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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

View larger version (44K): [in a new window]   Fig. 1. Map showing the San Timoteo Badlands and nearby Cajon Pass study locations. Stippled pattern designates upland regions.

View larger version (45K): [in a new window]   Fig. 2. Map of the San Timoteo study area, showing San Timoteo and Reche drainages, geomorphic surfaces (Q1, Q2, Q3), and location of pedons. The STC and RC designations refer to the pedons described on surfaces within the San Timoteo and Reche drainages, respectively.

View larger version (39K): [in a new window]   Fig. 3. Depth plots showing pedogenic silica content (shaded regions) in the studied pedons. The numerical value in each plot indicates the depth-weighted average of secondary silica. The silica content was integrated to a depth of 20 cm into the C or Cox horizon for each pedon (Table 1). The Fed (citrate-bicarbonate-dithionite-extractable Fe; solid lines) and percentage of clay (dotted lines) are plotted for comparison (data from Kendrick and McFadden, 1996).

View larger version (12K): [in a new window]   Fig. 4. Graphs showing variability associated with chronosequence analysis. (a) Plot of the relation between age estimate (ka) and horizon values of tiron-extractable Si (%) for pedons in Cajon Pass (CP) and San Timoteo Badland (STB) study areas. The results are arranged by surface, with the designation of CP or STB as modifiers to the surface designation (i.e., CP Qoac is the Qoac surface in Cajon Pass). The values for the Cajon Pass horizons are shown with a closed diamond, while San Timoteo soil horizon values are shown using an open diamond. Mean values for each pedon (sometimes more than one per surface) are plotted with a large "X" symbol. The data shown for each surface are from the following pedons: Surface CP Qoac: Pedon SB1; Surface STB Q1: Pedon STC 7; Surface CP Qoad: Pedon RW 11; Surface STB Q2: Pedons STC 9, RC 6; Surface STB Q3: Pedons RC 2, STC 10, STC 6. (b) Horizon values of Feo/Fed ratios for the soils in the Eastern Transverse Range chronosequence (McFadden, 1982; McFadden and Hendricks, 1985). (c) Horizon values of Feo/Fed ratios for the Cajon Pass chronosequence (McFadden and Weldon, 1987).

View larger version (15K): [in a new window]   Fig. 5. Depth plots showing primary Si as a function of age for the <2-mm fraction of soils in San Timoteo Canyon.

View larger version (17K): [in a new window]   Fig. 6. Plots of elements (K, Na, Ca, Fe, Al) versus Si from total elemental analysis of <2-mm fraction of horizons from STC pedons. Tabulated values are available in Kendrick (1999). Approximate surface ages are: STC = 47 ka; STC9 = 55 ka; STC6 = 500 ka.