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Hydrodynamically driven patterns of recent sedimentation in the shelf and upper slope off Southeast Brazil
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Continental Shelf Research 24 (2004) 1685–1697 www.elsevier.com/locate/csr
Hydrodynamically driven patterns of recent sedimentation in the shelf and upper slope off Southeast Brazil Michel Michaelovitch de Mahiquesa,, Moyse´s Gonsalez Tesslera, Aurea Maria Ciottia, Ilson Carlos Almeida da Silveiraa, Silvia Helena de Mello e Sousaa, Rubens Cesar Lopes Figueirab, Colombo Celso Gaeta Tassinaric, Valdenir Veronese Furtadoa, Raquel Fernanda Passosa,1 Institute of Oceanography of the University of Sa˜o Paulo, 05508-900 Prac- a do Oceanogra´fico 191, Sa˜o Paulo, SP Brazil b University Cruzeiro do Sul, Sa˜o Paulo, 05508-900 Prac- a do Oceanogra´fico 191, Sa˜o Paulo, SP Brazil c Institute of Geosciences of the University of Sa˜o Paulo, 05508-900 Prac- a do Oceanogra´fico 191, Sa˜o Paulo, SP Brazil
a
Received 10 April 2003; accepted 28 May 2004 Available online 23 August 2004
Abstract Sedimentological parameters of 206 box-core tops and 387 van Veen grab samples were analysed so as to understand the distribution, source and sedimentation rates in the shelf and upper slope of the southern Southeastern Brazilian margin. Most of the parameters (bulk organic matter constituents, d13C, and eNd) can be grouped on northern and southern sectors, separated by a sharp boundary. This suggests that sediment sources and sedimentary processes are different in both regions. The southern sector of the study area is more influenced by cold waters coming from the southern portion of the South American shelf and the organic fraction revealed an important role played by the primary productivity in the sedimentation. Also, eNd values indicate that part of the inorganic fraction of the southern sector sediments are allochtonous, probably coming from younger rocks of the Andean chain probably reaching the shelf through the La Plata river runoff. In the northern part of the study area the sedimentation is controlled almost exclusively by the meandering of Brazil Current. Compositional and isotope bulk organic parameters showed a more complex mixing of terrigenous and pelagic fractions. Also, eNd values are associated with the precambrian rocks of the Brazilian shield. The whole area exhibits sedimentation rates, varying from 5 to 660 mm kyr1. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sediment distribution; Sediment transport; Shelf dynamics; Ocean circulation; South atlantic; Brazil; Brazil current
Corresponding author. Institute of Oceanography of the University of Sa˜o Paulo, 05508-900 Prac- a do Oceanogra´fico 191, Sa˜o Paulo SP Brazil. E-mail address: [email protected] (M.M. de Mahiques). 1 Scholarship from the Fundac- a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo.
0278-4343/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2004.05.013
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1. Introduction Sedimentary processes on passive margins with limited fluvial discharges are mainly controlled by the displacement of water masses and currents, primary production rates, climate in the adjacent emerged lands and sea-level changes. All of these factors control the input and redistribution of the different sediment components. The lack of a dominant process leads to a multi-proxy approach for the characterization of the sources, transport and fate of the surface sediments (Stein, 1991; Meyers, 1994, 1997; Madureira et al., 1997). The characteristics of the surface sediments of the Brazilian margin and, more particularly of its southeastern portion, were intensively studied during the 1970s (Milliman, 1975, 1976, 1978; Rocha et al., 1975). Based on grain-size, calcium carbonate, bulk organic matter, light and heavy minerals and clay mineralogy analyses of a large number of grab samples, the southern and southeastern (SE) Brazilian inner and middle shelf modern sediments were classified as either lagoonal or beach relict sediments, bordered by a shelf-break carbonate facies. The main objective of this paper is to present an updated interpretation of sediment distribution and fate on the SE Brazilian upper continental margin as well as to better understand the role played by the ocean circulation, specially the Brazil Current, in the sedimentary regime of the area. Also, this paper represents a first attempt to characterize spatial distribution of sediments in relation to biological production in the area.
2. Study area The study area corresponds to the arc-shaped sector of the SE Brazilian upper continental margin, also known as Sa˜o Paulo Bight (Zembruscki, 1979), extending from Cabo de Santa Marta (281300 S–491000 W) to Cabo Frio (231000 S–421000 W). The continental shelf width varies from 73 to 231 km and its declivity ranges from 1:656 to 1:1333. The shelf break is located between 120 and 180 m depth. The isobaths approach the coast towards the north, the shelf close to Cabo Frio is narrower (Fig. 1).
The geological evolution of the Sa˜o Paulo Bight has its origin in the development of the Atlantic type margins, and is marked by the subsidence of the Santos Basin (Cretaceous to Recent) (Pereira and Feijo´, 1994; Meisling et al., 2001), by intensive Mesozoic alkaline magmatism and by the uplifting of the Serra do Mar range (Almeida and Carneiro, 1998). The alignment of the Serra do Mar in relation to the present coastline is reflected in the small width of the coastal plains and in the arrangement of the isobaths. As a consequence of Serra do Mar rising in Southeast Brazil, most of the drainage systems run towards the countryside, feeding the Parana´—La Plata river basins; only small rivers run directly towards the sea, draining mainly precambrian granites, gneisses and migmatites. During the Quaternary, the morphotectonic control on sedimentation was overprinted by the transgressive/regressive events related to the sealevel changes, mainly the dissecation and submersion of the shelf during the Last Glacial Cycle. On the Brazilian margin, Cabo Frio marks the transition between the tropical environment (to the north) and the subtropical (to the south) (Rocha et al., 1975). This transition has been pointed out by the authors as having been caused by different oceanographic conditions as well as distinct sedimentary dynamics, which produce mainly carbonate northward and terrigenous southward of Cabo Frio. Despite a relatively good coverage of bottom surface samples, the knowledge of the sedimentation on the SE Brazilian margin is mainly based on the grainsize and calcium carbonate content (Martins and Correa, 1996). In the study area sedimentation is essentially terrigenous, being comprised of quartzose sands and muds. Southward of Sa˜o Sebastia˜o Island (241300 S) there is a gradual increase of the mud content towards the deeper portions of the shelf. The northern part of the Bight is characterized by heterogeneity of grain-size, with significant deposition of mud on the inner shelf. The dichotomy between southern and northern sectors is also present in the clay mineralogy, with montmorillonite being predominant in the South and the kaollinite–illite–montmorillonite mixture prevailing in the North (Rocha et al., 1975). Some patches and belts of carbonate gravels are found
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Fig. 1. Location of the study area.
between the 100-m isobath and the shelf break. Most of the present sedimentary cover is said to be the result of the reworking of regressive sediments (Milliman, 1975; Rocha et al., 1975).
The inner and middle shelf dynamics are determined by the displacement of three water masses, which present strong seasonal variation (Castro et al., 1987). Between November and
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March, the South Atlantic Central Water (SACW) moves close to the bottom toward the coast leading to the oceanward displacement of the less dense Coastal Water (CW) and keeping the Tropical Water (TW) relatively distant from the coastline. This period also corresponds to the rainy season in SE Brazil and, thus, to the increase of the terrigenous flux to the coastal waters. The displacement of the CW is the most important factor in the transport of terrigenous organic matter towards the deeper areas of the shelf. From March to November, the retreat of the SACW leads to a greater influence of the TW in the shelf processes (Mahiques et al., 1999). Northward to Cabo Frio, the outer shelf and slope hydrodynamics and its role on the sedimentation has already been extensively described in several papers (Viana et al., 1998; Viana, 2002). In the study area the outer shelf and upper slope the flux is controlled by the southward displacement of the Brazil Current (BC), which shows a meandering pattern throughout its path. Meandering of (BC) and consequent eddy formation seems to be the factor responsible for geographic changes in the sedimentation rates in the area between Sa˜o Sebastia˜o Island and Cabo Frio (Mahiques et al., 2002). Northward to the study area, several papers have already emphasized the relationship between the circulation pattern and sedimentary processes.
3. Methods A total of 206 box-core tops and 387 Petersen grab samples, collected between 1992 and 1998 by the R.V. ‘‘Prof.W.Besnard’’ were included in this study (Fig. 1). For box-core samples, only the topmost 2 cm were considered for most of the analyses. Sediment for radiocarbon datings and calculations of sedimentation rates was collected at 8 cm. We assumed that the surface at the bottom was modern which was guaranteed by the presence of 210 Pbunsupported in all of the samples (Tessler, 2001). Sedimentation rate calculations were estimated using Dep-Age Software, developed by Dr. Louis Maher Jr., from the US Geological Survey.
Organic carbon, total nitrogen and sulphur contents were determined in an LECO CNS2000 Analyser. Approximately 500 mg of dry sediment from each of the samples was treated with 1 N HCl in order to remove calcium carbonate, freeze-dried and then analysed. LECO sulphametazin standards and blanks were analysed as controls for each set of 30 samples. Calcium carbonate was determined by the difference in weight of the sediment prior to and after acidification of 2 g of each sample with 1 N HCl. Values are reported as percentages of dry weight. Values of d13C (% PDB) and d15N (% air) of the sedimentary organic content were obtained by mass spectrometry at the Coastal Science Laboratories (Austin, TX, USA). C and N isotope analyses were performed by on line combustion with a Fisons NA1500 Elemental Analyser attached to a VG SIRA Model 10 mass spectrometer. Carbon was calibrated using NBS 22 oil standard (defined as 29.60% relative to PDB). Reproducibility for carbon was better than 0.2%. Nitrogen was calibrated using IAEA N-1, N-2 and N-3 (defined as+0.5%,+20.3% and +4.6%, respectively, relative to the Air). Reproducibility for nitrogen was better than 0.3%. Analyses of neodymium isotope, referred to as eNd, were carried out at the Geochronological Research Center of University of Sa˜o Paulo, Brazil. The Sm–Nd analyses were prepared by standard methods according to the analytical procedures described by Sato et al. (1995), involving HF-HNO3 dissolution plus HCl cation exchange, using a Teflon Powder column to separate ETR. No visible solid residues were observed after dissolution. Samples with incomplete dissolution were discarded. Nd ratios were normalized to a value of 146Nd/144Nd=.72190. The averages of 143Nd/144Nd for La Jolla and BCR-1 standards were .5118477.00005 (2s) and .5126627.00005 (2s), respectively. The blanks were less than .03 ng. Nd isotopic analyses were carried out with a multicollector Finnigan-MAT 262 mass spectrometer. SeaWiFS images were obtained from the NASA GSFC’s Distributed Active Archive Center (DAAC) acquired as Level 1A (Nadir Resolution
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5 9.82 9.71 0.85 10.73 8.54 34 34 20.4 6.3 20.3 6.3 0.8 0.7 19.2 5.0 22.6 8.1 106 0.4 0.2 0.6 0.04 5.66 106 0.8 0.6 0.6 0.01 2.07 106 6.7 5.3 4.9 0.50 18.87 363 16.3 14.7 13.2 1.5 91.9 351 351 70.9 3.82 85.6 3.45 32.3 1.32 0.1 0.30 100.0 7.65 Southern sector Number of samples 7 Mean 57.7 Median 43.0 Standard deviation 36.1 Minimum 29 Maximum 134
351 1.08 0.90 0.66 0.18 3.01
41 41 21.0 6.1 20.9 7.6 0.5 0.7 22.1 4.7 20.0 7.6 73 0.5 0.3 0.5 0.01 2.10 73 0.9 0.7 0.5 0.02 2.28 73 7.3 6.6 4.4 0.41 17.35 309 31.9 22.9 23.6 1.2 94.0 309 309 60.8 4.04 68.2 3.85 30.8 1.55 0.5 0.26 100.0 7.63
309 1.37 1.48 0.56 0.25 3.00
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Northern sector Number of samples 14 Mean 152.8 Median 60 Standard deviation 199.9 Minimum 5 Maximum 660
Due to the heterogeneous distribution of samples, this work will only emphasize the larger scales pattern of sediment distribution. In this sense almost all the parameters present a conspicuous dichotomy between the sectors northward and southward of Sa˜o Sebastia˜o island. Table 1 presents the statistics for each parameter in both sectors. Grain-size mean diameter distribution is presented on Fig. 2. The 100-m isobath controls the deposition of mud throughout the shelf; two large elongated zones of muddy sediment are found on the outer shelf separated by a sandy belt that extends towards the slope between longitudes 451W and 461W. Coarser sediments, described as relict sediments (Mahiques, 1987), are found on the inner shelf between longitudes 431300 W and 441 W. Textural heterogeneity is attested by the high values of sorting which predominate thoughout the shelf and upper slope as observed in Table 1. The calcium carbonate distribution (Fig. 3) shows an area of conspicuous carbonatic sedimentation between the 100 and 200-m isobaths and longitudes 431300 W and 451300 W. In general the remaining part of the study area presents lithoclastic sediments, according to the classification of Larsonneur et al. (1982). There are three main areas of higher organic carbon content (Fig. 4). Higher values are found in the southern sector, along the 100-m isobath between longitudes 451000 W and 471300 W. The two other regions of higher carbon content are the Cabo Frio upwelling zone and the vicinity of Sa˜o Sebastia˜o island. The C/N ratio pattern (Fig. 5) indicates one major area as source of terrigenous input corresponding to the coast southward to
Table 1 Statistics of northern and southern sectors of the study area obtained for each of the sedimentary parameters
4. Results
Sand Mean Sorting Calcium Organic Total Total d13C Sedimentation carbonate (%) carbon (mg g1) nitrogen (mg g1) sulphur (mg g1) rate (mm kyr1) (%) diameter (f) (j)
d15N eNd
of 1.1 km). Chlorophyll-a concentration (mg m3) was estimated using the standard Seadas 4.3 OC2 algorithm (for details see http://seadas.gsfc.nasa.gov). Standard masks and daily meteorological and ozone data (also obtained from DAAC) were used for the atmospheric correction. Images were remapped to the Mercator map projection.
14 13.27 12.75 1.87 17.13 9.91
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Fig. 2. Distribution of the mean diameter (f) in the surface sediments of the study area.
251000 S. A secondary area is located in the vicinities of the Sa˜o Sebastia˜o and extents up to the outer shelf/upper slope, between 251000 S and 26o000 S and 441000 W and 451000 W. The distribution of d13C in the organic fraction (Fig. 6) indicates once more the clear differentiation between the northern and southern sectors of the upper margin of SE Brazil. In general, the sediments of the southern sector show values higher than 21.0% (PDB) with a nucleus of values above 19.5% (PDB) along the 100-m isobath. Lighter isotopes are found as a projection towards deeper areas on the coast off the Sa˜o Sebastia˜o area. d15N distribution shows a rather complex pattern, the northern and southern complexes being divided by a strip with values of more than 6.0% (air). The distribution of eNd (Fig. 7) shows that the values northward from Sa˜o Sebastia˜o island range from 17.1 to 9.9; actually, 10 of the 12 samples of this sector present values below 12. On the other hand, all of the samples from the sector
southward from Sa˜o Sebastia˜o island give values of more than 11. The large distribution of eNd values around the 241S parallel is due to the orientation of the shelf in this area which is, in fact, SW–NE. Sedimentation rates varied from 5 to 660 mm kyr1 (Table 1), with lower sedimentation rates found associated with the shelf break between longitudes 421W and 441300 W. Higher rates are found off Cabo Frio, close to the 100-m isobath northward from Sa˜o Sebastia˜o Island and throughout the shelf southward to the 25th parallel. Fig. 8 presents SeaWiFS ocean colour images of estimated chlorophyl concentration. These images provide synoptical observation of biological processes, which are closely related to the circulation patterns. The seasonal examples illustrate a clear north–south gradient on phytoplankton abundance, which correlates positively with the organic carbon content and the zones of higher sedimentation rates.
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Fig. 3. Distribution of calcium carbonate (%) in the surface sediments of the study area.
Fig. 4. Distribution of organic carbon (mg g1 dry weight) in the surface sediments of the study area.
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Fig. 5. Distribution of the C/N ratio (weight) in the surface sediments of the study area.
Fig. 6. Distribution of the d13C values in the surface sediments of the study area.
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and southern, separated by a sharp boundary geographically associated to the Sa˜o Sebastia˜o island. In the southern sector, there is a clear trend of fining sediments towards offshore, with increasing amounts of silt and clay beyond the 100-m isobath. In general, sediments in the area can be classified either as lithoclastic or biolithoclastic, according to the classification of Larsonneur et al. (1982). The amount of organic matter follows a similar trend to that of grain-size. Stable isotope and C/N ratio values are indicative of pelagic organic matter. eNd values are always higher than 11. On the other hand, the sedimentation in the northern sector shows a more complex scenario, with the presence of a bioclastic belt, between 100 and 200 m which corresponds to a relict sediment facies. Also, the distribution of grain-size, organic matter and stable isotopes shows a patchy configuration. Most of the eNd values are lower than 12. 5.2. Source of the organic fraction
Fig. 7. Latitudinal (A) and longitudinal (B) variations in eNd in the study area. The vertical lines in the middle of the graphs indicate the position of Sa˜o Sebastia˜o Island.
5. Discussion 5.1. Sediment distribution Sediment distribution in the southeeastrn Brazilian continental shelf and upper slope between Cabo Frio and Cabo de Santa Marta is marked by the occurrence of two distinct sectors, northern
The differentiation in two sectors of the shelf is also represented by the deposition of a more pelagic organic matter in the south, where the primary productivity is more effective, in contrast to a mixed organic matter in the northern sector. The prevalence of high productivity conditions through time in the southern portion of the study area has been previously confirmed by high values of nitrate, and surface and subsurface phytoplanktonic biomass (Gaeta, 1999). The penetration of cold waters coming from the south in latitudes as low as 231S have already been reported elsewhere. Based on satellite images, Campos et al. (1996) have attributed the origin of the cold waters to the Brazil–Malvinas confluence (351S), or to further south, on the Argentinean shelf. In Sepetiba Bay, a 300 km2 area enclosed water body located at 231S, the presence of the benthic foraminifera Bucella peruviana and Quinqueloculina seminulum, the planktonic foraminifera Globigerina bulloides, together with ostracodes and micro bivalves typical of the Uruguayan and Argentinian shelves, were
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Fig. 8. Estimated surface chlorophyll concentration (mg m3) using OC2v4 SeaWIFS algorithm. Figures are high resolution (1 km) composites of 4 to 7 days representing (A) Spring, (B) Summer, (C) Autumn and (D) Winter of 2001 (Southern Hemisphere).
used by Stevenson et al. (1998) to suggest the penetration of an extension of the Malvinas Current towards the SE Brazilian shelf. Hydrodynamically it seems very unlikely that the Malvinas waters can reach the middle and inner portions of the shelf because of its barotropic character. More recently, Piola et al. (2000) built a hydrographic climatology of the South American shelf between 241S and 401S. Their results revealed that shelf dynamics drive coastal water northward during the winter season. This coastal water is mainly of subtropical region associated with the spreading of low-salinity water from the La Plata
river and the contribution of the Patos-Mirim Lagoonal system (located between 301S and 341S). The low temperature signal of such a flow is due to winter cooling of waters confined above the sharp halocline imposed by the northward movement of low-salinity plume. Therefore, we tend to assume that the organisms from the most southern portions of the South American shelf are incorporated in and transported northward by the low-salinity plume described by Piola et al. (2000). On the other hand, with exception of the Cabo Frio region, all of the northern sector of the study area shows oligotrophic conditions. In the northern
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sector, warm oligotrophic waters of the BC flow over the upper slope and outer shelf, with occasional penetrations of anticyclonic meanders onto the middle shelf. Higher productivity zones are then either found on the inner shelf or in areas where the BC meandering may also enhance the coastal winddriven upwelling such as that off Cabo Frio. 5.3. Source of the inorganic fraction The neodymium isotope ratio is considered one of the best proxies for source rocks of marine sediments and paleocirculation (Vroon et al., 1995; Abouchami et al., 1997, 1999; Vance and Burton, 1999; Rutberg et al., 2000). A comparison of our eNd data with those available for seawater, Mnnodules, red clays and metalliferous sediments from different oceans (Faure, 1986; DePaolo, 1988) shows a good correspondence of the northern sector samples with the Atlantic-type values. On the other hand, the sediments from the southern sector seem to show contribution from allochtonous sources. Assuming, then, the existence of a south-originated flow covering the southern sector of the study area we may consider that sediment from the upper south-eastern-most Brazilian margin is influenced by the La Plata drainage system, which delivers sediments derived from the younger Andean rocks to the SW Atlantic Ocean, agreeing with the hydrographic data presented in Piola et al. (2000). 5.4. Sedimentation rate and sedimentary processes The whole area presents sedimentation rates, ranging from 5 to 660 mm kyr1. The visual comparison between our data and the SeaWiFS images suggests that areas with lower sedimentation rates are associated with the main flux of the BC, in the northern sector of the study area. Also, a fairly close correlation is observed between the zones of higher sedimentation rates with the higher values of carbon, indicating that the sedimentation in the SE Brazil upper margin is controlled by the pelagic processes associated with the northward propagation of south-originated cold waters as well as with the upwelling processes related to the BC meandering as observed in the Cabo Frio region.
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6. Summary and conclusions The present sedimentation in the upper continental margin off Southeast Brazil is strongly dominated by the oceanic water mass dynamics and shelf circulation due to the lack of any considerable river input. These hydrodynamical processes control the primary productivity and sediment redistribution in the area, leading to the establishment of the differences in sedimentation rates and sedimentary facies observed in this work. The sedimentation rates vary from 5 to 660 mm kyr1 and are strongly correlated with the areas of higher primary productivity and/or of terrigenous input as well as with the flow of the BC. The area off Sa˜o Sebastia˜o Island marks the boundary between the two main sedimentary zones, which are characterized by differences both in the organic and inorganic fractions of the sediments. Southward from the island, the deposition processes are associated with the seasonal penetration of low-salinity, low-temperature plume of shelf water which has been recently related to the La Plata river runoff; its interaction with the meanders of the BC as well as with the shelf morphology favours the establishment of high productivity zones, very clearly identified by the bulk organic matter characteristics. The inorganic fraction represented by the eNd parameter suggests an as yet unidentified allochtonous origin for the sediments. Northward of Sa˜o Sebastia˜o Island, the relative heterogeneity of sediments indicate more complex hydrodynamics, especially on the middle and inner shelves. The inner shelf organic fraction distribution is linked to the Cabo Frio coastal upwelling as well as to other coastal processes. The middle and outer shelves sediment distribution is a result of a more vigorous meandering of the Brazil Current in the area.
Acknowledgments The authors are indebted to Professor Donn S. Gorsline (University of South California, USA), Professor Jean-Claude Fauge`res (Universite´ Bordeaux I, France), Professor Joa˜o Manoel
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Alveirinho Dias (Universidade do Algarve, Portugal) and Professor Alberto Garcia de Figueiredo (Universidade Federal Fluminense, Brazil) for the revision and improvement of the manuscript. The authors also wish to express their thanks to Messrs. Marcelo Rodrigues and Edilson Faria and Mrs. Samara Cazzoli y Goya, for the execution of the grain-size, calcium carbonate and bulk organic matter analyses, and to the staff of the Geocronological Research Centre of the University of Sa˜o Paulo, for the Neodymium isotope analyses. This work was financed by the Fundac- a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) via grant no. 01/01098-7, and by the Conselho Nacional de Desenvolvimento Cientı´ fico e Tecnolo´gico (CNPq) via grant no. 300650-9. This paper is a contribution to the IGCP464 (Continental Shelves during the Last Glacial Cycle) Project and to the Carbon, Hydrology and Global Environmental Systems (CHANGES) Programme.
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ARTICLE IN PRESS M.M. de Mahiques et al. / Continental Shelf Research 24 (2004) 1685–1697 Sato, K., Tassinari, C.C.G., Kawashita, K.E., Petronilho, L., 1995. A metodologia Sm-Nd no IGc-USP e suas aplicac- o˜es. Anais da Academia Brasileira de Cieˆncias 67, 313–336. Stein, R., 1991. Accumulation of Organic Carbon in Marine Sediments. Results from the Deep Sea Drilling Project/ Ocean Drilling Program. Springer, Berlin p. 217. Stevenson, M.R., Brito, D.D., Stech, J.L., Kampel, M., 1998. How cold water biota arrive in tropical bay near Rio de Janeiro, Brazil? Continental Shelf Research 13, 1595–1612. Tessler, M.G., 2001. Taxas de sedimentac- a˜o holoceˆnica na plataforma continental sul do Estado de Sa˜o Paulo. Associate Professor Thesis, University of Sa˜o Paulo, unpublished. Vance, D., Burton, K., 1999. Neodymium isotopes in planktonic foraminifera: a record of the response of continental weathering and ocean circulation rates to climate changes. Earth and Planetary Science Letters 173, 365–379.
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Viana, A.R., 2002. Seismic expression of shallow- to deep-water contourites along the south-eastern Brazilian margin. Marine Geophysical Researches 22, 509–521. Viana, A.R., Fauge`res, J.C., Kowsmann, R.O., Lima, J.A.M., Caddah, L.F.G., Rizzo, J.G., 1998. Hydrology, morphology and sedimentology of the Campos continental margin, offshore Brazil. Sedimentary Geology 115, 133–157. Vroon, P.Z., Vanbergen, M.J., Klaver, G.J., White, W.M., 1995. Strontium, neodymium, and lead isotopic and traceelement signatures of the east Indonesian sediments— provenance and implications for Banda arc magma genesis. Geochimica et Cosmochimica Acta 59, 2573–2598. Zembruscki, S.G., 1979. Geomorfologia da margem continental sul brasileira e das bacias oceaˆnicas adjacentes. In: Chaves, H.A.F. (Ed.), Geomorfologia da margem continental brasileira e a´reas oceaˆnicas adjacentes (relato´rio final). PETROBRA´S–DNPM-CPRM-DHN-CNPq, Rio de Janeiro, Brazil, pp. 129–177.
Continental Shelf Research 24 (2004) 1685–1697 www.elsevier.com/locate/csr
Hydrodynamically driven patterns of recent sedimentation in the shelf and upper slope off Southeast Brazil Michel Michaelovitch de Mahiquesa,, Moyse´s Gonsalez Tesslera, Aurea Maria Ciottia, Ilson Carlos Almeida da Silveiraa, Silvia Helena de Mello e Sousaa, Rubens Cesar Lopes Figueirab, Colombo Celso Gaeta Tassinaric, Valdenir Veronese Furtadoa, Raquel Fernanda Passosa,1 Institute of Oceanography of the University of Sa˜o Paulo, 05508-900 Prac- a do Oceanogra´fico 191, Sa˜o Paulo, SP Brazil b University Cruzeiro do Sul, Sa˜o Paulo, 05508-900 Prac- a do Oceanogra´fico 191, Sa˜o Paulo, SP Brazil c Institute of Geosciences of the University of Sa˜o Paulo, 05508-900 Prac- a do Oceanogra´fico 191, Sa˜o Paulo, SP Brazil
a
Received 10 April 2003; accepted 28 May 2004 Available online 23 August 2004
Abstract Sedimentological parameters of 206 box-core tops and 387 van Veen grab samples were analysed so as to understand the distribution, source and sedimentation rates in the shelf and upper slope of the southern Southeastern Brazilian margin. Most of the parameters (bulk organic matter constituents, d13C, and eNd) can be grouped on northern and southern sectors, separated by a sharp boundary. This suggests that sediment sources and sedimentary processes are different in both regions. The southern sector of the study area is more influenced by cold waters coming from the southern portion of the South American shelf and the organic fraction revealed an important role played by the primary productivity in the sedimentation. Also, eNd values indicate that part of the inorganic fraction of the southern sector sediments are allochtonous, probably coming from younger rocks of the Andean chain probably reaching the shelf through the La Plata river runoff. In the northern part of the study area the sedimentation is controlled almost exclusively by the meandering of Brazil Current. Compositional and isotope bulk organic parameters showed a more complex mixing of terrigenous and pelagic fractions. Also, eNd values are associated with the precambrian rocks of the Brazilian shield. The whole area exhibits sedimentation rates, varying from 5 to 660 mm kyr1. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sediment distribution; Sediment transport; Shelf dynamics; Ocean circulation; South atlantic; Brazil; Brazil current
Corresponding author. Institute of Oceanography of the University of Sa˜o Paulo, 05508-900 Prac- a do Oceanogra´fico 191, Sa˜o Paulo SP Brazil. E-mail address: [email protected] (M.M. de Mahiques). 1 Scholarship from the Fundac- a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo.
0278-4343/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2004.05.013
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1. Introduction Sedimentary processes on passive margins with limited fluvial discharges are mainly controlled by the displacement of water masses and currents, primary production rates, climate in the adjacent emerged lands and sea-level changes. All of these factors control the input and redistribution of the different sediment components. The lack of a dominant process leads to a multi-proxy approach for the characterization of the sources, transport and fate of the surface sediments (Stein, 1991; Meyers, 1994, 1997; Madureira et al., 1997). The characteristics of the surface sediments of the Brazilian margin and, more particularly of its southeastern portion, were intensively studied during the 1970s (Milliman, 1975, 1976, 1978; Rocha et al., 1975). Based on grain-size, calcium carbonate, bulk organic matter, light and heavy minerals and clay mineralogy analyses of a large number of grab samples, the southern and southeastern (SE) Brazilian inner and middle shelf modern sediments were classified as either lagoonal or beach relict sediments, bordered by a shelf-break carbonate facies. The main objective of this paper is to present an updated interpretation of sediment distribution and fate on the SE Brazilian upper continental margin as well as to better understand the role played by the ocean circulation, specially the Brazil Current, in the sedimentary regime of the area. Also, this paper represents a first attempt to characterize spatial distribution of sediments in relation to biological production in the area.
2. Study area The study area corresponds to the arc-shaped sector of the SE Brazilian upper continental margin, also known as Sa˜o Paulo Bight (Zembruscki, 1979), extending from Cabo de Santa Marta (281300 S–491000 W) to Cabo Frio (231000 S–421000 W). The continental shelf width varies from 73 to 231 km and its declivity ranges from 1:656 to 1:1333. The shelf break is located between 120 and 180 m depth. The isobaths approach the coast towards the north, the shelf close to Cabo Frio is narrower (Fig. 1).
The geological evolution of the Sa˜o Paulo Bight has its origin in the development of the Atlantic type margins, and is marked by the subsidence of the Santos Basin (Cretaceous to Recent) (Pereira and Feijo´, 1994; Meisling et al., 2001), by intensive Mesozoic alkaline magmatism and by the uplifting of the Serra do Mar range (Almeida and Carneiro, 1998). The alignment of the Serra do Mar in relation to the present coastline is reflected in the small width of the coastal plains and in the arrangement of the isobaths. As a consequence of Serra do Mar rising in Southeast Brazil, most of the drainage systems run towards the countryside, feeding the Parana´—La Plata river basins; only small rivers run directly towards the sea, draining mainly precambrian granites, gneisses and migmatites. During the Quaternary, the morphotectonic control on sedimentation was overprinted by the transgressive/regressive events related to the sealevel changes, mainly the dissecation and submersion of the shelf during the Last Glacial Cycle. On the Brazilian margin, Cabo Frio marks the transition between the tropical environment (to the north) and the subtropical (to the south) (Rocha et al., 1975). This transition has been pointed out by the authors as having been caused by different oceanographic conditions as well as distinct sedimentary dynamics, which produce mainly carbonate northward and terrigenous southward of Cabo Frio. Despite a relatively good coverage of bottom surface samples, the knowledge of the sedimentation on the SE Brazilian margin is mainly based on the grainsize and calcium carbonate content (Martins and Correa, 1996). In the study area sedimentation is essentially terrigenous, being comprised of quartzose sands and muds. Southward of Sa˜o Sebastia˜o Island (241300 S) there is a gradual increase of the mud content towards the deeper portions of the shelf. The northern part of the Bight is characterized by heterogeneity of grain-size, with significant deposition of mud on the inner shelf. The dichotomy between southern and northern sectors is also present in the clay mineralogy, with montmorillonite being predominant in the South and the kaollinite–illite–montmorillonite mixture prevailing in the North (Rocha et al., 1975). Some patches and belts of carbonate gravels are found
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Fig. 1. Location of the study area.
between the 100-m isobath and the shelf break. Most of the present sedimentary cover is said to be the result of the reworking of regressive sediments (Milliman, 1975; Rocha et al., 1975).
The inner and middle shelf dynamics are determined by the displacement of three water masses, which present strong seasonal variation (Castro et al., 1987). Between November and
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March, the South Atlantic Central Water (SACW) moves close to the bottom toward the coast leading to the oceanward displacement of the less dense Coastal Water (CW) and keeping the Tropical Water (TW) relatively distant from the coastline. This period also corresponds to the rainy season in SE Brazil and, thus, to the increase of the terrigenous flux to the coastal waters. The displacement of the CW is the most important factor in the transport of terrigenous organic matter towards the deeper areas of the shelf. From March to November, the retreat of the SACW leads to a greater influence of the TW in the shelf processes (Mahiques et al., 1999). Northward to Cabo Frio, the outer shelf and slope hydrodynamics and its role on the sedimentation has already been extensively described in several papers (Viana et al., 1998; Viana, 2002). In the study area the outer shelf and upper slope the flux is controlled by the southward displacement of the Brazil Current (BC), which shows a meandering pattern throughout its path. Meandering of (BC) and consequent eddy formation seems to be the factor responsible for geographic changes in the sedimentation rates in the area between Sa˜o Sebastia˜o Island and Cabo Frio (Mahiques et al., 2002). Northward to the study area, several papers have already emphasized the relationship between the circulation pattern and sedimentary processes.
3. Methods A total of 206 box-core tops and 387 Petersen grab samples, collected between 1992 and 1998 by the R.V. ‘‘Prof.W.Besnard’’ were included in this study (Fig. 1). For box-core samples, only the topmost 2 cm were considered for most of the analyses. Sediment for radiocarbon datings and calculations of sedimentation rates was collected at 8 cm. We assumed that the surface at the bottom was modern which was guaranteed by the presence of 210 Pbunsupported in all of the samples (Tessler, 2001). Sedimentation rate calculations were estimated using Dep-Age Software, developed by Dr. Louis Maher Jr., from the US Geological Survey.
Organic carbon, total nitrogen and sulphur contents were determined in an LECO CNS2000 Analyser. Approximately 500 mg of dry sediment from each of the samples was treated with 1 N HCl in order to remove calcium carbonate, freeze-dried and then analysed. LECO sulphametazin standards and blanks were analysed as controls for each set of 30 samples. Calcium carbonate was determined by the difference in weight of the sediment prior to and after acidification of 2 g of each sample with 1 N HCl. Values are reported as percentages of dry weight. Values of d13C (% PDB) and d15N (% air) of the sedimentary organic content were obtained by mass spectrometry at the Coastal Science Laboratories (Austin, TX, USA). C and N isotope analyses were performed by on line combustion with a Fisons NA1500 Elemental Analyser attached to a VG SIRA Model 10 mass spectrometer. Carbon was calibrated using NBS 22 oil standard (defined as 29.60% relative to PDB). Reproducibility for carbon was better than 0.2%. Nitrogen was calibrated using IAEA N-1, N-2 and N-3 (defined as+0.5%,+20.3% and +4.6%, respectively, relative to the Air). Reproducibility for nitrogen was better than 0.3%. Analyses of neodymium isotope, referred to as eNd, were carried out at the Geochronological Research Center of University of Sa˜o Paulo, Brazil. The Sm–Nd analyses were prepared by standard methods according to the analytical procedures described by Sato et al. (1995), involving HF-HNO3 dissolution plus HCl cation exchange, using a Teflon Powder column to separate ETR. No visible solid residues were observed after dissolution. Samples with incomplete dissolution were discarded. Nd ratios were normalized to a value of 146Nd/144Nd=.72190. The averages of 143Nd/144Nd for La Jolla and BCR-1 standards were .5118477.00005 (2s) and .5126627.00005 (2s), respectively. The blanks were less than .03 ng. Nd isotopic analyses were carried out with a multicollector Finnigan-MAT 262 mass spectrometer. SeaWiFS images were obtained from the NASA GSFC’s Distributed Active Archive Center (DAAC) acquired as Level 1A (Nadir Resolution
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5 9.82 9.71 0.85 10.73 8.54 34 34 20.4 6.3 20.3 6.3 0.8 0.7 19.2 5.0 22.6 8.1 106 0.4 0.2 0.6 0.04 5.66 106 0.8 0.6 0.6 0.01 2.07 106 6.7 5.3 4.9 0.50 18.87 363 16.3 14.7 13.2 1.5 91.9 351 351 70.9 3.82 85.6 3.45 32.3 1.32 0.1 0.30 100.0 7.65 Southern sector Number of samples 7 Mean 57.7 Median 43.0 Standard deviation 36.1 Minimum 29 Maximum 134
351 1.08 0.90 0.66 0.18 3.01
41 41 21.0 6.1 20.9 7.6 0.5 0.7 22.1 4.7 20.0 7.6 73 0.5 0.3 0.5 0.01 2.10 73 0.9 0.7 0.5 0.02 2.28 73 7.3 6.6 4.4 0.41 17.35 309 31.9 22.9 23.6 1.2 94.0 309 309 60.8 4.04 68.2 3.85 30.8 1.55 0.5 0.26 100.0 7.63
309 1.37 1.48 0.56 0.25 3.00
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Northern sector Number of samples 14 Mean 152.8 Median 60 Standard deviation 199.9 Minimum 5 Maximum 660
Due to the heterogeneous distribution of samples, this work will only emphasize the larger scales pattern of sediment distribution. In this sense almost all the parameters present a conspicuous dichotomy between the sectors northward and southward of Sa˜o Sebastia˜o island. Table 1 presents the statistics for each parameter in both sectors. Grain-size mean diameter distribution is presented on Fig. 2. The 100-m isobath controls the deposition of mud throughout the shelf; two large elongated zones of muddy sediment are found on the outer shelf separated by a sandy belt that extends towards the slope between longitudes 451W and 461W. Coarser sediments, described as relict sediments (Mahiques, 1987), are found on the inner shelf between longitudes 431300 W and 441 W. Textural heterogeneity is attested by the high values of sorting which predominate thoughout the shelf and upper slope as observed in Table 1. The calcium carbonate distribution (Fig. 3) shows an area of conspicuous carbonatic sedimentation between the 100 and 200-m isobaths and longitudes 431300 W and 451300 W. In general the remaining part of the study area presents lithoclastic sediments, according to the classification of Larsonneur et al. (1982). There are three main areas of higher organic carbon content (Fig. 4). Higher values are found in the southern sector, along the 100-m isobath between longitudes 451000 W and 471300 W. The two other regions of higher carbon content are the Cabo Frio upwelling zone and the vicinity of Sa˜o Sebastia˜o island. The C/N ratio pattern (Fig. 5) indicates one major area as source of terrigenous input corresponding to the coast southward to
Table 1 Statistics of northern and southern sectors of the study area obtained for each of the sedimentary parameters
4. Results
Sand Mean Sorting Calcium Organic Total Total d13C Sedimentation carbonate (%) carbon (mg g1) nitrogen (mg g1) sulphur (mg g1) rate (mm kyr1) (%) diameter (f) (j)
d15N eNd
of 1.1 km). Chlorophyll-a concentration (mg m3) was estimated using the standard Seadas 4.3 OC2 algorithm (for details see http://seadas.gsfc.nasa.gov). Standard masks and daily meteorological and ozone data (also obtained from DAAC) were used for the atmospheric correction. Images were remapped to the Mercator map projection.
14 13.27 12.75 1.87 17.13 9.91
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Fig. 2. Distribution of the mean diameter (f) in the surface sediments of the study area.
251000 S. A secondary area is located in the vicinities of the Sa˜o Sebastia˜o and extents up to the outer shelf/upper slope, between 251000 S and 26o000 S and 441000 W and 451000 W. The distribution of d13C in the organic fraction (Fig. 6) indicates once more the clear differentiation between the northern and southern sectors of the upper margin of SE Brazil. In general, the sediments of the southern sector show values higher than 21.0% (PDB) with a nucleus of values above 19.5% (PDB) along the 100-m isobath. Lighter isotopes are found as a projection towards deeper areas on the coast off the Sa˜o Sebastia˜o area. d15N distribution shows a rather complex pattern, the northern and southern complexes being divided by a strip with values of more than 6.0% (air). The distribution of eNd (Fig. 7) shows that the values northward from Sa˜o Sebastia˜o island range from 17.1 to 9.9; actually, 10 of the 12 samples of this sector present values below 12. On the other hand, all of the samples from the sector
southward from Sa˜o Sebastia˜o island give values of more than 11. The large distribution of eNd values around the 241S parallel is due to the orientation of the shelf in this area which is, in fact, SW–NE. Sedimentation rates varied from 5 to 660 mm kyr1 (Table 1), with lower sedimentation rates found associated with the shelf break between longitudes 421W and 441300 W. Higher rates are found off Cabo Frio, close to the 100-m isobath northward from Sa˜o Sebastia˜o Island and throughout the shelf southward to the 25th parallel. Fig. 8 presents SeaWiFS ocean colour images of estimated chlorophyl concentration. These images provide synoptical observation of biological processes, which are closely related to the circulation patterns. The seasonal examples illustrate a clear north–south gradient on phytoplankton abundance, which correlates positively with the organic carbon content and the zones of higher sedimentation rates.
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Fig. 3. Distribution of calcium carbonate (%) in the surface sediments of the study area.
Fig. 4. Distribution of organic carbon (mg g1 dry weight) in the surface sediments of the study area.
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Fig. 5. Distribution of the C/N ratio (weight) in the surface sediments of the study area.
Fig. 6. Distribution of the d13C values in the surface sediments of the study area.
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and southern, separated by a sharp boundary geographically associated to the Sa˜o Sebastia˜o island. In the southern sector, there is a clear trend of fining sediments towards offshore, with increasing amounts of silt and clay beyond the 100-m isobath. In general, sediments in the area can be classified either as lithoclastic or biolithoclastic, according to the classification of Larsonneur et al. (1982). The amount of organic matter follows a similar trend to that of grain-size. Stable isotope and C/N ratio values are indicative of pelagic organic matter. eNd values are always higher than 11. On the other hand, the sedimentation in the northern sector shows a more complex scenario, with the presence of a bioclastic belt, between 100 and 200 m which corresponds to a relict sediment facies. Also, the distribution of grain-size, organic matter and stable isotopes shows a patchy configuration. Most of the eNd values are lower than 12. 5.2. Source of the organic fraction
Fig. 7. Latitudinal (A) and longitudinal (B) variations in eNd in the study area. The vertical lines in the middle of the graphs indicate the position of Sa˜o Sebastia˜o Island.
5. Discussion 5.1. Sediment distribution Sediment distribution in the southeeastrn Brazilian continental shelf and upper slope between Cabo Frio and Cabo de Santa Marta is marked by the occurrence of two distinct sectors, northern
The differentiation in two sectors of the shelf is also represented by the deposition of a more pelagic organic matter in the south, where the primary productivity is more effective, in contrast to a mixed organic matter in the northern sector. The prevalence of high productivity conditions through time in the southern portion of the study area has been previously confirmed by high values of nitrate, and surface and subsurface phytoplanktonic biomass (Gaeta, 1999). The penetration of cold waters coming from the south in latitudes as low as 231S have already been reported elsewhere. Based on satellite images, Campos et al. (1996) have attributed the origin of the cold waters to the Brazil–Malvinas confluence (351S), or to further south, on the Argentinean shelf. In Sepetiba Bay, a 300 km2 area enclosed water body located at 231S, the presence of the benthic foraminifera Bucella peruviana and Quinqueloculina seminulum, the planktonic foraminifera Globigerina bulloides, together with ostracodes and micro bivalves typical of the Uruguayan and Argentinian shelves, were
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Fig. 8. Estimated surface chlorophyll concentration (mg m3) using OC2v4 SeaWIFS algorithm. Figures are high resolution (1 km) composites of 4 to 7 days representing (A) Spring, (B) Summer, (C) Autumn and (D) Winter of 2001 (Southern Hemisphere).
used by Stevenson et al. (1998) to suggest the penetration of an extension of the Malvinas Current towards the SE Brazilian shelf. Hydrodynamically it seems very unlikely that the Malvinas waters can reach the middle and inner portions of the shelf because of its barotropic character. More recently, Piola et al. (2000) built a hydrographic climatology of the South American shelf between 241S and 401S. Their results revealed that shelf dynamics drive coastal water northward during the winter season. This coastal water is mainly of subtropical region associated with the spreading of low-salinity water from the La Plata
river and the contribution of the Patos-Mirim Lagoonal system (located between 301S and 341S). The low temperature signal of such a flow is due to winter cooling of waters confined above the sharp halocline imposed by the northward movement of low-salinity plume. Therefore, we tend to assume that the organisms from the most southern portions of the South American shelf are incorporated in and transported northward by the low-salinity plume described by Piola et al. (2000). On the other hand, with exception of the Cabo Frio region, all of the northern sector of the study area shows oligotrophic conditions. In the northern
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sector, warm oligotrophic waters of the BC flow over the upper slope and outer shelf, with occasional penetrations of anticyclonic meanders onto the middle shelf. Higher productivity zones are then either found on the inner shelf or in areas where the BC meandering may also enhance the coastal winddriven upwelling such as that off Cabo Frio. 5.3. Source of the inorganic fraction The neodymium isotope ratio is considered one of the best proxies for source rocks of marine sediments and paleocirculation (Vroon et al., 1995; Abouchami et al., 1997, 1999; Vance and Burton, 1999; Rutberg et al., 2000). A comparison of our eNd data with those available for seawater, Mnnodules, red clays and metalliferous sediments from different oceans (Faure, 1986; DePaolo, 1988) shows a good correspondence of the northern sector samples with the Atlantic-type values. On the other hand, the sediments from the southern sector seem to show contribution from allochtonous sources. Assuming, then, the existence of a south-originated flow covering the southern sector of the study area we may consider that sediment from the upper south-eastern-most Brazilian margin is influenced by the La Plata drainage system, which delivers sediments derived from the younger Andean rocks to the SW Atlantic Ocean, agreeing with the hydrographic data presented in Piola et al. (2000). 5.4. Sedimentation rate and sedimentary processes The whole area presents sedimentation rates, ranging from 5 to 660 mm kyr1. The visual comparison between our data and the SeaWiFS images suggests that areas with lower sedimentation rates are associated with the main flux of the BC, in the northern sector of the study area. Also, a fairly close correlation is observed between the zones of higher sedimentation rates with the higher values of carbon, indicating that the sedimentation in the SE Brazil upper margin is controlled by the pelagic processes associated with the northward propagation of south-originated cold waters as well as with the upwelling processes related to the BC meandering as observed in the Cabo Frio region.
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6. Summary and conclusions The present sedimentation in the upper continental margin off Southeast Brazil is strongly dominated by the oceanic water mass dynamics and shelf circulation due to the lack of any considerable river input. These hydrodynamical processes control the primary productivity and sediment redistribution in the area, leading to the establishment of the differences in sedimentation rates and sedimentary facies observed in this work. The sedimentation rates vary from 5 to 660 mm kyr1 and are strongly correlated with the areas of higher primary productivity and/or of terrigenous input as well as with the flow of the BC. The area off Sa˜o Sebastia˜o Island marks the boundary between the two main sedimentary zones, which are characterized by differences both in the organic and inorganic fractions of the sediments. Southward from the island, the deposition processes are associated with the seasonal penetration of low-salinity, low-temperature plume of shelf water which has been recently related to the La Plata river runoff; its interaction with the meanders of the BC as well as with the shelf morphology favours the establishment of high productivity zones, very clearly identified by the bulk organic matter characteristics. The inorganic fraction represented by the eNd parameter suggests an as yet unidentified allochtonous origin for the sediments. Northward of Sa˜o Sebastia˜o Island, the relative heterogeneity of sediments indicate more complex hydrodynamics, especially on the middle and inner shelves. The inner shelf organic fraction distribution is linked to the Cabo Frio coastal upwelling as well as to other coastal processes. The middle and outer shelves sediment distribution is a result of a more vigorous meandering of the Brazil Current in the area.
Acknowledgments The authors are indebted to Professor Donn S. Gorsline (University of South California, USA), Professor Jean-Claude Fauge`res (Universite´ Bordeaux I, France), Professor Joa˜o Manoel
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Alveirinho Dias (Universidade do Algarve, Portugal) and Professor Alberto Garcia de Figueiredo (Universidade Federal Fluminense, Brazil) for the revision and improvement of the manuscript. The authors also wish to express their thanks to Messrs. Marcelo Rodrigues and Edilson Faria and Mrs. Samara Cazzoli y Goya, for the execution of the grain-size, calcium carbonate and bulk organic matter analyses, and to the staff of the Geocronological Research Centre of the University of Sa˜o Paulo, for the Neodymium isotope analyses. This work was financed by the Fundac- a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) via grant no. 01/01098-7, and by the Conselho Nacional de Desenvolvimento Cientı´ fico e Tecnolo´gico (CNPq) via grant no. 300650-9. This paper is a contribution to the IGCP464 (Continental Shelves during the Last Glacial Cycle) Project and to the Carbon, Hydrology and Global Environmental Systems (CHANGES) Programme.
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