TITLE:
Sedimentary Organic Matter Characterization on a Tropical Continental Shelf in Northeastern Brazil
AUTHORS:
Roberto Lima Barcellos, Maria Cristina da Silva Sales de Melo, Alcides Nobrega Sial, Valdir do Amaral Vaz Manso
KEYWORDS:
Biogenic Sediments, Total Nitrogen, δ13C Stable Isotopic Ratio, Geochemical Facies, Pernambuco
JOURNAL NAME:
International Journal of Geosciences,
Vol.11 No.6,
June
29,
2020
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 (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.