Geoquímica dos sedimentos de manguezais do nordeste do estado do Pará: um exemplo do estuário do rio Marapanim

Abstract

The northeast coast of Pará state was geologically built on fluvial valleys partially submersed during the Holocene, where the mangroves of Marapanim estuary were developed. Adjacent to the mangroves, iron sediments and Latosol of Barreiras Formation (Tertiary) are the main source of silt, clays and sands. Despite the ecological, social and economic mangrove significance, there is a lack of geologic information focusing the processes and the magnitude of mineralogical and geochemical transformations occurring in these ecosystems on the Brazilian north coast, which is the main goal of this research. To reach the purposed objectives topographic studies were run, as soon as a short description about the mangroves. Sediments were sampled in the end of both rainy and dry seasons and submitted to in situ interstitial salinity, Eh and pH measurements. To these samples were also run chemical analysis to determine SiO2, Al2O3, Fe2O3, TiO2, P2O5, Na2O, K2O, CaO, MgO, L.I. (lost on ignition) and granulometric analysis. Trace elements were determined by using ICPM-MS; minerals were determined by using X-ray diffraction and SEM techniques. Surface and interstitial water were sampled during neap and spring tides, in periods with both higher (March, June and July) and lower (September, November and December) pluvial precipitation. These samples were submitted to chemical analysis to determine H4SiO4, SO4 2-, ΣH2S, NH4 +, Cl-, PO4 3-, Na+, Mg2+, Ca2+, K+, total alkalinity and dissolved iron. Salinity, Eh and pH were also measured. The mangroves are typically from intertidal zones and are represented by a mixture of Rhyzophora mangle and Avicennia germinans developed under semidiurnal macro tides, totally submersed during the ebb-tides and weekly exposed during the spring-tides, under a rainy tropical weather, hot and humid with remarkable climatic seasonality. The mangrove sediments are predominantly silt-clayed, rich in organic matter (C: 1 to 4% grade). Those sediments were originally deposited over sand bars, which smooth morphology and sedimentological aspects promote vegetal colonization, drainage network evolution and sediments development, increasing its consistency. The tropical chemical weathering acts over the source areas producing quartz, low crystal kaolinite grains, iron oxides and other dissolved chemical substances like silica, aluminum and heavy metals which are incorporated to the mangrove sediments, with diatoms and Na+, K+, Ca2+ e Mg2+ ions from marine contributions. At the mangrove, the original material is reworked throughout intensive biological activity and geochemical processes developed in the presence of different organic matter grades and the reactive surface of biogenic silica (diatoms), originating minerals in total or partial equilibrium within the new conditions. The autigenic minerals are pyrite, smectite, K-feldspars, halite, gypsum, jarosite beyond quartz and remobilized iron oxy-hydroxides. Mangrove sediments present high grades of dissolved sulfide ions (6 to 40 mmol/L) while in depth, sulfate ions are consumed. Dissolved sulfide is formed from organic matter mineralization under bacteriological sulfate-reduction, which final product is the pyrite. In depth, total dissolved iron grades are reduced close to zero due to the reaction with part of the dissolved sulfide to form solid sulfide (pyrite). This reaction occurs in an extremely reductor chemical environment (-200<Eh<-400 mV), weakly acid to alkaline (6.5<pH< 8), followed by high grades of alkalinity (25 meq/L, max.), ammonia (close to 1200 μmol/L) and orthophosphate (170 μmol/L max) ions. The low crystalline smectite is formed due to the kaolinite degradation in contact with abundant diatoms and marine magnesium, while the K-feldspar can be formed beyond the potassium from the sea. Silica remobilization was evidenced in depth by the superficial dissolution of crystals of quartz and their re-crystallization as isolated and euhedric crystals, forming sometimes crystal agglomerates over the pre-existing grains of quartz. Due to a strong dessecation (including the biological action) or throughout the extensive sediment exposure during the spring tides, the halite is formed by the interstitial water saturation on the sediment surface while gypsum and jarosite are formed by the pyrite oxidation. The interstitial water movement, induced by the weather seasonality, allows the iron remobilization and precipitation on the sediments surface, like a coating that involves the grains of quartz. The obtained results show that mangroves from the Marapanim river estuary are formed from the tropical weathering in Tertiary sediments from the Barreiras Formation and derived soils. Clastic material (mainly quartz, kaolinite and iron oxides) transported and deposited over sand bars along the estuary are joined by marine contributions (alkaline ions), organic matter and diatoms. This mixed material is reworked by degradation and mineralogical formation processes, resulting in sulfides (pyrite) and aluminum-silicates (smectite and K-feldspars), beyond quartz and precipitated iron oxy-hydroxides. The mangrove morphology allows a frequent flood due to tidal and sedimentological deposition processes, associated to the colonization and development of vegetation. Physical properties and characteristics inherent to the drainage network prove that sediments evolutes with the mangroves. The short regional seasonality, allied to topography and tidal rhythm interfere positively to form high gradients of interstitial salinity, Eh and pH, associated to evaporatic phases (gypsum and halite), sulfide oxidation (jarosite) and iron oxy-hydroxides precipitation, similarly to preliminary results obtained to areas with dry weather around the world.