Variability of the Brazil Current During the Late Holocene

Home , Brazil Current, North Brazil Current

Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014) 28–36

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Variability of the Brazil Current during the late Holocene

Cristiano M. Chiessi a,⁎, Stefan Mulitza b, Jeroen Groeneveld b, Juliana B. Silva c, Marília C. Campos a,MarcioH.C.Gurgela a School of Arts, Sciences and Humanities, University of São Paulo, São Paulo, Brazil b MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany c Institute of Geosciences, University of São Paulo, São Paulo, Brazil article info abstract

Article history: Our understanding of the centennial-scale variability of the Brazil Current (BC) during the late Holocene is elusive Received 1 August 2013 because of the lack of appropriate records. Here we used the Mg/Ca and oxygen isotopic composition of plankton- Received in revised form 29 November 2013 ic foraminifera from two marine sediment cores collected at 27°S and 33°S off southeastern South America to as- Accepted 4 December 2013 sess the late Holocene variability in the upper water column of the BC. Our results show in phase ﬂuctuations of Available online 14 December 2013 up to 3 °C in sea surface temperatures (SST), and 0.8‰ in oxygen isotopic composition of surface sea water, a Keywords: proxy for relative sea surface salinity (SSS). Time-series analyses of our records indicate a cyclicity with a period ﬂ South Atlantic of ca. 730 yr. We suggest that the observed cyclicity re ects variability in the strength of the BC associated to Late Quaternary changes in the Atlantic meridional overturning circulation (AMOC). Positive (negative) SST and SSS anomalies Western boundary current are related to a strong (weak) BC and a weak (strong) AMOC. Moreover, periods of peak strength in the BC Stable oxygen isotopes occur synchronously to a weak North Brazil Current, negative SST anomalies in the high latitudes of the North Mg/Ca Atlantic, and positive (negative) precipitation anomalies over southeastern South America (equatorial Africa), Planktonic foraminifera further corroborating our hypothesis. This study shows a tight coupling between the variability of the BC and the high latitudes of the North Atlantic mediated by the AMOC even under late Holocene boundary conditions. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Here we address this issue using planktonic foraminiferal Mg/Ca and stable oxygen isotope ratios (δ18O) from two marine sediment cores In many regions of the globe, the Holocene was punctuated by collected off southeastern South America. Our data provide evidence centennial-scale climate variations. Such variations were recorded in for centennial-scale fluctuations in sea surface temperature (SST) and different oceans (e.g., Rimbu et al., 2004; Lamy et al., 2006; Hoogakker sea surface salinity (SSS) in the BC that are anti-correlated to changes et al., 2011) and continents (e.g., Russell et al., 2003; Turney et al., in the strength of the NBC. 2005; Strikis et al., 2011). But questions still persist on whether or not a unifying theory is able to explain their formation (Wanner et al., 2. Regional setting 2011). One of the factors that hampered the establishment of a theory is the small number of records from the Southern Hemisphere. The The upper water column of the study area is bathed by the BC, a weak tropical and subtropical western South Atlantic is one such region southward-flowing western boundary current that is associated with the where records are particularly scarce (Leduc et al., 2010). Despite the South Atlantic subtropical gyre (Fig. 1)(Peterson and Stramma, 1991; lack of studies, the western boundary currents (i.e., the Brazil Current Stramma and England, 1999). The BC originates between 10 and 15°S, (BC) and the North Brazil Current (NBC)) in the South Atlantic play an where the Southern South Equatorial Current bifurcates while ap- important role in the Atlantic meridional overturning circulation proaching the South American continental margin. At the bifurcation, (AMOC) (Peterson and Stramma, 1991). It was proposed, for instance, the SSEC feeds not only the BC but also the northward flowing NBC that the strength of the BC and the NBC should be anti-phased during (also termed the North Brazil Undercurrent (Stramma et al., 1995) abrupt climate change events (Arz et al., 1999; Crowley, 2011). Al- between the bifurcation and ca. 5°S). Around 37°S the BC encounters though some reconstructions are available for Marine Isotope Stages 2 the northward-flowing Malvinas Current (Olson et al., 1988), where and 3 (Arz et al., 1999; Carlson et al., 2008), there is so far no study both currents turn southeastward and flow offshore as the South Atlantic that investigates the late Holocene centennial-scale variability of the BC. Current and the northern branch of the Antarctic Circumpolar Current, respectively. The BC transports Tropical Water (TW) (N20 °C and N36 psu) in the upper ca. 100 m of the water column and South Atlantic Central Water ⁎ Corresponding author at: Av. Arlindo Bettio, 1000, CEP03828-000, São Paulo SP, Brazil. – – Tel.: +55 11 26480124; fax: +55 11 29439076. (SACW) (6 20 °C and 34.6 36 psu) from ca. 100 until 600 m water E-mail address: [email protected] (C.M. Chiessi). depth. Whereas the TW constitutes the mixed layer, the large

0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.12.005 C.M. Chiessi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014) 28–36 29

Fig. 1. Location of the marine sediment cores investigated in this study, and mean austral summer (A) and winter (B) sea surface temperatures (SST) (color shading, °C; Locarnini et al., 2010) plotted together with mean annual horizontal circulation at the surface (black arrows; Stramma and England, 1999). During austral summer (winter) the Brazil Current is strength- ened (weakened) and our study area experiences higher (lower) SST (Matano et al., 1993; Locarnini et al., 2010). ACC: Antarctic Circumpolar Current; BC: Brazil Current; NBC: North Brazil Current; SAC: South Atlantic Current; SSEC: Southern South Equatorial Current. This figure was partly produced with Ocean Data View (Schlitzer, 2012). See Table 1 for more information about the sediment cores. temperature and salinity ranges of the SACW characterize the perma- position and activity of the continental SACZ (Carvalho et al., 2004; nent thermocline. Chaves and Nobre, 2004). The BC shows a strong annual cycle in SST that increases from the bifurcation of the SSEC (amplitude of ca. 3 °C at 17.5°S/36.5°W) towards 3. Material and methods the confluence with the MC (amplitude of ca. 5 °C at 33.5°S/45.5°W), with warmer waters prevailing during austral summer (Fig. 1) 3.1. Marine sediment cores (Locarnini et al., 2010). On the other hand, changes in SSS show a rather small annual cycle (amplitude of ca. 0.2 psu on both positions along the We investigated multi-cores GeoB2107-5 (Bleil et al., 1993)and BC), with slightly higher values during austral winter as compared to GeoB6211-1 (Schulz et al., 2001) as well as part of the gravity core austral summer and no clear latitudinal trend (Antonov et al., 2010). In- GeoB6211-2 (Schulz et al., 2001; Wefer et al., 2001) collected from the strumental data and numerical modeling assigned the annual cycle in continental slope off southeastern South America (Table 1, Fig. 1). SST to both the annual cycle in insolation and variations in the transport Considering that (i) multi-core GeoB6211-1 shows a relatively high of the BC (Provost et al., 1992; Matano et al., 1993; Wainer et al., 2000). sedimentation rate and does not span the whole late Holocene (see Increased heat flux from the atmosphere during austral summer evi- Section 4.1. below), (ii) gravity coring frequently disturbs the upper- dently occurs synchronously with the peak in mass transport of the most centimeters of the sediment column, and (iii) the shallowest BC that is, in turn, related to intensified wind stress forcing (Peterson AMS 14C age from gravity core GeoB6211-2 was obtained at 18 cm and Stramma, 1991; Matano et al., 1993). core depth and may not adequately constrain the age of the uppermost The deficit in the southward BC transport relative to what would be 10–15 cm core depth of GeoB6211-2 (see Section 4.1. below), it was expected from the wind fields is due to the northward-directed upper necessary to produce a composite record (i.e., GeoB6211-1/2) by branch of the thermohaline circulation (Stommel, 1957; Peterson and assigning the base (i.e., 45 cm core depth) of GeoB6211-1 to 17 cm Stramma, 1991). Indeed, the formation of North Atlantic Deep Water core depth from GeoB6211-2, based on calibrated accelerator mass (NADW) requires a net transfer of thermocline water from the South spectrometer (AMS) 14C ages from both cores. Further details about Atlantic to the North Atlantic together with net northward fluxes of in- the preparation of composite record GeoB6211-1/2 are provided in termediate and bottom waters (Rintoul, 1991; Peterson and Stramma, Table 2 and Fig. 2. Because our focus here is the last 5 kyr, we analyzed 1991). Because of this condition, the NBC receives the largest the whole GeoB2107-5 and only down to 84 cm core depth from share (ca. 12 Sv) of the SSEC volume transport if compared to the BC composite core GeoB6211-1/2. Visual core inspection does not provide (ca. 4 Sv) (e.g., Stramma et al., 1990). evidence for disturbance in any of the cores analyzed here (Bleil et al., Sea surface temperatures (SST) in the western South Atlantic play an 1993; Schulz et al., 2001; Wefer et al., 2001). important role in precipitation over southeastern South America Multi-cores GeoB2107-5 and GeoB6211-1 were described and then (Robertson and Mechoso, 2000; Chaves and Nobre, 2004). Positive sliced onboard in 1 cm slices, and the samples were stored in plastic SST anomalies have been correlated with increased precipitation over vials at 4 °C. One meter sections of gravity core GeoB6211-2 were the northern sector of southeastern South America, i.e., to an intensification and northward shift of the South Atlantic Convergence Zone (SACZ). The SACZ is one of the main features of the South American Table 1 Marine sediment cores used in this study. monsoon system (SAMS) during austral summer (Zhou and Lau, 1998; Garreaud et al., 2009). It is an elongated NW-SE convective belt Core ID Coring Latitude Longitude Water depth Core length that originates in the Amazon Basin, and it extends above the northern device (°S) (°W) (m) (cm) sector of southeastern South America and the adjacent subtropical GeoB2107-5 Multi-core 27.18 46.46 1052 39 South Atlantic (Carvalho et al., 2004). It is still debated how far inland GeoB6211-1 Multi-core 32.51 50.24 654 45 GeoB6211-2 Gravity core 32.51 50.24 657 774 SST anomalies in the western South Atlantic are able to influence the 30 C.M. Chiessi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014) 28–36

Table 2 Accelerator mass spectrometer radiocarbon dates and calibrated ages used to construct the age models of cores GeoB2107-5 and GeoB6211-1/2. ⁎ Core depth (cm) Lab ID Species Radiocarbon age +/−1σ 2σ calibrated age Calibrated age, interpolated Source error (a BP) range (cal a BP) from 2σ range (cal ka BP)

GeoB2107-5 2Poz42358G. ruber 660 +/− 30 254–401 0.33 This study 14 Poz42359 G. ruber 1965 +/− 35 1402–1619 1.51 This study 25 Poz42360 G. ruber 3265 +/− 35 2968–3217 3.09 This study 36 Poz42361 G. ruber 4440 +/− 35 4505–4775 4.64 This study

GeoB6211-1 ⁎⁎ 2(2) Poz43428 G. ruber, G. sacculifer Modern Modern Modern This study ⁎⁎ 16 (16) Poz43429 G. ruber 585 +/− 25 137–284 0.21 This study ⁎⁎ 31 (31) Poz43431 G. ruber 1075 +/− 30 558–690 0.62 This study ⁎⁎ 45 (45) Poz43432 G. ruber 1575 +/− 30 1049–1227 1.14 This study

GeoB6211-2 ⁎⁎ 18 (47) KIA30528 G. ruber, G. sacculifer 1685 +/− 30 1169–1299 1.23 Chiessi et al. (2008) ⁎⁎ 35 (64) KIA35166 G. ruber, G. sacculifer 3170 +/− 40 2839–3105 2.97 Razik et al. (2013) ⁎⁎ 55 (84) KIA35165 G. ruber, G. sacculifer 4625 +/− 45 4763–4973 4.87 Razik et al. (2013)

⁎ KIA: Leibniz-Laboratory for Radiometric Dating and Stable Isotope Research, Kiel, Germany; Poz: Poznan Radiocarbon Laboratory, Poznan, Poland. ⁎⁎ Depths in parentheses refer to the depth of the composite core GeoB6211-1/2.

longitudinally split and described onboard, and then they were stored at corrections are typically necessary (Reimer and Reimer, 2001). All ages 4 °C. Onshore, the late Holocene section of GeoB6211-2 was sampled at are indicated as calibrated years before present (cal a BP; present is 1 cm intervals. All samples were wet sieved, and the residues from the 1950 AD), except where noted otherwise. To construct the age models, 150 μm size sieve were stored in glass vials. we linearly interpolated the calibrated ages (i.e., interpolated from 2σ ranges). 3.2. Age models

The age models of cores GeoB2107-5 and GeoB6211-1/2 are based on 3.3. Mg/Ca analyses 11 AMS radiocarbon ages of the shallow dwelling planktonic foraminifera Globigerinoides ruber (pink and white) and Globigerinoides sacculifer Around 40 tests of G. ruber [white, sensu stricto from Wang (2000)] – μ (Table 2, Fig. 2). Three radiocarbon ages were previously published by within the size range 250 350 m were hand-picked under a binocular Chiessi et al. (2008) and Razik et al. (2013) and eight additional ages microscope at every four cm in core GeoB6211-1 and at every cm in core are reported here for the first time. For each sample, we hand-picked GeoB6211-2 for Mg/Ca analyses. Different sampling intervals were applied to compensate for the lower sedimentation rates from under a binocular microscope around 10 mg of CaCO3 from the sediment fraction larger than 150 μm. Samples were analyzed either at the Poznan GeoB6211-2 as compared to GeoB6211-1 (see Section 4.1 below). Radiocarbon Laboratory, Poland, or at the Leibniz-Laboratory for Radio- After gently crushing the tests, shell fragments were cleaned according metric Dating and Stable Isotope Research, Germany (Table 2). All to the standard cleaning protocol for foraminiferal Mg/Ca analyses sug- fi radiocarbon ages were calibrated with the calibration curve Marine09 gested by Barker et al. (2003) and slightly modi ed by Groeneveld and (Reimer et al., 2009) with the software Calib 6.0 (Stuiver and Reimer, Chiessi (2011). Before dilution, samples were centrifuged for 10 min 1993). No additional marine reservoir correction was applied because (6000 rpm) to exclude any remaining insoluble particles from the anal- our core sites are located far from upwelling zones and significantly to yses. Samples were diluted with Seralpur water before analysis with an the north of the Brazil-Malvinas Confluence, both being places where ICP-OES [Agilent Technologies, 700 Series with autosampler (ASX-520 Cetac) and micro-nebulizer] at the MARUM – Center for Marine Environmental Sciences, University of Bremen, Germany. Instrumental precision of the ICP‐OES was monitored by analysis of an in‐house standard solution with a Mg/Ca of 2.93 mmol/mol after every five samples (long term standard deviation of 0.026 mmol/mol or 0.91%). To allow inter-laboratory comparison we analyzed an international limestone standard (ECRM752–1) with a reported Mg/Ca of 3.75 mmol/mol (Greaves et al., 2008). The long-term average of the ECRM752–1 standard, which was routinely analyzed twice before each batch of 50 samples in every session, is 3.78 mmol/mol (1σ = 0.066 mmol/mol). Analytical error based on three replicate measurements of each sample for G. ruber was 0.23% (1σ = 0.009 mmol/mol) for Mg/Ca. We mea- sured Mg/Ca in tests of G. ruber because it dwells in the uppermost water column and reflects mixed layer conditions (Chiessi et al., 2007). To convert Mg/Ca ratios into SST we used the calibration equation of Anand et al. (2003) for G. ruber (white) in the size range 250–350 μm with no pre-assumed temperature dependence constant [i.e., Mg/Ca = 0.34 exp (0.102 temperature)]. According to Hönisch et al. (2013), the small sensitivity of G. ruber Mg/Ca to changes in Fig. 2. Age model and sedimentation rates for marine sediment cores GeoB2107-5 and salinity (i.e., 3.3 +/− 1.7% per salinity unit) supports the use of this GeoB6211-1/2. Solid black line refers to core GeoB6211-1, and solid gray line refers to core GeoB6211-2. Cores GeoB6211-1 and GeoB6211-2 were used to produce the compos- paleotemperature proxy given the range of salinity change in our ite record GeoB6211-1/2. study area (see Section 4.3.below). C.M. Chiessi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014) 28–36 31

3.4. Stable oxygen isotope analyses (Torrence and Compo, 1998). Again, wavelet analyses were performed on the residuals from a second order polynomial fitfrom Ten hand-picked tests of G. ruber [white, sensu stricto from Wang both the G. ruber δ18O record from core GeoB2107-5, and from the 18 (2000)] within the size-range 250–350 μm from every cm of cores δ Oivc-ssw record from core GeoB6211-1/2. Before analyses, the re- GeoB2107-5 and GeoB6211-1/2 were used for δ18O analyses. The less cords were linearly interpolated. During analyses, we used a Morlet complex character of δ18O analyses as compared to Mg/Ca analyses wavelet. led us to analyze every cm from GeoB6211-1 and GeoB6211-2, unlike the larger sampling intervals applied for Mg/Ca. To reconstruct conditions in the permanent thermocline we also analyzed two globorotaliid 4. Results species known to record deeper portions of the upper water column in the western South Atlantic (Chiessi et al., 2007; Groeneveld and 4.1. Age models Chiessi, 2011). For these, we performed δ18Oanalysesonfive hand- picked specimens of Globorotalia truncatulinoides (dextral, sp. 2 from Core GeoB2107-5 shows an age of 4.64 cal ka BP at 36 cm core Vargas et al., 2001) within the size-range 500–650 μm from every cm depth, and core GeoB6211-1/2 records an age of 4.87 cal ka BP at of core GeoB2107-5 and from 1 to 45 cm from core GeoB6211-1. The 84 cm composite core depth (Table 2, Fig. 2). Sedimentation rates of relatively small abundance of G. truncatulinoides on core GeoB6211-2 core GeoB2107-5 show no major changes throughout the recorded pe- did not allow the selection of enough dextral specimens of sp. 2 within riod with a mean value of 8 cm/kyr (Fig. 2). In contrast, sedimentation the narrow size-range 500–650 μm for analysis. Considering the large rates from core GeoB6211-1/2 increase gradually towards the upper effect of the ontogenetic cycle on the chemical properties of deep- portion of the core from ca. 10 cm/kyr between 4.87 and 1.23 cal ka BP dwelling foraminiferal calcite (e.g., Groeneveld and Chiessi, 2011), we to more than 60 cm/kyr during the last 0.21 cal ka BP. Considering the selected tests of the more abundant Globorotalia inflata (3 chambers in sampling strategy and the sedimentation rates for both core records an- the final whorl, non-encrusted from Groeneveld and Chiessi, 2011) alyzed in this study, the mean temporal resolution from core GeoB2107- within the size range 315–400 μminsteadofG. truncatulinoides on 5 was ca. 130 yr for G. ruber and G. truncatulinoides δ18O. Because of the every cm of core GeoB6211-2. We used the δ18OofG. truncatulinoides higher sedimentation rates and a similar sampling strategy, the mean of core GeoB6211-1 together with the δ18OofG. inflata of core temporal resolution from core GeoB6211-1/2 was ca. 60 yr for the GeoB6211-2 to produce a 5 kyr-long record of the permanent thermo- G. ruber and the permanent thermocline δ18O records and ca. 100 yr 18 cline for our composite core GeoB6211-1/2, inasmuch as both species for SST and δ Oivc-ssw. calcify in similar water depths in the western South Atlantic and show no significant species-specificoffset(Niebler et al., 1999; Chiessi et al., 2007). All δ18O analyses were performed on a Finnigan MAT 252 mass 4.2. Mg/Ca analyses spectrometer equipped with an automatic carbonate preparation device at the MARUM – Center for Marine Environmental Sciences, University Mg/Ca ratios of the G. ruber samples from core GeoB6211-1/2 range of Bremen, Germany. Isotope results were calibrated relative to the from 2.93 to 4.50 mmol/mol (not shown here) and are equivalent to Vienna Peedee belemnite (VPDB) using NBS19 standard. The standard 21.1 and 25.3 °C (Fig. 3E), respectively. The mean SST obtained for the deviation of the laboratory standard was lower than 0.07‰ for the uppermost two samples of core GeoB6211-1/2 is 23.1 °C. This value measuring period. compares favorably with the modern mean summer SST in the top To calculate the δ18O of continental-ice-volume-corrected surface sea 20 m of the water column (i.e., 24.1 °C) from the closest grid cell (i.e., 18 water (δ Oivc-ssw), a proxy for relative SSS, we used our G. ruber Mg/Ca 32.5°S/49.5°W) of the World Ocean Atlas 2009, and differs considerably SST and δ18O, the paleotemperature equation from Mulitza et al. (2003) from modern mean winter SST (i.e., 17.8 °C) in the same location 18 18 for G. ruber (white) [i.e., temperature = −4.44 (δ OG. ruber - δ Osw)+ (Locarnini et al., 2010). Reconstructed SST show a long-term increase 14.20], the VPDB to Vienna Standard Mean Ocean Water (VSMOW) con- from ca. 22.3 °C at ca. 4.7 cal ka BP to ca. 23.8 °C at ca. 1.0 cal ka BP, version factor from Hut (1987), the sea level curve from Lambeck and followed by a decrease to ca. 23.1 °C to the present (Fig. 3E). 18 Chappell (2001), and the global average change in δ Osw since the Last Superimposed on the long-term trend, centennial-scale variability of Glacial Maximum from Schrag et al. (2002). up to 3 °C was found.

3.5. Time-series analyses 4.3. Stable oxygen isotope analyses To verify whether or not the fluctuations in our records were periodic, we performed time-series analyses on the frequency domain. G. ruber δ18O values from core GeoB2107-5 show a long-term increase To achieve this, the REDFIT algorithm (Schulz and Mudelsee, 2002) from ca. −0.7‰ at ca. 4.7 cal ka BP to ca. −0.4‰ at ca. 3.2 cal ka BP and was used with the software PAST (Hammer et al., 2001). The two a decrease to ca. −1.3‰ to modern times (Fig. 3A). Centennial-scale var- main advantages of this algorithm are: (i) it can be directly applied to iability of up to 0.6‰ is again present in this record. The δ18Oresultsob- temporally unevenly spaced records, which avoids tapering of high tained from the same species from core GeoB6211-1/2 show a roughly frequencies (particularly relevant in our study), and (ii) it estimates unidirectional long-term trend, i.e., a decrease from ca. −0.5 to −0.9‰ the red-noise spectrum, which allows the identification of significant and no clear centennial-scale variability (Fig. 3C). The δ18Orecordsfrom frequencies. Spectral analyses were performed on the G. ruber δ18Ore- our deep-dwelling species from both cores remain relatively stable 18 cord from core GeoB2107-5 and on the δ Oivc-ssw record from core throughout the recorded period (Fig. 3B,D). While G. truncatulinoides GeoB6211-1/2. This procedure allowed integrating the Mg/Ca-based from core GeoB2107-5 shows minor variability around ca. SST in the time series analyses. Before analysis, the long-term trend 0.8 +/− 0.1‰ (Fig. 3B), the permanent thermocline record from core was removed from both records by expressing them as residuals from GeoB6211-1/2 shows a mean δ18O value of ca. 1.0 +/− 0.1‰ with a a second order polynomial fit. During analyses, we used a Welch type negative excursion of ca. 0.2‰ around 1 cal ka BP (Fig. 3D). 18 spectral window. Ice-volume corrected δ Ossw values obtained from the G. ruber sam- In order to evaluate the stationarity of the periodicities, time-series ples of core GeoB6211-1/2 range from 1.0 to 2.0‰, with a mean value of 18 analyses were performed on the time-frequency domain. For this, our ca. 1.5‰ (Fig. 3F). Although δ Oivc-ssw values show no pronounced records were investigated using the online wavelet facility of the Uni- long-term changes, they do show a marked centennial-scale variability versity of Colorado (http://paos.colorado.edu/research/wavelets/) of up to 0.8‰. 32 C.M. Chiessi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014) 28–36

ca. 730 yr periodicity was positively checked in both records via wavelet analyses (Fig. 4B,D). It is noteworthy that the periodic character of the 18 18 δ O G. ruber record from core GeoB2107-5 and the δ Oivc-ssw record from core GeoB6211-1/2 already stands out by the simple comparison to a 730 yr sinusoid (Fig. 5F).

5. Discussion

5.1. Regional paleoceanographic periodicities

Centennial-scale periods of high G. ruber δ18O values in core 18 GeoB2107-5 (Fig. 3A) coincide with increased δ Oivc-ssw (i.e., increased SSS) and SST in GeoB6211-1/2 (Fig. 3F,E, respectively). It is notable that both permanent thermocline records (Fig. 3B,D) remain relatively sta- 18 18 ble (as compared to GeoB2107-5 G. ruber δ O, GeoB6211-1/2 δ Oivc- ssw and SST records) throughout the analyzed period, indicating that the centennial-scale signal is restricted to the mixed layer of the BC (i.e., TW). Thus, SST and SSS in the TW of the BC in our study area oscillated in phase and synchronously with a period of ca. 730 yr (Figs. 3,4). Oscillations in SST in our study area could be related to changes in the strength of the BC. Similarly to the modern annual SST cycle that accompanies variations in the strength of the BC (Fig. 1)(Matano et al., 1993; Locarnini et al., 2010), centennial-scale periods of higher SST as evidenced at site GeoB6211 (Fig. 3E) could be related to sustained periods of a strong BC. The modern annual SST cycle in the BC ranges from 3 to 5 °C and is in agreement with the amplitude of the centennial-scale oscillations in SST (up to 3 °C) reconstructed for the BC at site GeoB6211. Nevertheless, the reconstructed synchronous 18 changes in δ Oivc-ssw (i.e., SSS) at site GeoB6211 require an additional 18 driver because the modern annual δ Ossw cycle in the BC shows only minimal changes (ca. 0.1‰) [calculated based on seasonal salinity values from Antonov et al. (2010) and the regional salinity-δ18Osw equation from LeGrande and Schmidt (2006)]. These small changes could not be responsible for the reconstructed centennial-scale oscillations of up to 0.8‰.

5.2. A possible role for the Atlantic meridional overturning circulation

During the last deglaciation, the δ18O of the BC's upper water column showed two periods of striking increase (Carlson et al., 2008). Such conditions were reconstructed for Heinrich Stadial 1 and the Younger Fig. 3. Proxy records for the upper Brazil Current spanning the late Holocene based on ma- Dryas, which are both associated with decreases in the strength of the 18 rine sediment cores GeoB2107-5 and GeoB6211-1/2. (A) GeoB2107-5 Globigerinoides AMOC (McManus et al., 2004). The two periods of increase in δ Oivc-ssw 18 18 ruber white (w) δ O; (B) GeoB2107-5 Globorotalia truncatulinoides dextral (d) δ O; reported for the BC were also reconstructed for other sites of the (C) GeoB6211-1/2 G. ruber w δ18O; (D) GeoB6211-1/2 permanent thermocline tropical and subtropical western Atlantic along the AMOC upper return (i.e., GeoB6211-1 G. truncatulinoides d and GeoB6211-2 Globorotalia inflata) δ18O; (E) fl GeoB6211-1/2 G. ruber w Mg/Ca sea surface temperatures (SST); and (F) GeoB6211-1/2 ow (Schmidt et al., 2004; Weldeab et al., 2006; Carlson et al., 2008). It ice volume corrected (ivc) surface seawater (ssw) δ18O, a proxy for local sea surface salin- was potulated that, because of a decrease in strength, the ability of the ity. Note that (A) and (C), and (B) and (D) are plotted on the same y-axis scale to facilitate AMOC to transport salt to the high latitudes of the North Atlantic was – comparison. In (A) (D) the black lines represent a three-point running average. inhibited. In agreement with a decrease in strength of the AMOC, Arz et al. (1999) suggested that the transport of the NBC was weaker while the BC experienced a strengthening during these two abrupt climate change events. We hypothesize that a similar mechanism may 4.4. Time-series analyses have also occurred repeatedly during the late Holocene, being responsi- 18 ble for the reconstructed oscillations in δ Oivc-ssw at site GeoB6211. It is The δ18O G. ruber record from core GeoB2107-5, and the SST and noteworthy that a strong BC related to a weak AMOC (Stramma et al., 18 δ Oivc-ssw records from core GeoB6211-1/2 reveal pronounced short- 1990; Peterson and Stramma, 1991; Crowley, 2011) could account not 18 term variability at centennial-scale during the late Holocene (Fig. 3A,E, only for the high δ Oivc-ssw (Arz et al., 1999) but also for the positive F). In order to determine whether or not these fluctuations were period- anomalies in SST (Matano et al., 1993), in agreement with our recon- ic we performed time-series analyses in the frequency domain. These structions (Fig. 3E,F). results are presented in Fig. 4A(δ18O G. ruber from core GeoB2107-5) The hypothesized mechanism requires anti-phased changes in 18 18 and 4C (δ Oivc-ssw from core GeoB6211-1/2). Both the G. ruber δ O transport of the BC and the NBC to have happened also during the 18 and the δ Oivc-ssw show comparable spectra with the main periodicity late Holocene. Indeed, Arz et al. (2001) showed that the stratification at ca. 730 yr (820–641 yr in the G. ruber δ18O spectrum, and of the upper water column of the NBC oscillated with a period of 770– 18 815–643 yr in the δ Oivc-ssw spectrum) above the 95% confidence 690 yr during the Holocene and was forced by changes in the intensity level. For the G. ruber δ18O spectrum, the frequency band 820–641 yr of the NBC and of the trade-winds (Table 3, Fig. 5C). Therefore, is significant even at the 99% confidence level. The stationarity of the centennial-scale periods of a shallow thermocline in the NBC were C.M. Chiessi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014) 28–36 33

Fig. 4. Time series analyses of the Globigerinoides ruber δ18O record (see Fig. 3A) from core GeoB2107-5 (A and B), and of the ice volume corrected surface seawater δ18O record (see Fig. 3F) from core GeoB6211-1/2 (C and D). Time series analyses were performed on the residuals from a second order polynomial fit from both records. For wavelet analyses, the records were additionally linearly interpolated. (A) and (C): spectral analyses performed with the REDFIT algorithm (Schulz and Mudelsee, 2002) and the software PAST (Hammer et al., 2001); peaks that exceed the 95% confidence level are labeled with their periods (in years); the bandwidth (BW) determines the frequency resolution; stippled line depicts the red-noise spectrum; smooth lines depict 95% and 99% confidence levels. (B) and (D): wavelet analyses performed with the online facility of the Program in Atmospheric and Oceanographic Science at the Uni- versity of Colorado at Boulder (http://paos.colorado.edu/reseach/wavelets/)(Torrence and Compo, 1998); black contour is the 99% confidence level, using a red-noise background spectrum; gray-shaded contour levels are selected so that 75%, 50%, 25% and 5% of the wavelet power is above each level, respectively. related to a weakening in the transport of the NBC and to a southward model uncertainties. Indeed, based on modern climatology (Robertson displacement of the Intertropical Convergence Zone (ITCZ) (Arz et al., and Mechoso, 2000; Chaves and Nobre, 2004), it was expected that pos- 2001). Phases of a weak NBC appear to be coincident with periods of itive SST anomalies in the western South Atlantic would be related to an 18 high δ Oivc-ssw and SST (i.e., strong BC) in our records, within age intensification and northward shift of the SACZ, and thus to a strength- model uncertainties, and to support our hypothesis. However, it is ening in the SAMS. For equatorial Africa, cold North Atlantic sea surface worthy of note that the availability of only two 14C ages (i.e., 0.2 and temperatures during times of weakened AMOC probably induced a 3.9 cal ka BP) constraining the age model of the NBC record (Arz southward shift of the West African monsoon in conjunction with an et al., 2001) hinder a one-to-one comparison with our BC records. intensification and southward expansion of the African Easterly Jet Periodicities within the 820–640 yr range have actually been de- (Mulitza et al., 2008). Moreover, both the South American and the scribed in a large number of Holocene records from the Atlantic Ocean African records are coherent with the atmospheric changes reconstruct- and the adjacent continents (Table 3). In the high latitudes of the ed by Arz et al. (2001) for the equatorial Atlantic (Fig. 5C) and with our North Atlantic, for instance, the flow strength of deep water masses BC records (Fig. 5E,F). that are part of the lower branch of the AMOC oscillated with periods Periodicities in the climate system within the range 820-640 yr have between 700 (Bianchi and McCave, 1999; Dima and Lohmann, 2009) also been described outside the Atlantic realm. Examples come, for in- and 800 yr (Hoogakker et al., 2011). At the sea surface in the same re- stance, from the South China Sea (Wang et al., 1999), the western gion, Berner et al. (2008) characterized centennial-scale oscillations North Pacific(Jian et al., 2000), and the Black Sea–northern Red Sea (period of 1000–600 yr) in SST (Fig. 5A). The authors correlated the Ho- (Lamy et al., 2006). Marine archives recorded changes in the activity locene cold events to weak deep-water flow in the North Atlantic and of the East Asian monsoon, in the upper structure of the Kuroshio Cur- thus to a weakened AMOC. Interestingly, most of the cooling events of rent, and in hydroclimatic conditions around the Black Sea–northern the late Holocene described by Berner et al. (2008) (Fig. 5A) coincide Red Sea, respectively. Interestingly, all the mentioned periodicities 18 with periods of high δ Oivc-ssw and SST from our records (Fig. 5E,F), fur- were related to SST changes in the high latitudes of the North Atlantic, ther corroborating our hypothesis that changes in the strength of the similar to our records. Thus, the widespread occurrence of periodicities AMOC are responsible for producing the reconstructed variability in within the range 820-640 yr that are related to the same climatic fea- the BC during the late Holocene. ture suggests the manifestation of a pacing with near-global expression during the late Holocene. 5.3. Paleoclimatic consequences of the marine periodicities At least two different forcings have been claimed to be responsible for periodicities in the climate system within the range 820-640 yr Precipitation over southeastern South America and equatorial Africa during the Holocene (Table 3). They involve changes in (i) solar output also oscillated with periods within the range 820-640 yr (Table 3, Fig. (e.g., Lamy et al., 2006) or (ii) ocean circulation (e.g., Bianchi and 5B,D) (Russell et al., 2003; Strikis et al., 2011). Although the South McCave, 1999). This is a particularly intense debate, and it is beyond American record was related to changes in the intensity of the SAMS, the ability of this study to determine the ultimate forcing of the the African record was interpreted to reflect shifts in the position centennial-scale fluctuations. We note, however, that evidence for of the equatorial rainbelt away from the equator. Both records are both lines of explanation have been published recently. State of the anti-phased with each other, and positive (negative) precipitation art reconstructions of total solar irradiance show a periodicity of anomalies over southeastern South America (equatorial Africa) relate ca. 710 yr (e.g., Wanner et al., 2008; Steinhilber et al., 2012). Although 18 to periods of high δ Oivc-ssw and SST from our records, within age the ca. 710 yr cyclicity lies below the 95% significance level, it is 34 C.M. Chiessi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014) 28–36

Fig. 5. Centennial-scale variability of the Brazil Current during the late Holocene compared to circum-Atlantic climate records. (A) Low sea surface temperatures (SST) events from marine sediment core LO09-14 collected in the subpolar North Atlantic at 58.94°N/20.41°W (Berner et al., 2008); (B) Detrended Mg percentage in calcite from lacustrine composite sediment core E96-1P/E96-5 M/E96-6 M collected in eastern equatorial Africa at 0.29°S/29.71°E (Russell et al., 2003; Russell and Johnson, 2005); (C) Δδ18O Globigerinoides sacculifer – Globorotalia tumida from marine sediment core GeoB3910-2 collected in the western South Atlantic at 4.25°S/36.35°W (Arz et al., 2001); (D) δ18O from composite stalagmite LG3/LG11 collected in central- eastern Brazil at 14.42°S/44.37°W (Strikis et al., 2011); (E) Globigerinoides ruber white (w) δ18O from marine sediment core GeoB2107-5 collected in the western South Atlantic at 27.18°S/46.46°W (black line represents a three-point running average) (this study); (F) ice volume corrected (ivc) surface seawater (ssw) δ18O from composite marine sediment core GeoB6211-1/2 collected in the western South Atlantic at 32.51°S/50.24°W (black line) (this study) together with a sinusoid with period of 730 yr (gray line). See Table 3 and the Discussion for more information about records (A)–(D).

particularly marked during the late Holocene. On the other hand, multi- of our records reveal comparable spectra with a main period of 18 millennial model runs with late Holocene boundary conditions show ca. 730 yr. We hypothesize that the oscillations in SST and δ Oivc-ssw centennial-scale variability in ocean circulation (e.g., Park and Latif, can be explained by changes in the strength of the BC that are, in turn, 2008; Delworth and Zeng, 2012). Although the amplitude of the related associated with changes in the strength of the AMOC. Positive (nega- 18 climatic signals is relatively weak, the spatial patterns compare tive) SST and δ Oivc-ssw anomalies are related to a strong (weak) BC favorably with archive-based reconstructions. Finally, for both forcings and a weak (strong) AMOC. Additionally, periods of a strong BC are syn- (i.e., solar output and ocean circulation) it seems that the AMOC played chronous with periods of (i) a weak NBC, (ii) negative SST anomalies in a central role in the ampliﬁcation and propagation of the climatic signals the high latitudes of the North Atlantic, and (iii) positive (negative) pre- (e.g., Vellinga and Wood, 2002; Denton and Broecker, 2008; Prange cipitation anomalies over southeastern South America (equatorial et al., 2010). Africa) and corroborate our hypothesis. Moreover, the presence of a similar periodicity in proxies for the ﬂow strength of deep water masses 6. Conclusions in the high latitudes of the North Atlantic and in records collected outside the Atlantic realm suggests a coherent pattern of centennial-scale Our mixed layer records from the BC show in phase centennial-scale climatic oscillations with period of ca. 730 yr operating during the late 18 oscillations in SST and δ Oivc-ssw (a proxy for relative SSS) of up to 3 °C Holocene. Although the ultimate forcing responsible for these oscilla- and 0.8‰, respectively, during the late Holocene. Time-series analyses tions remains uncertain (main candidates are changes in solar output C.M. Chiessi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 415 (2014) 28–36 35

Table 3 Overview of paleoclimatic records from the Atlantic Ocean and adjacent continents that show cyclicities with period between 820 and 640 yr during the Holocene.

Region Lat (°N) Lon (°E) Proxy and interpretation Spectral Possible mechanism and forcing Reference peak (yr)

Subpolar North Atlantic 58.94 −20.41 Diatom counts; sea 1000-600 Variations in the upper circulation of the Berner et al. (2008) Ocean–Reykjanes Ridge surface temperatures subpolar North Atlantic; solar forcing ⁎ Eastern North Atlantic Ocean– 56.37 −27.81 Sortable silt mean size; ca. 700 Variations in the velocity of the Iceland– Bianchi and McCave Gardar Drift near-bottom water flow Scotland Overflow Water; oceanic forcing (1999) speed Ireland 55.38–51.43 10.48–15.44 Tree populations; 800 Variations in the latitude and intensity of the Turney et al. (2005) surface moisture zonal atmospheric circulation; no linear response to solar forcing ⁎⁎ Western North Atlantic 50.21 −45.69 Sortable silt mean size; ca. 800 Variations in the velocity of the Northwest Hoogakker et al. Ocean–Orphan Knoll near-bottom water flow Atlantic Bottom Water; solar or oceanic (2011) speed forcing Tropical Atlantic Ocean, 20.75–−20.40 −61.25–117.38 Alkenone unsaturation ca. 700 Variations in the Arctic Oscillation/North Rimbu et al. (2004) tropical Indian Ocean and index; sea surface Atlantic Oscillation; internal (i.e., South China Sea temperatures atmospheric, oceanic, coupled atmospheric- oceanic processes or sea-ice system) or external forcing (i.e., volcanic activity, solar irradiance) Equatorial Africa–Lake Edward −0.29 29.71 Mg percentage in calcite; ca. 725 Variations in the position of the equatorial Russell et al. (2003); lacustrine water balance rainbelt; the authors do not indicate a Russell and Johnson forcing (2005) ⁎⁎⁎ Western South Atlantic −4.25 −36.35 Planktonic foraminiferal 770-690 Variations in trade-wind intensity and in the Arz et al. (2001) Ocean–off northeastern Δδ18O; depth of the strength of the North Brazil Current; internal South America mixed layer oscillations in the ocean–atmosphere system Southeastern South America– −14.42 −44.37 Stalagmite δ18O; rainfall ca. 820 Variations in the South American summer Strikis et al. (2011) Central-eastern Brazil amount monsoon system; oceanic forcing Western South Atlantic −27.18–−32.51 −50.24–−46.46 Planktonic foraminiferal ca. 730 Variations in the strength of the Brazil This study Ocean–off southeastern Mg/Ca and ice volume Current; solar or oceanic forcing South America corrected surface sea water δ18O

⁎ Dima and Lohmann (2009). ⁎⁎ Most of the changes in sortable silt mean size that characterize the ca. 800 yr peak fall close to the analytical precision of the method, and should be interpreted with care. ⁎⁎⁎ Arz et al. (2001) only reported coherent peaks of a cross-spectral analysis. Thus, the peaks reported here relate to our spectral analysis of the GeoB3910-2 record. Methodological details are described in Materials and Methods. No detrending was performed. Only peaks above the 95% confidence level were listed. and ocean circulation), it seems likely that the AMOC played a central Barker, S., Greaves, M., Elderfield, H., 2003. A study of cleaning procedures used for fora- fi miniferal Mg/Ca paleothermometry. Geochemistry Geophysics Geosystems 4, 8407. role in the ampli cation and propagation of the signals even under http://dx.doi.org/10.1029/2003GC000559. late Holocene boundary conditions. Berner, K.S., Koç, N., Divine, D., Godtliebsen, F., Moros, M., 2008. A decadal-scale Holocene sea surface temperature record from the subpolar North Atlantic constructed using diatoms and statistics and its relation to other climate parameters. Paleoceanography Acknowledgments 23. http://dx.doi.org/10.1029/2006PA001339 (PA2210). Bianchi, G.G., McCave, I.N., 1999. Holocene periodicity in North Atlantic climate and deep- fl – We thank M. Segl and S. Pape for their help with the isotope and ocean ow south of Iceland. Nature 397, 515 517. Bleil, U., Cruise Participants, 1993. Report and preliminary results of Meteor Cruise M23/2, trace element analyses, respectively. Logistic and technical assistance Rio de Janeiro–Recife, 27.02.–19.03.1993. Berichte, Fachbereich Geowissenschaften, was provided by the Captain and Crew of the R/V Meteor. We also 43. Universität Bremen, Bremen (133 pp.). thank two anonymous Reviewers and P.A. Meyers for their constructive Carlson, A.E., Oppo, D.W., Came, R.E., LeGrande, A.N., Keigwin, L.D., Curry, W.B., 2008. fi Subtropical Atlantic salinity variability and Atlantic meridional circulation during comments. CMC acknowledges the nancial support from FAPESP the last deglaciation. Geology 36, 991–994. (grants 2010/09983-9 and 2012/17517-3). SM was funded through Carvalho, L.M.V., Jones, C., Liebmann, B., 2004. The South Atlantic Convergence Zone: in- the DFG Research Center/Cluster of Excellence “TheOceanintheEarth tensity, form, persistence, and relationships with intraseasonal to interannual activity ” and extreme rainfall. Journal of Climate 17, 88–108. System . This study was performed during a stay of CMC at the Chaves, R.R., Nobre, P., 2004. Interactions between sea surface temperature over the Hanse Institute for Advanced Study in Delmenhorst, Germany. Sample South Atlantic Ocean and the South Atlantic Convergence Zone. Geophysical Research material has been provided by the GeoB Core Repository at the Letters 31. http://dx.doi.org/10.1029/2003GL018647 (L03204). ‐ MARUM – Center for Marine Environmental Sciences, University of Chiessi, C.M., Ulrich, S., Mulitza, S., Pätzold, J., Wefer, G., 2007. Signature of the Brazil Malvinas Confluence (Argentine Basin) in the isotopic composition of planktonic Bremen, Germany. The data reported in this paper will be archived in foraminifera from surface sediments. Marine Micropaleontology 64, 52–66. Pangaea (www.pangaea.de). Chiessi, C.M., Mulitza, S., Paul, A., Pätzold, J., Groeneveld, J., Wefer, G., 2008. South Atlantic interocean exchange as the trigger for the Bølling warm event. Geology 36, 919–922.

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