Sedimentary Organic Matter in Cores of the Cananéia - e-Geo

Journal of Coastal Research
SI 56
1335 - 1339
ICS2009 (Proceedings)
Portugal
ISSN 0749-0258
Sedimentary Organic Matter in Cores of the Cananéia-Iguape
Lagoonal-Estuarine System, São Paulo State, Brazil
R.L. Barcellos†; P.B. Camargo‡; A. Galvão† and R.R. Weber†
†Department of Physical Oceanography,
Institute of Oceanography, University of São
Paulo, São Paulo, 05508-900, Brazil
[email protected], [email protected],
[email protected]
‡ Isotopic Energy in Agriculture Center of
São Paulo University (CENA-USP), ESALQ
Campus, Piracicaba (SP), 13400-970 Brazil.
[email protected]
ABSTRACT
BARCELLOS, R.L.; CAMARGO, P.B.; GALVÃO, A. and WEBER, R.R., 2009. Sedimentary Organic Matter in Cores
of the Cananéia-Iguape Lagoonal Estuarine System, São Paulo State, Brazil. Journal of Coastal Research, SI 56
(Proceedings of the 10th International Coastal Symposium), 1335 – 1339. Lisbon, Portugal, ISSN 0749-0258.
Sedimentary organic matter is a good tool for environmental evaluation where the sediments are deposited. We
determined the elemental and C- and N-isotopic compositions of 211 sub-surface sediment samples from 13
cores (ranging from 18 to 46cm), collected in the Cananéia-Iguape estuarine-lagoonal system. The aim of this
research is to evaluate the environmental variations of this tropical coastal micro-tidal system over the last
decades, through SOM distribution. The studied parameters show differences between the cores located in the
northern (sandy-silt sediments) and southern (sand and silty-sand) portions. The whole area presents a mixed
organic matter origin signature (local mangrove plants: < -25.60/00PDB/ phytoplancton 13C values: 19.40/00PDB). The northern cores, which submitted higher sedimentation deposition (1.46cm year-1), are more
homogenous, presenting lower 13C (< -25.20/00PDB) and higher C/N values (in general >14), directly related to
the terrestrial input from Ribeira de Iguape River (24,000 km2 basin). The southern portion presents lower
sedimentation rates (0.38cm year-1) and is associated to a small river basin (1,340 km2), presenting values of
13C: -25.0 to 23.00/00PDB and of C/N ratio: 11 to 15. In general, the elemental contents in the 15 cores may be
considered from low to medium (< 2.0% C - < 0.1% N), compared to similar environments. Although a greater
marine influence is observed in the southern system portion, the majority of the cores present an elevated
increase of continental deposition, most likely related to the strong silting process that the area has been
subjected to since the 1850s, when an artificial channel was built linking, directly, the Ribeira River to the
estuarine-lagoonal system.
ADITIONAL INDEX WORDS: Estuarine sediments, organic material, 13C- 15N isotopes ratios, Holocene
INTRODUCTION
Sedimentary organic matter distribution is a good tool for the
environmental evaluation where the sediments are deposited
(Romankevich, 1984; Lamb et al., 2006). The sedimentary organic
matter character of coastal marine areas is used to correlate several
oceanographic processes, such as water masses dynamics,
sedimentation rates, sedimentary processes associated to local
hydrodynamics, oxi-reduction potential and land-derived materials
input (Meyers, 1997). High organic matter contents are related to
high mud contents directly linked to low hydrodynamic areas, or a
high primary productivity region (Pettijohn, 1975; Tyson, 1995).
The nature of the organic matter still allows analysis about its
sources, terrigenous or marine, through the use of C/N ratio (Saito
et al., 1989). Stable carbon isotope has proven to be an effective
indicator of material sources and also to trace organic matter flow
in many coastal studies (Emery et al., 1967; Ruttenberg & Goñi,
1997; Barcellos et al., 2005a). It is observed, in estuaries, a linear
increase in 13C values from the inner estuary to its oceanic
portion (Thornton & McManus, 1994). The 15N isotopic
signatures are more affected by biogeochemical processes than
13C (Ogrinc et al., 2005). Carbonate contents can also be used as
a parameter that allows inferences about land-derived material
input influences in a depositional site (Paraopkari et al., 1991).
The aim of this work is to evaluate the environmental variations
of a tropical coastal micro-tidal system over the last decades,
through sedimentary organic matter (elemental C and N; 13C and
15N stables isotopes), calcium carbonate contents and sediments
distribution in shallow-cores (0.00-0.46m).
STUDY SITE
The research site is located in the southern coast of São Paulo
State, south-eastern Brazil (25°S/48°W) (Fig. 1). This system is 75
km long, composed of four brackish lagoon-like channels (Mar
Pequeno, Mar de Cananéia, Mar de Cubatão and Trapandé Bay),
and two main inlets (Cananéia and Icapara Inlets), separated from
the ocean by Comprida Island, inside the largest São Paulo State
coastal plain (2,500km2). Its origin is genetically related to
Quaternary sea-level fluctuations (Suguio and Martin, 1978).
Well-developed mangroves (200km2), and extensive pristine areas
of the Tropical Atlantic Rain Forest are present. Sedimentary
geomorphological features, such as spits, meanders, bars, islands,
sand and mud banks, are found in the channels’ domains. Human
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Sedimentary Organic Matter in Cores of the Cananéia-Iguape Lagoonal-Estuarine System, São Paulo State, Brazil
settlements are more prominent in the northern part of the system,
although environmental impacts are still incipient.
The tidal wave propagation is the process which leads to the
hydrodynamic circulation within the system and promotes the mix
between oceanic and estuarine waters, although secondary
influences, such as the fresh water inputs and wind stress, also
contribute to it. Spring tides present a mean range of 1.20m and
the system may be considered as a semi-diurnal micro-tidal
estuarine system (Myiao et al., 1986). The Ribeira de Iguape
River, the largest in São Paulo State coastal river (24,000km2
basin), is the greatest contributor of terrestrial material to the
estuarine system, mainly in its central and northern portions.
Moreover, the southern part of the system is associated to small
river basins (1,340km2), presenting a limited fresh water input
capacity, except in the huge rainfall events, due to the proximity to
the Serra do Mar mountain chain slopes (700-1,000m high).
It is observed in the majority of the estuarine system a sandy
sediment domain (80% of the samples) (Tessler, 1982; Barcellos
et al., 2005b) according to the Shepard (1954) classification.
Sedimentation rates are high in the whole area, varying from 0.23
to 1.46cm year-1 (Saito et al., 2001). The estuarine-lagoonal
system presents a eutrophic character (Braga, 1995).
Valo Grande Artificial Channel
Valo Grande is an artificial channel that connects the Ribeira de
Iguape River with the Cananéia-Iguape lagoonal-estuarine system,
in the Mar Pequeno portion next to Iguape City. It was built in the
19th Century (1827-1852) on the sands of the Cananéia Formation
and became the preferential river pathway (70% of the Ribeira
River flux), introducing 2.6x109ton year-1 of sedimentary
particulated material directly to the Mar Pequeno Channel
(GEOBRÀS, 1966). The eroded material from its margins is
responsible for the great amount of sediments on the system,
causing an accelerated silting process (Freitas et al., 2008). This
channel was closed in 1978 due to the construction of a dam, and
was definitively re-opened in 1995.
METHODS
On the Research Vessel “Albacora”, 211 sub-surface sediment
samples were generated from 13 cores, collected with a minimultiple-corer (5.0 inches in diameter) in November, 2007. The 2
by 2cm sub-surface samples were kept frozen and later freezedried for 48 hours. Grain size was analyzed by a sieving and
pipeting method (Müller, 1967) and the calcium carbonate
contents were determined by the weight difference prior to and
after acidification, with 1 N HCl (Ingram, 1971). The organic
contents (C and N elemental and isotopic) were obtained through a
Carlo Erba (CHN-1110) elemental analyzer linked to a Finnigan
Delta Plus mass spectrometer. About 0.5g of dried and weighed
sediment were decarbonated with 1 M solution of HCl, washed 3
times with deionized water, filtered and freeze-dried again, before
being placed in the analyzer. Due to the homogeneity of the cores,
these analyses were performed on 54 samples between 10 by
10cm intervals: 0-2cm, 10-12cm, 20-22cm, 30-32cm, 40-42cm.
RESULTS AND DISCUSSION
Spatial Distribution of Sedimentary Organic
Matter in the Cananéia-Iguape EstuarineLagoonal System
The spatial analysis of sedimentary organic matter distribution
and the grain size parameters (Shepard, 1954; Folk & Ward, 1957)
show differences between the cores located in the northern (sand,
Figure 1. Study area location and the sampling stations
clayey-sand and sandy-silt sediments), central (sand sediments)
and southern (clayey-sand and silty-sand) system portions. The
whole area presents a mixed organic matter origin signature, based
on local mangrove plants (< -25.60/00PDB) and phytoplankton
13C values (-19.40/00PDB) (Barcellos, 2005).
The northern cores (Fig. 2), which submitted a higher
sedimentation deposition (1.46cm year-1; Saito et al., 2001), are
more homogenous in the organic matter contents, presenting lower
13C (< -25.20/00PDB) and higher C/N (in general, >14) and C/P
values (> 200 according to Barcellos et al., 2008)), directly related
to the terrestrial organic matter input from the Ribeira de Iguape
River (24,000km2 basin). Organic carbon (OC), total nitrogen
(TN), 13C, 15N, CaCO3 and C/N ratio value ranges for this area
are: 0.39-3.44%, 0.03-0.26%, -25.50/00 to -27.70/00PDB, 4.10/00 to
5.60/00Air, 3.9-12.6% and 11.6-16.7, respectively. The higher
organic matter contents (> 3,0%C and 0,26% N) are found at the
top samples (0-2cm) of the Valo Grande (ST-20) and Pedra do
Tombo (ST-74) cores. These contents are expected, considering
that the ST-20 core is located in the area that receives the majority
of the sedimentary land-derived material input for the whole study
area. For the ST-74 core, the higher SOM contents are directly
related to the low hydrodynamic conditions induced by the
encounter of high-tide waves that come from the Cananéia and
Icapara Inlets, printing for this area the best conditions for silt and
clay deposition within the estuarine-lagoonal system.
In the central cores area (Fig. 3), the sedimentation rates are
around 0.50cm year-1 (Saito et al., 2001). The main source of
sediments is the reworking process of quaternary sand deposits
located at the bottom and in the marginal erosive sandy slopes of
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Barcellos et al.
the Mar Pequeno lagoonal channel. As in the northern cores, the
samples also present lower 13C and higher C/N and C/P values (<
-25.70/00PDB, > 15.7, > 227). Sandy sedimentation and low
organic matter contents are observed, due to the hydrodynamic
behavior of the currents that present higher velocities in this
portion of the system (Myiao et al., 1986). OC, TN, 13C, 15N,
CaCO3 and C/N ratio value ranges for the central cores are: 0.410.84%, 0.02- 0.05%, -25.70/00 to -27.90/00PDB, 3.70/00 to 5.30/00Air,
2.6-4.2% and 15.7-20.8, respectively. In this area, the Mar
Pequeno lagoon presents a narrowing of the channel section,
inducing velocity acceleration in the tide current flow and
avoiding the deposition of land-derived mud. These silts and
clays, whose main source is the Ribeira de Iguape River, are
exported further to the south and are responsible for the mud
deposits present in low hydrodynamic areas located in the
southern system portion, as in convex sides of the Mar de Cubatão
and Mar de Cananéia channel meanders, and in the deeper parts of
the Trapandé Bay.
The southern cores (Fig. 4), though, are submitted to lower
sedimentation rates (0.23 to 0.38cm year-1) (Saito et al., 2001),
and are associated to a small river basin (1,340km2) and subjected
to a higher marine influence (in general, 13C: -25.0 to
23.00/00PDB and for C/N: 11 to 15). The calcium carbonate also
presents higher contents, especially at the cores located near the
Cananéia Inlet (ST-166 and ST-194), which indicate an increase
of the marine sedimentation for the southern portion of the system.
Clayey-sand and silty-sand sedimentation are observed in this
collection of cores whose organic matter contents ranges from OC,
TN, 13C, 15N, CaCO3 and C/N ratio: 0.31-3.51%, 0.03-0.29%, 23.90/00 to -26.40/00PDB, 3.10/00 to 5.40/00Air, 1.6-24.6% and 10.117.4, respectively. The C/P ratio also indicates an increase in
marine sedimentation for these samples exhibiting lower values (<
200) than in the central and northern areas (Barcellos et al., 2008).
For this same collection of 54 samples, the total, inorganic,
organic phosphorus contents (TP, IP and OP) and C/P ratios varies
from 0.9 to 38.9µMolg-1, 1.2 to 43.5µMolg-1, 0.2 to 7.6µMolg-1,
and 100.8 to 432.7, respectively (Barcellos et al., 2008).
In general, the elemental contents in the cores may be
considered from low to medium (< 2.0% C - < 0.1% N- <
20.0µMolg-1, TP), compared to other world tropical estuarine
environments (Andrews et al., 1998; Ruiz-Fernandez et al., 2002;
Bouillion et al., 2002).
Temporal Distribution of Sedimentary Organic
Matter in the Cananéia-Iguape EstuarineLagoonal System
The distribution of organic matter in the study area presents
trends that can be related to the temporal and weather dynamics of
the last two centuries, main factor of the sediment deliverance and
distribution for the land-derived sediments to the system. It is
observed that the natural dynamic equilibrium of the estuarinelagoonal system was seriously altered by human actions, with the
end of the construction of Valo Grande artificial channel in 1852.
Nowadays, this artificial channel is responsible for 70% of the
total flux of the Ribeira de Iguape River and 2.6,106tons year-1 of
particulate material, that is introduced directly in the estuarinelagoonal waters of the Mar Pequeno Channel (GEOBRÀS, 1966;
Bérgamo, 2000).
In the majority of the estuarine system the cores presented a
higher increase of mud sediments and organic matter contents
(OC, TN, TP, IP, OP), except in the central cores (ST-102 and ST213) and ST-32, ST-166 and ST-289 cores. Previous studies
performed in 9 cores sampled in the area (Duleba, 1997; Teles,
1997) indicate the same trend, also observing a fine-grain increase
in the direction of the core surface sediments. In the core ST-20,
located adjacent to the Valo Grande mouth (fig. 2), for example,
the 40-42cm sediment sample presents 76.2% of sand, while the
surficial sediment (0-2cm) presents 90.5% of mud, and the (1012cm) layer presents 46.8% of sand. High IP contents probably
associated to the Ribeira de Iguape River nutrient input are
observed in the 6 northern cores, with IP increase of 2.5 to 7 times
from the base to the top of the cores (Barcellos et al., 2008). The
increase of OC and TN contents are also observed in some cores
(ST-20, ST-74, ST-257, ST-148), indicating temporal changes of
the land-derived input material for the whole lagoonal system, and
may also indicate a gradual eutrophization process.
The organic matter origin indicated by 13C, 15N and C/N, on
the other hand, did not present clear temporal differences in the
specific quality of organic matter deposited, derived from the
mixed character of the SOM sources in the area.
CONCLUSIONS
Although a greater marine influence is observed in the southern
system portion, the majority of the cores present an increase of
continental deposition, probably related to the strong silting
process that the area has been subjected to since the 1850s, when
the Valo Grande artificial channel was built linking, directly, the
Ribeira River to the estuarine-lagoonal system.
These proxies indicate a terrigenous material influence to the
northern part of the system and a higher marine influence in its
southern portion. Temporally, these differences are not so marked.
The results and discussions presented in this work are an
important part of the preliminary phase of the current research.
Some complementary data, such as 210Pb sedimentation rates (for
ST-148, ST-166 cores), hydrocarbons, metallic elements, coarse
fraction analysis (Shepard & Moore, 1957), micropaleontology
and a statistical treatment, will refine the collection of data leading
to a wide interpretation about the recent sedimentary process in
the Cananéia-Iguape estuarine-lagoonal System.
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ACKNOWLEDGEMENTS
The authors wish to express their thanks to Valdenir Veronese
Furtado from the Oceanography Institute of São Paulo University,
for their discussion over the organic matter behaviour and
sedimentary processes on the Cananéia-Iguape EstuarineLagoonal system. Thanks are also due to Mr. Gustavo Lima
Barcellos, for the revision of the text. They are also indebted to the
São Paulo State Foundation for Research (FAPESP) for financial
support given for the projects 06/59331-2 and 06/05675-2.
Journal of Coastal Research, Special Issue 56, 2009
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Barcellos et al.
Figure 2. Northern cores (represented by core ST-20) lithology, SOM and grain-size parameters
Figure 3. Central cores (represented by core ST-102) lithology, SOM and grain-size parameters
Figure 4. Southern cores (represented by core ST-166) lithology, SOM and grain-size parameters
Journal of Coastal Research, Special Issue 56, 2009
1339