Saturday, 15 July 2006

Soil Landscape Relations in the Brazilian Fernando de Noronha Oceanic Island, Southern Atlantic Ocean.

Mateus Rosas Ribeiro, Flavio Adriano Marques, Sheila Maria B. Bittar, and Jose Fernando W. F. Lima. Univ Federal Rural De Pernambuco, Rua Dom Manoel De Mederos S/Nº. Dois Irmaos, Recife, PE, 52171-900, Brazil

The Brazilian Fernando de Noronha Archipelago, located 345 km off the Brazilian cost, at latitude 3º 51'S and longitude 32º 26' W, consists of a main island and several islets, with a total area of about 18.4 km2. It is the top of a great volcanic pile with a basal diameter on the ocean floor of 60 km at a depth of 4,000 meters. The main island, which gives the name to the Archipelago, has 17.6 km2 and is the unique Brazilian oceanic island inhabited during 500 years. The Archipelago has been studied in relation to several aspects, particularly, geology geomorphology and biology. However, despite the recent advances in the scientific field, the National Park do not have a satisfactory knowledge in terms of soils, their behavior and landscape distribution. The objective of this work was to study the properties and distribution of the soils of the main island aiming to support actions and decisions towards the sustainable management of this ecological–touristic complex. In a general geological description Fernando de Noronha is formed by a basal group of pyroclastic rocks (tuffs, breccias and agglomerates) invaded by diques of phonolites and trachytes and capped by a post–erosional flow of ankaratrites. A central plateau, 50 to 70 m high, forms the main island, together with hills, and is limited by steep slopes and scarps, sometimes directly submitted to marine abrasion. In some parts a coastal plain, formed by a narrow sandy strip of beaches, marine terraces and sand dunes can be observed. The climate is tropical semi-arid with well-defined dry and humid seasons (Aw'). The mean annual precipitation is 1,274 mm, with a rainy season between February and July, and potential evapotranspiration of 1,942 mm per year. The vegetation is a tree–shrub deciduous forest similar to the caatinga vegetation of the semi-arid Northeast Brazil. The dry tropical climate with oceanic influence and the recent volcanic or sedimentary parent material are responsible for the small weathering of the island soils. The relief determines the local variations. According to the World Reference Base for Soil Resources (WRB) the soils belong to the major groups of Leptosols, Vertisols, Cambisols and Arenosols. The main WRB soil units identified are:Calcaric Arenosols, Mollic Leptosols,Eutric Leptosols, Salic Vertisols, Sodic Vertisols, Chromic Vertisols, Dystric Cambisols, Vertic Cambisols and Chromic Cambisols . The Calcaric Arenosols are deep and sandy soils developed on marine recent sedimentary deposits of the coastal plain areas. The mineralogy of the sand grains is, essentially, composed by calcareous materials from reefs, algae and see animals. The topography of this soil unit is predominantly level with small undulations. Leptosols are related to the steep slopes of the central plateau borders and hills, with a strongly rolling to hilly topography, stoniness and rockiness. Vertisols are related with lower and slightly depressed areas, imperfectly to poorly drained and susceptible to occasional brief floods during the rainy season. Sodic and Chromic Vertisols are related to depressed areas in the top of the central plateau, developed on parent materials from tuffs and ankaratrites, and the Salic Vertisols can be found in the low plain of Atalaia beach, derived from alluvial quaternary sediments influenced by sea water. The Dystric Cambisols are related to parent material from alkaline under-saturated rocks and occur in the lower slopes of the phonolitic hills, with high levels of stoniness and rockiness. The Chromic Cambisols have ferric properties and are related to the most preserved parts of the central plateau, developed from the weathering of alkaline basic rocks (ankaratrites). The good conditions of relief and depth, and the high fertility status, give to these soils a great potential for agricultural activities. The Vertic Cambisols are related to the gently slopes in the transition between Chromic Cambisols and Vertisols. The Vertic Cambisols have a double parent material derived from tuffs capped by the material from the ankaratritic flow.

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