Pedogenic Iron Oxide Trends in a Marine Terrace Chronosequence

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 d 110 line. The intensity of the goethite d 110 line increased with terrace age. Degree of Al substitution in goethite was calculated from d 111 and d 110 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.