes. We sampled a lithosequence of four parent materials (rhyolite, granite, basalt, dolostone/volcanic cinders) under Pinus ponderosa in mesic and ustic soil moisture and temperature regimes of central and southern Arizona, USA to quantify the contribution of parent material to chemical weathering and elemental mass flux. We quantified chemical weathering and mass flux using a combination of quantitative X-ray diffraction and elemental mass balance. Mass flux calculations were confounded by the addition of volcanic cinders in the dolostone soils and addition of eolian materials in both the basalt and dolostone soils. These variations in parent material were accounted for using a combination of refractory element indices and X-ray diffraction. Results indicated significant differences in profile characteristics and chemical weathering among parent materials. Chemical mass loss from the basalt and dolostone soils were balanced or exceeded by addition of eolian materials, leading to positive and highly variable mass fluxes of 14 ± 48 kg m − 2 and − 10 ± 22 kg m − 2 , respectively. Rhyolite and granite soils exhibited large differences in chemical mass flux despite nearly identical elemental and mineralogical compositions of the respective parent materials. Total chemical mass flux from the granite soils averaged − 173 ± 31 kg m − 2 , whereas mass flux from the rhyolite soils was much larger, on the order of − 930 ± 71 kg m − 2 . These large differences result from the variation of parent material grain size and bulk density. The data demonstrate strong control of parent material on chemical weathering and mass flux in cool, semiarid forested ecosystems.