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Pedogenic Iron Oxide Trends in a Marine Terrace Chronosequence

Dep. of Soil Science, Makerere Univ., Kampala, Uganda

Michael J. Singer^*

Dept. of Land, Air, and Water Resources, Univ. of California, Davis, 95616

* Corresponding author.

ABSTRACT

Depth to maximum concentration and maximum horizon concentration of whole-soil dithionite-extractable Fe (Fe_d) and Al (Al_d) increased from the youngest (105 000 yr) to oldest (600 000 yr) terrace in a California marine chronosequence. The ratio Fe_d/Fe_t (total Fe) for clays extracted from horizons of maximum pedogenic expression also increased with terrace age, while the clay Fe_o (oxalate extractable)/Fe_d decreased. These data suggest that there is a progressive transformation of ferrihydrite to crystalline Fe oxides with time. Relative concentrations of goethite in the clay samples were estimated from the integrated intensity of the d₁₁₀ line. The intensity of the goethite d₁₁₀ line increased with terrace age. Degree of Al substitution in goethite was calculated from d₁₁₁ and d₁₁₀ peak positions. Aluminum substitution in the geothites increased with terrace age. The relative concentrations increased from the youngest to next-to-oldest terrace, then decreased, but the degree of crystallinity increased from youngest to oldest terrace. Goethite was the dominant pedogenic Fe oxide in these four soils. Mean annual temperature of 14 °C, moderate precipitation (716 mm yr^–1), and frequent fog appear to favor the transformation of ferrihydrite to goethite.

Contribution from Dept. of LAWR, Univ. of California, Davis. The work reported here was funded, in part, by The U.S. Geological Survey Contracts 14-08-0001-20481 and 14-08-0001-21972.

Received for publication April 11, 1989.