Characteristics of the sedimentary organic matter on the inner and middle continental shelf between Guanabara Bay and São Francisco do Sul, southeastern Brazilian margin

Continental Shelf Research 19 (1999) 775 — 798

Characteristics of the sedimentary organic matter on the inner and middle continental shelf between Guanabara Bay and Sa o Francisco do Sul, southeastern Brazilian margin Michel Michaelovitch de Mahiques *, Yasufumi Mishima
, Marcelo Rodrigues Department of Physical Oceanography, Institute of Oceanography, University of SaJ o Paulo, 05508-900 Praia do Oceanogra& fico, 191 SaJ o Paulo SP, Brazil
Chugoku National Industrial Research Institute, 2-2-2 Hiro, Suehiro, Kure, Hiroshima, Japan Received 31 October 1997; received in revised form 4 June 1998; accepted 5 October 1998

Abstract A total of 111 Petersen grab sediment samples, collected during two cruises in 1991 and 1993, on the inner and middle continental shelf between Guanabara Bay and Sa o Francisco do Sul, on the southeastern Brazilian margin, were analyzed for the purpose of understanding the nature and distribution of the organic matter in the surface sediments. The analysis of the distribution of the different parameters related to the organic matter revealed the existence of an important distinction between the sediments to the south and to the north of the Sa o Sebastia o Island. Six sedimentary zones characterized by different mean values of the organic matter parameters can be identified. The analysis of the dC and C/N ratios distribution suggest an increasing contribution of land derived organic matter toward the north. The distribution of the organic matter in the area can be explained qualitatively by a model of water mass dynamics, which acts over the southeastern shelf of Brazil.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction The characteristics of the organic matter deposited in the superficial sediments of marine areas are being widely used in the correlation of several oceanographic

* Corresponding author. E-mail: [email protected]  Supported by the Sa o Paulo State Foundation for Research (FAPESP) Project No. 95/3970-0. 0278—4343/99/$ — See front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 02 7 8— 4 34 3 ( 98 ) 0 01 0 5— 8

776

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

processes, such as surface-water productivity, input of land-derived materials into the ocean, dynamics of water masses, oxi-reduction potential and sedimentation rates (Mu¨ller and Suess, 1979; Stein, 1991; Faganeli et al., 1994; Meyers, 1997). Stein (1991) also observes that, in spite of the potential use of different organic matter parameters as environmental indicators, misinterpretations can occur according to the parameters used for it. The application of bulk organic matter parameters in the determination of its origin is, at least, contradictory. The C/N ratio has been used for decades as a parameter that could be easily determined for the evaluation of the relative influence of terrestrial or marine organic matter (Bordovskiy, 1965; Inte`s and LeLoeuf, 1986; Saito et al., 1989). On the other side, the utilization of dC and dN values, based on the different signatures of these isotopes in C and C plants, benthic organisms and phyto- and   zooplancton, led to a drastic increase in the number of studies on present sedimentary dynamics in coastal areas and shelves, the input of terrigenous organic matter and paleoenvironmental analysis (Anderson and Arthur, 1983; Kaplan, 1983; Wada et al., 1987; Sackett, 1989). Also, dC and dN parameters have been widely used in food web studies (Haines and Montague, 1979; Rau et al., 1983; Wada, 1986; Matsuura and Wada, 1995). More recently, Holmes et al. (1996) associates the differences in dN in surface sediments of the Angola Basin with variations in nitrate utilization and suggested that no correlation could be established between dN values and the surface productivity or terrestrial inputs. Lately, after the publication of papers on molecular biomarkers (Hedges and Parker, 1976; Hedges and Mann, 1979), a new approach to studies on the origin of organic matter was achieved and a re-evaluation of the use of bulk sedimentary organic matter parameters as organic matter source indicators then became necessary. Meyers (1994), analyzing sediments from several lacustrine and marine areas, observes that the C/N and dC parameters appear to retain the signatures of their sources for thousands of years. Prahl et al. (1994), analyzing TOC, n-alkanes, lignin phenols, dC and C/N from Washington (USA) margin sediments, used the values of dC"!27.8  and C/N "14.8 as terrestrial and dC"!20.1  and   C/N "8.11 as marine endmembers for these parameters.   A two-endmember model, based on C/P and dC of bulk sediment, was proposed, by Ruttenberg and Gon i (1997), to explain the variability in terrestrial-marine influence in the Arctic and in a region of the Gulf of Mexico. The same model failed to explain the origin of the organic matter on the Amazon shelf. Gon i et al. (1997), propose a re-evaluation of the importance of the terrigenous organic matter in marine sediments. According to the authors, the C vegetation contribution to the organic  matter of the sediments in the Gulf of Mexico is not recognized, leading to an underestimation of the terrigenous influence in the offshore areas, based on dC values. Westerhausen et al. (1993), analyzing samples from the equatorial East Atlantic observed an unexpected distribution of the C/N ratio values, the highest occurring on the Mid-Atlantic Ridge and lowest on the upper slope. The purpose of this study is to understand the distribution pattern of the organic matter deposited in the surface sediments of a region of the southeastern Brazilian shelf (Fig. 1) as well as to assess the

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

777

Fig. 1. Location of the study area and sampling stations.

utilization of bulk sediment organic matter parameters in the evaluation of the origin of the organic matter in the area.

2. Area of study According to Zembruscki (1979), the southeastern Brazilian shelf is within the Embayment of Sa o Paulo. The continental shelf in this sector of the Brazilian continental margin presents a variable width of between 73 and 231 km, declivity of between 1 : 656 and 1 : 1333 and a shelf break depth between 120 and 180 m. In a general way, the isobaths approach each other towards the north, and close to Cape Frio the shelf becomes narrower. Zembruscki (1979) defined, for the area, the terms inner, middle and outer shelf, characterized by changes in the declivity. Northward to Sa o Sebastia o Island, the middle shelf gives place to a middle slope. In general, the middle shelf as well as the middle slope is limited by the 100-m isobath. Sa o Sebastia o Island marks a notable inflection of the coastline as well as of the submerged morphology, passing from a SW—NE general orientation, to a W—E direction. Also, to the north of the island, a meandering coastline, a more irregular contour of the isobaths, and the presence of several islands reveals the greatest complexity of the local geomorphology. Sa o Sebastia o Island also marks a conspicuous modification in the pattern of the distribution of sediments (Rocha et al., 1975; Kowsmann and Costa, 1979; Martins and Correˆa, 1996). To the south of Sa o Sebastia o Island the inner and middle shelf is

778

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

characterized by the prevalence of fine to very fine sands, well sorted and very poor in mud. In this sector a gradual increase in the mud content occurs in the direction of the outer shelf. To the north of the island, a mixture of grain size terms is observed, with a significant deposit of mud in the inner shelf and a complex pattern of sedimentary features, marked by several sedimentary patches. This differentiation in terms of grain size parameters can be observed, also, in the clay mineralogy. To the south of Sa o Sebastia o Island montmorillonite is the predominant clay mineral. To the north, the sediments show a mixture of kaollinite— illite—montmorillonite. In the adjacent continental areas the drainage basins have their extension limited by the relative distance of the slopes of the Serra do Mar mountain chain to the coastline. Northward from the Sa o Sebastia o Island, the crystalline rocks of the Serra do Mar mountain chain dip almost directly into the ocean, leading to the occurrence of small coastal plains and pocket beaches. To the south, the greater distance of the mountain chain from the coast allows the development of larger coastal plains and, consequently, of more highly developed drainage systems. The Ribeira River, which is the larger fluvial system in the area, presents an average discharge of 435 m s\ at its intermediate course. The humid tropical and subtropical climate as well as the absence of great river basins draining into the area, give the rainfall regime very great importance in the contribution of freshwater from the continental areas to the ocean. Landslides are considered as having been very important processes in the evolution of the Serra do Mar mountain chain during the Quaternary. A tropical rainforest covers the slopes of the mountain chain. In the estuaries and lagoon borders, mangrove swamps and Spartina banks are common. Halodule banks, referred to by Matsuura and Wada (1995) as important sources of organic matter to the benthos, are limited to small areas. The water masses dynamic that acts over the shelf is strongly influenced by the advance and retreat of the South Atlantic Central Water (SACW) (Castro Filho et al., 1987). During the austral summer, the SACW advances from the bottom up to the coastal areas, leading to the displacement of the Coastal Water (CW) towards the open ocean and generating a seasonal upwelling. A marked thermocline can be observed even in areas shallower than 20 m. From March to November, the retreat of the SACW to the outer shelf allows the advance of the Tropical Water on the largest part of the shelf. Also, due to the diminishing of the freshwater input, the CW exerts less influence within the area. A more homogeneous water column is observed in the inner and middle sectors of the shelf. Over the year the CW has its main longshore flux deviated offshore due to the presence of Sa o Sebastia o Island. Also, the meandering longshore flux of the Brazil Current in the area leads to the occurrence of vortices, which are frequent to the southeast of Sa o Sebastia o Island.

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

779

3. Materials and methods 3.1. Sampling Two research cruises were undertaken, on the board RV Prof. W. Besnard, in December of 1991 and January of 1993. On the first 82 samples were collected and on the second 29, according to the plan presented in Fig. 1. The samples were collected with the aid of a Petersen grab sampler, kept frozen and later freeze-dried for 48 h. 3.2. Grain size analysis Samples were homogenized with distilled water and sodium dimetaphosphate, and analyzed according to sieving and pipetting techniques. 3.3. Calcium carbonate content About 1 g of dried and weighed sediment samples was acidified overnight with 1 N hydrochloric acid, washed three times with distilled water, filtered and dried again. The calcium carbonate content of the samples was calculated by the weight difference prior to and after acidification. 3.4. TOC, nitrogen and sulfur contents About 500 mg of dried and weighed sediment were decarbonated with 1 M solution of hydrochloric acid, washed 3 times with deionized water, filtered, freeze-dried again and then analyzed in a LECO CNS2000 analyzer. 3.5. Carbon and nitrogen stable isotopes About 500 mg of dry sediment were weighed and placed in test tubes, with 2—3 ml of 1 M hydrochloric acid solution. Samples were then dried in a centrifuge evaporator at 40°C. This procedure was repeated 3 times for each sample. Samples were analyzed in a Finnigan MAT 252 mass spectrometer. Working standards of cellulose (dC"!27.8 vs. PDB), and alanine (dC"!20.6 vs. PDB, dN "7.8 vs. Air), were used to calibrate sample values. 3.6. Data treatment — multivariate analysis In order to identify the different sedimentary zones, a cluster analysis was carried out, using the contents of clay, calcium carbonate, organic carbon, nitrogen and sulfur from the 1991 cruise samples as variables. The clusters were determined by the calculation of the Euclidean distances on the normalized data. The WPGMA grouping technique was used for the construction of the dendrogram.

780

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

4. Results Results are presented in Appendices A and B and in Fig. 2. The distribution in clay (Fig. 2a) presents a clear differentiation between the inner and middle continental shelf. To the south of Sa o Sebastia o Island, the sediments of the inner shelf show values smaller than 2% in clay. On the other hand, there is a remarkable increase in clay content towards the outer shelf, where samples with more than 50% of clay occur. In the inner shelf northward of Sa o Sebastia o area the clay content slightly increases and values close to 5% in clay are observed. According to the distribution map of calcium carbonate (Fig. 2b), the sediments of the study area can be classified, in a general way, as lithoclastic (according to the classification of Larsonneur et al., 1982). There is a strong increase in calcium carbonate content in a restricted section in the most external portion of the middle shelf. These carbonate-rich sediments correspond to the internal limit of a long belt located in the outer shelf, which extends outs to the shelf break. The distribution map of organic carbon (Fig. 2c) presents a tendency similar to the distribution in clay. The highest values in organic carbon ('12.0 mg/g) are located in the south, in the most external portions of the area. An inflection of the isovalue lines

Fig. 2. (A) Distribution map of clay content (% dry weight); (B) distribution map of calcium carbonate content (% dry weight); (C) distribution map of organic carbon content (mg/g dry weight); (D) distribution map of organic nitrogen content (mg/g dry weight); (E) distribution map of the C/N ratio (weight).

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

781

Fig. 2. (Continued.)

is observed associated with Sa o Sebastia o Island and a new increment in the carbon content can be observed in the northern portion of the area. The distribution map of nitrogen (Fig. 2d) is very similar to the distribution of carbon, indicating a strong correlation among these two parameters. The same

782

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

Fig. 2. (Continued.)

inflection of the isovalues is to be seen on the nitrogen map, associated with the presence of Sa o Sebastia o Island. The C/N ratio distribution map (Fig. 2e) shows a complex patchy distribution on the inner shelf. Higher values are associated with the mouths of the estuarine and

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

783

lagoonal complexes of Canane´ia, Paranagua´ and Sa o Francisco do Sul, in the southernmost part of the area, but also present close to the Ribeira River mouth, and off Sa o Sebastia o island. This latter patch extends up to the outer parts of the study area. The faciology of the sediments, determined from the cluster analysis, permitted the recognition of six main zones (Fig. 3a and b), whose characteristics are summarized in Table 1. It can be observed that, to the south of Sa o Sebastia o Island there is a gradient, in the direction of the edges of the shelf, of richer terms in organic matter (Zones 5, 4, and 2, respectively). This gradation is practically not observed in the north, where a more complex distribution of the organic matter parameters occurs. The distribution of dC (Fig. 4a) indicates that the cross-shore tendency is superimposed by a north—south differentiation. Actually, samples with lower values of dC ((!20.80 PDB) are present on the inner shelf but also at the 100-m isobath, northwards from Sa o Sebastia o Island. On the other hand, in the southern sector of the area most of the samples show values higher than !20.00 PDB. The distribution of dN (Fig. 4b) shows a more complex pattern. Despite the existence of a weak southward trend to increasing values, which do not seem significant, its distribution by depth is completely random, suggesting the occurrence of patches of high dN values.

5. Discussion 5.1. Distribution of the organic matter The inner and middle continental shelf between Guanabara Bay and Sa o Francisco do Sul presents a marked dichotomy, with respect to the characteristics of the organic matter. Two main sectors can be identified, to the north and the south, respectively, of Sa o Sebastia o Island. This dichotomy reflects the geomorphological and sedimentological configuration and, therefore, the oceanographic characteristics prevailing on this section of the Brazilian continental margin. To the south of Sa o Sebastia o Island, the absence of any great modifications in the submarine relief leads to a more effective wave action over the inner shelf, making the deposition of muddy sediments and, consequently, of organic matter difficult. This leads to the occurrence of a gradient in the content of organic matter towards the shelf break. Northward of Sa o Sebastia o Island, the complexity of the morphological characteristics of the shelf leads to the significant attenuation of the prevailing hydrodynamic processes (waves and wave-driven currents) allowing the deposition of larger amounts of fine sediments and organic matter. The zones identified in the cluster analysis reveal that besides the differentiation between the north—south sectors, an inshore—offshore pattern can also be described. As expected, due to the interaction of wave trains and the seabed, the inner shelf sediments are poorer in organic matter than the middle shelf samples.

784

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

Fig. 3. (A) Dendrogram based on cluster analysis of the 82 samples from the 1991 cruise. (B) Distribution map of the zones generated from the cluster analysis of the samples from the 1991 cruise.

Clay (%) CaCO (%)  Org. C (mg/g) Org N (mg/g) S (mg/g) C/N

23.9 41.3 10.49 1.31 0.46 8.1

6.2 9.3 1.70 0.26 0.10 0.7

ZONE 1 n"5 Mean Std.dev. 41.8 19.9 11.22 1.40 0.44 8.1

4.6 2.3 1.26 0.22 0.07 0.7

ZONE 2 n"11 Mean Std.dev. 20.4 19.1 7.89 0.89 0.91 9.0

10.3 4.5 1.52 0.25 0.22 0.9

ZONE 3 n"3 Mean Std.dev.

Table 1 Statistical parameters of the different zones established from the cluster analysis

6.4 22.0 4.00 0.48 0.19 8.8

5.7 8.8 1.92 0.24 0.08 2.2

ZONE 4 n"33 Mean Std.dev. 0.3 6.0 1.12 0.12 0.17 32.7

0.8 2.7 0.45 0.09 0.11 68.3

ZONE 5 n"28 Mean Std.dev.

12.3 79.4 3.03 0.33 0.08 9.0

0.7 6.8 0.82 0.00 0.01 2.5

ZONE 6 n"2 Mean Std.dev.

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798 785

786

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

Fig. 4. (A) Distribution of dC ( PDB).; (B) distribution of dN ( Air).

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

787

Most of the parameters show a distribution pattern relating to depth which can be explained in terms of an exponential function (Fig. 5). The C/N value shows a perceptible decrease in values towards deeper areas. Nevertheless, the C/N ratio’s high values in zone 5, identified through the cluster analysis, must be critically interpreted,

Fig. 5. Distribution by depth of the parameters analyzed in the 1991 and 1993 samples. (A) clay; (B) calcium carbonate; (C) organic carbon; (D) nitrogen; (E) dC; (F) dN and; (G) C/N. Circles correspond to samples from the 1991 cruise and crosses correspond to samples from the 1993 cruise.

788

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

Fig. 6. Scatter plot of clay content (in % d.w.) vs. organic carbon content (in mg/g).

since some of the values may be influenced by low values of organic matter. Bader (1955) had already observed a relationship between low amounts of organic matter and high C/N values. dN was the only parameter that did not present any clear tendency in its distribution. The north—south trend that seems to occur does not show statistical significance (for a(0.05). In fact, it seems that this parameter presents a patchy distribution. In this case, we may suppose that significant changes in these parameter are much more related to blooms, due to the action of vortices, which stir up the seabed and are temporary and localized in the area (Gaeta et al., 1994). A plot of organic carbon content vs. clay content shows a fairly good correlation (r"0.86) between these parameters (Fig. 6), indicating that the distribution of organic carbon is mainly controlled by the texture of the sediments. Nevertheless, the existence of several samples that are outside of the significance level of the plot reveals that other factors, such as the paucity of the primary production or the presence of large-size organic matter particles, can act as causes for the distribution of the organic carbon. The relative organic and inorganic contribution of nitrogen can be easily estimated by plotting the values of total nitrogen and organic carbon (Ruttenberg and Gon i, 1997; Andrews et al., 1998). The plot generated by our data (Fig. 7) shows a very good correlation between these parameters (r"0.97). A negligible value of N"0.02 mg/g intercepts the C "0.00, which means that almost of the totality of nitrogen is of  organic origin. This fact agrees with data from Braga and Mu¨ller (1998), who observe

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

789

Fig. 7. Scatter plot of organic carbon content (in mg/g) vs. total nitrogen content (in mg/g).

that the lack of inorganic nitrogen in the shelf waters is a limiting factor for the primary production in the area (Braga and Mu¨ller, 1998) 5.2. Origin of the organic matter Differently from many areas of North America, Europe, and even of the Amazon shelf, the southeastern Brazilian continental margin presents very little information on parameters indicative of the origin of the organic matter in marine sediments. Results relating to natural isotope composition are restricted to Matsuura and Wada (1995), Mahiques et al. (1997), and this paper. Also, information on n-alkanes distribution in sediments is limited to Zanardi’s (1996) thesis. Matsuura and Wada (1995) analyzed the dC and dN of plankton, benthos, land—soil, and sediments on the inner shelf northward from the Sa o Sebastia o area. The reported phytoplankton carbon isotope values of !20.5 to !21.1, and nitrogen isotope values ranging from 6.9 to 9.0 must be more appropriately stated for phyto- and microzooplankton, since no separate procedure was used for these samples. One sample of tree leaves showed values of !26.1 and 7.9, for carbon and nitrogen isotopes, respectively. Two land—soil samples presented values of !19.0 and !21.6 for carbon and 0.49 and !1.5 for nitrogen. In this way, more reliable marine and terrestrial endmembers, using dC only, or even a dC vs. dN scatter plot cannot be satisfactorily used for the area.

790

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

Fig. 8. Scatter plot of C/N vs. dC. Data from Mahiques et al. (1997) (A) and this paper (B).

Mahiques et al. (1997) analyzed organic carbon, nitrogen, phosphorus and dC values in samples from a core collected in a mangrove swamp in the Bertioga region (Sa o Paulo state). Significant correlation (r"0.75, n"32) was obtained as between the C/N and dC values.  Zanardi (1996) analyzed hydrocarbon parameters in sediment samples from the Sa o Sebastia o region, identifying three different zones according to the degree of petroleum contamination. In the petroleum-poor sediment area, the C — n-alkanes   parameter showed values ranging from 93.9 to 850.6 ng/g\ of total sediment. The odd/even ratio for C n-alkanes, also used as indicative of terrestrial organic matter,  presented values ranging from 2.7 to 9.8, demonstrating the occurrence of some exportation of terrestrial organic matter to areas as deep as 50 m. The distribution of the different parameters according to depth (Fig. 5) shows that the C/N and dC values seem to maintain the signature of the origin of the organic matter, despite the possibility of some mixing with C plant organic matter and of  nitrogen changes due to phytoplankton utilization or bacterial activity. A scatter plot of C/N vs. dC values, in the light also of the results obtained by Mahiques et al. (1997) and, gives an exponential curve, with a very good correlation (r"0.98; explained variance of 97%) (Fig. 8). Average values of 24 for C/N and  !26.0 for dC can be given for the mangrove swamp sediments (Mahiques et al., 1997) and C land plants (Matsuura and Wada, 1995), and can also be considered as a  good hypothetical approach to a terrestrial endmember for the area. On the other hand, the establishment of a marine endmember, even when the dC values obtained by Matsuura and Wada (1995) are taking into consideration is much more difficult, since most of the sediment samples of the shelf present dC values which enter the plankton range, and C/N values are not available for marine organisms from the area. Also, dN values did not prove to be helpful in discriminating between the terrestrial and marine sources of the sediment samples. Nevertheless, considering the general range commonly accepted for C/N and dC values of marine

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

791

end-members (Stein, 1991), we may assume average values of 6 and !19.00, for C/N and dC parameters. More reliable values for terrestrial and marine end-members can be determined only after further work is undertaken on molecular biomarkers and on the actual significance of the considerably high values of dC for the soil samples. Considering the above end-members as valid, the organic matter deposited in the study area seems to present a prevalence of marine contribution. However, in spite of the absence of any great rivers draining into the ocean, it is possible to observe an increasing continental contribution towards to the north and it is also possible to identify some areas of the 100-m isobath where there is still some terrestrial influence in the organic matter. The northward increase of terrestrial organic matter is to be explained by the progressive narrowing of the shelf, and a consequent decrease in the mixing of marine organic matter with land-derived sediments. On the other hand, the presence of land-derived organic matter, even in more distant areas of the shelf, and the absence of important fluvial sources that might explain this presence, makes the establishment of a qualitative model for the understanding of this input necessary. 5.3. Transport of continental organic matter to the continental shelf The exportation of terrestrial organic matter offshore can be understood in terms of the water mass model prevailing on the southeastern Brazilian continental shelf, proposed by Castro Filho et al. (1987), and described above. In accordance with this model, based of the wind dynamics pattern prevailing on the southeastern coast of Brazil, the penetration of the South Atlantic Central Water (SACW) induces the displacement of the Coastal Water (CW) and its outward offshore movement in the neighborhood of Sa o Sebastia o Island. According to the authors, the model is frequently disturbed by the occurrence of cyclonic vortices close to the 80-m isobath, caused by the meandering of the Brazil Current. The occurrence of these vortices may explain some anomalies in the dN values, as well as a discontinuity in the dC and C/N pattern in the area off Sa o Sebastia o Island.

6. Conclusions (1) The distribution of sedimentary organic matter on the inner and middle continental shelf between Guanabara Bay and Sa o Francisco do Sul presents a distribution pattern strongly conditioned by the differences in the geomorphological, oceanographic and sedimentological factors prevailing to the north and to the south of Sa o Sebastia o Island. (2) To the south of Sa o Sebastia o Island a marked increasing gradient in organic matter towards deepest portions of the shelf is to be observed. The highest values in organic carbon occur close to the 100-m isobath. (3) To the north of Sa o Sebastia o Island, the values of the various parameters analyzed reflect the complexity of the sedimentology and geomorphology of the area.

792

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

(4) There is a marked increase in the contribution of land-derived organic matter towards the north of the area. This pattern may be explained by a progressive narrowing of the shelf, which leads to a decrease in the mixing of marine with terrestrial organic matter. (5) The texture of the sediment seems to be the most important factor in determining the distribution of the organic matter. (6) Nitrogen content in the sediments of the area is almost totally organic. (7) Despite the possibility of some influence of C plants and nitrogen content  changes due to bacterial activity or phytoplankton utilization, it seems that the dC and C/N parameters still maintain the signature of the origin of organic matter. Difficulties in determining relative marine-terrestrial contributions are mainly related to the problem of determining more reliable marine end-member values. (8) The values of C/N"24 and dC "!26.00 can be assumed as good estimates for terrestrial end-members for the organic matter. Marine end-member values can be determined only after further analysis of molecular biomarkers. (9) The exportation of land-derived organic matter to offshore may be explained in terms of a model of the dynamics of water masses, conditioned by the action of the wind, which leads to the outward movement of the Coastal Water. These processes are frequently altered by the occurrence of vortices, which are caused by the displacement of the Brazil Current, and cause disturbances on the seabed.

Acknowledgements The authors wish to express their thanks to Dr. Valdenir Veronese Furtado and Dr. Salvador Airton Gaeta, of the Institute of Oceanography, for their serious discussion with them of sedimentation and primary productivity on the continental shelf; and to Dr. Mario Katsuragawa, of the Institute of Oceanography, for the facilities used during the sampling cruises. Thanks are also due to Mr. Arthur Anthony Boorne, for the revision of the text. They are also indebted to the Sa o Paulo State Foundation for Research (FAPESP) for the financial support given for the undertaking of project 95/3970-0.

Appendices

Depth (m)

106 60 24 43 65 102 106 58 24 44 62 100 108 60 44 37 50 76 106 126 91 78 46 21 20 40 70 80 104 103

Sample

5521 5522 5523 5524 5525 5526 5527 5528 5529 5530 5531 5532 5533 5534 5535 5536 5537 5538 5539 5540 5541 5542 5543 5544 5545 5546 5547 5548 5549 5550

!43.07 !43.13 !43.17 !43.51 !43.46 !43.41 !43.74 !43.82 !43.89 !44.23 !44.16 !44.07 !44.40 !44.50 !44.57 !44.92 !44.86 !44.79 !44.72 !44.93 !45.02 !45.08 !45.16 !45.24 !45.58 !45.53 !45.43 !45.36 !45.27 !45.58

Long. (W) !23.34 !23.18 !23.00 !23.10 !23.26 !23.42 !23.58 !23.35 !23.11 !23.23 !23.47 !23.81 !23.93 !23.62 !23.40 !23.49 !23.69 !23.93 !24.17 !24.68 !24.44 !24.20 !23.96 !23.72 !23.85 !24.04 !24.33 !24.57 !24.85 !25.11

Lat. (S) 18.64 3.60 0.00 0.00 4.84 23.08 40.09 5.83 0.00 0.00 3.81 17.57 11.74 6.23 0.00 2.67 2.46 0.00 7.46 12.77 11.79 1.77 8.59 25.10 0.00 0.00 3.57 5.17 15.21 53.60

Clay (% weight) 5.66 3.62 1.19 1.02 3.80 6.10 6.58 2.21 0.87 0.88 2.68 5.56 4.07 3.85 3.07 1.80 3.42 2.78 2.89 5.23 3.73 2.77 4.64 5.90 3.47 3.23 3.11 3.02 4.68 7.65

1.95 0.97 0.53 0.58 1.10 1.69 1.84 2.39 0.46 0.93 1.78 2.04 2.67 1.39 0.81 2.02 1.23 0.85 2.12 1.80 1.73 0.96 1.64 1.95 0.23 0.24 0.86 1.25 2.16 1.40

Mean Diam. Sorting (phi) (phi) 42.2 11.8 3.2 10.3 14.0 28.8 20.1 46.0 4.6 19.9 17.3 35.3 28.5 15.0 38.7 21.9 17.0 20.1 31.1 84.2 74.6 43.7 14.8 18.7 4.3 5.6 14.7 21.3 38.8 21.6

CaCO  (% weight)

Appendix A Position, grain size and compositional data of the samples collected during the 1991 cruise

8.88 3.51 0.41 2.10 4.56 11.90 10.56 4.33 0.53 1.80 4.02 10.25 6.11 6.20 1.97 2.41 6.05 2.40 3.07 2.45 3.60 2.66 7.57 6.56 0.78 2.10 2.38 2.81 6.08 12.80

Org. C (mg/g) 1.18 0.55 0.09 0.34 0.68 1.58 1.47 0.58 0.11 0.24 0.52 1.26 0.69 0.84 0.30 0.36 0.80 0.32 0.43 0.34 0.34 0.28 0.75 0.74 0.13 0.31 0.32 0.38 0.82 1.82

N (mg/g) 0.42 0.23 0.01 0.12 0.29 0.48 0.49 0.16 0.03 0.06 0.18 0.63 0.19 0.36 0.15 0.33 0.34 0.07 0.07 0.07 0.09 0.08 0.75 1.16 0.20 0.20 0.19 0.19 0.24 0.58

S (mg/g) 7.5 6.4 4.6 6.3 6.7 7.5 7.2 7.4 4.8 7.5 7.7 8.1 8.9 7.4 6.6 6.7 7.6 7.5 7.1 7.2 10.7 9.5 10.0 8.9 6.1 6.9 7.4 7.4 7.4 7.0

C/N (weight)

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798 793

Depth (m)

68 64 61 41 20 22 34 55 66 90 114 108 84 60 52 38 23 22 38 46 66 95 84 62 46 32 20 20 33 56 32

Sample

5551 5552 5553 5554 5555 5556 5557 5558 5559 5560 5561 5562 5563 5564 5565 5566 5567 5568 5569 5570 5571 5572 5573 5574 5575 5576 5577 5578 5579 5580 5581

Appendix A (Continued.)

!45.67 !45.74 !45.82 !45.89 !45.97 !46.29 !46.22 !46.14 !46.07 !45.99 !45.92 !46.26 !46.33 !46.40 !46.48 !46.56 !46.63 !46.95 !46.88 !46.80 !46.72 !46.64 !46.98 !47.06 !47.13 !47.21 !47.28 !47.58 !47.49 !47.35 !47.61

Long. (W) !24.82 !24.58 !24.35 !24.11 !23.85 !24.08 !24.32 !24.56 !24.79 !25.04 !25.28 !25.43 !25.20 !24.96 !24.72 !24.48 !24.23 !24.47 !24.71 !24.94 !25.19 !25.52 !25.65 !25.40 !25.17 !24.93 !24.68 !24.96 !25.20 !25.52 !25.42

Lat. (S) 12.37 3.91 5.92 0.00 0.00 2.22 0.00 0.00 10.32 44.33 40.98 31.78 27.56 8.49 3.67 0.00 0.00 1.06 0.00 0.00 6.55 34.80 20.71 3.90 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Clay (% weight) 4.78 3.38 3.50 3.27 3.56 3.53 3.17 3.21 3.82 7.24 7.14 6.82 6.30 3.68 3.61 3.15 3.04 3.45 3.04 3.07 3.35 6.52 5.16 3.07 3.04 2.99 3.04 3.22 3.28 3.26 1.37

2.16 1.24 1.08 0.27 0.50 0.68 0.32 0.83 1.88 1.50 1.49 1.50 2.03 2.01 0.61 0.45 0.48 0.54 0.52 0.55 1.53 2.18 2.69 1.06 0.52 0.44 0.61 0.38 0.24 0.78 0.85

Mean Diam. Sorting (phi) (phi) 25.0 19.5 12.3 5.8 8.0 6.0 4.9 18.2 27.5 23.0 21.8 51.1 23.7 27.0 15.8 7.3 5.4 6.0 7.4 14.2 21.7 17.5 15.5 19.6 10.0 5.8 4.5 5.6 5.2 19.1 8.7

CaCO  (% weight) 8.26 3.79 3.41 1.53 1.17 1.16 1.06 2.58 4.61 11.08 13.14 12.54 9.55 3.35 1.94 1.19 1.19 2.10 1.20 0.94 4.44 9.94 6.99 2.23 1.20 1.00 1.22 1.11 1.13 1.88 0.80

Org. C (mg/g) 1.07 0.45 0.36 0.19 0.15 0.18 0.21 0.44 0.49 1.53 1.65 1.55 1.17 0.37 0.17 0.10 0.13 0.20 0.14 0.07 0.49 1.17 0.76 0.24 0.29 0.22 0.13 0.12 0.11 0.15 0.03

N (mg/g) 0.34 0.17 0.19 0.17 0.26 0.31 0.09 0.14 0.18 0.34 0.37 0.35 0.81 0.12 0.17 0.14 0.28 0.50 0.18 0.09 0.17 0.46 0.30 0.08 0.15 0.16 0.43 0.29 0.20 0.13 0.04

S (mg/g) 7.7 8.3 9.5 8.2 8.0 6.4 5.0 5.8 9.4 7.2 8.0 8.1 8.2 9.0 11.4 11.7 9.2 10.3 8.5 12.6 9.1 8.5 9.2 9.2 4.1 4.6 9.4 9.7 10.5 12.7 23.5

C/N (weight)

794 M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

5582 5583 5584 5585 5586 5587 5588 5589 5590 5591 5592 5593 5594 5595 5596 5597 5598 5599 5600 5601 5602

20 21 38 57 72 102 95 70 56 36 20 18 40 60 82 112 110 84 61 45 21

!47.87 !48.08 !47.83 !47.57 !47.32 !47.13 !47.30 !47.56 !47.82 !48.08 !48.34 !48.43 !48.18 !47.92 !47.67 !47.40 !47.53 !47.78 !48.03 !48.29 !48.55

!25.32 !25.60 !25.70 !25.79 !25.88 !25.98 !26.24 !26.16 !26.07 !25.97 !25.87 !26.20 !26.30 !26.39 !26.48 !26.58 !26.90 !26.81 !26.71 !26.62 !26.53

0.00 0.00 0.00 5.10 18.68 39.07 43.16 20.59 6.12 0.00 0.00 0.00 3.76 8.62 41.82 40.38 40.72 40.60 28.47 2.55 0.00

2.93 2.11 3.05 3.08 4.49 6.91 7.13 5.83 3.62 2.94 2.83 2.54 3.68 4.09 7.10 7.11 6.96 7.06 6.01 3.15 2.53

0.45 0.70 0.47 1.30 3.01 1.65 1.62 2.08 1.61 0.65 0.52 0.47 1.06 1.95 1.55 1.51 1.76 1.55 2.26 1.15 0.50

2.0 2.4 5.8 19.8 16.1 17.7 19.5 19.1 16.1 6.3 3.2 2.8 9.5 17.2 19.9 21.0 17.5 18.8 49.1 18.2 2.9

0.97 0.50 1.16 2.95 6.32 11.58 11.60 8.97 3.13 1.30 0.75 0.64 1.52 4.76 11.50 10.53 8.71 11.97 8.86 2.12 0.68

0.04 0.01 0.04 0.26 0.66 1.41 1.34 0.99 0.27 0.05 0.01 0.01 0.10 0.43 1.33 1.34 0.98 1.41 0.96 0.13 0.01

0.15 0.11 0.16 0.13 0.21 0.50 0.38 0.30 0.12 0.17 0.13 0.07 0.20 0.22 0.46 0.43 0.37 0.52 0.45 0.14 0.05

25.0 50.2 28.5 11.5 9.6 8.2 8.6 9.1 11.8 28.5 75.2 63.9 14.6 11.1 8.7 7.9 8.9 8.5 9.2 16.6 67.6

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798 795

Depth (m)

106 62 98 106 102 108 106 124 92 74 44 18 104 104 66 84 112 108 90 78 57 70 102 96 112 110 86 60 44

Sample

5778 5779 5783 5784 5789 5790 5796 5797 5798 5799 5800 5801 5806 5807 5816 5817 5818 5819 5829 5830 5842 5843 5844 5845 5854 5855 5856 5857 5858

!43.06 !43.13 !43.41 !43.74 !44.08 !44.40 !44.72 !44.93 !45.02 !45.08 !45.16 !45.24 !45.27 !45.58 !46.07 !45.99 !45.92 !46.25 !46.63 !46.98 !47.57 !47.32 !47.07 !47.31 !47.40 !47.53 !47.78 !48.03 !48.28

Long. (W) !23.34 !23.18 !23.42 !23.58 !23.81 !23.93 !24.17 !24.68 !24.44 !24.20 !23.96 !23.72 !24.85 !25.11 !24.79 !25.04 !25.27 !25.42 !25.52 !25.65 !25.78 !25.88 !25.97 !26.24 !26.58 !26.90 !26.81 !26.70 !26.62

Lat. (S) 12.00 5.72 13.78 11.76 13.32 11.39 5.61 4.86 2.20 1.50 9.59 7.78 5.26 16.25 4.81 7.36 15.67 13.91 13.73 8.56 2.96 7.96 11.93 11.05 12.26 10.79 13.52 8.75 1.88

Org. C (mg/g)

Org N (mg/g) 1.48 0.70 1.78 1.47 1.73 1.49 0.76 0.58 0.33 0.24 1.15 0.83 0.64 2.19 0.64 0.99 2.17 1.92 1.91 1.11 0.39 1.05 1.65 1.45 1.57 1.40 1.78 1.12 0.27

Appendix B Position and compositional data of the samples collected during the 1993 cruise

8.1 8.1 7.7 8.0 7.7 7.7 7.4 8.4 6.8 6.3 8.3 9.4 8.2 7.4 7.6 7.4 7.2 7.2 7.2 7.7 7.6 7.6 7.2 7.6 7.8 7.7 7.6 7.8 6.9

C/N !20.89 !21.06 !20.64 !20.62 !20.37 !20.44 !20.84 !20.42 !20.26 !20.33 !21.03 !21.95 !20.33 !19.95 !20.70 !20.07 !19.68 !19.67 !19.61 !19.81 !20.25 !19.87 !19.71 !19.38 !19.31 !19.49 !19.21 !19.56 !20.01

C13/C1 ( PDB) 5.39 4.94 5.78 5.70 5.74 5.76 5.79 5.64 6.11 5.79 5.93 5.95 4.95 5.79 5.32 5.84 6.09 6.20 6.14 5.56 5.56 5.49 6.43 6.28 5.58 6.00 6.54 6.31 6.50

N15/N13 ( air)

796 M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

797

References Anderson, T.F., Arthur, M.A., 1983. Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems. In: Arthur, M.A. (Ed.), Stable Isotopes in Sedimentary Geology. SEPM Short Course No. 10. Dallas, SEPM, pp. 1-1—1-151. Andrews, J.E., Greenaway, A.M., Dennis, P.F., 1998. Combined carbon isotope and C/N ratios as indicators of source and fate of organic matter in a poorly flushed, tropical estuary: Hunts Bay, Kingston Harbour, Jamaica. Estuarine, Coastal and Shelf Science 46, 743—756. Bader, R.G., 1955. Carbon and nitrogen relations in surface and subsurface marine sediments. Geochimica et Cosmochimica Acta 7, 205—211. Bordovskiy, O.K., 1965. Accumulation of organic matter in bottom sediments. Marine Geology 3, 33—82. Braga, E.S., Mu¨ller, T.J., 1998. Observation of regeneration of nitrate, phosphate and silicate during upwelling off Ubatuba, Brazil, 23°S. Continental Shelf Research 18, 915—922. Castro Filho, B.M., Miranda, L.B., Miyao, S.Y., 1987. Hydrographic conditions on the continental shelf offshore off Ubatuba: seasonal and middle scale variations. Boletim do Instituto Oceanogra´fico, Sa o Paulo, 35, 135—151 (in Portuguese). Faganeli, J., Pezdic, J., Ogorelec, B., Misic, M., Najdek, M., 1994. The origin of sedimentary organic matter in the Adriatic. Continental Shelf Research 14, 365—384. Gaeta, S.A., Brino, O.L., Ribeiro, S.M.M.S., 1994. Distributions of NO\, chlorophyll a and primary  productivity in the southwestern region of the South Atlantic during summer. Southwestern Atlantic Physical Oceanography Workshop, Report. Instituto Oceanogra´fico da Universidade de Sa o Paulo, Sa o Paulo, pp. 57—60. Gon i, M.A., Ruttenberg, K.C., Eglinton, T.I., 1997. Sources and contribution of terrigenous organic carbon to surface sediments in the Gulf of Mexico. Nature 389, 275—278. Haines, E.B., Montague, C.L., 1979. Food sources of estuarine invertebrates analyzed using C/C ratios. Ecology 60, 48—56. Hedges, J.L., Mann, D.C., 1979. The lignin geochemistry of marine sediments from the southern Washington coast. Geochimica et Cosmochimica Acta 43, 1809—1818. Hedges, J.L., Parker, P.L., 1976. Land-derived organic matter in surface sediments from the Gulf of Mexico. Geochimica et Cosmochimica Acta 40, 1019—1029. Holmes, M.E., Mu¨ller, P.J., Schneider, R.R., Segl, M., Pa¨tzold, J., Wefer, G., 1996. Stable nitrogen isotopes in Angola Basin surface sediments. Marine Geology 134, 1—12. Inte`s, A., LeLoeuff, P., 1986. Les anne´lides polyche`tes de Coˆte d’Ivoire. IV. Relations faune-sediments. Oceanographie Tropicale, ORSTOM 21, 53—88. Kaplan, I.R., 1983. Stable isotopes of sulfur, nitrogen and deuterium in recent marine environments. In: Arthur, M.A. (Ed.), Stable Isotopes in Sedimentary Geology. SEPM Short Course No. 10. Dallas, SEPM, pp. 2-1—2-108. Kowsmann, R.O., Costa, M.P.A., 1979. Quaternary sedimentation of the Brazilian continental margin and adjacent oceanic areas (final report). REMAC Project Series, no. 8. Rio de Janeiro, PETROBRA¨S, 55 pp. Larsonneur, C., Bouysse, P., Auffret, J.P., 1982. The superficial sediments of the English Channel and its western approaches. Sedimentology 29, 851—864. Mahiques, M.M., Tessler, M.G., Hoshika, A., Mishima, Y., Suguio, K., Kawana, K., 1997. Infra-annual variations in the characteristics of the organic matter from Bertioga channel, southeastern Brazil. 6th Congress of the Brazilian Association on Quaternary Research. Extended Abstracts, Curitiba, ABEQUA. pp. 94—98. (in Portuguese). Martins, L.R., Correa, I.C.S. (Eds.), 1996. Atlas Morphology and Sedimentology of the Southwest Atlantic Coastal Zone and Continental Shelf from Cabo Frio (Brazil) to Peninsula Valde´s (Argentina). Porto Alegre, UFRGS, 4pp., 20 maps. Matsuura, Y., Wada, E., 1995. Carbon and nitrogen stable isotope ratios in marine organic matters of the coastal ecosystem in Ubatuba, southern Brazil. Cieˆncia e Cultura 46, 141—146. Meyers, P.A., 1994. Preservation of elemental and isotopic source identification of sedimentary organic matter. Chemical Geology 144, 289—302.

798

M. Michaelovitch de Mahiques et al./Continental Shelf Research 19 (1999) 775—798

Meyers, P.A., 1997. Organic geochemical proxies of paleoceanographic, paleolimnologic and paleoclimatic processes. Organic Geochemistry 27, 213—250. Mu¨ller, P.J., Suess, E., 1979. Productivity, sedimentation rate, and sedimentary organic matter in the oceans. I. — organic matter preservation. Deep-Sea Research 36, 191—209. Prahl, F.G., Ertel, J.R., Gon i, M.A., Sparrow, M.A., Eversmeyer, B. 1994. Terrestrial organic carbon contributions to sediments on the Washington margin. Geochimica et Cosmochimica Acta 58, 3035—3048. Rau, G.H., Meurns, A.J., Yong, D.R., Olson, R.J., Schafer, H.A., Kaplan, I.R., 1983. Animal C/C correlates with trophic level in pelagic food webs. Ecology 64, 1314—1318. Rocha, J., Milliman, J.D., Santana, C.I., Vicalvi, M.A., 1975. Southern Brazil. Contribution to Sedimentology. V. 4: upper continental margin sedimentation off Brazil. Stuttgart, p. 117—150. Ruttenberg, K.C., Gon i, M.A., 1997. Phosphorous distribution, C:N:P ratios, and dC in arctic, temperate, and tropical coastal sediments: tools for characterizing bulk sedimentary organic matter. Marine Geology 139, 123—145. Sackett, W.M., 1989. Stable carbon isotope studies on organic matter in the marine environment. In: Fritz, P., Fontes, J.Ch. (Eds.), Handbook of Environmental Isotope Geochemistry. Elsevier, Amsterdam. pp. 139—170. Saito, Y., Nishimura, A., Matsumoto, E., 1989. Transgressive sand sheet covering the shelf and upper slope off Sendai, Northeast Japan. Marine Geology 89, 245—258. Stein, R., 1991. Accumulation of organic carbon in marine sediments. Results from the Deep Sea Drilling Project/Ocean Drilling Program. In: Bhattacharji, S., Friedman, G.M., Neugebauer, H.J., Seilacher, A. (Eds.), Lecture Notes in Earth Sciences, Vol. 34. Springer, Berlin. 217pp. Wada, E., 1986. N and C abundances in marine environments with emphasis on biogeochemical structure of food web. In 4th Working Meeting on Isotopes in Nature. Proceedings. Leipzig, Germany, September, pp. 639—646. Wada, E., Minagawa, M., Mizutani, H., Tsuji, T., Imaizumi, R., Karasawa, K., 1987. Biogeochemical studies on the transport of organic matter along the Otsuchi River watershed, Japan. Estuarine, Coastal and Shelf Science 25, 321—336. Westerhausen, L., Poynter, J., Eglinton, G., Erlenkeuser, H., Sarnthein, M., 1993. Marine and terrigenous origin of organic matter in modern sediments of the equatorial East Atlantic: the dC and molecular record. Deep-Sea Research I 40, 1087—1121. Zanardi, E., 1996. Hydrocarbons in the Sa o Sebastia o Channel and adjacent inner shelf - influence of the 1994 oil split. M.Sc. thesis. Institute of Oceanography, University of Sa o Paulo, Sa o Paulo,112pp. (in Portuguese). Zembruscki, S.G., 1979. Geomorphology of the southern Brazilian continental margin and adjacent oceanic basins. In: Chaves, H.A.F. (Ed.), Geomorphology of the Brazilian Continental Margin and Adjacent Oceanic Basins (Final Report). REMAC Project Series, no. 7 Rio de Janeiro, Petrobra´s, pp. 129—177 (in Portuguese).