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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 São Paulo. Soils from 11 mangroves distributed along the coastal plain of the State of São 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 fresh 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 São Paulo coast appear to regulate mineral distribution in mangrove soils. [ABSTRACT FROM AUTHOR]

Abstract: Five zones along a transect of 180 m were selected for study on the Island of Pai Matos (Sao Paulo, Brazil). Four of the zones are colonised by vascular plants (Spartina SP, Laguncularia LG, Avicennia AV and Rhizophora RH) and were denominated soils, and the other zone, which lacks vegetation, was denominated sediment (SD). The geochemical conditions differed significantly in soils and sediment and also at different depths. The soils were oxic (Eh>350 mV) or suboxic (Eh: 350–100 mV) at the surface and anoxic (Eh<100 mV) at depth, whereas in the sediment anoxic conditions prevailed at all depths, but with a lower concentration of sulphides in the pore water and pyrite in the solid fraction. Under these geochemical conditions Fe is retained in the soils, while the Mn tends to be mobilized and lost. The most abundant form of iron oxyhydroxide was lepidocrocite (mean concentration for all sites and depths, 45±19 µmol g−1), followed by goethite (30±19 µmol g−1) and ferrihydrite (19±11 µmol g−1), with significant differences among the mean concentrations. There was a significant decrease with depth in all the types of Fe oxyhydroxides measured, particularly the poorly crystalline forms. The pyrite fraction was an important component of the free Fe pool (non-silicate Fe) in all soils as well as in the sediment, especially below 20 cm depth (mean concentration for all sites and depths, 60±54 µmol g−1). Furthermore, the mean concentration of Fe-pyrite for all sites and depths was higher than that obtained for any of the three Fe oxyhydroxides measured. The Fe-AVS was a minor fraction, indicating that the high concentrations of dissolved Fe in the soils in the upper area of the transect result from the oxidation of Fe sulphides during low tide. Mössbauer spectroscopy also revealed that most of the Fe(III) was associated with silicates, in this case nontronite. The presence of crystals of pyrite associated with phyllosilicates in samples from the upper layer of the soils may indicate that pyritization of this form of Fe(III) is more rapid than usually reported for ocean bed sediments. The sequential extraction of Mn did not reveal any clearly dominant fraction, with the Mn-carbonate fraction being the most prevalent, followed by exchangeable Mn and oxides of Mn, whereas pyrite-Mn and Mn associated with crystalline Fe-oxides were present at significantly lower concentrations. The high concentration of dissolved Mn found in the soils in the lower part of the transect is consistent with the fact that the solubility is determined by the carbonate fraction. Unlike for Fe, in the soils in the higher zone, which are subject to intense drainage during low tide, there was loss of Mn, as reflected by the concentration of total Mn. [Copyright &y& Elsevier]

Abstract: Solid-phase analysis was undertaken to obtain information on the effects of bioturbation (activity of plants and macrofauna) on the distribution of different forms of iron and sulfur in a mangrove ecosystem in Brazil. A seasonal study involving four compartments from a mangrove forest (UN: an unvegetated mudflat; SP: a low intertidal zone colonized by Spartina alterniflora; RH: monospecific forest of Rhizophora mangle and; AV: monospecific forest of Avicennia schauerianna) was carried out. Pyrite-Fe, oxyhydroxides-Fe, acid volatile sulfide (AVS), degree of pyritization (DOP), pH and Eh were determined. The results show an intense effect of mangrove vegetation on soil redox processes. Higher live root densities (56–132 g m−2) in surface layers (0–10 cm) from vegetated sites led to suboxic/oxic conditions (129–374 mV), pyrite oxidation, and acidic (3.8–5.5) conditions, showing the role of plant activity on iron sulfide biogeochemistry. The combination of more oxidizing conditions and pyrite oxidation process also favored oxyhydroxide precipitation, as indicated by higher values of oxyhydroxides-Fe found in superficial layers. In contrast, vegetated zones presented higher values of pyrite-Fe than UN, indicating that root exudades may stimulate the activity of sulfate-reducing bacteria. The results show a clear seasonal variation in AVS and pyritic fractions. In summer, the elevated temperatures cause higher evapotranspiration rates and plant activity which intensified the oxidation of iron sulfides. Thus, at all sites the AVS and pyrite-Fe values were substantially lower during summer (especially in superficial layers). A clear seasonal pattern was also observed in the unvegetated mudflat compartment. The lowest pyrite-Fe, DOP and AVS values, observed in summer, appear to be related to increased crab activity (Uca spp), probably because of more intense foraging and reproductive activity. [Copyright &y& Elsevier]

Abstract: Morphological and analytical data corresponding to several profiles from two mangrove systems in the Brazilian state of São Paulo were examined for evidence of pedogenetic processes. Plant activity exerted a strong effect on the processes occurring in mangrove substrates, especially on the intensity of these processes and, therefore, constitutes one of the major factors involved in the differentiation of sediments and soils. Colonization of substrates by vascular plants leads to drastic changes in physicochemical conditions. The redox processes were much more diverse in the soil than in the sediment (substrate without vegetation); in the former they varied with depth, with oxic or suboxic conditions being observed at the surface and anoxic conditions observed below 30 cm, whereas in the sediment, anoxic conditions were observed throughout. Likewise, the acid–base conditions were more variable in the soils, ranging from strongly acid to neutral, and in the sediment they were close to neutral. Furthermore, different pedogenetic processes were identified: (1) addition of organic matter by accumulation of vegetable debris and dead roots, with formation – in some cases – of a histic epipedon, (2) transfer of soluble iron (Fe2+) towards the surface and precipitation in the form of Fe oxyhydroxide, and transfer of the mineral particles due to the bioturbation caused by activity of crabs, (3) transformation of elements such as iron (gleization) and sulphur (sulphidization). Changes undergone in the mangrove substrates were also characterized by simultaneity and intergradations between pedogenesis and diagenesis, especially in the lowermost layers in which authigenesis appears to intensify. Thus, the presence of smectite minerals in the clay fraction may be attributed to authigenic processes that take place in both soils and sediments. [Copyright &y& Elsevier]