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Wetland soils, especially those under a semi-arid climate, are among the least studied soils in the tropics. The hypersaline tidal flats on the north-eastern Brazilian coast, locally named apicum, are coastal wetland ecosystems in the peripheral portions of semi-arid estuaries. Despite their great ecological importance, they have been highly impacted by anthropogenic activities. Morphological and analytical data of six soil profiles, representative of the different coastal compartments (mangroves, apicum and coastal tablelands) of the north-eastern Brazilian coast, were examined to better understand the pedogenesis of apicum soils. The hypersaline tidal flat soils were classified as Typic Fluvaquents and Typic Sulfaquents with the following main characteristics: predominance of sand fraction (62-77%); presence of high- activity clays (> 24 [cmol.sub.c] [kg.sup.-1] clay); clay fraction comprising kaolinite, illite, smectite and an interstratified smectite/illite; exchangeable complex dominated by [Na.sup.+] (ESP [greater than or equal to] 15%); elevated levels of salinity (electrical conductivity, EC 25-44 dS [m.sup.-1]); alkaline pH values (7.5-9.5). The sandy texture and quartz-dominated composition of the hypersaline, tidal flat soils indicate a pedogenesis associated with the superficial addition of mineral material. This upbuilding process would have lowered the watertable (relatively to the ground level) and decreased the flooding frequency by the tides, favouring salinisation and solonisation processes at the hypersaline tidal flats. Furthermore, the still-existing hydromorphism would have promoted the maintenance of gleisation and sulfidisation. The presence of pyrite on the hyper-saline tidal flat soils further corroborates the formation of apicum soils from/over buried mangroves. Additional keywords: coastal wetlands, mineralogy, soil genesis. Received 16 October 2012, accepted 22 October 2013, published online 13 March 2014, Introduction The terms salt flats (Ridd and Stieglitz 2002), supratidal flats, hypersaline tidal flats and unvegetated flats (Hadlich et al. 2009) are used to refer to coastal ecosystems on the [...]

The minerals of the clay fraction in estuarine plains are mainly detrital, being a mixture of marine and continental sediments, but can also be authigenic. Because of the importance of mangrove ecosystems in tropical estuarine areas and the relatively few existing studies of the mineralogical composition of soils in these environments, the aim of this study was to determine the mineralogical assemblage and identify potential contrasts along the coast of the State of Sao Paulo. Soils from 11 mangroves distributed along the coastal plain of the State of Sao Paulo were sampled at depths of 0 to 20 and 60 to 80 cm, and samples of suspended sediments from the Ribeira do Iguape River were collected for analysis. Mineralogical analyses were performed on the clay and silt fractions by x-ray diffraction (XRD) and transmission electron microscopy, and flesh soil samples were analyzed by scanning electron microscopy-energy dispersive spectrometry and suspended sediments by XRD. The silt fraction contained quartz, feldspars, gibbsite, kaolinite, illite, and vermiculite, and the clay fraction contained smectite, kaolinite, illite, gibbsite, quartz, and feldspars. Locally, vermiculite, biotite, anatase, halloysite, and goethite may occur because of recent transport of sediments to the system. Pyrite was identified in fresh samples. The allochthonous minerals found either were terrestrial and transported by rivers or had originated from the continental platform by past transgressive events. We suggest that the neoformation of smectite and kaolinite occurs in mangrove soils. Different geomorphological settings along the Sao Paulo coast appear to regulate mineral distribution in mangrove soils. Abbreviations: IGUA, Caranguejo Island; ITA, Itapanhau River; PM, Pal Matos Island; RE, Escuro River; RF, Fazenda River; XRD, x-ray diffraction.

Pore-water and solid-phase analysis was undertaken to study the effects of different environmental conditions on redox processes in mangrove soils on the southern coast of Brazil. Three Rhizophora mangle (L.) mangrove forests were studied in different physiographic positions--riverine, fringe, and basin forest--and substrate compositions. Our results indicate different geochemical conditions in soils from different physiographic positions. In the fringe soils, the low acid-volatile sulfide and pyrite Fe contents, associated with high values of [Fe.sup.2+] in pore water, indicate oxic and suboxic conditions in surface layers (0-10 cm). Below this depth, anoxic conditions prevail and sulfate reduction becomes the dominant process of organic matter decay, as indicated by higher concentrations of dissolved H[S.sup.-] and pyrite Fe. In the basin forest, long periods of tidal flooding and low rates of litter export maintained anoxic conditions and high organic matter contents, necessary for anoxic respiration to take place, as shown by the low [Fe.sup.2+] and H[S.sup.-] concentrations in pore water, along with high degree of Fe pyritization values and high concentrations of pyrite Fe. In the riverine soils, the lowest concentrations of pyrite Fe and the highest concentrations of H[S.sup.-] in pore water were obtained in response to low availability of reactive Fe (0-10cm, 31.8 [+ or -] 8.0 mmol [kg.sup.-1]; 20-30 cm, 43.2 [+ or -] 32.3 mmol [kg.sup.-1]) due to its sandy texture, which limits pyrite synthesis. Our results indicate that the participation and intensity, of each process may vary within different physiographic positions because of the effects on the frequency and duration of tidal flooding and soil composition. Our results also show that R. mangle is present on substrates with different geochemical conditions (oxic, suboxic, and anoxic). Abbreviations: AVS, acid-volatile sulfide; BRS, bacterial reduction of sulfate; DOP, degree of Fe pyritization; TOC, total organic carbon.