25th International Meeting on Organic Geochemistry IMOG 2011

P-395
Source characterization using molecular distribution and stable
carbon isotopic composition of n-alkanes in sediment cores
from Mundaú-Manguaba estuarine-lagoon system, Brazil
Thaís Silva, Silvia Lopes, Débora Azevedo
Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil (corresponding author:thaisreis@iq.ufrj.br)
Ocean waters, marine organisms and sediments
contain hydrocarbons derived from many sources.
The wide variability in the hydrocarbon composition
from different sources makes the study of these
compounds a useful tool in geochemical
investigations [1]. The Mundaú-Manguaba estuarinelagoon
system (MMELS) constitutes one of the most
representative ecosystems of the State of Alagoas,
NE-Brazil. It has been subject to multiple
anthropogenic activities, mainly from urbanization,
sugarcane agriculture and chemical industries [2].
This study aims to provide temporal distributions of nalkanes,
together with their compound-specific � 13 C
values in core sediments from the MMELS, in order to
characterize the contribution and the sources of these
compounds over the past 40 years. The core samples
were collected at three sites: Manguaba Lagoon
(C03), Mundaú Lagoon (C07) and Mundaú River
(C08) in 2007. The cores were sectioned at 2 cm
intervals. About 4 g of each dry sediment was
extracted with dichloromethane:methanol 8:2. The
extracts were concentrated and fractionated using an
activated silica gel column. The analyses were carried
out by GC-FID and GC/C/IRMS.
The MMELS cores had a total organic carbon
(TOC) ranging from 1.88 to 3.35%. The C/N values
fluctuated between 9.6 and 13.2 for Manguaba and
Mundaú lagoon indicating mainly algal, bacterial and
non-vascular plant origins. For Mundaú River the
values ranged from 11.7 to 17.8 except for two
section of the top of the core that presented values
25.3 and 22.0. The n-alkanes ranged from C15 to C35,
with total aliphatic hydrocarbon (TAH) concentration
in the range 27.8–139.5 µg g -1 . The unresolved
complex mixture (UCM) was observed in all
sediments. The distribution of individual n-alkanes in
each core was different. For C03, the predominance
of C31 suggested terrestrial higher plant input. The
prominance of C18 in C07 suggested contribution of
marine algae and bacteria and the higher abundance
of C16, C18, C27 and C29 in C08 indicated mixture of
these sources. The terrigenous/aquatic ratio (TAR)
and carbon preference index (CPI) values showed
that the terrigenous input was predominant in
samples. The average chain lengths (ACL) do not
show significant variations (28.5 to 29.4), suggesting
stronger terrestrial influences [3].
The � 13 C values of individual n-alkanes (C16–33)
varied between -22.7‰ and -35.8‰. Individual nalkanes
carbon isotope values of Manguaba Lagoon
core (C03) are showed in Fig. 1. The � 13 C values for
the long-chain n-alkanes (C27, C29, C31 and C33) are
characteristic for n-alkanes biosynthesized by the C3
plants and coastal mangrove sources [3].
� 13 C (‰)
� 13 C (‰)
-20
-22
-24
-26
-28
-30
-32
-34
-36
-38
-20
-22
-24
-26
-28
-30
-32
-34
-36
-38
Carbon number
15 17 19 21 23 25 27 29 31 33
C03-1 C03-3 C03-5 C03-7 C03-9
15 17 19 21 23 25 27 29 31 33
C03-11 C03-13 C03-15 C03-17 C03-21
Fig.1. Individual n-alkanes carbon isotope values of
Manguaba Lagoon core (C03). The sections C03-1, C03-3,
C03-5, C03-7, C03-9, C03-11, C03-13, C03-15, C03-17 and
C03-21 correspond to the depths 1, 5, 9, 13, 17, 21, 25, 29,
33 and 41cm, respectively.
References
[1] Harada, N., Handa, N., Fukushi, M., Ishiwatari, R.
(1995) Org. Geochem. 23, 229-237.
[2] ANA/CELMM. (2006) Agência Nacional de Águas
Brazil.
[3] Sikes, E. L., Uhle, M. E., Nodder, S. D., Howard, M. E.
(2009) Mar. Chem. 113, 149-163.
[4] Silva, L. S. V., Piovano, E. L., Azevedo, D. A., Aquino
Neto, F. R. (2008) Org. Geochem. 39, 450-464.
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