Sedimentary Organic Matter Characterization on a Tropical Continental Shelf in Northeastern Brazil

Abstract

In continental shelf areas works where the focus is Geochemistry are widely relevant, due to the vast complexity and uses of these relief features (social, environmental and economic). On the Brazilian Northeast tropical shelves, with emphasis on the Pernambuco Continental Shelf (PCS), the published studies are limited and have aimed at portions closer to the coastline. The objective of the present work is the description of the characteristics and origin of sedimentary organic matter (SOM) in the inner and middle Pernambuco shelf, defining and classifying the local geochemical sedimentary facies. The sampling stations (136) were collected in the study area, and the grain-size, contents of total organic matter (TOM), calcium carbonate (CaCO 3 ) were analyzed. The samples were processed in an elemental analyzer coupled with a mass spectrometer after the elimination of calcium carbonate. The obtained data were the grain-size, TOM, CaCO 3 , elemental (C, N) and the stable isotopic ratios of δ 13 C e δ 15 N of SOM. Based on the measured values of carbon and nitrogen, the C/N ratio, associated to δ 13 C e δ 15 N, is observed that in the PCS predominates an organic matter of marine origin along its entire length: C < 1%, N < 1%, C/N < 5, δ 13 C > -21 PDB e δ 15 N > 8 Ar . The PCS presents patches of continental origin SOM, associated to the coastal zones adjacent to estuarine systems mouths and covering the paleochannels, which may indicate that this material was carried from the coastal rivers to the offshore areas, possibly by the paleo-valleys that also act as traps of fine sediments and SOM. The statistical analysis indicates the existence of 6 different sedimentary facies and a prevalence of one of them, characterized by poorly-sorted bioclastic sandy-gravel, with low to medium organic contents of marine origin. This indicates that the coastal sedimentary material contributions are low and the local cover is autochthonous of biogenic origin, and deposited according to the surficial topography, morphosedimentary processes and meteoceanographycal conditionings of the study area, typical of moderate hydrodynamic energy environments.