Thick iron oxide pans in soils of Taranaki, New Zealand

Abstract

Thick iron oxide pans are a distinctive feature of some soils in Taranaki, New Zealand, which occur on the ringplain, or on terraces of valleys draining the ringplain of Mount Egmont. The pans tend to form in the boundary area between layers of differing texture within the zone of water table fluctuations. The pans are indurated, brittle, and vesicular, and have a black or reddish brown appearance with a shiny black fracture. They are up to 50 cm thick and form lenticular deposits sometimes several metres across at depths ranging from a few centimetres to about 1 m. Analysis of seven samples of pan gave 34-45% elemental Fe, 3-5% A1 and 4-10% Si, consistent with about 55-70% iron oxides, together with entrapped and adhering soil particles. V and Mo are enriched in the pan samples and probably occur as anionic species strongly adsorbed on the iron oxide surfaces. X-ray powder diffraction, Moessbauer spectroscopy and acid-oxalate dissolution indicate that the dominant iron oxides present are goethite and ferrihydrite. The relative proportion of these two minerals varies widely without any noticeable change in the nature of the pan materials. Microstructures in one sample were examined by scanning electron microscopy. The pans are considered to have formed from the aeration of groundwaters (rich in ferrous ions) moving laterally through the soils. Such groundwaters are formed on Mount Egmont from the reaction of meteoric water, sometimes containing dissolved volcanic carbon dioxide, with ferromagnesian minerals. Positive tests for ferrous ions (using �,�'-dipyridyl as indicator) were obtained from groundwaters presently associated with the pans.

Geochemical fluxes and weathering of volcanic terrains on high standing islands: Taranaki and Manawatu-Wanganui regions of New Zealand

“…Favorable conditions for allophane and imogolite formation occur where rainfall (and usually leaching, depending on the drainage conditions) is moderate to high (Parfitt, 1990). Allophane has been previously identified in the clay fraction of Taranaki region soils (Neall, 1977;Russell et al, 1981;Childs et al, 1990), and is attributed to the region's deep soil profiles, thick volcaniclastic deposits, and abundant precipitation (up to 6500 mm yr À1 ) (Neall, 1977).…”

“…Mineralogically, the soil fines contain quartz (<3% in the upper layer), feldspar (present mostly as calcic andesine), andestic glass (with microlites of feldspar, mafic minerals, and titatomagnetite in a glassy matrix), rhyolitic glass, and the mafic minerals augite, hornblende, and hypersthene (Stewart et al, 1977) along with high amorphous clay allophane content ($70% of the clay fraction) (Russell et al, 1981). The presence of iron oxide pans is an additional characteristic feature of the Egmont andisols (Childs et al, 1990). Studies of volcanic soils in the Ruapehu region are few.…”

The Primacy of Temporal Dynamics in Driving Spatial Self-organization of Soil Redox Patterns

“…Soils with more crystalline Fe(III) or plinthic-like horizons often have diminished nutrient and water retention properties (31, 32, 41) and these can limit root growth. Additionally, highly crystalline Fe(III)— plinthite, often observed in soil redoximorphic features, is hard and brittle, forming a barrier to root penetration (27, 33). When lower crystallinity Fe(III) dominates the Fe pool, roots usually can effectively penetrate soils by various morphological and chemical adaptations (42, 43).…”

“…While high Fe(III) concentrations do not lead to increases in crystallinity per se , under repeated redox oscillations over time, increases in Fe(III) aggregation and in the crystallinity of Fe(III) usually coincide (26, 83). Aggregated Fe(III) precipitates can lead to coarse soil texture, low soil water content, and low nutrient availability, and highly crystalline Fe(III) aggregates (e.g., plinthitic material) create high mechanical impedance for root growth (4, 32, 33, 84). [ Fe ( III )] ng denotes the threshold Fe(III) concentration (mol m -3 ), above which Fe(III) will negatively affect root growth.…”

“…Crystalline Fe(III) weakens soil aggregation (30), which reduces soil water and nutrient retention (31, 32). Furthermore, hard and brittle plinthite-like material forms a barrier to roots (33), especially during dry periods when resistance to root penetration increases markedly (34). Together these processes form the long-range negative feedback that inhibit root expansion.…”