Geological controls on evolution of evaporative precipitates on soil-free substrates and ecosystems, southern New Zealand

Abstract

Soil-free bare substrates have formed by natural and human-induced processes in a semi-arid rain shadow, in the lee of actively rising mountains. These substrates have developed evaporative salt encrustations with a wide range of minerals controlled by local substrate permeability and mineralogy. Many of these small (hectare scale) sites also host endemic salt-tolerant ecosystems that are currently endangered by weed incursion. This study characterises the differing mineralogy and geochemistry of these rare ecosystem-hosting substrates. Impermeable substrates, especially clay-rich schist basement, are dominated by NaCl from marine aerosols in rain. The high Na at these sites further enhances impermeability of substrates by promoting surficial clay mobility, so that even Pleistocene-Recent sediments derived from schist basement develop evaporative crusts. The Na/Cl ratio of some of these substrates, especially sediments, has been increased by alteration of albite. However, mineralogically similar greywacke-derived sediments do not develop saline encrustations. Penetration of rainwater into substrates facilitates water-rock interaction reactions that yield entirely different evaporative salt mineralogy, as the NaCl component is overshadowed by constituents dissolved from the rocks. Schist basement and limestone-rich substrates develop carbonate-dominated evaporites, especially calcite, and associated waters are typically supersaturated with respect to carbonate minerals. Some sulphate-rich evaporites form where rock pyrite has been oxidised. Hydrothermally altered schist basement is locally enriched in Mg-bearing carbonates, sulphides and gold. Bare substrates on these sites are variably permeable and develop evaporative salts with carbonates, sulphates, ferric oxyhydroxide (some arsenic-bearing), and minor brucite, after extensive water-rock interaction. Most of the sites are alkaline, and pH locally exceeds 10, as a result of the combinations of evaporative processes and water-rock interactions. The specialist plants have evolved to tolerate the relatively high salinities and pH of these chemically distinctive sites.

Keywords: Calciflora; Evaporation; Geoecology; Halophyte; Limestone; Rain shadow; Salt; Sodicity.