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

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 (CaCO3) 
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, CaCO3, elemental (C, N) and the 
stable isotopic ratios of δ13C 
e δ15N of SOM. Based on 
the measured values of carbon and nitrogen, the C/N ratio, associated to δ13C e δ15N, is observed that in the PCS predominates an 
organic matter of marine origin along its entire length: C δ13C > -21 PDB e δ15N > 8Ar. 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.