Post-LGM sedimentation on the outer shelf–upper slope of the northernmost part of the São Paulo...

Post-LGM sedimentation on the outer shelf^upper slope ofthe northernmost part of the Sa¬o Paulo Bight,

southeastern Brazil

Michel Michaelovitch de Mahiques a;*, Ilson Carlos Almeida da Silveira a,Silvia Helena de Mello e Sousa b, Marcelo Rodrigues a

a Department of Physical Oceanography, Institute of Oceanography, University of Sa¬o Paulo, 05508-900 Prac°a do Oceanogra¢co,191, Sa¬o Paulo, SP, Brazil

b Department of Geology, Federal University of Parana, P.O. Box 19011, 81531-990 Curitiba, Parana, Brazil

Received 25 July 2000; accepted 24 July 2001

Abstract

Sedimentary and isotope data as well as 14C AMS dating on 15 box cores were used to identify the main post-LastGlacial Maximum (LGM) processes in the outer shelf and upper slope of the northernmost part of the Sa¬o Paulo Bight,southeastern Brazil. Results show slight but significant variations in calcium carbonate, total sulphur and nitrogencontents as well as carbon and oxygen isotope ratios. Sedimentation rates, varying from 5 to 184 mm kyr31 arecontrolled by the shelf and upper slope morphology, the Brazil Current meander dynamics, and the Coastal Wateroffshore motion. ß 2002 Elsevier Science B.V. All rights reserved.

Keywords: sedimentation; Last glacial maximum; Holocene; stratigraphy; Brazil

1. Introduction

The Quaternary development of continentalmargins is the result of a complex set of geologicaland oceanographic parameters. Even on relativelystable passive margins the interaction amongbasement structure, subsidence, paleogeography,sea-level changes, and present oceanographic con-ditions frequently result in the development of

sedimentary processes, which cannot be inter-preted in terms of simple evolutionary models(Pitman and Golovchenko, 1983; Mienert et al.,1997; Nittrouer, 1999).

The knowledge of the late Quaternary sedimen-tary processes on the continental shelf and slopeo¡ the Brazilian continental margin is still rela-tively poor. Due to the importance of its potentialand known oil reserves, the Campos Basin is theonly extensively studied part of the southeasternBrazilian margin (Caddah et al., 1998; Viana etal., 1998). More recently, however, the easternand northeastern Brazilian margins have becomethe object of more detailed research (Knoppersand Ekau, 1999).

0025-3227 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 5 - 3 2 2 7 ( 0 1 ) 0 0 2 2 5 - 0

* Corresponding author.E-mail address: [email protected]

(M. Michaelovitch de Mahiques).

MARGO 2986 28-3-02 Cyaan Magenta Geel Zwart

Marine Geology 181 (2002) 387^400

www.elsevier.com/locate/margeo

In this paper we present a ¢rst interpretation ofpost-Last Glacial Maximum (LGM) sedimenta-tion on the outer shelf and upper slope of thenorthernmost part of the Sa¬o Paulo Bight, south-eastern Brazil. Main attention was given to therole played by the dominant water masses dynam-ics on the Holocene sedimentation.

2. Morphology and oceanography of the study area

The study area comprises the northernmostpart of the Sa¬o Paulo Bight, southeastern Brazil(Fig. 1). The Sa¬o Paulo Bight is the arc-shapedpart of the southeastern Brazilian margin extend-ing from 23³S to 28³S (Zembruscki, 1979). In this

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


M. Michaelovitch de Mahiques et al. / Marine Geology 181 (2002) 387^400388

paper we analyze the sedimentary processes thatprevail eastward to Sa¬o Sebastia¬o area (46³W).

The ocean £oor shows a rather complex mor-phology involving channels, canyons, and impor-tant variations in the slope morphology (Furtadoet al., 1996). The shelf break is located at a depthof approximately 140 m with the upper slopeshowing an average gradient of approximately1:55. In the southeastern sector o¡ Sa¬o Sebastia¬oIsland close to the 600-m isobath (25³S, 44³30PW),the upper slope changes into a fan-shaped featureat the end of the Buzios Channel.

The hydrodynamic conditions in the inner shelfare strongly dominated by wind- and wind wave-driven currents. North of Sa¬o Sebastia¬o Island,the combination of dynamic processes and com-plex bottom and coastline morphologies yields amuddy sedimentation pattern.

In the middle and outer shelves as well as in theupper slope sedimentary processes seem to be in-£uenced by the Brazil Current (BC) £ow alongthe south-western Atlantic continental margin(Campos et al., 1995, 2000; Mu«ller et al., 1998;Stramma and England, 1999). The BC transportsTropical Water (TW, T = 25.0³C, S = 37.1) atupper levels and South Atlantic Central Water(SACW, T = 14.0³C, S = 35.5) at pycnocline levels(Miranda, 1985; Silveira et al., 2000). The BCcore is centered on the 1000-m isobath (Gar¢eld,1990).

According to Campos et al. (2000), the BC de-velops a convoluted pattern of meanders on CapeFrio surroundings (Fig. 2A). The reason is thechange in the Brazilian coastline orientation,which dynamically favors the formation of aclockwise meander on BC in an attempt to con-serve angular momentum. The consequence is avortical structure with upwelling associated to itssouthernmost part and downwelling in its north-ernmost part.

A counterclockwise meander is often founddownstream of the Cape Frio meander and atopographic Rossby wave pattern is observed in-side the Sa¬o Paulo Bight (Fig. 2B). The wavepattern is not stationary and is thought to beadvected southward by the BC, but the structuredepicted in Fig. 2B is recurrent.

The inner shelf is characterized by the interac-

tion of TW and SACW with the longshore ando¡shore displacement of the Coastal Water (CW,T = 22.0³C, S6 35.0) (Castro et al., 1987; Signo-rini et al., 1989).

A seasonal cycle on the prevailing wind direc-tion in the Cape Frio area establishes a seasonalEkman-driven upwelling region (Castro et al.,1987; Campos et al., 2000). During the summer,the dominant NW wind direction favors the CWmotion o¡shore and subsurface penetration of theSACW to shallower areas (Castro, 1996). Theupwelled SACW increases local biological pro-ductivity and consequential deposition of organicmatter (Gaeta et al., 1994). In the Sa¬o Sebastia¬oIsland area, the CW motion o¡shore causes thetransport of suspended terrigenous material todeeper areas of the shelf (Mahiques et al., 1999).

The distribution of surface sediments has beenextensively studied during the 1970's and is de-scribed in the papers of Rocha et al. (1975) andKowsmann and Costa (1979). In general, thepresent sea£oor is covered by siliciclastic very¢ne sands and silts, with variable amounts ofclay and calcium carbonate. Coarser sedimentsand carbonate gravel and boulder facies, whichrepresent less than 5% of the present bottom sedi-ments are generally related to relict sediments,deposited under lower sea-level conditions.

3. Materials and methods

Fifteen box cores were collected during the1997/1998 austral summer on board the R.V.Prof. W. Besnard (Fig. 1, Table 1). Each corewas described and sampled continuously at inter-vals of 2 cm. Samples were frozen and laterfreeze-dried. Only box cores out of the relictcarbonate facies and without any evidence ofsediment reworking or bioturbation were con-sidered for this work. Also, in order to guaranteethat relict sediments were not analyzed, only boxcores which core-tops showed the presence of210Pbunsupported were considered.

Due to the lack of enough suitable carbonatematerial (e.g. monospeci¢c foraminifer tests) wehave used the organic fraction of sediment sam-ples from the middle and bottom of each core in


M. Michaelovitch de Mahiques et al. / Marine Geology 181 (2002) 387^400 389

Fig. 2. (A) AVHRR image of the southeastern Brazilian margin, showing the temperature signature associated with the BrazilCurrent front. Edited arrows indicate current direction (adapted from Campos et al., 1996). (B) Schematic representation of theBrazil Current meandering in the study area. The schematic 1000-m isobath separates the dark blue and light blue zones in thediagram.



order to make radiocarbon (14C) dating. Datingwas done by BETA Analytic (Miami, FL, USA),by means of accelerated mass spectrometry(AMS). Before Present calibrated ages were cal-culated with the aid of the Calib Software,which uses the IntCal98 procedures (Stuiver etal., 1998). Due to the lack of Marine ReservoirCorrection Data for the SW Atlantic we decidedto use the Caribbean average vR and uncertaintyto calibrate the samples (Paula J. Reimer, TheQueen's University of Belfast, personal communi-cation).

All of the core-tops were considered to repre-sent present age (0 yr BP). Interpolation of agesof the intermediate samples as well as calculationof the core-top sedimentation rates was donethrough the use of the cubic spline interpolation,with the aid of the Dep-Age Software, developedby Louis Maher Jr., which is available at the FileBoutique of the Working Group on Data Han-dling Methods of the INQUA ^ Commission forthe Study of the Holocene (www.geology.wisc.edu/Vmaher/inqua.html). For the calculation ofthe sedimentation rates, only the three topmostsamples of each core were considered.

Organic carbon, total nitrogen and sulphurwere analyzed in a LECO CNS2000 Analyzer.Approximately 500 mg of dry sediment of each

sample was treated with 1 N HCl in order toremove calcium carbonate and then freeze-driedand analyzed. LECO sulfamethazine standardsand blanks were analyzed as controls for eachset of 30 samples.

Calcium carbonate was determined by weightdi¡erence of the sediment prior and after acidi¢-cation of 2 g of each sample with 1 N HCl. Valuesare reported in dry weight percentage.

Values of N13C and N15N of the sedimentaryorganic content as well N13C and N18O of testsof the planktonic foraminifer Globigerinoides ruber(white+pink) were obtained by mass spectrom-etry at the Coastal Science Laboratories (Austin,TX, USA). C and N isotope analysis was per-formed by online combustion with a FisonsNA1500 elemental analyzer attached to a VGSIRA Model 10 mass spectrometer. Carbon wascalibrated using NBS 22 oil standard (de¢ned as329.60x relative to PDB). Reproducibility forcarbon was better than 0.2x. Nitrogen was ref-erenced to IAEA N-1, N-2 and N-3 (de¢ned as+0.5x, +20.3x and +4.6x, respectively, rela-tive to the air). Reproducibility for nitrogen wasbetter than 0.3x.

Oxygen isotope was measured on the VG SIRAModel 12 mass spectrometer. Calibration for oxy-gen was made with laboratory standards and

Table 1Position and description of each box core

Sample Latitude Longitude Depth Recovery Description(³S) (³W) (m) (cm)

6539 23.08 41.98 98 30 mud6541 23.60 41.71 143 22 muddy ¢ne sand with mollusk

shells6542 23.95 41.61 1226 25 mud6553 23.82 42.79 227 20 ¢ne sand6554 23.89 42.76 496 25 mud with very ¢ne sand6561 23.70 44.00 99 16 mud6571 24.21 44.99 78 10 ¢ne sand with mud and mollusk

shells6573 24.91 44.62 501 30 muddy sand6625 24.48 43.62 980 38 mud6626 24.23 43.75 205 37 mud6627 23.97 43.88 133 33 mud6664 24.45 44.17 472 37 mud6670 24.69 44.38 503 32 mud6678 24.77 45.19 100 26 mud with mollusk shells6680 25.25 44.88 258 29 mud with carbonate sands



reference materials such as NBS 19. Reproducibil-ity for oxygen is better than 0.2x.

Carbon and oxygen isotope values are reportedas N13C (x PDB) and N18O (x PDB), respec-tively, and nitrogen isotope values are referred asN15N (x air).

4. Results

Dating results and core-top sedimentation ratesare shown in Table 2. All of the cores show verylow sedimentation rates, with average core valuesranging from 5 mm kyr31 at station 6553, withlower sedimentation rates coinciding with steeperareas of the continental slope, to 184 mm kyr31 at

sample 6539, in the vicinity of the upwelling areanear Cape Frio. Cores 6553, 6573, 6625, 6626 and6680 were the only that passed the Pleistocene/Holocene boundary.

Sediment and isotope variations along the sedi-mentary column are shown in Figs. 3^6. The re-sults show a trend of increasing values, over thelast 15 000 to 16 000 yr, for calcium carbonate,total nitrogen, N13Csediment, and N13CG: ruber. De-creasing values are observed in total sulphur andN18OG: ruber. The N15Nsediment values do not showany clear tendency.

On the other hand, in cores that present highersedimentation rates, we observe a tendency in de-creasing N13Csediment and N15Nsediment for the last4000 yr.

Table 214C dating and core-top sedimentation rate of each box

Sample Depth interval Radiocarbon age Cal age Sedimentation rate(yr BP) (yr BP) (mm kyr31)

6539 10^12 930 þ 40 580 18420^22 1 410 þ 50 975

6541 08^10 2 190 þ 40 1 805 6116^18 3 250 þ 40 3 095

6542 10^12 2 180 þ 40 1 790 5018^20 2 870 þ 40 2 670

6553 04^06 9 210 þ 40 10 230 514^16 14 740 þ 50 17 080

6554 10^12 2 470 þ 50 2 150 5020^22 3 530 þ 30 3 420

6561 08^10 1 350 þ 60 905 9010^12 1 420 þ 50 98512^14 1 560 þ 40 1 125

6573 10^12 9 920 þ 80 11 070 1024^26 13 960 þ 100 16 190

6625 06^08 3 650 þ 50 3 560 1932^34 11 750 þ 80 13 275

6626 12^14 3 880 þ 60 3 785 3830^32 12 800 þ 80 14 930

6627 08^10 1 360 þ 60 920 9726^28 2 810 þ 60 2 590

6664 10^12 2 310 þ 40 1 935 5430^32 3 340 þ 40 3 450

6669 10^12 1 460 þ 40 1 010 10626^28 2 380 þ 40 2 025

6670 06^08 2 260 þ 50 1 880 3724^26 5 390 þ 50 5 790

6678 10^12 2 110 þ 40 1 720 5418^20 2 230 þ 40 1 855

6680 10^12 9 210 þ 50 10 230 1020^22 13 920 þ 60 16 140



5. Discussion

The sedimentation rates obtained for the studyarea are signi¢cantly lower than those presentedby Arz et al. (1999a). The two main areas of high-er sedimentation rates occur in the area of upwell-ing at Cabo Frio and to the south of the BuziosChannel area.

The spatial distribution of sedimentation rates

correlates well with shelf^slope morphology andthe recurring meandering pattern of the BrazilCurrent (Fig. 7A). On analyzing the core-topsalong a transect parallel to the shelf break(Fig. 7B) lower sedimentation rates are associatedwith the dominant onshore £ow due to the BCmeandering. On the other hand, higher rates arerelated to dominant o¡shore £ow. In other words,the cross-shore £ow related to the BC meandering

Fig. 3. Sedimentary and isotope variations along cores 6539, 6541, and 6542.



controls the sedimentary process on the outershelf^upper slope within Sa¬o Paulo Bight.

Therefore, it seems reasonable to assume thatsedimentary processes over the middle and mainlyouter shelf are altered by the BC meandering,which passes along the sea£oor.

In Sa¬o Sebastia¬o Island surroundings, in addi-tion to the dominant BC meander o¡shore £ow,the CW o¡shore £ow enhances the suspendedsediment transport towards outer shelf regions.

In some areas of the eastern and southeasternBrazilian continental margin the LGM coastlinerepresented either by the 130-m or the 160-m iso-baths, shows a di¡erent orientation than the

present coastline. This fact was responsible fordramatic sedimentary changes in the southern Ab-rolhos Bank region (Sousa et al., 1997), locatednorthward to the study area.

During the LGM the coastline in the study areacorresponded roughly to the present shelf break.Thus, the slope topography must have been themain factor that then controlled the water massesdynamics. In this period the east^west orientationof the coastline of the study area, together withthe absence of a £at, wider shelf and eventually astronger trade winds regime (Wyputta andGrieger, 1999; Wol¡ et al., 1999; Abrantes,2000; Flores et al., 2000), may have been respon-

Fig. 4. Sedimentary and isotope variations along cores 6553 and 6554.



Fig. 5. Sedimentary and isotope variations along cores 6561, 6627, 6626, and 6625.



sible for changes in the sedimentary patternthrough variations in the Brazil Current meanderdynamics in the study area.

Arz et al. (1999a, 1999b) have shown that theabrupt changes in the sedimentary record in theWestern Equatorial and Tropical Atlantic re£ecttwo important climatic changes, which were cor-

related with the end of the last glacial maximumand with the Younger Dryas. In the study area,with the exception of the total sulphur data and aslight break in the N13CG: ruber and N18OG: ruber

curves observed at ca. 13 000 yr BP, most param-eters seem to show slight, rather than abruptchanges. Nevertheless, sulphur changes can be re-

Fig. 6. Sedimentary and isotope variations along cores 6573 and 6670.



Fig. 7. Sedimentary and isotope characteristics of the core-tops of a transect along the shelf break in the study area.



lated much more to post-depositional processes,in a reducing environment than to syn-depositio-nal environmental conditions. Also oxygen iso-tope changes can be associated with the ice-volume e¡ect rather than with sea surface temper-ature changes (Wefer et al., 1999). Changes incarbon isotope between present and LGM canbe attributed to the e¡ect of nutrient subsurfaceenrichment and to reduced air^sea exchange (Mu-litza et al., 1999).

For the last 18 000 yr, the enrichment in calci-um carbonate (Core 6553, Fig. 4, and Core 6626,Fig. 5) and organic carbon (Cores 6625 and 6626,Fig. 5) towards the Recent suggest that during theLGM the shelf waters were more in£uenced bythe Coastal Water. This fact re£ects a narrowersurface of the LGM continental shelf and conse-quently a more e¡ective input of terrigenous ma-terial to the continental slope.

Results also show that after the LGM and par-ticularly along the Holocene climatic changeswere also responsible for variations in the organicmatter as well as in the input of terrigenous sedi-ments towards the ocean. According to Behling(1997), the climate in the southeastern Brazilianplateau was markedly cooler and drier during thelast glacial period (ca. 35 000^17 000 yr BP); awarmer and more humid condition was reachedonly during the late Holocene. Actually, it seemsthat after a progressively increasing in£uence ofmarine organic matter up to 5000 yr BP, repre-sented by the enrichment in N13Csediment, calciumcarbonate, and organic carbon, an inversion ofthe trend occurred, leading to higher terrigenousinput and expressed by decreasing N13Csediment andN15Nsediment.

For the last ca. 5000 yr such slight changes thata¡ected sedimentation in the outer shelf can berelated either to climate or to the Holocene sea-level changes. According to Angulo et al. (1999),who worked south of the area, the sea-levelreached a maximum of at least 2.1 m above thepresent sea-level during the Holocene. During thisperiod the southeastern Brazilian coastline mayhave advanced to the base of the Serra do Mar,leading to the complete drowning of the formerdrainage system and to the trapping of terrige-nous sediments close to the coastline. During

the following lowering of sea-level, a small bute¤cient drainage network was re-established (Su-guio et al., 1993).

6. Conclusions

Post-LGM sedimentation on the outer shelfand upper slope of Sa¬o Paulo Bight is a responseto the combination of the bottom morphology,the cross-isobath £ow associated to the BrazilCurrent meandering and the CW seaward trans-port.

The steeper continental slope areas coincidewith the dominant onshore £ow of the BC mean-ders, and are characterized by low sedimentationrates. Sedimentation rates are greater near CapeFrio and southeast of Sa¬o Sebastia¬o Island. In theformer case, the interaction between coastlinechange and wind direction favors the upwellingof the SACW and, consequently, more conspicu-ous organic matter deposition. In the latter casethe dominant o¡shore £ow of the BC meanders isenhanced by the CW motion, and consequent ter-rigenous sediment transport towards the outershelf. This depositional process is probably re-sponsible for the development of an arc-shapeddeposit located on the upper slope. Further shal-low seismic and coring studies are needed to con-¢rm this hypothesis.

During the Last Glacial Maximum the combi-nation of shelf morphology and stronger tradewinds may have favored a widening of the CapeFrio upwelling area as well as modi¢cations in thedisplacement of the Brazil Current. In additionthe progressive Holocene sea-level rise was re-sponsible of a perceptible decrease in the CoastalWater in£uence on the outer shelf and slope, ex-pressed by a lowering of terrigenous sedimenta-tion. Finally, data show that for the last 5000 yrthe shelf of the study area was subjected to anincrease in the terrigenous input.

Acknowledgements

This work was funded by the Fundac°a¬o deAmparo a© Pesquisa do Estado de Sa¬o Paulo



(FAPESP) via grant nos. 1998/02439-8 and 1998/0572-2, and by the Conselho Nacional de Pesquisae Desenvolvimento Tecnologico (CNPq) via grantnos. 300650/94-9, 300582/96-0 and 910018/98-7.Thanks to Dr. Paula Reimer (The Queen'sUniversity of Belfast) for the information aboutthe Marine Reservoir Correction Database and forDr. Jean-Claude Fauge©res (Universite Bordeaux I)for his critical reading and suggestions. Thanks aredue also to Prof. Dr. Yasunobu Matsuura (Uni-versity of Sa¬o Paulo), coordinator of the PADCT-CNPq/IOUSP Project, and Prof. Dr. CarmenLucia Del Bianco Rossi-Wongtschowski (Univer-sity of Sa¬o Paulo), coordinator of the REVIZEEProject, in which the samples were collected.Thanks to the crew and researchers who partici-pated in the R.V. Prof. W. Besnard cruises. Thanksto Mr. Edilson Faria, who executed most of thelaboratory analyses, as well as to Mr. LeandroCalado, who prepared the drawings. This paper isa contribution to the Universidade de Sa¬o Paulo(Brazil) 9 Universite de Bordeaux I (France) Co-operation Programme in Oceanography.

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Post-LGM sedimentation on the outer shelf–upper slope of the northernmost part of the São Paulo...

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