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DIVISION S-5—PEDOLOGY

Pedogenic Silica Accumulation in Chronosequence Soils, Southern California

^a U.S. Geological Survey, 525 S. Wilson Ave., Pasadena, CA 91106
^b Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521

^* Corresponding author (kendrick{at}usgs.gov).

Chronosequential analysis of soil properties has proven to be a valuable approach for estimating ages of geomorphic surfaces where no independent age control exists. In this study we examined pedogenic silica as an indicator of relative ages of soils and geomorphic surfaces, and assessed potential sources of the silica. Pedogenic opaline silica was quantified by tiron (4,5-dihydroxy-1,3-benzene-disulfonic acid [disodium salt], C₆H₄Na₂O₈S₂) extraction for pedons in two different chronosequences in southern California, one in the San Timoteo Badlands and one in Cajon Pass. The soils of both of these chronosequences are developed in arkosic sediments and span 11.5 to 500 ka. The amount of pedogenic silica increases with increasing duration of pedogenesis, and the depth of the maximum silica accumulation generally coincides with the maximum expression of the argillic horizon. Pedogenic silica has accumulated in all of the soils, ranging from 1.2% tiron-extractable Si (Si_tn) in the youngest soil to 4.6% in the oldest. Primary Si decreases with increasing duration of weathering, particularly in the upper horizons, where weathering conditions are most intense. The loss of Si coincides with the loss of Na and K, implicating the weathering of feldspars as the likely source of Si loss. The quantity of Si lost in the upper horizons is adequate to account for the pedogenic silica accumulation in the subsoil. Pedogenic silica was equally effective as pedogenic Fe oxides as an indicator of relative soil age in these soils.

Abbreviations: Fe_d, dithionite-extractable Fe • Fe_o, oxalate-extractable Fe • ka, thousands of years before present • IROSL, near-infrared optical stimulation luminescence • Si_tn, tiron-extractable Si • TL, thermoluminescence

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