PEDOGENIC IRON-OXIDE TRENDS IN A MARINE TERRACE CHRONOSEQUENCE

Depth to maximum concentration and maximum horizon concentration of whole-soil dithionite-extractable Fe (Fed) and Al (Ald) increased from the youngest (105 000 yr) to oldest (600 000 yr) terrace in a California marine chronosequence. The ratio Fed /Fet (total Fe) for clays extracted from horizons of maximum pedogenic expression also increased with terrace age, while the clay FeO (oxalate extractable)/Fed decreased. These data suggest that there is a progressive transformation of ferrihydrite to crystalline Fe oxides with time. Mean annual temperature of 14°C moderate precipitation (716 mm yr-1), and frequent fog appear to favour the transformation of ferrihydrite to goethite. -from Authors