Island beaches morphological and sedimentary short term variations – the case of SE Fernando de Noronha Island, South Atlantic, Brazil

Abstract

The Fernando de Noronha archipelago is comprised of 21 volcanic islands (26km2 ) located in the south equatorial Atlantic (4°S/32°W), 540km from Recife, Pernambuco state, Brazil. This study evaluates the short term morphological variations, the distribution and characteristics of surface sediments from the tropical meso-tidal Sueste bay, beach, mangrove, lagoon and dune systems, located in the southern coast of the main island. Eight beach profiles and 80 surface sediment samples were collected in January, April, June and October of 2010. The research methods included grain-size, calcium carbonate, total, elemental and isotopic organic matter analysis and monitoring of two beach profiles. The collected samples are fine sandy (> 99% of sand in 87,5% of samples), well to moderately sorted and bioclastic, presenting high carbonate contents (> 94% in 52,5% of the samples). Despite the homogeneity of fine sandy bioclastic moderately sorted sedimentary properties, this morphodynamic study shows spatial and temporal differences between the sediments of the west and the east side of the Sueste beach, as well as in each compartment of the Sueste beach/bay system. Multitemporal topographic profiles of the beach reveal seasonal erosion/accretion due to oceanographic-climatic processes. They also show the strong silting process in course on the Sueste mangrove and lagoon. The main factor responsible for this modern deposition and the beach physiography, along with the distribution of sediments in the Sueste beach shoreface and backshore, is the SE wind (Southern Hemisphere Trade Wind). This pattern accumulates sediments in the beach west side, occasionally removed for the Sueste bay when the mangrove lagoon breaks in the huge rain events, carrying fresh water and sediments to the foreshore.