Soil morphology and distribution of iron and sulfur fractions in a coastal flood plain toposequence

Abstract

A transect of soils across a toposequence on the coastal flood plain of the Shoalhaven River (N.S.W.) was described. The toposequence comprised a backswamp, toe of levee, and levee, and was underlain by a pyritic estuarine deposit. The development of an acid sulfate horizon along the toposequence was related to drainage as influenced by the topography. The backswamp profile had no acid sulfate horizon, whereas the soils of the toe of levee had well-developed acid sulfate horizons. The levee profiles had thin acid sulfate horizons. The distribution and form of the products of oxidation of the pyritic deposit were studied in profiles of each topographic unit by chemical fractionation of soil iron and sulfur. The levee soil showed marked redistribution of iron within its profile. Its acid sulfate horizon contained little jarosite, and its pH was higher than that found in the well-developed acid sulfate horizon of the toe of the levee soil. The latter horizon contained up to 24.3 m.e. sulfur/100 g as jarosite and had pH values near 3.9. The backswamp profile had little iron redistribution, and the estuarine sediment had been only partially oxidized. The distribution of various iron and sulfur fractions and soil pH, were discussed in relation to the oxidation of the pyritic sediment, migration of Fe2+, long term movements in the water table, hydrolysis of jarosite to free iron oxide under the influence of accreting alluvium, and periodic surface flooding.