INTERPRETATION OF SOIL FEATURES PRODUCED BY ANCIENT AND MODERN PROCESSES IN DEGRADED LANDSCAPES .5. DEVELOPMENT OF SALINE SULFIDIC FEATURESIN NON-TIDAL SEEPAGE AREAS

Saline sulfidic soils are usually associated with tidal flushing zones . However, less developed saline sulfidic soils have recently been ide ntified in non-tidal seepage and marsh areas strongly affected by wate rlogging, dryland salinity and erosion in the Mediterranean climatic r egion (>600 mm per annum winter rainfall maximum) of the Mt Lofty Rang es, South Australia. A conceptual sub-model is developed to explain th e formation of these saline sulfidic soils by using data from detailed pedological, mineralogical, hydrological and physico-chemical investi gations. The following three saline sulfidic features were recognised both within the subsurface soil layers and on the soil surface: (i) bl ack sulfidic materials, (ii) iron-rich gelatinous precipitates and (ii i) salt-iron crusts. The abundance of these features and their distrib ution in relation to each other were used to establish a chronological order of formation. For each saline sulfidic feature, specific minera ls (e.g. pyrite framboids, ferrihydrite and schwertmannite) were ident ified 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 sh ows how these two water-how systems and three biomineralization proces ses lead to the formation of these saline sulfidic marsh soils. It ill ustrates how the following three main conditions control the formation of these non-tidal saline sulfidic marsh soils: (i) the development o f aquic and saline conditions throughout the solum, (ii) the accumulat ion of organic matter from which Fe and S reducing/oxidising bacteria derive their energy, and (iii) a continuous supply of Fe and S in grou ndwater aquifers. In the Mt Lofty Ranges, such conditions are due to r ising saline sulfatic groundwater tables following land clearing since European settlement and contemporary weathering of pyrite lenses in t he underlying rock.