Quantificação e caracterização de matéria orgânica em sedimentos de mangue, visando estudo geoquímico (Baía de Inajá, São João de Pirabas NE do Pará)

Abstract

The good functioning of the ecosystem depends on important vital factors, among which, one includes the soil stability, the salinity and an adequate supply in fresh- water and nutrients. The selected physiografic area is the Inajá Bay, situated in the municipal district of São João de Pirabas, in Northeast of Pará. Mangrove sediments sampling was accomplished at Itarana's island, located in this bay, and receiving direct influence of the Atlantic Ocean waters. Analytical results showed seasonal variation in the transportation of the fixed material in mangroves or exported to coastal waters. While pH are close to 7 in the rainy period, it to can reach values higher to 8 (maximum of 8,55) in the dry period; also specific conductivity (maximum in 33,9 ms.cm –1 in the rainy period) can reach 50 ms.cm –1 in the dry period, revealing an elevated electrolytic content due to the influence of oceanic waters. The suspended particulate material exhibits higher concentrations in the rainy period (maximum of 178 mg.L –1), in concordance with turbidity (maximum of 150 UNT) and with the index of colour (maximum in 60 UC). Organic matter, however was lower in the dry period (maximum content observed in 2,06 mg C/ L), it was higher in the rainy period (maximum in 3,4 mg C/L) , suggesting larger material contribution transposed by the rivers. While silicate contents are higher in the rainy period (maximum in 1,63 mg SiO2 / L) , the total organic and inorganic phosphate reached a maximum (1,46 mg PO4 3- /L) in the dry period, revealing probable oceanic influences in the middle of Inajá Bay, where, fishing captures take place. The ratios C / N, C / P and N / P (maximum at 126,7, 1039 and 83,2; minimum at 1,75, 101 and 17,2, respectively) suggest deficiency of N and P and enrichment in cellulose decomposition material, of vegetation origin. Else, the carbonous material of “furo” Grande exhibits a C/N ratio of 126,7 that indicates long period of maturation. The interpretation of the absorption spectra in the infrared region can identify the hydrophilic groups (OH alcoholic and phenolic); of functional groups methyl ( -CH3 ) and methylene ( -CH2 - ) of aliphatic chains; of carboxylic and/or carbonyl groups; and their derivatives (carboxylate and complexes, more probably with Fe and Al) ; and they confirm geopolymer presence through the identification bands owed to silicate. These considerations to come suggest to the occurrence of humic material whether in the water (perhaps more recent) or in the sediments. Being of more refractory material, these humic substances can be aggregated to the geological sediments and promote the necessary sedimentation to facies formation, mostly in the carbonic material collected in the “furo” Grande (15,29 % of organic matter ) and in the material collected in the Pilões sandstone (9, 62% of organic matter ). Samples with higher organic matter content (15,29% and 9,62%) showed higher CEC (30,12 meq/100g and 35,66 meq/100g) respectively. The large quantity of organic matter in the environment of this study seems to be much more associated with the tidal cycle, with the poor drainage in the sediment (whether in mangrove or in the sandstone), elevated bioturbation (plant detrius, marine organisms action) and drainage of ground water, that originate humics substances in podzol soils, that is to say, the organic matter is from two origins: one, alloctonous – material transported by the rivers; and other, authoctonous – resultant of the incorporation of roots, spineless microorganisms and perhaps mostly of the litter resultant of mangrove metabolism.