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PLANKTONIC FORAMINIFERAL VARIATIONS IN THE SOUTHWESTERNATLANTIC SINCE THE LAST GLACIAL–INTERGLACIAL CYCLEAuthor(s): RODRIGO DA COSTA PORTILHO-RAMOS, CÁTIA FERNANDES BARBOSA, andARISTÓTELES MORAES RIOS-NETTOSource: PALAIOS, 29(1):38-44. 2014.Published By: Society for Sedimentary GeologyURL: http://www.bioone.org/doi/full/10.2110/palo.2012.104

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PALAIOS, 2014, v. 29, 38–44

Research Article

DOI: http://dx.doi.org/10.2110/palo.2012.104

PLANKTONIC FORAMINIFERAL VARIATIONS IN THE SOUTHWESTERN ATLANTIC SINCE THE LASTGLACIAL–INTERGLACIAL CYCLE

RODRIGO DA COSTA PORTILHO-RAMOS,1 CATIA FERNANDES BARBOSA,1 AND ARISTOTELES MORAES RIOS-NETTO2

1Departamento de Geoquımica, Universidade Federal Fluminense, Outeiro de Sao Joao Baptista, s/n, CEP: 24020-141, Centro, Niteroi, Rio de Janeiro, Brazil2Departamento de Geologia, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos 274, CCMN, Bloco G, Rio de Janeiro, R.J., CEP21941-916, Brazil

e-mail: catia@geoq.uff.br

ABSTRACT: Relative abundances of planktonic foraminiferal species from two cores from the subtropical, southwestern Atlanticindicate changes in oceanic conditions during the last glacial–interglacial cycle. During interglacial intervals (biozone X or MarineIsotopic Stage 5 (MIS-5) and biozone Z or MIS-1), the relative abundance of the intermediate water-dwelling species ofmenardiform plexus was high, whereas those of deep-dwelling species Globorotalia truncatulinoides and Globorotalia inflata wererelatively low, suggesting high temperature conditions ($22uC) and/or increased upper-water stratification during this period oftime. In contrast, the absence of menardiform plexus and high abundances of cold-water species (G. truncatulinoides and G.inflata) during the glacial interval (biozone Y [MIS-4, MIS-3, and MIS-2]) suggest cold-water conditions (#22uC) and/or areduction of upper-water stratification. Two intervals of moderate temperature and/or low salinity during the last glaciation,however, are suggested by the increase in abundance of Pulleniatina plexus. Foraminiferal fauna have suggested a difference of 1to 2uC between the last glacial interval and the late Holocene. Millennial-scale paleoceanographic events have been identifiedduring the last interglacial interval (MIS-5), suggesting that warm conditions and/or a stratified water column were replaced byshort intervals of cooler water and reduced upper-water stratification, as indicated by changes in abundances of menardiformplexus and G. truncatulinoides. The short interval during which menardiform plexus disappeared during the Holocene suggests atemperature decrease and can be related to the widespread stadial event in the Holocene at 8.2 ka.

INTRODUCTION

The distribution of planktonic foraminifera in the oceans is influencedby such environmental parameters as temperature, salinity, productivity,and upper-ocean structure (Ravelo et al. 1990; Boltovskoy et al. 1996;Mulitza et al. 1997; Steph et al. 2009; Rigual-Hernandez et al. 2012).Planktonic foraminifera live at different depths in the water column, withdifferent groups recording different environmental conditions. In thewestern subtropical South Atlantic, foraminifera directly indicate thetemperature and/or salinity conditions of surface waters, with a dominanceof warm-water species north of the Brazil–Malvinas Confluence (BMC)and cold-water and/or transitional species south of the BMC (Boltovskoyet al. 1996; Niebler and Gersonde 1998; Chiessi et al. 2007). In this context,planktonic foraminifera have been used as a powerful tool to reconstructpast oceanic temperature (Niebler and Gersonde 1998; Hayes et al. 2005;Kucera et al. 2005; Rouis-Zargouni et al. 2010), salinity (Xu et al. 2005;Toledo et al. 2007a; Carlson et al. 2008), productivity (Toledo et al. 2007b),upper–water-column stratification (Wolff et al. 1999; Huang et al. 2003),paleocirculation (Baohua et al. 1997; Eberwein and Mackensen 2008;Toledo et al. 2008), and upwelling intensity (Little et al. 1997; Huang et al.2002; West et al. 2004; Souto et al. 2011).

The South Atlantic Ocean plays a crucial role in global heat transportbecause it (1) links the Indian, Pacific, and North Atlantic oceans; (2) isthe main source of heat to the North Atlantic; and (3) is an area of deep-water formation (Stramma et al. 1990; Curry and Oppo 2005; Schott et al.2005). Few studies, however, have described the planktonic foraminiferalassemblages in the western subtropical South Atlantic during the lastglacial–interglacial cycle (Toledo et al. 2007a, 2007b) and during the

present interglacial (Souto et al. 2011). Thus, this study aims toreconstruct past ocean conditions recorded in two cores retrieved fromthe subtropical southwestern Atlantic through the relative abundance ofplanktonic foraminiferal species.

STUDY SITES

The surface ocean circulation of the southern Brazilian continentalmargin, which includes the region studied in this work, is under theinfluence of the Brazil Current (BC), which is characterized by warm,salty, and oligotrophic waters, consisting mainly of tropical waters(temperature .20uC, salinity .36 psu; Silveira et al. 2000). The BCoriginates from the bifurcation of the South Equatorial Current (SEC),which flows toward the west. Near 10u to 11uS, the SEC divides into theNorth Brazil Current (NBC), which flows to the north, and the BC, whichflows to the south (Stramma et al. 1990; Schott et al. 2005; Barbosa et al.2012) (Fig. 1). The intensity of both currents varies seasonally, dependingon the wind intensity and the Intertropical Convergence Zone position,with the strengthening of the BC occurring during the austral spring andsummer (Chang et al. 1997; Johns et al. 1998). The volume transported bythe BC does not change significantly between 10u and 20uS (Stramma etal. 1990; Peterson and Stramma 1991). At 20uS, however, the BC receivescontributions from South Atlantic Central Water (SACW), withtemperatures from 6 to 20uC and salinities from 34.6 to 36 psu, andbecomes deeper, reaching 750 m in thickness at 28uS (Silveira et al. 2000).

To the south of our studied area, the BC encounters the MalvinasCurrent (MC), a cold, relatively fresh, and nutrient-rich current that flowsnorthward, adjacent to the Argentine continental margin, transporting

Published Online: June 2014

Copyright E 2014, SEPM (Society for Sedimentary Geology) 0883-1351/14/029-038/$03.00

subantarctic waters (with temperatures from 5 to 9uC and salinities of 33–34 psu). The MC originates from the Antarctic Circumpolar Current,which flows northward after crossing the Drake Passage (Wainer et al.2000; Wainer and Venegas 2002) (Fig. 1). The MC and the BC meet in theBMC. The BMC is located between 35uS and 40uS (Wainer et al. 2000;Chiessi et al. 2007), but the BMC shifts seasonally to the north and southduring the austral winter and summer, respectively (Boltovskoy et al.1996; Wainer and Venegas 2002).

MATERIALS AND METHODS

The cores used in this study were collected from the southerncontinental slope of the Brazilian margin (Fig. 1). The piston coresJPC-17 (27u41.839S, 46u29.649W, at a water depth of 1627 m) and JPC-95(27u31.649S, 46u33.159W, at a water depth of 2485 m) were collectedduring the KNORR 159-5 cruise from Woods Hole OceanographicInstitution (WHOI, Massachusetts). These cores were sampled continu-ously at a resolution of 10 cm. The samples were washed through sieveswith a 62-mm mesh size, dried, and split into mesh sizes of 125 and 250 mm.Classification was performed by counting a minimum of 300 specimensper sample, from which significant paleoceanographic species were

selected. The taxa identification was based on Bolli and Saunders(1985) and Stainforth et al. (1975).

The age model of the cores was based on accelerator mass spectrometry(AMS 14C dating) and planktonic biostratigraphy. All radiocarbon ageswere corrected for a reservoir age of 400 years (Butzin et al. 2005) andconverted into years before present using the Software Fairbanks0805(Fairbanks et al. 2005) (Table 1). For portions of the core lying beyondthe range of the radiocarbon method (50 cal kyr and older), we used theQuaternary planktonic foraminiferal biostratigraphy proposed by Eric-son and Wollin (1968) and Vicalvi (1997) to estimate the sediment ages.

RESULTS

Biostratigraphy and Chronology

The frequency pattern (presence or absence) of menardiform plexus inmarine cores has been widely applied as an indicator of paleoclimaticfluctuations of the late Quaternary, and menardiform plexus is the maingroup of planktonic foraminiferal species used in the biostratigraphiczonation of this interval (Ericson and Wollin 1968; Martin et al. 1993;Vicalvi 1997; Slowey et al. 2002; Xu et al. 2005; Portilho-Ramos et al. 2006).The interval from the base of core JPC-95 to 920 cm depth was identified asbelonging to the biozone X (corresponding to MIS-5; Fig. 2). Thisidentification was confirmed by the presence of menardiform plexus(.5%), including Globorotalia flexuosa, a species characteristic of thisbiozone. The species Globorotalia truncatulinoides and Globorotalia inflatarevealed a rather uniform frequency pattern during the entire interval, withaverage percentages of 1.6 and 2.8%, respectively. The species Orbulinauniversa, Neogloboquadrina dutertrei, and Globorotalia crassaformis exhibitedlow abundances, with average percentages of 0.1, 1, and 0.4%, respectively.

There were fluctuations in the frequency of menardiform plexus alongbiozone X (JPC-95; Fig. 2), with fluctuation frequencies corresponding tosubzones X1 to X8, as proposed by Vicalvi (1997). Thus, intervals ofmenardiform plexus presence correspond to the subzones X7, X5, X3,and X1, whereas intervals of its absence correspond to subzones X8, X6,X4, and X2. The intervals in which menardiform plexus is present (oddsubzones) occur simultaneously with low abundances of G. truncatuli-noides, whereas intervals in which menardiform plexus is absent (evensubzones) are accompanied by increases in G. trucatulinoides (Fig. 2). Theboundary between subzone X (MIS-5) and subzone Y (MIS-4), with anage estimated by Vicalvi (1997) of 84 ka, was found only in JPC-95(between 920 and 910 cm), and this boundary is characterized by thedisappearance of menardiform plexus.

The biostratigraphical intervals between 910 and 20 cm in core JPC-95and between 770 and 70 cm in core JPC-17 correspond to biozone Y(MIS-2, MIS-3, and MIS-4), which is characterized by the absence or rareoccurrence of menardiform plexus, by the increase in the relativeabundance of such deep-dwelling species as G. truncatulinoides, N.dutertrei ($3%), G. inflata (.8%), and G. crassaformis (15%), and by thedisappearance and reappearance of Pulleniatina plexus (Figs. 2–3).Pulleniatina plexus was found in two intervals (<1%) during the lastglaciation, which correspond to subzones Y4 and Y2. These subzones aretwo of five subzones (Y1, Y2, Y3, Y4, and Y5). The Y5–Y4 boundarywas found at 725 cm in core JPC-17 (at 795 cm in JPC-95), and the

FIG. 1.—Location of cores JPC-17 and JPC-95, in the southwestern AtlanticOcean. Schematic representation of South Equatorial Current (SEC), North BrazilCurrent (NBC), Malvinas Current, and Brazil–Malvinas Confluence.

TABLE 1.— 14C ages obtained by accelerator mass spectrometry (AMS) dating of bulk of planktonic foraminifera.

Core Lab ID Depth (cm) Material 14C age (kyr) Error (yr) Cal age (kyr) Error (yr) Lab

JPC-17 HYOS-27531 56 Globorotalia ruber 10.0 660 10.967 6147 WHOISacA18294 190–191 BPF 32.95 6270 37.95 6312 LMC14

JPC-95 HYOS-34254 16.6 Globigerinoides ruber 21.80 6130 25.68 6217 WHOI

BPF 5 Bulk of planktonic foraminifera.

UPPER QUATERNARY BIOSTRATIGRAPHY 39P A L A I O S

Y4–Y3 boundary was found at 575 cm in JPC-17 (665 cm in JPC-95),with ages estimated between 67 and 74 ka. The Y3–Y2 boundary wasidentified at 435 cm in JPC-17 (195 cm in JPC-95), and the Y2–Y1boundary was at 225 cm in JPC-17 (65 cm in JPC-95). The Y2–Y1boundary is characterized by the last disappearance of Pulleniatina

plexus, with a relative age estimated between 42 and 45 ka (Vicalvi 1997).The relative age provided by biostratigraphy can be confirmed and thuscalibrated by AMS 14C dating of JPC-17, indicating an absolute age of37.95 6 312 ka at a depth of 190 cm. Linear extrapolation produces anage at the Y2–Y1 boundary of 45 ka, which is in agreement with that ofVicalvi (1997) (Fig. 3).

The return of menardiform plexus and the drop in abundances of G.

truncatulinoides and G. inflata, which were observed at the top of the

cores characterizing biozone Z (MIS-1), were identified at 65 cm inJPC-17 and at 16.5 cm in JPC-95, with absolute ages (AMS 14C dating)of 11 6 146 and 26.2 6 178 kyr, respectively (Table 1). The analysis ofcore JPC-95 at 16.5 cm depth revealed a sedimentation rate of 0.4 cm/kyr between 26.2 and 11 kyr (between 16.5 and 10 cm depths). Theinterval from 26 to 11 kyr includes the last glacial maximum, whichcorresponds to a time interval during which sea level was <105 mbelow the present level (Siddall et al. 2003), implying an increase in thesedimentation rate due to greater terrigenous input. Our results suggestthat the final portion of the Pleistocene (the last glacial period)sediments and a large part of the Holocene sediments from core JPC-95may have been lost as a result of gravitational episodes such asslumping or turbidity currents.

FIG. 2.—Abundance of planktonic foraminiferal taxa along the core JPC-95, showing the biozones X, Y, and Z, the subzones X1 to X8, Y1 to Y5 and the three stagesof regional disappearance of the Pulleniatina plexus (Pulleniatina obliquiloculata biohorizons: YP.1, YP.2, and YP.3 with relative ages of 84 ka, 67–74 ka, and 42–45 ka,respectively). Black arrow shows the AMS 14C dating (16.6 cm 5 25.7 6 217 cal kyr before present).

FIG. 3.—Planktonic foraminiferal frequency of the upper 7.7 m of core JPC-17, showing the biozone Y and Z and the subzones Y1 to Y5 and the two stages of regionaldisappearance of the Pulleniatina plexus (Pulleniatina obliquiloculata biohorizons: YP.2 and YP.3 with relative ages of 67–74 ka and 42–45 ka, respectively). Black arrowsshow the AMS 14C dating.

40 R.C. PORTILHO-RAMOS ET AL. P A L A I O S

DISCUSSION

Glacial–Interglacial Upper-Ocean Conditions

Different planktonic foraminifera live at different depths in the watercolumn, recording environmental conditions. Relative abundances ofsuch deep-dwelling species as G. truncatulinoides and G. inflata and ofsuch intermediate-dwelling species as Globorotalia menardii have beenused to reconstruct the temperature and element profiles of the upperocean during the past (Mulitza et al. 1997; Wolff et al. 1999; Huang et al.2002, 2003). Mulitza et al. (1997) suggested that the opposite abundancepattern of these species, compared with those patterns typically found inthe water column, reflect changes in thermocline depth in the Caribbeanregion during the last glacial cycle. The reduction of upper-waterstratification may have favored an increase in the abundance of G.truncatulinoides during the last glacial period, whereas increasing upper-water stratification during the Holocene (MIS-1) may have favored theproliferation of G. menardii. These responses are explained by the factthat an increase in upper-water stratification hampers the penetration ofG. truncatulinoides juveniles into the photic zone, reducing theirabundance (Mulitza et al. 1997; Wolff et al. 1999). The same responseshave been suggested by Huang et al. (2003), using G. inflata in cores fromthe South China Sea during the Miocene, a period during which highabundances of G. menardii and low abundances of G. inflata indicate awarm and well-stratified water column.

The abundance of menardiform plexus was high in biozones X and Z(MIS-5 and MIS-1), whereas the abundance of G. truncatulinoides and G.inflata was relatively low during these time periods. These patternssuggest warm-water conditions and/or an increase in upper-waterstratification during these intervals (Figs. 2–3). In biozone Y (MIS-4and MIS-2), menardiform plexus disappeared completely from ourrecords, whereas the abundance of G. truncatulinoides and G. inflataincreased. These findings suggest cold-water conditions and/or decreasedupper-water stratification. The species G. inflata recorded in both westernSouth Atlantic cores analyzed in this study indicated paleoceanographicthermal stratification changes that occurred during the last glacial–interglacial cycle.

Glacial–Interglacial Paleotemperature Estimation

In the South Atlantic, G. trucatulinoides and G. inflata are found livingin regions where temperatures are lower than 22uC, whereas menardiformplexus species are found living to the north of the subtropical gyre, wherethe temperature is higher than 22uC (Niebler and Gersonde 1998). Thetemperature in the subtropical southwestern Atlantic was, therefore,likely higher than 22uC during the interglacial intervals (biozone X and Z)and lower than 22uC during the last glacial interval (biozone Y). A higherabundance of menardiform plexus in biozone X relative to biozone Zsuggests that the period represented by biozone X was warmer than theHolocene interglacial (Vicalvi 1997; Portilho-Ramos et al. 2006). Toestimate exactly how warm the period represented by biozone X was,however, is impossible.

The sea surface temperature (SST) of the glacial South Atlanticsubtropical gyre has been estimated to be between 20 and 23uC during thecold season and between 22 and 24uC during the warm season (Kucera etal. 2005), which is consistent with our interpretations based on the biotain the two cores analyzed in this study. The current annual temperatureaverage is approximately 23 to 24uC (World Ocean Atlas 2009 (WOA09),Locarnini et al. 2010), suggesting a difference of 1 to 2uC between the lastglacial period and the Holocene. This difference is consistent with that ofCarlson et al. (2008), who suggested an increase of 1uC during the lastdeglaciation.

Paleoceanographic records have suggested a temperature difference of2 to 3uC between the last glaciation and the Holocene in the equatorial

region (Lear et al. 2000; Barker et al. 2005; Weldeab et al. 2006) and adifference of approximately 3 to 4uC near Antarctica (Brathauer andAbelmann 1999; Gersonde et al. 2005). This cooling could have causedan advance of Antarctic sea ice (Gersonde et al. 2005) and an advance ofpolar and subpolar fronts 3u northward toward the equator (Brathauerand Abelmann 1999) during the last glaciation. Latitudinal migrationsof the subtropical front (subtropical gyre) could have had a stronginfluence on the distribution of planktonic foraminifera during the lastglacial–interglacial cycle that was recorded in the studied cores.Southward displacements of the subtropical front and of the Westerliesduring the interglacial period facilitated the entrance of warm and saltywaters from the Indian Ocean into the South Atlantic through theAgulhas leakage (the warm route) (Beal et al. 2011). This processincreased the temperature and salinity of the South Atlantic, whichfavored the growth of warm tropical species such as G. menardii on thewestern side of the ocean, as can be noted in our records. Alternatively,northward displacement of the subtropical front and of the Westerliesduring glacial intervals reduced the Agulhas leakage (Beal et al. 2011),decreasing South Atlantic temperature and salinity and favoring theoccurrence of such cold and/or transitional water species as G.truncatulinoides and G. inflata.

Millennial-Scale Events in Biozone X (MIS-5)

The last interglacial interval of the Pleistocene (MIS-5) was a period ofhigh temperatures (<2uC warmer than present, Turney and Jones 2010)during which atmospheric concentrations of CO2 (<280 ppmv) and CH4

(<700 ppbv) were high (Petit et al. 1999) and the ocean exhibited anactive thermohaline circulation (Schmidt et al. 2004b). This interval,however, was marked by millennial-scale climatic instabilities that wererecorded in Greenland Ice cores (Dansgaard et al. 1993) and in NorthAtlantic and Mediterranean sediment cores (McManus et al. 1994, 2002;Kukla et al. 1997; Heusser and Oppo 2003; Martrat et al. 2004; Sprovieriet al. 2006).

These abrupt paleoclimatic events are indicated by penetrations of coldpolar water into subpolar regions; by oscillations in North Atlantic andMediterranean sea surface temperatures (McManus et al. 1994, 2002;Kukla et al. 1997; Martrat et al. 2004; Sprovieri et al. 2006); by changes inpollen composition (vegetation) in Grande Pile in France (Kukla et al.1997); and by oscillations of Pinus (pine) and Quercus (oak) forests in thesoutheastern United States (Heusser and Oppo 2003). Paleoclimaticrecords from the North Atlantic (McManus et al. 1994) and theMediterranean Sea (Sprovieri et al. 2006) have implicated six oceaniccold events (C20–C25), with SST reductions up to 27.7uC (Martrat et al.2004) that are closely related to millennial oscillations in d18O recorded inGreenland ice cores (Dansgaard et al. 1993).

In this study, MIS-5 was identified only in core JPC-95, and theplanktonic foraminiferal assemblage found during this period of timesuggests warm-water conditions and/or strong upper-water stratification.Short intervals of menardiform plexus disappearances, accompanied byhigh abundances of G. truncatulinoides, however, were observed inbiozone X, indicating that warm water and/or strong upper-oceanstratification conditions were replaced by short intervals of cold waterand/or reductions in upper-water stratification. Such short-term coolingevents and reduction in upper-water stratification in our records can mostlikely be associated with subtle periods of worldwide climate changeduring MIS-5 (McManus et al. 1994, 2002; Kukla et al. 1997; Heusser andOppo 2003; Sprovieri et al. 2006); however, linking the JPC-95 events tothose recorded in the Northern hemisphere is not yet possible because ofour limited age model. Nevertheless, our results indicate that millennial-scale events have also occurred in the southwestern subtropical AtlanticOcean during the last interglacial period, as indicated by changes inupper-ocean conditions.

UPPER QUATERNARY BIOSTRATIGRAPHY 41P A L A I O S

Millennial-Scale Events in Biozone Y (MIS-4 to MIS-2)

Fluctuations in Pulleniatina plexus abundance during the last glaciation(biozone Y, equivalent to MIS-2, 3, and 4) were found in both cores(Figs. 2–3). Pulleniatina plexus is populated by thermocline-dwellingspecies that are generally found in tropical and subtropical regionsdominated by warm and salty currents (Be 1976; Boltovskoy et al. 1996;Baohua et al. 1997; Schmidt et al. 2004a; Xu et al. 2005; Rincon-Martınezet al. 2011). In the western Pacific Ocean, the highest (lowest) abundanceof Pulleniatina plexus reflects a strengthening (weakening) of theKuroshio Current during the last 20 kyr (Baohua et al. 1997).

Pulleniatina abundance is also closely related to ocean salinity (Prelland Damuth 1978; Xu et al. 2005). In the North Atlantic, thedisappearance of Pulleniatina developed in a diachronic and progressivefashion approaching the Equator (60, 50, and 35 ka in the Gulf ofMexico, West Caribbean, and Equatorial Atlantic, respectively). Thedisappearance of Pulleniatina was associated with changes in localsalinity, with disappearances occurring first in areas of higher salinity(Prell and Damuth 1978). Xu et al. (2005), however, observed an increasein Pulleniatina obliquiloculata abundances in the South China Sea duringglacial intervals over the last 850 kyr. These increases in P. obliquiloculataabundances were caused by salinity increases associated with changes inthe regional oceanic circulation.

In South Atlantic records of the last glaciation, events known as P.obliquiloculata biohorizons (YP) are identified based on the frequency ofthe Pulleniatina plexus disappearances and reappearances (Vicalvi 1997;Portilho-Ramos et al. 2006). According to these authors, Pulleniatinaplexus species are more tolerant to temperature changes than thosespecies from the menardiform plexus, which could explain the occurrenceof Pulleniatina plexus during glacial intervals, suggesting mild heatingand/or a reduction in salinity. In this context, the occurrence ofPulleniatina plexus during the last glacial interval (biozone Y) character-izes subzones Y4 and Y2 and suggests a period of less cold and/or lesssaline surface waters despite glacial conditions, whereas its absencecharacterizes subzones Y5, Y3, and Y1 and suggests colder and saltierwaters relative to subzones Y4 and Y2 (Figs. 2–3).

Millennial-Scale Events in Biozone Z (MIS-1)

Climatic variability was also recorded during the Holocene, ascharacterized by the disappearance of menardiform plexus at approxi-mately 7 to 8 ka (Fig. 3). An increase in the abundance of G. inflata and/or G. truncatulinoides is absent during this interval, suggesting apaleoceanographic event distinct from those that occurred along biozoneX (MIS-5). Thus, the absence of menardiform plexus suggests thepaleoceanographic phenomenon of low sea surface temperature, whichdeserves greater attention in future studies in this region. This event wasassociated possibly with the widespread Holocene cold event thatoccurred at 8.2 ka (Alley et al. 1997). Further studies of other Holocenesediment cores, however, are necessary to test this hypothesis.

The 8.2 ka cold event was the strongest and most prominent cold anddry climate event recorded during the last 11 kyr (Alley et al. 1997; Barberet al. 1999; Maslin et al. 2001). This event, which had a global impact, wascharacterized by (1) reductions in atmospheric CO2 concentrations(Indermuhle et al. 2000; Wagner et al. 2002); (2) cooling in the SouthAtlantic (Ljung et al. 2007); and (3) sea ice advance in Antarctica (Bianchiand Gersonde 2004). In Brazil, this 8.2 ka cold event was associated withan intensification of the South America Summer Monsoon (Cheng et al.2009).

Globorotalia crassaformis Optimum Event

Globorotalia crassaformis is a deep-dwelling species (Huang et al. 2003)that calcifies its shells at depths between 125 and 800 m and is directly

related to the photic zone depth (Ravelo and Fairbanks 1992; Steph et al.2009). In the South Atlantic, G. crassaformis generally lives south of thesubtropical gyre, where temperatures vary between 3 and 24uC, andconstitutes ,1% of the planktonic foraminiferal assemblage (Ravelo andFairbanks 1992; Niebler and Gersonde 1998). The abundance of G.crassaformis, however, increases dramatically to 5 to 8% when cold waterspenetrate into the photic zone (Ravelo and Fairbanks 1992).

In the present study, G. crassaformis was usually absent from thesediment cores analyzed, but an interval of high abundance (<15%) of G.crassaformis was observed at the beginning of the last glaciation (Figs. 2–3). This high abundance event was not accompanied by any changes inthe abundances of G. truncatulinoides, G. inflata, and menardiform andPulleniatina plexus (Figs. 2–3), indicating that this event was not causedby changes in sea surface temperature and/or salinity in water-columnstratification (thermocline variability). A low abundance of N. dutertreiduring this period suggests low productivity during this event. The G.crassaformis Optimum Event occurs as a biostratigraphical marker in thisregion and deserves attention in future studies. To identify the spatialextent and timing of this event in the South Atlantic, more analyses ofd18O and planktonic foraminiferal assemblages in sediment cores atdifferent depths are necessary.

CONCLUSION

Changes in the relative abundances of planktonic foraminiferal speciesfrom two cores from the southwestern subtropical Atlantic indicatechanges in ocean conditions during the last glacial–interglacial cycle. Inbiozone X (MIS-5) and biozone Z (MIS-1), the relative abundance of theintermediate-dwelling menardiform plexus species was high, whereasabundances of deep-dwelling species G. truncatulinoides and G. inflatawere relatively low, suggesting warm-water conditions and/or increasedupper-water stratification during these intervals. A higher abundance ofmenardiform plexus (17%) in biozone X (MIS-5) compared with itsabundance during the Holocene (2%) indicates higher sea surfacetemperature and/or stronger upper-ocean stratification during thepenultimate interglacial period, suggesting different ocean conditionsbetween the last two interglacial intervals. In contrast, the absence ofmenardiform plexus and a high abundance of cold-water species (G.truncatulinoides and G. inflata) in biozone Y (MIS-4 to MIS-2) suggestcold-water conditions and/or a reduction in upper-water stratification.Additionally, two moderate-temperature and/or low-salinity intervalsduring the last glaciation are suggested by an increase in Pulleniatinaplexus abundance.

The SST was likely higher than 22uC during interglacial intervals andlower than 22uC during the last glacial interval, based on the presentdistribution of planktonic foraminifera in the southwestern subtropicalAtlantic. Our SST estimation shows a difference of approximately 2uCbetween the Holocene and the last glacial interval, which is consistentwith previous ocean temperature estimates.

Millennial-scale paleoceanographic events recorded in the planktonicforaminiferal biostratigraphy could be identified in biozones X and Z.Short intervals of cold water and/or a less stratified upper-water columninterrupted the warm water and/or strong upper-water stratificationconditions in biozone X (MIS-5). The identification of these eventsindicates that millennial-scale events have also occurred in the south-western Atlantic Ocean during this interval.

The disappearance of menardiform plexus during the Holocenesuggests a temperature decrease and can be related to the widespreadstadial event in the Holocene that occurred at 8.2 ka. Further high-resolution studies of cores in this area, however, are necessary to verifythis hypothesis. In our two cores, an interval of high abundance of G.crassaformis was observed at the beginning of the last glacial period. Thisinterval may be a potential biostratigraphical marker and must be further

42 R.C. PORTILHO-RAMOS ET AL. P A L A I O S

studied to determine its geographical extent and temporal amplitude. Theresults in this study can be compared with subtle periods of worldwideclimate events during the last glacial–interglacial cycle in the SouthwestAtlantic and can be used in predictive models as well.

ACKNOWLEDGMENTS

Thanks to Delia Oppo, William B. Curry, and Jerry F. McManus fromWHOI-USA for providing studied cores (JPC-17 and JPC-95, KNORR 159-5), and to Ellen Roosen who sampled the cores. We are grateful to Bruno J.Turcq from IRD-France and the Laboratoire de Mesure du Carbone 14 fromUMS 2572 LMC14 (CEA-CNRS-IRD-IRSN-MCC) for the AMS 14C datingfor core JPC-17. The repository of foraminiferal samples of this study is theDepartamento de Geoquımica, Universidade Federal Fluminense. We thankGuilherme Martins for improving the English of the manuscript. We alsothank the editor and anonymous reviewers for their constructive comments.This study was funded by the CNPq-Brazil Fellowship granted to RCPR.

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Received 10 October 2012; accepted 20 September 2013.

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