Radiocarbon geochronology of the sediments of the São Paulo Bight (southern Brazilian upper margin)

Radiocarbon geochronology of the sediments of the São Paulo Bight (southern Brazilian upper margin)

Michel M. Mahiques; Silvia H.M. Sousa; Leticia Burone; Renata H. Nagai; Ilson C.A. Silveira; Rubens C.L. Figueira; Rafael G. Soutelino; Leandro Ponsoni; Daniel A. Klein

Instituto Oceanográfico da Universidade de São Paulo, Praça do Oceanográfico, 191, 05508-120 São Paulo, SP, Brasil

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ABSTRACT

The aim of this work was to generate an inventory of the data on radiocarbon datings obtained from sediments of the São Paulo Bight (southern Brazilian upper margin) and to analyze the data in terms of Late Quaternary sedimentary processes and sedimentation rates. A total of 238 radiocarbon datings from materials collected using differents ampling procedures was considered for this work. The sedimentation rates varied from less than 2 to 68 cm.kyr^-1. The highest sedimentation rate values were found in a low-energy (ría type) coastal system as well as in the upwelling zones of Santa Catarina and Cabo Frio. The lowest rates were found on the outer shelf and upper slopes. Our results confirm the strong dependency of the shelf currents, with an emphasis to the terrigenous input from the Río de La Plata outflow which is transported via the Brazilian Coastal Current, as well as of the coupled Brazil Current - Intermediate Western Boundary Current (BC-IWBC) dynamics on the sedimentary processes. At least three indicators ofthe paleo sea level were found at 12200 yr BP (conventional radiocarbon age) (103 meters below sea level - mbsl),8300-8800 cal yr BP (13 mbsl) and 7700-8100 cal yr BP (6 mbsl).

Key words: continental margin, quaternary, radiocarbon, sea-level, sedimentation.

RESUMO

O objetivo deste trabalho foi a geração de um inventário dos dados de datação de radiocarbono obtidos de sedimentos do Embaiamento de São Paulo (Margem Continental Superior do Sul do Brasil) e analisar os dados em termos de processos sedimentares quaternários e taxas de sedimentação. Um total de238 datações ao radiocarbono de materiais coletados com autilização de procedimentos amostrais diferentes foi considerado neste trabalho. As taxas de sedimentação variaram de menor que 2 a 68 cm.kyr^-1. As taxas de sedimentação mais altas foram encontradas em um sistema costeiro de baixa energia (tipo ría), bem como nas zonas de ressurgência de Santa Catarina e Cabo Frio. As taxas mais baixas foram encontradas na plataforma externa e talude superior. Nossos resultados confirmam a forte dependência do sistema de correntes de plataforma, com ênfase no aporte terrígeno oriundo do Rio daPrata, transportado através da Corrente Costeira do Brasil, e da dinâmica da Corrente do Brasil - Corrente de Contorno Intermediária (CB-CCI) nos processos sedimentares. Pelo menostrês indicadores de paleo-níveis marinhos foram encontrados a 12200 anos AP (idade radiocarbônica convencional) (103 metros abaixo do nível do mar atual - manm), 8300-8800 cal anos AP (13 manm) e 7700-8100 cal anos BP (6 manm).

Palavras-chave: margem continental, quaternário, radiocarbono, nível do mar, sedimentação.

INTRODUCTION

The northernmost part of the southern Brazilian marginis known as the São Paulo Bight, an arc-shaped embayment extending from 23ºS to 28ºS (Zembruscki 1979). Due to the absence of important adjacent fluvial sources, Late Quaternary depositional processes on the São Paulo Bight have been considered for decades to be a result of the reworking of sediments that had been previously deposited at sea level lowstands duringthe Late Pleistocene. More recently, a series of papershas reevaluated the sedimentary processes on the continental shelf and upper slope in terms of hydrodynamic factors and the input of terrigenous sediments. The latter is especially related to the transport of the Río de La Plata sediments to the Brazilian margin (Mahiqueset al. 2004, 2008, Campos et al. 2008).

The hydrodynamic control, together with the relative tectonic stability and the absence of post-glacial rebound, makes the area a favorable site for investigations of the Late Quaternary climatic changes of the southwestern Atlantic. The area has a big potential forstudying changes related to the Last Climatic Cycle, sub-Milankovitch variations such as latitudinal shifts ofthe Intertropical Convergence Zone (ITCZ) (Haug et al.2001), and variations in the El Niño Southern Oscillation (ENSO) (Woodroffe et al. 2003).

On the other hand, due to flourishing oil and gas exploration activities and related engineering and environmental aspects such as the installation of pipelinesand platforms, studies on sedimentary processes are of great importance.

In the last decade, a set of more than two hundred radiocarbon datings have been obtained by the authorsusing sediment samples from the São Paulo Bight. Most of these datings has never been published, although someare available in the scientific literature (Mahiques et al. 2002, 2005, 2007, 2009, Nagai et al. 2009).

Here we present an inventory of the data and analyze the radiocarbon datings in terms of Late Quaternary sedimentary processes and sedimentation rates. In addition, these data provide indications of sea level stabilization periods that occurred prior to the Holocene Climatic Optimum, a subject that is not well understood in thisarea of study.

Different aspects have been described by other authors that could make it difficult or even impossible toutilize radiocarbon datings in studies of sedimentary and stratigraphical processes in the São Paulo Bight. In particular, inconsistencies between radiocarbon datings and other geological or geochronological indicators may affect the usefulness of this method for examining such processes.

Carroll et al. (2003) analyzed the radiocarbon ages of brachiopod shells found in the first 10 cm of sediment in cores collected from the inner shelf (between5 and 23 meters). The authors identified a wide range of ages among these samples, varying from 540-410 to 2420-2240 cal yr BP. In another study comparing radiocarbon datings with ²¹⁰Pb and ¹³⁷Cs data, Figueira etal. (2007) identified a difference of one order of magnitude in estimates of the sedimentation rates in sediment samples from the continental shelf of the area. Finally, Angulo et al. (2008) observed the occurrence of radiocarbon age inversions in the coastal plain adjacent to the study area. The authors therefore defined two sets of radiocarbon ages, e.g., those representing in situ or poorly transported samples, and those indicating allochthonous, highly transported materials.

STUDY AREA

Figure 1 presents the study area. The ocean floor ofthe São Paulo Bight shows a rather complex morphology involving channels, canyons, and considerable variations in slope morphology (Furtado et al. 1996). Thes helf break is located at a water depth of approximately 140 meters, with the upper slope showing an average gradient of approximately 1:55.

The distribution of surface sediments on the southeastern Brazilian margin was extensively studied in the decade of 1970 and is described in the papers of Rocha et al. (1975) and Kowsmann and Costa (1979). In general, the present sea floor is covered by very fine siliciclastic sands and silts with variable amounts of clayand calcium carbonate. Coarser terrigenous sediments, carbonate gravel and boulder facies found on the outer shelf represent less than 5% of the present bottom andare generally related to relict sediments that are deposited under lower sea level conditions.

Sedimentary processes in the area have been reevaluated in recent papers in terms of the controlling hydrodynamic processes. On the inner shelf, the sedimentation is mainly determined by the displacement of the Brazilian Coastal Current (Souza and Robinson 2004), which carries sediments from the Río de La Plata and, to a lesser extent, from the southern Brazilian coastal lagoons (Campos et al. 2008, Möller Jr. etal. 2008, Mahiques et al. 2008). On the middle and outer shelves, as well as on the upper slope, the sedimentary processes are mainly influenced by the southward flow of the Brazil Current (BC) along the western Atlantic continental margin (Mahiques et al. 2002,2004).

METHODS

In this study, we organized all of the information obtained by the authors on radiocarbon datings performedon sediment samples collected along the São Paulo Bight(Fig. 1). A total of 238 radiocarbon datings obtainedfrom materials collected with different sampling procedures (scuba diving, box cores, piston cores, vibracores) (see Tables I to IV for details) were considered for this work.

With few exceptions, the content of suitable carbonate materials in the collected cores was insufficient for radiocarbon dating, and at several sampling sites the bulk organic fraction was used for dating. Almost all of the samples were AMS dated at Beta AnalyticInc. (Miami, USA). Calibrated ages of marine samples (preserved bivalve shells, corals, specimens of the planktonic foraminifer Globigerinoides ruberon core 7485 and the benthic foraminifer Elphydium sp on core SSB01) were calculated using Calib software, version 5.0.2 html, available at http://calib.qub.ac.uk/calib/. Weemployed the standard marine correction of 408 yearsand a regional reservoir effect of Δ R = 12 ± 60, corresponding to the average value of eleven samples reported for the SE Brazilian coast between the latitudes 23ºS and 28ºS (Nadal de Masi 1999, Eastoe et al. 2002, Angulo et al. 2005) and the Marine 04 Calibration Dataset (Hughen et al. 2004). For terrestrial material, the Southern Hemisphere Calibration Curve (McCormacet al. 2004) was used. Finally, for organic matter samples, the Mixed-Marine calibration curve was used. Estimates of marine versus terrestrial percentages are based on δ¹³C end-members for the São Paulo Bight (-19.00% PDB for marine and -26.00% PDB) (Mahiques et al. 1999).

Average sedimentation rates were obtained by linear interpolation of calibrated ages only for those samples that presented at least three datings with Holocene ages, assuming that the surface sediment was modern (0 kyr). For each of these average rates, the significance level was determined as shown in Tables I to III.

 

RESULTS

Tables I to III present the results of the radiocarbon datings. In order to develop a better comprehension ofthe sedimentary processes in the context of these data,the samples were divided into three different geomorphological sectors, i.e., "Coast", comprising the submerged coastal environments up to the 20-meter isobath; "Inner and Middle Shelf", comprising the samples located from the 20-meter to the 100-meter isobath; and "Outer Shelf/Upper Slope", beyond the 100-meter isobath.

The samples from the Coast sector comprise 69 datings that were performed on seven vibracores, fivebox cores and four beach rock samples collected by scuba diving (Table I). In this sector, most of the datings showed Mid-to Late Holocene ages with the exception of the base of one core (SS1), which showed a coherent sequence of datings (foraminifers and organic matter) with conventional radiocarbon ages ranging from ca. 39,300 yr BP to 10,240 yr BP. Age inversions are not rare in core samples from this sector, and incoherencies between carbonate-based (mollusk) and organic matter-based radiocarbon ages were also observed. Due to these complications, the estimates of sedimentation rates were performed at only three vibracore and two box core stations, providing a range varying from 12 ± 2 cm.kyr^-1to 68 ± 2 cm.kyr^-1. The highest sedimentation rate value was obtained from avibracore sampled in the Mamanguá inlet (23º17.40'S044º38.88'W), a coastal system with geomorphological and sedimentological characteristics that are very similar to those present in the northwestern Iberian Rías(Méndez and Vilas 2005). The lowest sedimentation rate values were observed in cores collected in the São Sebastião Channel (23º45.12'S-045º22.62'W) and in Santos Bay (24º00.00'S-046º20.58'W), two coastal systems in which the wave action is more effective.

Samples from the Inner and Middle Shelf sector (Table II) correspond to 98 datings from 17 box coresand 5 piston cores. Most of these show a rather coherent pattern of radiocarbon dating, with only a few age inversions at the base of some cores and a prevalence of Mid-to Late-Holocene calibrated ages. One of the piston core samples (7616, 25º05.88'S-045º38.64'W) presented an erosional contact at its lowermost levels. Corresponding radiocarbon hiatuses detected at these levels showed indications of the transition from Late Pleistocene sandy beach facies, with conventional radiocarbon ages ranging from 13370 ± 70 to 12170 ± 70 yr BP, to a Mid-Holocene muddy shelf sediment.

Sedimentation rate estimates were calculated basedon 11 box cores and 5 piston cores; the highest values(40 ± 9 and 62 ± 10 cm.kyr^-1) were found in samples from the Cabo Frio and the Santa Catarina upwelling zones. Lower sedimentation rate values (lower than15 cm.kyr^-1) were found in two samples located close to the 100-meter isobath (samples 6561 and 7605).

Samples from the Outer Shelf/Upper Slope sector (Table III) comprise 71 datings from 21 box cores and 3 piston cores. This sector is characterized by very low sedimentation rate values that were present in several samples. These values indicate the presence of relictterms such as those found in samples 6573, 6626, 6652 and 6680, in which Pleistocene ages were reached ata thickness of few centimeters of sediment (see Table III for location). Also, a 4.5-meter long core (7607,27º22.14'S-47º08.40'W), which was collected at 287 meters below sea level (mbsl) and was composed exclusively of massive mud, exhibited a sedimentary column with a completely fuzzy pattern and ages ranging between 30000 and 40000 yr BP These features lead usto consider this latter core as an evidence of a relict slump. The highest sedimentation rates were found inthe shallowest samples of the sector, as well as in one sample located in the 485-meter isobath (Sample 6630,25º49.20'S-45º17.88'W).

DISCUSSION

SEDIMENTARY PROCESSES

As a rule, the Southwestern Atlantic upper margin is marked by low sedimentation rates. Nevertheless, when compared with its SE counterpart, the South African margin, marked by intense upwelling and terrigenous input (Herbert and Compton 2007), the pattern observed in our study area shows important similarities, such as the presence of a starving surface on the outer shelf (Compton and Wiltshire 2009, Compton et al. 2010). In the Holocene mudbelt of the western margin of South Africa, Herbert and Compton (2007) estimated sedimentation rate values ranging from 25 to 240 cm.kyr^-1, whereas on the slope, the values varied from 4 to22 cm.kyr^-1.

The sedimentation rates calculated here never exceeded the limit of 70 cm.kyr^-1, and estimates from the Coast sector have the same order of magnitude than those from the Inner and Middle Shelf sector (Fig. 2).Outer Shelf/Upper Slope sector sediments present sedimentation rate values that are negligible, indicating the relict character of part of this sector.

Cores from the Coast sector also exhibit several age inversions, which may represent both sediment reworking due to hydrodynamic factors or biological activity. This aspect has been previously analyzed in the scientific literature (Kinoshita et al. 2002, Wood et al. 2006).

The highest sedimentation rates on the shelf werefound on the Cabo Frio (around 23ºS) and Santa Catarina (around 27ºS) upwelling zones, which may be anindication that the marine productivity matter may represent a significant process on the sedimentary processes. On the other hand, these two areas are located closer to the main potential allochthonous sources of terrigenous sediments in the area, e.g., the Paraíba do Sul and La Plata rivers.

Figure 2 also shows a marked latitudinal break at 25ºS, with an abrupt decrease in sedimentation rates northward to São Sebastião Island. In fact, this break marks the limit of influence of the sediments originating from the Río de La Plata as stated by Campos et al.(2008) and Mahiques et al. (2008).

There is a significant difference in terms of sedimentation rates when comparing the Middle Shelf sector with the Outer Shelf/Upper Slope sector sediments. Figure 3 shows a cross-shelf shallow seismic (chirp) profile in which it is possible to recognize a marked breakin the sedimentary processes. The sea bottom belowthe 140-meter isobaths is marked by a strong roughness that, in conjunction with the low sedimentation rates obtained, indicates the effectiveness of the Brazil Current(BC) moving over the outer shelf and upper slope inreworking sediments (Silveira et al. 2001). This aspecthas been locally pointed out in previous papers (Macarioet al. 2004, Mahiques et al. 2007) but, considering our data, it acquires a regional importance that extends allalong the outer shelf and upper slope, between 23ºSand 27ºS.

A representative cross-margin transect of current speeds is shown in Figure 4. The strong correlation between both speed and sedimentation rate can be seen. In this sense, the slight increase in sedimentation rate at the 450-and 500-meter isobaths is coincident with the transition between the southward flow of the Brazil Current (BC) and the northward flow of the Intermediate Western Boundary Current.

EVIDENCE OF SEA LEVEL STABILIZATIONS PRIOR TO THE MID-HOLOCENE MAXIMUM

The scarcity of datings and reliable sea level indicatorsis the main characteristic of the sea level curves of the southern Brazilian shelf prior to 7,000 cal yr BP. Mostof the papers published on the subject is restricted tocorrelations of the morphose dimentary features such assubmerged terraces with global sea level curves. Thestudy by Correa (1996) is one of the exceptions and, according to this report, stabilization periods occurred at 9,000 cal yr BP (between 32 and 45 meters below sea level) (mbsl) and 8,000 cal yr BP (between 20 and25 mbsl). We found some indication of these paleo-sealevels. A core collected on the coast off São Sebastião (SS1, see Table I) represents a sequence of sediments varying from a mixohalyne environment, dated at 39000 cal yr BP (organic matter and benthic foraminifers) at 300 cm (6 meters bmsl) to fresh water sediments atresents a paradox when the worldwide sea level curves136 cm dated at 10240 ± 40 yr BP. This depth also are considered. In this sense, one hypothesis to explainmarks an erosional contact, and sediments located above these anomalous ages would be a general contaminationit exhibit properties of the Mid-to Late-Holocene ages of organic matter originally deposited during the Iso-Due to the location of the core at the present watertope Stage 5e by young carbon (Hanebuth et al. 2006).depth of 3 mbsl, this evidence of a coastal environment On the other hand, the coherency of the data along thein such shallow waters during the Isotope Stage 3 rep-core is noteworthy. These data do not represent the first evidence of high sea level during the Isotope Stage 3; in fact, other indications of sea level highstands of the same age have been found worldwide (Mausz and Hassler 2000, Rodriguez et al. 2000, Hanebuth et al. 2006, Angulo et al. 2008, among others). Nevertheless, further work is needed to improve our understanding of the significance of these data.

Sedimentological evidence of past sea levels was also found in cores FLT01 (Fig. 5) and 7616 (see Tables I and II for further information). Both cores exhibit a passage from sandy beach sediments, attributed to intense bioturbation from Callichirus major (Crustacea, Thalassinidea), to sandy mud sediments. In core 7616, located at 100 mbsl, the top of the sandy facies was dated at 12170±70 yr BP (beyond the limit of calibration). In core FLT01, collected at a water depth of 3 mbsl, the top of the facies was dated at 7470±60 yr BP (7734-8116 cal yr BP), as previously reported by Mahiques and Souza (1999).

Finally, a beach rock located at 13 mbsl presented a consistent set of four datings obtained from different materials (total rock, bivalve and fecal pellets), presenting an average age of 8470 ± 110 cal yr BP (Fig. 6).

Although the assumption that beach rocks represent the sea level during the time of their formation may not be valid (Kelletat 2006), these data may indicate theminimum depth of a sea level stabilization that occurred immediately before the Melt Water Pulse I-C, as reported in the scientific literature (Clark et al. 2001).

CONCLUSIONS

In this paper we summarize all of the information on radiocarbon datings compiled by the authors in the SãoPaulo Bight (southern Brazilian upper margin). Our results confirm the strong dependency of the dynamics ofthe shelf current system, as well as those of the Brazil Current-Intermediate Western Boundary Current (BC-IWBC) system, in the sedimentary processes of the area.

The sediments from the Coast sector exhibit sedimentation rates that may vary from 12 to 68 cm.kyr^-1.Several cores from this data set exhibit characteristics of reworking and/or bioturbation, as has been previously observed by other authors.

The sediments from the Inner and Middle Shelf sector present rates that are equivalent to those of the Coast. The highest values of sedimentation rates werefound in the zones that are more favorable for upwelling processes, suggesting that organic production may actas an important source for particulate sediments. Nevertheless, the proximity of these areas to the main source of terrigenous input must not be neglected.

The sediments from the Outer Shelf/Upper Slopesector are those that are directly affected by the BC-IWBC system. As a rule, there is a clear relationship between current speeds and sedimentation rates. A Transition Zone between the cores of these two main flows is also recognizable in the sedimentation rate values.

At least three indicators of the paleo sea level at12200 yr BP (conventional radiocarbon age) (103 mbsl),8300-8800 cal yr BP (13 mbsl) and 7700-8100 cal yr BP(6 mbsl) were found. A Marine Isotope Stage 3 highstand at 6 mbsl found in the coast off the São Paulo State will require further study for confirmation.

ACKNOWLEDGMENTS

This paper is a contribution to the IGCP526 Project (Risks, Resources, Record of the Past on the Continental Shelf). It also represents a contribution from the researchers of the Institute of Oceanography of the University of São Paulo, to the Brazilian Program of Marine Geology and Geophysics (PGGM). Financial support was provided by Fundação de Amparo à Pesquisado Estado de São Paulo (FAPESP) (grants 03/107400 and 07/54657-0) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (300593/2007-4).

Manuscript received on October 14, 2009; accepted for publication on October 14, 2010

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Publication Dates

• Publication in this collection
  29 July 2011
• Date of issue
  Sept 2011

History

• Accepted
  14 Oct 2010
• Received
  14 Oct 2009