Distribuição temporal de matéria orgânica, nitrogênio, alumínio, densidade e porosidade em dois perfis sedimentares do Baixo Jaguaribe

Abstract

Paleoclimatology studies represent an important source for the understanding of the phenomena that occurred in various regions of the planet and tend, gradually, modified the biota composition, as well the local geomorphology. This work aimed to evaluate the content of Total Organic Carbon (MO), Total Nitrogen (NT), Aluminum (Al), Porosity and density of two points of the Baixo Jaguaribe Basin, the former in a lake near Aracati-CE city and the latter in an area of mangrove swamp near the mouth of the estuary. The data were analyzed through 14C dating results, in order to analyze the variations between allochthonous and autocthonous contributions of Organic Matter (MO) and minerals in the areas, according to possible climatological phenomena in geological time. In addition to that, the data were also used to infer the input of the continental or oceanic contribution of organic matter, according to the C/N ratio. Testimony collected near Aracati-CE that have been submitted in a geological middle time (about 6.000 years), showed a finer grain size due to the low density and high porosity. Using the variation of MO, the amount of Al, and bibliographical references, it was possible to split the testimony in three geological periods: the first between 0 to 90 cm, presents lower concentrations of MO and Al, the second between 90 and 170 cm shows an increase of MO, the first two intervals showed a probable autoctonous and allochthonous contribution, due to stable concentration of Al, and the third range characterized by an increase of Al, probably due to some pluvial event of allochtonous origin, likely due to a change of the Intertropical Convergence Zone (ITCZ), for the southern latitudes between 3.000 to 6.000 years ago. The testimony collected in mangrove area presented three intervals and a variation of Al similar to the Lake zone, with a geological time between 2.000 years. The values of OM and TN showed higher indices in shallow areas, and below average indexes in deeper areas from cm, probably due to a sea level rise between 2.100 to 1.000 years ago, which must have provided lower rates of MO caused for the contribution of sediments with highest particle size by the action of the tides. In the range 170 to 230 cm, there was an increased rate of Al, which probably is due to the rainfall input, before the sea level rise. In addition, three peaks of C/N ratio have been displayed which indicated a contribution of MO by higher plants. In spite of the results and the contextualization with the climatic phenomena that probably occurred at the time, it is necessary to improve the procedure of dating to a better reliability of data. In conclusion, the analysis of other parameters such as the carbon and nitrogen isotopes (13C e 15N) can lead to a better understanding of the source of these elements to the ecosystem.