Research papers

Hydrodynamic controls on the distribution of surface sediments fromthe southeast South American continental shelf between 231S and 381S

R.H. Nagai a,n, P.A.L. Ferreira a, S. Mulkherjee b,1, M.V. Martins c,d, R.C.L. Figueira a, S.H.M. Sousa a,M.M. Mahiques a

a Instituto Oceanográfico, Universidade de São Paulo, Praça do Oceanográfico, 191, 05508-120, Cidade Universitária, São Paulo/SP, Brazilb Geological Survey of India, 172 Kid Street, Kolkata, Indiac GeoBioTec, Departamento de Geociências, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugald CESAM, Departamento de Geociências, Universidade de Aveiro, Portugal

a r t i c l e i n f o

Article history:Received 17 January 2013Received in revised form10 September 2013Accepted 16 September 2013

Keywords:SE South American shelfMineralogyRadionuclideRio de la PlataBrazil Current

a b s t r a c t

This study aims to explore the hydrodynamic factors that influence the depositional processes on theupper continental margin of southeastern South America. Mineralogy and the activities of natural (40K)and artificial (137Cs) radionuclides from surface sediment samples were used to evaluate the provenanceand oceanographic controls on depositional processes. High values of the smectite/illite (S/I) ratioindicate that the Rio de la Plata (RdlP) is a major source of terrigenous sediments to the continental shelfbetween 381S and 271S and that its importance decreases towards lower latitudes. This is alsocorroborated by the high radionuclide 40-K activity found in muddy sediments. The kaolinite/illite(K/I) ratio highlights the interplay between two hydrodynamic controls in the sediment distribution ofthe study area: (i) the Brazil Current, which supplies the shelf mostly in the northern portion of the studyarea with kaolinite-enriched clay minerals from tropical regions, and (ii) the Brazil Coastal Current,which transports the clay minerals, mainly illite and smectite, from the RdlP to the southern portion ofthe study area (up to 271S). This is also corroborated by high 137-Cs activities. In the deeper parts(4300 m) of the study area, high calcite content indicates the progressive decrease in terrigenous inputtowards the outer shelf and upper slope.

& 2013 Published by Elsevier Ltd.

1. Introduction

The systematic study of the southern Brazilian shelf beganin the 1960s with the bathymetric, seismic and sedimentologicalstudies of the REMAC Project (Global Reconnaissance of the BrazilianContinental Margin). These studies still represent the only availablesource of information about the geology and geophysics of some areasof the Brazilian margin (see Milliman and Barreto, 1975 and relatedpapers). In these studies, the surface sediments of the southernBrazilian shelf were considered to be a product of reworked particlesthat were deposited during sea level lowstands due to the absence ofsignificant fluvial input between 221S and 341S, which limited theinput of modern terrigenous sediments.

Over the last decade, sedimentological (Mahiques et al., 2004;Campos et al., 2008), geochemical (Mahiques et al., 1999, 2004,2008), and micropalaeontological (Eichler et al., 2008; Burone

et al., 2010) studies of the southern Brazilian upper margin, someof which include the northern Argentinean and Uruguayan shelf,have been published. These studies have allowed a re-evaluationof the sedimentary processes in the area, especially those relatedto the role of the RdlP as a modern source of sediments to thesouthwestern Atlantic margin.

In a recent study, Campos et al. (2008) analyzed the mineralogyof the surface sediments between 281S and 381S, focusing mainlyon the clay versus non-clay constituents and the smectite con-tents. These authors suggest that the Subtropical Shelf Front (STSF)is effective in blocking the transport of coarse sediments originat-ing from the RdlP towards the Brazilian shelf, but this blockage iseven less effective for clay sediments.

Mahiques et al. (2008) used Pb and Nd isotopes of surfacesediments to identify the present relative contribution of sedi-ments derived from the RldP to at least 271S, thereby confirmingthe effectiveness of this large river as a source of terrigenoussediments for the modern sedimentary processes on the southernBrazilian shelf.

Clay mineralogy is useful for inferring the distribution, sources,and dispersal routes of fine-grained sediments (e.g., Hein et al.,2003; Griggs and Hein, 1980; Karlin, 1980; Park and Khim, 1990),

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/csr

Continental Shelf Research

0278-4343/$ - see front matter & 2013 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.csr.2013.09.016

n Correspondence to: IOUSP Praça do Oceanográfico, 191 (sala 171A), 05508-120,Cidade Universitária, São Paulo/SP, Brazil. Tel.: þ55 11 3091 6655.

E-mail addresses: renatanagai@usp.br, renatanagai@gmail.com (R.H. Nagai).1 Presently, Guest Faculty, Jadavpur University, Kolkata 700032, India.

Please cite this article as: Nagai, R.H., et al., Hydrodynamic controls on the distribution of surface sediments from the southeast SouthAmerican continental shelf between 231S and 381S. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.09.016i

Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎

but also the dispersal patterns or transport pathways of the bulksediment (Naidu et al., 1995; Petschick et al., 1996; Oliveira et al.,2002). However, the presence of multiple sources and transportprocesses complicates the assessment of the main source area of agiven clay mineral assemblage (Fagel, 2007). The combination ofclay mineralogy and radionuclide measurements, such as 40K and137Cs, places further constraints on the identification of sedimentsource area (Meijer and Donoghue, 1995; Ligero et al., 2001;Yeager and Santschi, 2003; Carvalho et al., 2011).

The present study characterizes the mineralogy and radiogenicactivity of surface sediments from the southeastern South Americaupper continental margin, between Cabo Frio (Brazil) and themouth of the RldP (Argentina), extending the analysis previouslypublished by Campos et al. (2008) to the north by incorporatingdata from between 231S and 281S and by including novel radio-activity data. Furthermore, it explores and discusses the hydro-dynamic factors that influence modern depositional processes inthe study area.

2. Study area

The southeastern South American continental margin, with oneof the largest and widest shelves in the world, is a typical exampleof a siliciclastic passive margin (Fig. 1).

In the northern part of the SE South American continentalmargin, between 221S and 351S, middle and outer shelves, as wellas on the upper slope, circulation is dominated by the BrazilCurrent (BC), which flows southward and meanders around the200 m isobaths (Souza and Robinson, 2004). While the inner shelfis mainly influenced by the northward flow of the Brazil Coastal

Current (BCC) which transports low-salinity waters derived fromthe RdlP (Möller et al., 2008; Souza and Robinson, 2004). The BCCalso transports sediments from the RdlP (Mahiques et al., 2008),and controls seasonal variation in the primary productivity of thearea (Ciotti et al., 1995). In the vicinity of 331S a strong thermoha-line front at depths higher than 50 m marks the location of theSubtropical Shelf Front (STSF – Piola et al., 2000, 2008). South ofthe STSF the water column below 50 m is dominated by Sub-Antarctic Shelf Water while north of the front the dominant watermass is the Subtropical Shelf Water.

Modern sediment distribution exhibits latitudinal changes andbathymetric control (Fig. 1). According to Mahiques et al. (2004), inthe SE Brazilian shelf the São Sebastião Island marks a boundarybetween two main sedimentary zones; north of 241S, surfacesediments distribution is relatively more complex and consists ofa patchy sedimentary composite. On the south of São SebastiãoIsland, a clear trend in fine sediments towards offshore, withincreased amounts of silt and clay beyond the 100 m isobaths isobserved (Mahiques et al., 2004).

Between 281 and 361S sand and gravel accounts for more than50% of the surface sediments of the inner shelf. On the middleshelf silts and clays are predominant between the 50 and 100 misobaths. Sand and gravel contents increase on the outer shelf, andin some places, they represent more than 75% of the sediment(Mahiques et al., 2004; Urien and Ewing, 1974).

The inner RdlP Estuary is covered with sands and its middlesector with very fine sediments (Martins et al., 2003; Violanteand Parker, 2004; Urien and Ewing, 1974). In the outer estuary,the deposits coarsen again to sands. Around 65% of the continentalshelf of the southern sector (4361S) is covered with fine sands(Urien and Ewing, 1974; Parker et al.,1997) and minor contents

Fig. 1. Modern clay mineralogy provinces (upper right panel) and the ternary concentration diagram (lower panel) featuring the definition of the clay–mineral assemblages,according to the proportions of the three terrigenous clay mineral groups kaolinite, chlorite and illite (modified from: Diekmann et al., 2004). Upper left panel shows in detailthe location map of the study area showing spatial distribution of sand, mud and gravel (modified from: Figueiredo and Tessler, 2004 – from 221 to 281S – and Martins et al.,2003 – from 301 to 381S) and sampling sites for mineralogy (circles – this study dataset; pentagons – Campos et al. (2008) dataset) and for radioactivity analysis (crosses andsquares).

R.H. Nagai et al. / Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎2

Please cite this article as: Nagai, R.H., et al., Hydrodynamic controls on the distribution of surface sediments from the southeast SouthAmerican continental shelf between 231S and 381S. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.09.016i

of very fine and medium sand are observed in the vicinity of theRldP Estuary.

According to Parker et al. (1997), the Argentinian shelf is mainlysiliciclastic and is covered by relict sediments that were depositedin littoral, barrier, and estuarine environments and reworkedduring the several Cenozoic transgressive–regressive events.Approximately 65% of the Argentinian shelf is covered by sands,which mainly consist of well-sorted fine sands with subsidiaryvery fine and medium fractions and variable gravel and mudcontent. Mud consisting of clayey–silts that is sometimes resus-pended covers 8% on the inner shelf off the RdlP estuary mouth.

The mineralogical characteristics of the Argentinian shelf hasbeen studied by several authors (Etchichury and Remiro, 1960,1963; Cortelezzi et al., 1971; Gelós and Chaar, 1988; Campos et al.,2008), all of whom agree on the influence of pyroclastic andvolcanic sediments on the sedimentary processes. The influence ofbasalts of the Paraná Magmatic Province as a potential source rock,especially on the northern Argentinean shelf, has been empha-sized by Campos et al. (2008) and Mahiques et al. (2008).

In terms of radionuclide activity, the S/SE Brazilian shelf haslow background of artificial radionuclide (137Cs) levels due toglobal fallout in the 1960s (Figueira et al., 2006; Saito et al.,2001), which has been estimated between 0.4 and 1.8 Bq kg�1

(Cunha et al., 1999), and the 40K activity is approximately 1000-fold greater than the 137Cs activity (Alencar and Freitas, 2005).

3. Methods

3.1. Mineralogy of surface sediments

Two datasets were used to evaluate the mineralogy of thesurface sediments. The first set consists of 14 core surface samplesthat were collected by the R/V NoC “Prof. W. Besnard” on severalsurveys between latitudes 22156.52′S and 28124.60′S from waterdepths between 44 and 511 m (Table 1; Fig. 1).

The second dataset consists of 19 samples collected with aclamshell-type snapper by Campos et al. (2008) on board the R/VNoC “ANTARES”. Only mineralogical information common to bothdatasets was considered in the present study.

Unoriented powder mounts including sediments 44 μm of bothsets of samples were analyzed using powder X-ray diffraction (XRD).Unoriented powder mounts of each sample were acquired in an M/SSiemens model D5000 X-ray diffractometer at the Institute ofGeosciences of the University of Sao Paulo. The operating voltageand current ratings were at 40 kV and 40 mA. Step scanning wasused with a scanning rate of 0.05 s�1 and temperature ranging from3 1C to 80 1C in order to confirm the presence of smectite. To identifythe phases, reduction of diffraction data were performed with thesoftware DIFFRAC AT jointly with the database PDF2 (release 2001).For the semi-quantitative analysis, a Diffraction Function cAt basedon the height of the peaks and on the intensity I/Icorindon was used.Peak profile estimation was done using the same program tocalculate phase crystallinity. The main mineralogical phases identi-fied in the samples were quartz, calcite, feldspar, hematite, andamphiboles, which wewill here refer to as the non-clay phases (NCP;sizes 44 μm), and kaolinite, illite, and smectite, which we willreferred to as clay phases (CP; sizes o4 μm).

After identifying the clay phases (which had similar crystal-linity), quantitative estimation was done using the peak heightdata (3–4% accuracy). To distinguish the clay phases, glycolationtreatment was performed and the shift of 12–14 Å peak to higherd-value confirms smectite group of minerals (e.g., montmorilonite)and no alteration of 10 Å peak confirms illite. In addition, on XRD-scanning the sample, heated for 2 h at 500 1C, the absence of 7.18 Åpeak confirms the presence of kaolinite.

A principal components analysis (PCA) was carried out for theordination of the samples based on a correlation matrix of quartz,feldspars, calcite and clay contents, and smectite/kaolinite (S/K),smectite/illite (S/I), and kaolinite/illite (K/I) ratios. The PCA permitsthe identification of the eigenvalues and eigenvectors of thecorrelation matrix, the first yields a measure of the varianceaccounted for the corresponding eigenvectors. This analysis canbe graphically represented by two axes that explain the highestamount of variation in the data set. As the variables are measuredin different units, we chose to use the correlation to standardizethe data. The analyses were performed using the MultivariateStatistical Package (MVPS) software developed by Kovach (1999).

3.2. Radionuclide activity (40K and 137Cs) measurements

Two groups of surface sediment samples collected with a boxcorer were analyzed for the radioactivity study (Table 1; Fig. 1).The first group comprised more than 80 samples from the southernBrazilian margin that were collected by the R/V NoC “Prof.W. Besnard” between latitudes 231S and 29130′S from depths between50 and 300 m. The second group consisted of 20 samples that werecollected in the estuary of the RdlP by the R/V “Miguel Oliver”.

The 40K and 137Cs activities were determined by high-resolutiongamma-ray spectrometry with an EGandG ORTEC hyperpure Gedetector model GMX50P (resolution of 1.91 keV for the 1332.35 keV60Co peak) and coupled electronics. Certified reference materials(CRMs) from IAEA were used to evaluate the detector's efficiencyand the precision/accuracy parameters. The analysis followed that ofFigueira et al. (2006) and exhibited good precision and accuracy, withstandard deviation (RSD) and relative error (RE) values less than 5%(Table 2).

A one-way analysis of variance (one-way ANOVA) was per-formed on the 137Cs data to identify differences in its distributionin the study area. The ANOVA assumptions of normality andhomocedasticity of data residues were tested and confirmed withShapiro–Wilk and Levene's tests, respectively. As a complement tothe one-way ANOVA Tukey's LSD method was also run to assessgroup means that are significantly different from each other,comparing all possible pairs of means.

4. Results

4.1. Mineralogy of surface sediments

The abundance and spatial distribution of the mineralogicalphases identified in this study are presented in Table 1 andFigs. 2 and 3.

The non-clay phases (NCPs) are mainly composed of quartz,feldspars, calcite, and in smaller quantities hematite (only identi-fied in the Campos et al., 2008 dataset). Lower values (o50% and50–75%) of this sediment fraction were observed between PatosLagoon and 301S, whereas higher values were detected in latitudeslower than 301S and near the mouth of RdlP (Fig. 2a). Quartz wasthe main component of the NCP; it was present in all of thesamples and makes up 420% of most of the samples (Fig. 2b),followed by feldspars found in higher amounts near the mouth ofthe RdlP and near São Sebastião Island. Bioclastic calcite concen-trations between 25% and 40% were observed in outer shelf andslope samples collected in latitudes lower than 271S (Fig. 2d).

The kaolinite/illite ratio (K/I) showed a progressive increasefrom the RdlP mouth towards northern latitudes, with maximumvalues (�12) at the northernmost station (7620; Fig. 3b).

The smectite/kaolinite (S/K) ratio showed higher values (41) atlatitudes greater than 281S and in the northernmost part of the study

R.H. Nagai et al. / Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 3

Please cite this article as: Nagai, R.H., et al., Hydrodynamic controls on the distribution of surface sediments from the southeast SouthAmerican continental shelf between 231S and 381S. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.09.016i

Table 1Locations of sample sites (where, b and c stand for the sampling devices box-corer and clamshell-type snapper, respectively), XRD diffraction data and radionuclide activity measurements. oMDA¼ lower than the minimumdetectable activity; ND¼data not measured.

Sample Latitude (1S) Longitude (ºW) Depth (m) NCP (%) Clay (%) Quartz Feldspar Calcite KI ratio SK ratio 40K (Bq kg�1) 137Cs (Bq kg�1)

6573b 24.92 44.63 155 ND ND ND ND ND ND ND 360.10 1.536577b 25.26 45.08 142 ND ND ND ND ND ND ND 49.28 oMDA6595b 26.38 46.64 175 ND ND ND ND ND ND ND 176.96 2.406611b 28.41 47.36 197 48.90 5.00 16.10 0.00 32.80 1.12 0.24 ND ND6626b 24.23 43.75 205 ND ND ND ND ND ND ND 231.31 2.236627b 23.97 43.88 133 ND ND ND ND ND ND ND 301.09 1.796631b 25.77 45.48 164 ND ND ND ND ND ND ND 327.14 1.926635b 27.17 47.46 129 ND ND ND ND ND ND ND 243.98 1.276641b 26.25 46.88 130 ND ND ND ND ND ND ND 291.75 1.556643b 25.42 46.07 114 ND ND ND ND ND ND ND 517.75 0.826647b 25.68 45.38 157 ND ND ND ND ND ND ND 199.09 1.286685b 25.71 45.20 280 ND ND ND ND ND ND ND 339.44 2.256690b 27.02 46.68 285 ND ND ND ND ND ND ND 396.42 oMDA6691b 26.45 47.02 140 ND ND ND ND ND ND ND 393.67 2.956695b 26.29 46.70 153 ND ND ND ND ND ND ND 200.58 oMDA6698b 26.18 46.33 242 ND ND ND ND ND ND ND 321.12 1.446703b 25.67 46.23 133 ND ND ND ND ND ND ND 311.43 0.516706b 25.82 45.76 183 ND ND ND ND ND ND ND 206.71 1.796742b 23.99 43.16 215 ND ND ND ND ND ND ND 224.86 0.966751b 23.10 41.42 93 ND ND ND ND ND ND ND 400.50 oMDA6755b 23.08 42.02 101 ND ND ND ND ND ND ND 379.67 2.216759b 23.33 41.36 96 ND ND ND ND ND ND ND 88.44 0.276761b 23.49 41.16 284 ND ND ND ND ND ND ND 151.27 1.396762b 23.44 41.23 126 ND ND ND ND ND ND ND 101.87 oMDA6763b 23.13 41.02 101 17.50 7.00 3.10 0.00 14.40 0.08 0.18 ND ND6765b 23.16 40.95 247 ND ND ND ND ND ND ND 67.91 0.596776b 26.79 47.58 100 ND ND ND ND ND ND ND 539.55 1.896779b 26.83 46.76 200 ND ND ND ND ND ND ND 259.81 2.196780b 27.12 47.74 102 ND ND ND ND ND ND ND 503.09 1.206793b 27.78 47.67 138 ND ND ND ND ND ND ND 326.44 1.256794b 28.08 48.10 100 ND ND ND ND ND ND ND 526.05 2.376807b 28.89 47.80 225 ND ND ND ND ND ND ND 256.51 0.696808b 28.81 48.00 141 ND ND ND ND ND ND ND 137.18 0.276809b 28.69 48.30 104 ND ND ND ND ND ND ND 425.43 oMDA6817b 29.48 48.15 210 ND ND ND ND ND ND ND 263.80 1.556945b 24.01 46.35 10 ND ND ND ND ND ND ND 478.14 2.666947b 24.03 46.36 9 ND ND ND ND ND ND ND 399.73 1.166948b 24.00 46.34 8 ND ND ND ND ND ND ND 535.66 2.126949b 23.05 42.01 84 ND ND ND ND ND ND ND 335.01 2.006950b 23.07 41.98 94 ND ND ND ND ND ND ND 342.00 1.406951b 23.01 41.97 47 ND ND ND ND ND ND ND 334.20 2.756953b 23.03 41.54 73 ND ND ND ND ND ND ND 390.59 0.616954b 23.05 41.95 87 ND ND ND ND ND ND ND 363.49 0.926956b 23.15 41.90 106 ND ND ND ND ND ND ND 363.44 0.757222b 23.22 44.45 29 ND ND ND ND ND ND ND 350.23 1.247225b 23.36 44.33 34 ND ND ND ND ND ND ND 185.67 1.917230b 23.55 44.22 68 ND ND ND ND ND ND ND 361.92 1.347231b 23.53 44.60 53 ND ND ND ND ND ND ND 296.19 oMDA7232b 23.50 44.92 24 ND ND ND ND ND ND ND 589.09 1.617233b 23.44 45.00 8 ND ND ND ND ND ND ND 214.63 0.837234b 23.63 44.67 58 ND ND ND ND ND ND ND 323.27 0.867235b 23.67 45.33 8 ND ND ND ND ND ND ND 387.18 0.997236b 23.72 45.30 8 ND ND ND ND ND ND ND 516.53 1.097237b 23.91 45.21 8 ND ND ND ND ND ND ND 394.62 2.96

R.H.N

agaiet

al./Continental

ShelfResearch

∎(∎∎∎∎)

∎∎∎–∎∎∎

4

Pleasecite

this

articleas:

Nagai,R

.H.,et

al.,Hyd

rodyn

amic

controls

onthedistribu

tionof

surface

sedim

ents

fromthesou

theast

South

American

contin

ental

shelf

between

231S

and38

1S.Con

tinen

talSh

elfResearch

(2013),http

://dx.d

oi.org/10.1016/j.csr.2013.09.016i

7605b 27.10 47.80 93 ND ND ND ND ND ND ND 547.34 1.187606b 26.99 48.08 60 66.40 20.00 47.30 15.90 3.20 1.25 0.38 492.70 2.187607b 27.37 47.14 287 65.50 17.00 28.40 13.20 23.90 1.19 0.78 329.41 0.377608b 25.94 46.14 179 64.00 5.00 12.00 13.00 39.00 1.53 0.89 194.92 1.567609b 26.02 46.04 308 83.30 7.00 51.00 9.30 23.00 1.60 0.35 ND ND7610b 25.51 46.63 89 ND ND ND ND ND ND ND 489.15 3.317611b 25.66 46.45 127 77.80 8.00 41.00 23.80 13.00 1.26 0.48 ND ND7612b 24.62 46.19 57 86.40 3.00 66.80 11.10 8.50 0.39 0.24 205.16 oMDA7613b 24.97 45.80 88 ND ND ND ND ND ND ND 430.24 2.407617b 24.06 44.56 121 79.20 3.00 29.40 32.80 17.00 2.17 0.65 321.00 0.437618b 23.85 42.87 231 60.60 17.00 41.00 14.90 4.70 0.85 0.65 ND ND7619b 23.94 42.82 511 69.50 15.00 28.20 14.20 27.10 4.21 1.30 ND ND7620b 22.94 41.98 44 56.40 30.00 35.70 3.00 17.70 12.21 6.02 387.42 1.837622b 24.31 43.75 430 66.00 27.00 22.60 10.60 32.80 3.95 1.36 ND ND7623b 24.49 44.24 458 64.40 31.00 22.60 9.00 32.80 4.68 0.81 ND NDAb 34.86 56.89 ND ND ND ND ND ND ND ND 532.28 1.99Bb 34.91 56.69 ND ND ND ND ND ND ND ND 540.09 1.65Cb 34.95 56.49 ND ND ND ND ND ND ND ND 673.38 1.41Db 34.99 56.29 ND ND ND ND ND ND ND ND 613.06 3.00Eb 35.02 56.11 ND ND ND ND ND ND ND ND 570.03 2.35Fb 35.06 55.92 ND ND ND ND ND ND ND ND 623.54 2.72Gb 35.10 55.72 ND ND ND ND ND ND ND ND 538.19 2.51Hb 35.14 55.52 ND ND ND ND ND ND ND ND 512.83 1.99Ib 35.18 55.33 ND ND ND ND ND ND ND ND 440.34 1.43Jb 35.22 55.12 ND ND ND ND ND ND ND ND 397.28 oMDAKb 35.26 54.90 ND ND ND ND ND ND ND ND 367.33 0.49Lb 35.15 54.73 ND ND ND ND ND ND ND ND 539.20 2.12Mb 35.05 54.58 ND ND ND ND ND ND ND ND 825.05 4.61Nb 34.93 54.42 ND ND ND ND ND ND ND ND 601.65 2.40Ob 34.83 54.27 ND ND ND ND ND ND ND ND 543.17 2.42Pb 34.73 54.12 ND ND ND ND ND ND ND ND 313.57 1.61Qb 34.62 53.95 ND ND ND ND ND ND ND ND 458.53 0.75Rb 34.52 53.80 ND ND ND ND ND ND ND ND 546.66 0.30Sb 34.42 53.63 ND ND ND ND ND ND ND ND 371.98 0.59Tb 34.30 53.48 ND ND ND ND ND ND ND ND 314.61 1.4212c 37.62 55.17 112 70.90 28.00 22.20 22.70 26.00 0.26 1.17 ND ND15c 37.17 56.08 40 51.40 42.10 14.40 22.20 14.80 1.06 1.02 ND ND21c 36.14 55.32 25 58.70 17.20 29.90 21.10 7.70 1.25 NDa ND ND23c 36.41 54.72 49 62.90 32.50 26.60 20.70 15.60 0.79 0.37 ND ND30c 35.62 53.47 54 65.60 31.50 17.90 32.00 15.70 1.14 1.12 ND ND33c 35.26 54.13 27 59.70 36.90 26.60 21.40 11.70 0.36 1.97 ND ND38c 34.25 53.09 34 66.43 31.95 46.10 17.90 2.43 0.95 0.82 ND ND41c 34.53 52.63 60 68.40 29.00 43.00 14.40 11.00 0.46 1.84 ND ND42c 33.59 51.04 137 69.80 25.70 36.20 11.00 22.60 1.28 1.14 ND ND51c 33.37 51.88 64 31.50 68.50 24.40 3.70 3.40 0.89 0.76 ND ND53c 33.61 51.52 92 33.40 66.60 19.80 8.00 5.60 0.27 1.24 ND ND57c 32.97 50.57 103 45.90 51.40 23.10 14.20 8.60 0.53 1.13 ND ND59c 32.73 51.06 64 28.40 71.60 19.80 4.50 4.10 5.72 0.89 ND ND61c 32.50 51.51 46 44.20 95.39 27.40 9.80 7.00 0.25 1.50 ND ND65c 31.09 50.38 74 45.60 54.40 23.40 17.30 4.90 0.34 2.15 ND ND67c 29.82 49.38 70 61.70 38.30 27.60 23.00 11.10 0.53 1.39 ND ND75c 29.68 49.51 47 45.60 53.40 22.00 18.80 4.80 0.90 1.59 ND ND78c 28.67 48.52 75 49.50 49.70 44.20 0.40 4.90 0.60 0.74 ND ND79c 28.76 48.32 109 50.70 48.60 27.40 15.20 8.10 0.82 1.20 ND ND

R.H.N

agaiet

al./Continental

ShelfResearch

∎(∎∎∎∎)

∎∎∎–∎∎∎

5

Pleasecite

this

articleas:

Nagai,R

.H.,et

al.,Hyd

rodyn

amic

controls

onthedistribu

tionof

surface

sedim

ents

fromthesou

theast

South

American

contin

ental

shelf

between

231S

and38

1S.Con

tinen

talSh

elfResearch

(2013),http

://dx.d

oi.org/10.1016/j.csr.2013.09.016i

area (latitudes less than 251S; Fig. 3c). S/K ratios of approximately1 were observed in the area between Itajaí (271S) and São SebastiãoIsland (241S).

The first two axes of the principal components analysis (PCA)accounted together for 52.6% of the total variance (Fig. 4a).The plot of such axes allowed us to identify three main groups

Table 240K and 137Cs radioactivity values (in Bq kg�1) in the certified reference materials (CRMs), precision and accuracy of the method.

Nuclide CRM Certified activity (Bq kg�1) Obtained activity (Bq kg�1)a Precisionb (%) Accuracyc (%)

40K IAEA-300 1059.00 1129.00722.84 2.0 6.6IAEA-326 580.00 592.98716.94 2.9 2.2IAEA-327 621.00 623.94711.01 1.8 0.5IAEA-375 417.00 402.86712.16 3.0 3.4

Mean: 2.4 3.2

137Cs IAEA-300 1066.60 1080.2577.22 0.7 1.3IAEA-326 NRd – – –

IAEA-327 24.90 24.3170.50 2.1 2.4IAEA-375 5280.00 5304.9179.54 0.2 0.5

Mean: 1.0 1.4

a Values expressed in terms of mean and associated error of determination.b Precision measured with the relative standard deviation (RSD) value (in %).c Accuracy measured with the relative error (RE) value (in %).d It is not a CRM for the analyzed element.

Fig. 2. Spatial distribution of the (a) non-clay particle (NCP) contents and main NCP components, (b) quartz, (c) feldspar and (d) calcite along the SE South Americancontinental margin.

R.H. Nagai et al. / Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎6

Please cite this article as: Nagai, R.H., et al., Hydrodynamic controls on the distribution of surface sediments from the southeast SouthAmerican continental shelf between 231S and 381S. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.09.016i

of samples with distinct mineralogical characteristics (Fig. 4a).Group 1 consisted of samples collected south of 271S with highcontents of clay fraction, high S/I and S/K values, and low calcite

and quartz contents. Group 2 samples were concentrated in theinner and middle shelf north to 271S, with the exception of twosamples that are located on the outer shelf off the mouth of theRdlP, and showed low clay content, high quartz, and feldsparcontents. Group 3 was composed of samples rich in calcite andwith high K/I ratios. The samples of this group were all located inthe deeper parts (4300 m) of the study area.

4.2. Radionuclide activities (40K and 137Cs)

The spatial distribution of the radionuclides throughout thesouthern Brazilian margin and the estuary of the RdlP is shown inFig. 5. Both 40K and 137Cs showed higher activity values in thesouthern part of the Brazilian continental shelf and mainly atdepths o200 m (Fig. 5). 137Cs activities in the estuary of the RdlPhave activity ranges similar to those of the southern part (between221S and 271S) of the southeastern South American continentalshelf (Table 3).

5. Discussion

The mineralogy and radionuclide activities of sands and mudsfrom surface sediments across the southeastern South Americancontinental shelf allowed us to identify sources and oceanographiccontrols on the depositional processes between 231S and 381S.

In general, higher percentages of NCP are observed in thesouthernmost and northernmost parts of the southeastern SouthAmerican continental shelf.

According to Campos et al. (2008) the STSF would act as abarrier to the penetration of RdlP derived NCP north of 311S, beinga less effective barrier to the clay fraction. In fact, NCP valuesdecrease in latitudes lower than 311S until approximately 261S.This higher amounts of NCP found in the northernmost latitudesare probably related to the lack of input of clay material to thispart of the study area and, in the outer shelf and upper slope, tothe BC floor-polishing effect leading to low sedimentation rates(Mahiques et al., 2004) and preventing fine material deposition.

The illite found in the sediments of the southeastern SouthAmerican shelf is mainly formed by degradation of primary mineralsin the hydrographic basin of Southern Brazil (Campos et al., 2008),which is drained by the rivers that compose the RdlP drainage basinand the Patos Lagoon. Meanwhile, kaolinite is the dominant claymineral in the oceanic sediments at low latitudes, where thecontinental sources are affected by intense chemical weathering(Biscaye, 1965; Petschick et al., 1996). This implies that the presenceof illite in the Brazilian continental shelf north of 301S is the result ofthe transport of this clay phase by the BCC, whereas the presence ofkaolinite along the southern coast of Brazil represents the latitudinaltransport of river-derived clay material along the South Americanshelf by the BC. Thus, the kaolinite/illite ratio (K/I) potentiallyrepresents different clay mineral sources for the Brazilian continentalshelf. Illite being derived from the continental area of southern Brazil,and kaolinite, mainly derived from N/NE Brazil. It also highlights theinterplay between two different oceanographic controls over thedepositional processes in the study area.

It is important to notice the presence of a high value of K/I ratiooff the coast of southernmost Brazil found in a sample from theCampos et al. (2008) dataset. According to these authors the PatosLagoon would be the main source of kaolinite in this area;however, this is not the pattern shown by the surroundingsamples, thus it seems more reasonable to assume this high K/Iratio value as an outlier.

The smectite/kaolinite ratio (S/K) can also be applied to betterunderstand the oceanographic controls on the sedimentationprocesses on the shelf due to the presence of different potential

Fig. 3. Distribution of the (a) kaolinite/illite and (b) smectite/kaolinite ratios alongthe SE South American continental margin.

R.H. Nagai et al. / Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 7

Please cite this article as: Nagai, R.H., et al., Hydrodynamic controls on the distribution of surface sediments from the southeast SouthAmerican continental shelf between 231S and 381S. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.09.016i

source areas. As previously noted, kaolinite is brought to the studyarea from equatorial latitudes by the BC. In addition, the basaltsfrom the Paraná Magmatic Province are the primary source ofsmectite in southeastern South America (Fagel, 2007) and are alsoa source rock for the sediments derived from the RdlP (Mahiqueset al., 2008). Thus, the smectite found in shelf sediments in thesouthern portion of the study area is likely derived from the RdlP(Campos et al., 2008).

Although the S/K ratio does not show a clear a pattern as that ofthe K/I ratio, it indicates that the southernmost part of the studyarea appears to be dominated by the influence of sediment inputfrom the RdlP; this influence decreases continuously up to 271S.In the area between Itajaí (271S) and São Sebastião Island (241S),the S/K ratio is close to 1, and, in contrast to the K/I ratio, the S/Kratios increase towards northern latitudes. Thus, this area might beconsidered a transitional area between the dominance of the BCand the BCC. Apparently there is also an important smectitecontribution to the northern portion of the study area, possibly

transported by the BC. In the continental area, rich in smectitesoils are also found in the semi-arid region of NE Brazil (Melfiet al., 2004). However, at this moment, we are unable to trace thesource of this mineral given the latitudinal extent of our samples.

The PCA of the mineralogy data supports the existence ofdifferent sources for the clay minerals in the study area. Group1 corresponds to very fine (clay) sediments samples with high S/Kvalues, showing a direct influence of the RdlP outflow, extendingfrom 361S to 271S carried by the northward flow of the BCC alongthe southeastern South American continental margin. The pre-sence of coarser sediments (Group 2 samples) in the outer shelf offthe mouth of the RdlP reinforces the northward transport of theRdlP outflow (Piola et al., 2000). Also, this group composed by lowclay and high quartz contents samples, mainly located north of271S, corroborates to the fact that the area between 271S and 241S(São Sebastião Island) is a transitional zone between the dom-inance of the BC and the BCC. While, rich in biogenic calcite andkaolinite samples (Group 3), from the deeper parts of the study

Fig. 4. (a) PCA ordination diagram of sampling based on standardised data of the selected factors (quartz, feldspars, calcite and clay contents, and smectite/illite andkaolinite/illite ratios). The first two axes of the PCA accounted for 52.6% of the total variance. (b) Spatial distribution of the three distinct groups identified by the PCA analysisaxes plot.

R.H. Nagai et al. / Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎8

Please cite this article as: Nagai, R.H., et al., Hydrodynamic controls on the distribution of surface sediments from the southeast SouthAmerican continental shelf between 231S and 381S. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.09.016i

area, highlight the progressive decrease in terrigenous inputtowards the outer shelf and upper slope with a stronger BCinfluence; for the northern part of the study area this interpreta-tion is also supported by the lower 40K and 137Cs activity values inthe deeper stations.

The radioactivity found in the surface sediments of the S/SEBrazilian shelf also highlights the contribution of the RdlP to thesedimentation processes of the Brazilian continental shelf. The 40Kisotope, which is found in feldspars, clay minerals, and finesediments, can be used as a sedimentary tracer (Ligero et al.,2001). On the Brazilian continental shelf, the 40K activity and fine-sediment fraction showed a statistically significant correlation(r¼0.69, po0.05). There is a negative correlation between grainsize and radionuclide concentration, as a result, muddier sedi-ments exhibit higher concentrations (He and Walling, 1996; Ligeroet al., 2001). Thus, the concentrations of both radionuclides can berelated to sediment grain size, and the high activity values foundfor 40K and 137Cs in the southern portion of the study area areassociated with the presence of fine sediments (Fig. 1).

As a further study of the relationship between 40K and 137Csactivities and fine sediments, the one-way ANOVA shows, with alevel of significance of 5%, that there is statistically significantdifferences between sediment samples from the northern andsouthern parts of the study area and the RdlP estuary (p¼0.007).Also, the Tukey method highlights that the northern and southernparts of the study area are statistically different, but the southernpart and the RdlP estuary are statistically similar (α¼5%) in termsof 137Cs activity (Table 4). Because there is no other significantfluvial input to the study area and because there is no naturalsource for 137Cs, the similarity between the 137Cs activities foundin the RdlP estuary sediments and those from the south Brazilian

continental shelf also confirms the RdlP as the major source of finesediments to the Brazilian shelf up to 271S.

The hydrodynamic processes, oceanic water mass dynamics andshelf circulation, are the main controlling factors for sedimentdistribution in the upper continental margin off S/SE Brazil, leadingto the establishment of differences in sedimentation rates (Mahiqueset al., 2011) and sedimentary facies (Mahiques et al., 2004) related tosediment source (Mahiques et al., 2008 and this study).

6. Conclusions

The mineralogy of the surface sediments between Cabo Frio(231S) and the mouth of the Rio the la Plata (381S) highlights theinterplay between two oceanographic controls in the study area:the Brazil Current, which supplies the shelf with clay mineralsfrom equatorial regions, and the Brazil Coastal Current, whichtransports the sediments derived from the RdlP. The new miner-alogical and radionuclide data obtained in this study also confirmthe RdlP as a major source of terrigenous sediment up to 271S ofthe southeastern South American continental shelf with decreasein importance towards northern latitudes. Sediment distributionpoints to the area between Itajaí (271S) and São Sebastião Island(241S) as a transitional zone between the dominance of the BC andthe BCC.

Acknowledgments

The first author was supported by a PhD scholarship providedby FAPESP (Proc. 2009/01594-6). Sample acquisition was also

Fig. 5. Spatial distribution of (a) 40K and (b) 137Cs activities (in Bq kg�1) in sediments from the southern Brazilian margin and the mouth of the Rio de La Plata.

Table 3Mean and standard deviation of 40K and 137Cs activities (in Bq kg�1) for thenorthern (between 271S and 381S) and southern (between 221S and 271S)compartments of the southeastern South American continental shelf and theestuary of Rio de la Plata.

Nuclide Northerncompartment

Southerncompartment

Rio de la Plataestuary

40K 290.127105.41 364.647119.40 516.147126.20137Cs 1.2070.57 1.8170.63 1.8871.02

Table 4The Tukey method for grouping post-ANOVA. Sample groups that do not sharea grouping letter are significantly different.

Sample group 137Cs activity (Bq kg�1) Grouping letter

A B

Northern compartment 1.2070.57 XSouthern compartment 1.8170.63 XRio de La Plata estuary 1.8871.02 X

R.H. Nagai et al. / Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 9

Please cite this article as: Nagai, R.H., et al., Hydrodynamic controls on the distribution of surface sediments from the southeast SouthAmerican continental shelf between 231S and 381S. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.09.016i

supported by FAPESP (Proc. 2003/10740-0). The authors wouldalso like to acknowledge projects LAPLATA (FAPESP Proc. 2004/01950-3) and RECOBRIAS (CNpq 490806/2007-4) for the supportin the visiting of Dr. Swapna Mulkherjee to Brazil. This is a GEO-SEDex contribution (No. 003), with institutional support of theUniversity of São Paulo (PrPesq).

References

Alencar, A.S., Freitas, A.C., 2005. Reference levels of natural radioactivity for thebeach sands in a Brazilian southeastern coastal region. Radiation Measure-ments 40, 76–83.

Biscaye, P.E., 1965. Mineralogy and sedimentation of recent deep-sea clay in theAtlantic Ocean and adjacent seas and oceans. Geological Society of AmericaBulletin 76, 803–832.

Burone, L., Sousa, S.H.M., Mahiques, M.M., Valente, P., Ciotti, A., Yamashita, C., 2010.Benthic foraminiferal distribution on the southeastern Brazilian shelf andupper slope. Marine Biology 158, 159–179.

Campos, E.J.D., Mulkherjee, S., Piola, A.R., Carvalho, F.M.S., 2008. A note on amineralogical analysis of the sediments associated with the Plata River andPatos Lagoon outflows. Continental Shelf Research 28, 1687–1691.

Carvalho, C., Anjos, R.M., Veiga, R., Macario, K., 2011. Application of radiometricanalysis in the study of provenance and transport processes of Brazilian coastalsediments. Journal of Environmental Radioactivity 102, 185–192.

Ciotti, A.M., Odebrecht, C., Fillmann, G., Möller Jr., O.O., 1995. Freshwater outflowand subtropical convergence influence on phytoplankton biomass on thesouthern Brazilian continental shelf. Continental Shelf Research 15, 1737–1756.

Cortelezzi, C.R., Colado, U.R., Mouzo, F.H., Robbiano, J.A., 1971. Resultados de lascampañas oceanográficas Mar del Plata I–V, Revista del Museo de la Plata.Geología, Tomo VII - no. 59.

Cunha, I.I.L., Figueira, R.C.L., Saito, R.T., 1999. Application of radiochemical methodsand dispersion models in the study of environmental pollution in Brazil. Journalof Radioanalytical and Nuclear Chemistry 239, 477–482.

Diekmann, B., Fütterer, D.K., Grobe, H., Hillenbrand, C.D., Kuhn, G., Michels, K.,Petschick, R., Pirrung, M., 2004. Terrigenous sediment supply in the polar totemperate South Atlantic: land–ocean links of environmental changes duringthe late Quaternary. In: Wefer, G., Mulitza, S., Ratmeyer, V. (Eds.), The SouthAtlantic in the Late Quaternary. Springer, Berlin Heidelberg, pp. 375–399.

Eichler, P.P.B., Sen Gupta, B.K., Eichler, B.B., Braga, E.S., Campos, E.J.D., 2008. Benthicforaminiferal assembleges of the South Brazil, relationship to water masses andnutrient distributions. Continental Shelf Research 28, 1674–1686.

Etchichury, M.C., Remiro, J., 1960. Muestras de fondo de la plataforma continental,Revista do Museo Argentino Bernardino Rivadavia, Cs. Geológicas. Tomo VI- no. 4.

Etchichury, M.C., Remiro, J.R., 1963. La corriente de Malvinas y los sedimentosPampeano-patagónicos. Comunicaciones del Museo Nacional de Historia Nat-ural ``Bernardino Rivadavia'', Geol., Tomo I 20, 1–11.

Fagel, N., 2007. Clay minerals, deep circulation and climate. In: Hillaire-Marcel, C.,De Vernal, A. (Eds.), Developments in Marine Geology, vol. 1 – Proxies in theLate Cenozoic Paleoceanography. Elsevier, Amsterdam, pp. 139–184.

Figueiredo, Jr. A.G., Tessler, M.G., 2004. Topografia e composição do substratomarinho da região Sudeste-Sul do Brasil. In: Rossi-Wongtschowski, C.L.D.B.(Ed.), Série documentos Revizee : Score Sul. Instituto Oceanográfico, São Paulo,64 pp.

Figueira, R.C.L., Tessler, M.G., Mahiques, M.M., Cunha, I.I.L., 2006. Distribution of137Cs, 238Pu and 239þ240Pu in sediments of the southeastern Brazilian shelf-SWAtlantic margin. Science of the Total Environment 357, 146–159.

Gelós, E., Chaar, E., 1988. Estudio sedimentológico–mineralógico de testigos de laplataforma continental argentina entre los paralelos de 391 y 421 Lat. Sur.Segundas Jornadas Geológicas Bonaerenses. Actas, Bahía Blanca, pp. 440–441.

Griggs, G.B., Hein, J.R., 1980. Sources, dispersal, and clay mineral composition offine-grained sediment off central and northern California. Journal of Geology88, 541–566.

He, Q., Walling, D.E., 1996. Interpreting particle size effects in the adsorption of137Cs and unsupported 210Pb by mineral soils and sediments. Journal ofEnvironmental Radioactivity 30, 117–137.

Hein, J.R., Dowling, J.S., Schuetze, A., Lee, H.J., 2003. Clay–mineral suites, sources,and inferred dispersal routes, Southern California continental shelf. MarineEnvironmental Research 56, 79–102.

Karlin, R., 1980. Sediment sources and clay mineral distributions off the Oregoncoast. Journal of Sedimentary Petrology 50, 543–560.

Kovach, W.L., 1999. MVSP – a multivariate statistical package for windows, ver. 3.1.Kovach Computing Services, Pentraeth, Wales, UK.

Ligero, R.A., Ramos-Lerate, I., Barrera, M., Casas-Ruiz, M., 2001. Relationshipsbetween sea-bed radionuclides activities and some sedimentological variables.Journal of Environmental Radioactivity 57, 7–19.

Mahiques, M.M., Mishima, Y., Rodrigues, M., 1999. Characteristics of the sedimen-tary organic matter on the inner and middle continental shelf betweenGuanabara Bay and São Francisco do Sul, south-eastern Brazilian margin.Continental Shelf Research 19, 775–798.

Mahiques, M.M., Tessler, M.G., Ciotti, A.M., Silveira, I.C.A., Sousa, S.H.M., Figueira,R.C.L., Tassinari, C.C.G., Furtado, V.V., Passos, R.F., 2004. Hydrodynamically-drivenpatterns of recent sedimentation in the shelf and upper slope off southeast Brazil.Continental Shelf Research 24, 1685–1697.

Mahiques, M.M., Tassinari, C.C.G., Marcolini, S., Violante, R.A., Figueira, R.C.L.,Silveira, I.C.A., Burone, L., Sousa, S.H.M., 2008. Nd and Pb isotope signatureson the southeastern South American upper margin, implications for sedimenttransport and source rocks. Marine Geology 250, 51–63.

Mahiques, M.M., Sousa, S.H.M., Burone, L., Nagai, R.H., Silveira, I.C.A., Figueira, R.C.L.,Soutelino, R.G., Ponsoni, L., Klein, D.A., 2011. Radiocarbon geochronology of thesediments of the Sao Paulo Bight (southern Brazilian upper margin). Anais daAcademia Brasileira de Ciências 83, 817–834.

Martins, L.R., Martins, I.R., Urien, C.M., 2003. Aspectos sedimentares da plataformacontinental na área de influência do Rio de La Plata. Gravel 1, 68–80.

Meijer, R.J., Donoghue, J.F., 1995. Radiometric fingerprinting of sediments on theDutch, German and Danish coasts. Quaternary International 26, 43–47.

Melfi, A.J., Montes, C.R., Carvalho, A., Forti, M.C., 2004. Use of pedological maps inthe identification of sensitivity of soils to acidic deposition, Application toBrazilian soils. Anais da Academia Brasileira de Ciências 76, 139–145.

Milliman, J.D., Barreto, H.T., 1975. Relict magnesian calcite oolite and subsidence ofAmazon shelf. Sedimentology 22, 137–145.

Möller, O.O., Piola, A.R., Freitas, A.C., Campos, E.J.D., 2008. The effects of riverdischarge and seasonal winds on the shelf off southeastern South America.Continental Shelf Research 28, 1607–1624.

Naidu, A.S., Han, M.W., Mowatt, T.C., Wajda, W., 1995. Clay minerals as indicators ofsources of terrigenous sediments, their transportation and deposition, BeringBasin, Russian–Alaskan Arctic. Marine Geology 127, 87–104.

Oliveira, A., Rocha, F., Rodrigues, A., Jouanneau, J., Dias, A., Weber, O., Gomes, C.,2002. Clay minerals from the sedimentary cover from the Northwest Iberianshelf. Progress in Oceanography 52, 233–247.

Park, Y.A., Khim, B.K., 1990. Clay minerals of the recent fine-grained sediments onthe Korean continental shelves. Continental Shelf Research 10, 1179–1191.

Parker, G., Paterlini, C.M., Violante, R.A., 1997. El Fondo Marino. In: Boschi, E.E. (Ed.),El mar Argentino y sus Recursos Pesqueros. INIDEP, Mar del Plata, pp. 65–87.

Petschick, R., Kuhn, G., Gingele, F., 1996. Clay mineral distribution in surfacesediments of the South Atlantic, sources, transport, and relation to oceano-graphy. Marine Geology 130, 203–229.

Piola, A.R., Campos, E.J.D., Möller Jr., O., Charo, M., Martinez, C., 2000. Subtropicalshelf front off eastern South America. Journal of Geophysical Research 105,6565–6578.

Piola, A.R., Romero, S.I., Zajaczkovski, U., 2008. Space– time variability of the PlataPlume inferred from ocean color. Continental Shelf Research 28, 1556–1567.

Saito, R.T., Figueira, R.C.L., Tessler, M.G., Cunha, I.I.L., 2001. 210Pb and 137Csgeochronologies in the Cananéia-Iguape estuary (São Paulo, Brazil). Journal ofRadioanalytical and Nuclear Chemistry 249, 257–261.

Souza, R.B., Robinson, I.S., 2004. Lagrangian and satellite observations of theBrazilian Coastal Current. Continental Shelf Research 24, 241–262.

Urien, C.M., Ewing, M., 1974. Recent sediments and environment of southern Brazil,Uruguay, Buenos Aires, and Rio Negro continental shelf. In: Burk, C.A., Drake,C.L. (Eds.), The Geology of Continental Margins. Springer, Berlin, pp. 157–177.

Violante, R.A., Parker, G., 2004. The post-last glacial maximum transgression in thede la Plata River and adjacent inner continental shelf, Argentina. QuaternaryInternational 114, 167–181.

Yeager, K.M., Santschi, P.H., 2003. Invariance of isotope ratios of lithogenic radio-nuclides, more evidence for their use as sediment source tracers. Journal ofEnvironmental Radioactivity 69, 159–176.

R.H. Nagai et al. / Continental Shelf Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎10

Please cite this article as: Nagai, R.H., et al., Hydrodynamic controls on the distribution of surface sediments from the southeast SouthAmerican continental shelf between 231S and 381S. Continental Shelf Research (2013), http://dx.doi.org/10.1016/j.csr.2013.09.016i