Journal of Coastal Research SI 39 1073 - 1077 ICS 2004 (Proceedings) Brazil ISSN 0749-0208 BARCELLOS, R.L. and FURTADO, V. V., 2006. Organic matter contents and modern sedimentation at São Sebastião channel, São Paulo state, south-eastern Brazil. Journal of Coastal Research, SI 39 (Proccendigs of the 8 International Coastal Symposium), 1073 - 1077. Itajaí, SC, Brazil, ISSN 0749-0208. The present study was performed in São Sebastião Channel, located in the inner continental shelf of São Paulo State, south-eastern Brazil. The aim of this work is understand the organic matter distribution and behaviour associated to the modern sedimentary processes in the São Sebastião Channel. The studied parameters used in this present work were the organic carbon, total nitrogen and total sulphur contents, C/N and C/S ratios, grain size data and calcium carbonate contents. A hundred and two surface sediments samples were collected with a grab sampler. Fifty-one of them in 1997 summer and fifty-one in 1997 winter. Higher organic matter and mud contents are observed in the central and northern continental side, and in small bays located in the northern insular side, due to a low bottom current circulation. No significant spatial and temporal variations occurs. Occasional variations can be associated to the seasonal oceanographic and climatic dynamics of 1997 “El-Niño” year. A good correlation was observed between carbon, nitrogen and clay, indicating similar sources of organic matter for these elements. Sulphur contents presented a different behaviour, probably due to influences of organic matter inputs derived of human activities. th ADDITIONAL INDEX WORDS: Sedimentary organic matter, sedimentary processes, São Sebastião Channel (SP,Brazil). ABSTRACT Organic Matter Contents and Modern Sedimentation at São Sebastião Channel, São Paulo State, South-Eastern Brazil R. L. Barcellos† andV.V. Furtado† INTRODUCTION Organic matter distribution is a good tool for the environmental evaluation where the sediments are deposited 1984). Analysis on sedimentary organic matter distribution allows the evaluation of coastal sedimentary processes associated to the local circulation. High organic matter contents are related to highly mud contents, directly linked to low hydrodynamic areas 1975; 1995). The nature of organic matter still allows analysis about its sources, through the use of C/N ratio 1955). Lower values of C/N (< 6) ratio are related to a marine organic matter origin and higher values (> 24) indicates the increase of terrestrial organic material. Values between 8 and 12, reveals the mixing of continental and oceanic inputs ., 1989; ., 1999). The presence of sulphur compounds in sediments has been broadly used as an indicator of anoxic environmental conditions ., 1998). In addition, the C/S ratio is used to evaluate the sedimentary deposits oxi-reduction potential 1991; ., 1998). Low C/S values (< 3) are related to anoxic environments, while higher values (> 10) are associated to oxic marine environments. The aim of this work is understand the organic matter distribution and behaviour associated to the modern sedimentary processes in the São Sebastião Channel, south- eastern Brazil. The São Sebastião Channel is a feature located in the inner continental shelf of São Paulo State, south-eastern Brazil (23°43'/23°54'S, 45°20'/45°28'W) (Fig.1). It presents a SW-NE direction, and its higher depths axis is dislocated to the São Sebastião Island side. This bottom morphology is irregular (Fig. 2). The maximum depth (>45m) occurs in the channels' axis central region, where the harbour, submarine sewage outfall and oil terminal are adjacently located. The northern and southern mouths presents, respectively, isobaths of 20 and 25m, and lower depths occurs in the continental side of the channel, from its central part to north (0 to 7m). The São Sebastião Channel's sediments distribution is heterogenous, represented by several grain size classes and the modern sedimentary processes are directly related to the hydrodynamic circulation and the bottom topography. The wind generated currents are the most effective hydrodynamic phenomena, responsible for the bottom sedimentary processes 1995). The geomorphological and geological characteristics are controlled by the presence of granites, migmatites, quartzites and alcaline rocks of two mountains chains located at each side of the channel. These mountain chains reach directly the shore, the drainage systems are incipient and the Quaternary coastal plains are small and narrow. The humid tropical climate associated to the absence of greater river basins draining into the area, give to the rainfall regime a huge importance in the contribution of freshwater from the continent to the ocean, mainly in the rainy season (summer). A tropical preserved rain forest covers the slopes and coastal plains. The channel water masses dynamics is directly related to the shelf processes. The advances of South Atlantic Central Water (SACW) (S.: 35,5; T.: <16°C) in the spring-summer period, from the bottom up to the coast, causes the displacement of the Coastal Water (CW) (S: 34; T.: >22°C) towards the open ocean, generating a seasonal upwelling ( ., 1987). The autumn-winter period, is dominated by Tropical Water (TW) (S.: 36; T.: > 22°C) associated to the diminishing of freshwater input, leading to a more homogenous water column conditions. (ROMANKEVICH, (PETTIJOHN, TYSON, (BADER, (SAITO, MAHIQUES (BORREGO (STEIN, BORREGO (FURTADO, CASTRO FILHO et al et al et al et al et al STUDY SITE METHODS A total of 102 surface sediments samples were collected, using a grab sampler, in March (summer) and September (winter) of 1997. The samples were kept frozen and later freeze- dried for 48 hours. Grain size was analysed according to sieving and pipetting techniques 1967), and the calcium carbonate contents were determined by the weight difference prior to and after acidification, with 1 N HCl 1971). The total organic carbon, nitrogen and sulphur contents were obtained through the LECO CNS-2000 analyser. About 0,5g of (M LLER, (INGRAM, Ü † Department of Physical Oceanography, Institute of Oceanography, University of São Paulo, São Paulo, 05508-900, Brazil rlb@usp.br, vfurtado@usp.br Journal of Coastal Research Special Issue 39, 2006 ,