Interpretation of soil features produced by ancient and modern processes in degraded landscapes: V. Development of saline sulfidic features in non-tidal seepage areas

Abstract

Saline sulfidic soils are usually associated with tidal flushing zones. However, less developed saline sulfidic soils have recently been identified in non-tidal seepage and marsh areas strongly affected by waterlogging, dryland salinity and erosion in the Mediterranean climatic region ( > 600 mm per annum winter rainfall maximum) of the Mt Lofty Ranges, South Australia. A conceptual sub-model is developed to explain the formation of these saline sulfidic soils by using data from detailed pedological, mineralogical, hydrological and physico-chemical investigations. The following three saline sulfidic features were recognised both within the subsurface soil layers and on the soil surface: (i) black sulfidic materials, (ii) iron-rich gelatinous precipitates and (iii) salt-iron crusts. The abundance of these features and their distribution in relation to each other were used to establish a chronological order of formation. For each saline sulfidic feature, specific minerals (e.g. pyrite framboids, ferrihydrite and schwertmannite) were identified and their conditions of formation (e.g. Eh and pH) established. Their development is closely associated with two water-flow systems: a rising saline sulfatic groundwater aquifer and the seasonal discharge of fresh water via a perched water table. The conceptual sub-model shows how these two water-flow systems and three biomineralization processes lead to the formation of these saline sulfidic marsh soils. It illustrates how the following three main conditions control the formation of these non-tidal saline sulfidic marsh soils: (i) the development of aquic and saline conditions throughout the solum, (ii) the accumulation of organic matter from which Fe and S reducing/oxidising bacteria derive their energy, and (iii) a continuous supply of Fe and S in groundwater aquifers. In the Mt Lofty Ranges, such conditions are due to rising saline sulfatic groundwater tables following land clearing since European settlement and contemporary weathering of pyrite lenses in the underlying rock.