This study explores the mechanisms behind the high glacial
productivity in the southern Brazilian margin (SBM) during the last 70 kyr using
planktonic foraminifera assemblage and subsurface temperature information derived using the modern analogue technique. We show that enhanced glacial
productivity was driven by the synergy of two mechanisms operating in
different seasons: (i) enhanced productivity in the upwelling region
during short austral summer events; and (ii) the persistent presence of the
Plata Plume Water (PPW) due to prolonged austral winter conditions. We suggest
that the upwelling systems in the southern Brazilian margin were more
productive during the last glacial period due to the enhanced Si supply for
diatom production by high-Si thermocline waters preformed in the
Southern Ocean. We hypothesize that orbital forcing did not have a major
influence on changes in upwelling during the last glacial period. However,
the more frequent northward intrusions of the Plata Plume Water were
modulated by austral winter insolation at 60∘ S via changes in the
strength of alongshore southwesterly winds. After the Last Glacial Maximum, the reduced
Si content of thermocline waters decreased upwelling productivity, while
lower austral winter insolation decreased the influence of the Plata Plume
Water over the southern Brazilian margin, reducing regional productivity.
Introduction
Continental margins are regions of relatively high biological productivity
and long-term carbon storage due to high nutrient flux (i.e., continental
discharge and upwelling) and shallow seafloor depths (i.e., interception of
sinking particulate organic matter)
(Bianchi et al., 2005; Wang et al., 2015; Abrantes et al., 2016; Ito et al., 2016; Brandini et al., 2018). The high biological productivity and exportation of
particulate organic carbon to the seafloor, the so-called “biological
pump” (Turner, 2015), play a
paramount role in removing CO2 from the atmosphere
(Bianchi et al., 2005; Muller-Karger et al., 2005). It is estimated that ca. 0.06 Pg C yr-1 is buried in continental margins, accounting for
> 40 % of the carbon storage in the oceans (Muller-Karger et al., 2005).
In contrast to this drawdown, the upwelling of CO2-rich thermocline waters
along continental margins can release CO2 to the atmosphere
(Bianchi et al., 2005; Ito et al., 2016). Thus, continental margins have a great
potential with respect to modulating the drawdown and emission of atmospheric CO2,
influencing the Earth's climate system.
Surface salinity in the southwestern Atlantic Ocean (Locarnini et al.,
2010) during (a) austral summer (December–February; DJF) and (b) winter
(June–August; JJA) showing the location of cores JPC-17 (27∘53′ S, 46∘55′ W; this study),
GeoB2107-3 (27∘17′ S, 46∘45′ W; Gu et al., 2017),
GL-1090 (24∘92′ S, 42∘51′ W; Santos et al., 2017) and
GL-75 (21∘9′ S, 40∘1′ W; Portilho-Ramos et al., 2015). Blue arrows indicate the western
boundary Brazil Current (BC) and the gray arrows show the regional
prevailing alongshore wind direction (northeast – NE and southwest – SW). The figure was created using the Ocean Data View software (ODV; version
4.7.10; https://odv.awi.de/, last access: 20 March 2018).
The southwestern Atlantic (southern Brazilian margin – SBM) is generally an
oligotrophic area bathed by the nutrient-poor, warm, salty, tropical waters of the Brazil Current (BC) (Fig. 1). However, upwelling zones and riverine
discharge inject nutrients into the photic zone
(Campos et al., 2000, 2013; Garcia and Garcia, 2008; Möller
et al., 2008; Brandini et al., 2018) resulting in confined areas and seasons with higher
concentrations of phytoplankton and zooplankton biomass
(Brandini et al., 2014; Rodrigues et al., 2014). During austral summer, upwelling
zones are intensified due to the prevailing alongshore northeasterly winds
and the cyclonic meanders of the BC induced by the interaction of the
current with the morphology of the continental margin
(Campos et al., 2000; Castelao et al., 2004; Aguiar et al., 2014). Winter conditions
of vigorous alongshore southwesterly winds and a relatively weakened BC, allow the
northward intrusion of low-salinity waters from the Plata River along the
SBM (Garcia and Garcia, 2008; Möller et
al., 2008). Both processes increase local productivity and lead to distinct
changes in planktonic community
(Brandini et al., 2014; Rodrigues et al., 2014), which are preserved in seafloor
sediments and can be used to reconstruct changes in productivity in the SBM
over time (Portilho-Ramos et al., 2015; Gu et al., 2017; Lessa et al., 2017).
Previous paleoceanographic studies provide evidence of an extremely intense
primary productivity in the SBM probably related to an upwelling system
during part of Marine Isotope Stage 5 (∼90–130 kyr) forced
by strengthened northeasterly winds and the BC
(Portilho-Ramos et al., 2015; Lessa et al., 2017). During the last glacial period
(Marine Isotope Stages 2–4, ∼11.7–71 kyr), primary
productivity weakened but was still significantly higher than that
occurring in the Holocene (Portilho-Ramos et al., 2015). It has been
suggested that the upwelling systems of the SBM were reduced during the last
glacial period, and may have been limited to short intervals of the austral
summer due the prolonged winter-like conditions of prevalent alongshore
southwesterly winds and frequent cold front passages (Portilho-Ramos et al., 2015).
However, the high abundance of eutrophic dinoflagellate cysts suggests increased
primary productivity in the SBM during specific intervals of the last
glacial period (Gu et al., 2017). These high productivity periods would be
triggered by the input of local (i.e., Itajaí River) and remote riverine
nutrient-rich freshwater (i.e., Plata River plume) (Gu et al., 2017).
Additionally, a recent study proposed that the periods of expansion and
contraction of the upwelling zones of the SBM are modulated by eccentricity
(Lessa et al., 2017), providing yet another mechanism to explain the
evolution of primary productivity in the SBM. In summary, these studies show
that different oceanographic mechanisms may have triggered high primary
productivity in the SBM over time. These mechanism are, however, poorly
understood.
Here we used planktonic foraminifera assemblage records and associated
subsurface temperature reconstructions derived using the modern analogue
technique (MAT) from piston core JPC-17 (27∘52.73′ S,
46∘55.25′ W) to understand the paleoceanographic processes
controlling changes in biological productivity in the SBM over the last 70 kyr. The comparison of our results to previously published records from the
SBM allowed us to recognize two different mechanisms modulating past
productivity changes over the last glacial–interglacial cycle.
Regional setting
The SBM is an oligotrophic margin under the influence of warm (≥25∘C), salty (≥35 psu) Tropical Water that flows southward
within the BC (Fig. 1). Interactions of the BC with the morphology of the
margin (i.e., changes in the orientation of the margin and the presence of a
barrier represented by the Abrolhos Bank) generates cyclonic meanders and
eddies that bring cold (≤20∘C), nutrient-rich thermocline
waters (South Atlantic Central Water – SACW) to shallower depths where they
are subjected to alongshore northeasterly winds
(Campos et al., 2000; Rodrigues and
Lorenzzetti, 2001; Castelao et al., 2004; Aguiar et al., 2014). Once over the shelf, wind stress and the Ekman dynamics
bring the SACW to the surface creating mature upwelling zones in the SBM
(Castelao et al., 2004; Aguiar et al., 2014). These processes boost biological
productivity in specific portions of the SBM such as off Vitória
(∼18∘ S), Cabo Frio (22–23∘ S) and Cape Santa
Marta (27–29∘ S) during the austral summer (Fig. 1). Marine
sediment core JPC-17 investigated in this study was collected off Cape Santa
Marta (Fig. 1).
In the vicinity of Cape Santa Marta, local productivity is also enhanced by
the injection of nutrients from the freshwater discharge of local (i.e.,
Itajaí River) and remote (i.e., Plata River and Patos/Mirim Lagoon
complex) sources (Garcia and Garcia, 2008;
Möller et al., 2008). During the summer (Fig. 1a), the upwelling
favorable northeasterly winds and the strong BC block the northward penetration of the
Plata Plume Water (PPW) (at ca. 32∘ S) (Möller et al., 2008; Campos et al., 2013). During austral winter (Fig. 1b), the weakened BC and the prevailing
alongshore southwesterly winds increase the northward intrusion (up to ca. 27∘ S)
of the nutrient-rich, cold (≤18∘C), low salinity (≤33.5 psu) PPW (Möller et al., 2008; Campos et al., 2013).
Both upwelling and freshwater inject large amounts of nutrients into the
oligotrophic SBM, seasonally modulating the biological productivity as well
as the plankton community in the region
(Garcia and Garcia, 2008; Rodrigues et al., 2014). In the vicinity of the Cape Santa
Marta, diatoms are the dominant group of phytoplankton, accounting for
29 %–90 % of phytoplankton and 31 %–90 % of the carbon biomass during the
summer upwelling, whereas dinoflagellates dominate the phytoplankton during
the winter intrusion of the PPW (Brandini et al., 2014).
Material and methods
Piston core KNR159-5-17JPC (27∘52,73′ S, 46∘55,25′ W)
recovered 15 m, from which the uppermost 350 cm was investigated in this
study. The core was raised from a water depth of 1627 m during R/V Knorr cruise 159-5 from Woods Hole Oceanographic Institution (WHOI, USA) (Fig. 1). The
uppermost 350 cm of the core consisted of dark gray carbonate sediments. This
section was sampled continuously every 10 cm, and 2 g of sediment per sample was washed using a 62 µm sieves.
Planktonic foraminifera assemblage
Planktonic foraminifera from core JPC-17 were dry picked from the >150 µm size fraction and quantified in relative abundances from splits
containing more than 300 specimens per sample. The taxonomy was based on
Stainforth et al. (1975). We assumed the effect of
dissolution in our planktonic foraminiferal faunal composition to be
negligible as core JPC-17 was collected at a water depth of 1627 m, well above
the modern and glacial lysocline (Volbers and
Henrich, 2004). Here we do not distinguish between the white and pink varieties of Globigerinoides ruber,
and also counted Globigerinoides sacculifer and Globigerinoides trilobus together as G. sacculifer as they are genetically the same
species (André et al.,
2013). Considering the taxonomic ambiguity involved with distinguishing the small (with respect to organism size, i.e., ≤250µm) specimens of Globigerinella calida and Globigerinella siphonifera (de Vargas
et al., 2002), we counted them together as G. siphonifera.
Subsurface temperature reconstruction
We reconstructed subsurface temperatures at a water depth of 100 m using the
modern analog technique (MAT) following Portilho-Ramos et al. (2015). The
MAT was performed on the C2 software (Juggins, 2007), and the
basic assumption was that the temperature of ambient seawater is the primary
control on foraminiferal assemblages (Morey et al., 2005). The planktonic
foraminiferal calibration dataset used here comprises 1052 surface samples
from the Atlantic Ocean: from these samples, 891 were previously published in
Kucera et al. (2005a) and 161 from North
Atlantic eastern boundary upwelling zones were previously published in
Salgueiro et al. (2014). The modern annual temperature
values at a water depth of 100 m from WOA 2009 (Locarnini et al., 2010) were
extracted and used to calibrate the MAT. For the MAT transfer function, the
squared chord distance was applied as a similarity measure. Additionally, when
reconstruction results were evaluated, the weighted mean of the best 10
modern analogs was used (Kucera et al., 2005b). Using
the leave-one-out cross-validation method, the root-mean-square error of
prediction of the transfer function was 0.95 ∘C (R2=0.98).
Chronology of core JPC-17 obtained by accelerator mass spectrometry
(AMS) 14C dating on planktonic foraminifera shells (Tessin and Lund,
2013; Portilho-Ramos et al., 2014a) and stable oxygen isotope (δ18O) tie points tuned to the LS16 stack from Lisiecki and Stern (2016).
Depth14C age1σ errorCalibrated age1σ errorNotesδ18OEstimatedReference(cm)(years)(years)(ka cal BP)(years)tie pointserror104140804228283Tessin and Lund (2013)186970207463201Tessin and Lund (2013)2698952510 784233Tessin and Lund (2013)3010 5552511 795298Tessin and Lund (2013)3411 9553013 454195Tessin and Lund (2013)3812 8703014 550341Tessin and Lund (2013)4213 6503516 001390Tessin and Lund (2013)504190154281264ReversalTessin and Lund (2013)5414 0803516 569338Tessin and Lund (2013)5610 0006010 967147ReversalPortilho-Ramos et al. (2014)5810 9903512 371273ReversalTessin and Lund (2013)6610 7902512 091273ReversalTessin and Lund (2013)7418 01010020 922282Tessin and Lund (2013)8216 1208018 994199ReversalTessin and Lund (2013)9018 1006020 975282Tessin and Lund (2013)9819 0207022 074292Tessin and Lund (2013)19032 55027037 946312Portilho-Ramos et al. (2014)26057 6143870This study35070 0002260This studyAge model
An age model for core JPC-17 has been previously published on the basis of
calibrated radiocarbon AMS 14C ages, δ18O in both
planktonic and benthic foraminifera as well as regional planktonic
foraminifera biostratigraphy
(Portilho-Ramos et al., 2014a; Tessin and Lund, 2013). Reversals in radiocarbon ages from ca. 16 to 21 calibrated
kiloannum before present (ka cal BP; i.e., 1950 AD) were detected by Tessin
and Lund (2013). These authors excluded four radiocarbon ages (i.e., 50, 54,
58 and 82 cm core depth) from 17 dated samples from core JPC-17 due to
reversals (Table 1). The radiocarbon age obtained at 56 cm from
Portilho-Ramos et al. (2014a) is placed within this
interval and also seems to be reversed. Considering these age reversals, the
chronology of the JPC-17 core was improved using the R script “Bacon” version 2.2, which uses Bayesian statistics to reconstruct accumulation histories for
sedimentary deposits and considers a Student t model to address outlying
(reversed) ages (Blaauw and Christen, 2011).
Thus, as explained by Blaauw and Christen (2011), Bacon version 2.2 is not
affected by outlying ages. For the upper 190 cm of core JPC-17, all AMS
14C ages (Table 1) were calibrated using the IntCal13 calibration curve
(Reimer et al., 2013) with a reservoir correction
age of 400±100 years (1σ error). For the core section that
extrapolates the radiocarbon range (i.e., 191–350 cm), two additional
tie points (Table 1) were obtained by aligning the benthic foraminifera
stable oxygen isotopes (δ18O) record from JPC-17 to benthic
δ18O of nearby core GL-1090 (Santos et al., 2017) (Fig. 1) and
to the intermediate-depth South Atlantic benthic δ18O stack
LS16 (Lisiecki and Stern, 2016) using the AnalySeries 2.0.5.2 software
(Paillard et al., 1996) (Fig. 2). The benthic δ18O curve from
JPC-17 is a combination of published Cibicidoides spp. (Tessin and Lund, 2013) and
unpublished Cibicidoides spp. δ18O provided by WHOI (Fig. 2). The latest data follow the methodology applied in Curry and Oppo (2005).
Error estimations of the δ18O tie points followed Santos et al. (2017), which take into account the mean resolution of the JPC-17 benthic
δ18O record around the tie-point depth, the mean resolution of
the reference curve around the tie-point age, a matching error visually
estimated when defining tie points and the absolute age error of the
timescale used for the reference record. In addition to the default parameters of
the software, the following settings were used: mem.mean =0.4, acc.shape =0.5. and t.a =9/t.b =10. A total of 10 000 age–depth realizations
were used to calculate the median age and the 1σ analytical
uncertainty at a 5 mm resolution (Fig. 2b). The chronology of core JPC-17 was
additionally supported by planktonic foraminifera biostratigraphy
(Ericson and Wollin, 1968; Portilho-Ramos et al., 2014b), where the presence of Globorotalia menardii and the low abundance
of Globorotalia inflata indicate Biozone Z (Holocene), whereas the absence of G. menardii and the high
abundance of G. inflata characterize the glacial Biozone Y (last glacial period) (Fig. 2).
Age model of core JPC-17. (a) Comparison between the benthic
foraminifera δ18O record of JPC-17 – composed of published
Cibicidoides spp. (Tessin and Lund, 2013; T&L, 2013) and unpublished Cibicidoides spp. data from
Woods Hole Oceanographic Institution (WHOI) – to the benthic foraminifera
δ18O record of core GL-1090 (Santos et al., 2017) as well as to
the intermediate-depth South Atlantic benthic δ18O stack LS16
(Lisiecki and Stern, 2016). Asterisks represent the
calibrated radiocarbon ages published in (green) Tessin and Lund (2013) and
(black) Portilho-Ramos et al. (2014b), whereas yellow
stars represent the δ18O tie points shown in Table 1. (b) Abundance of the main biostratigraphical planktonic foraminifera species
Globorotalia menardii and Globorotalia inflata from core JPC-17. Marine Isotopic Stages 1–4 (MIS1–4; MIS2 and MIS4
indicated by vertical blue bars) are shown at the top, whereas “Z”
(vertical grey bar) and “Y” in the bottom correspond to biostratigraphical
biozones from Ericson and Wollin (1968). (c) Age–depth model based on Bacon v. 2.2 (Blaauw and Christen, 2011). The symbols represent the positions of the
calibrated AMS 14C ages benthic δ18O tie points listed in
Table 1. Error estimations of the δ18O tie points follow Santos et
al. (2017) and take into account the mean resolution of the JPC-17 benthic
δ18O record around the tie-point depth, the mean resolution of
the reference curve around the tie-point age, a matching error visually
estimated when defining tie points and the absolute age error of the
timescale used for the reference record. The vertical yellow bar marks the
interval with reversed radiocarbon ages listed in Table 1.
Results
In contrast to a previous chronology
(Portilho-Ramos et al., 2014a), the new age model
for the upper 350 cm of core JPC-17 spans the last ca. 70 kyr (Fig. 2).
The benthic oxygen isotope records from core JPC-17 display a clear
glacial–interglacial pattern, which is comparable to the benthic δ18O
record of nearby core GL-1090 as well as that of the intermediate-depth
South Atlantic benthic δ18O stack LS16 (Lisiecki
and Stern, 2016) (Fig. 2).
The planktonic foraminifera assemblage is composed of 28 species and subspecies.
The following six species accounted for more than 70 % of total planktonic
assemblage: G. ruber (39 %), Globigerina glutinata (13 %), G. bulloides (11.2 %), G. inflata (8.8 %),
G. sacculifer (5 %) and G. siphonifera
(2.5 %). The abundance of G. menardii (0.7 %), Pulleniatina obliquiloculata (0.3 %), Orbulina universa (0.3%),
Globorotalia crassaformis (0.3 %),
Neogloboquadrina dutertrei (5.5 %) and Globorotalia truncatulinoides (3.9 %) were published in
Portilho-Ramos et al. (2014a).
In general, the distribution of the most abundant species follows the
glacial–interglacial pattern over the last 70 kyr (Fig. 3). The abundance of
the non-spinose species G. bulloides and G. inflata were higher during the last glacial period
(mean of 12 % and 9.6 %, respectively) and lower during the Holocene
(mean of 4.6 % and ∼2 %, respectively; Fig. 3a, b,
respectively). In contrast, the abundance of spinose species displayed the
opposite behavior. The abundance of G. ruber ranged from 25 % to 50 % (mean of
39 %) during the glacial and increased after 40 ka cal BP towards the
Holocene (mean of 47; Fig. 3c). The abundance of G. sacculifer and G. siphonifera display similar
patterns (Fig. 3d, e, respectively), and ranged between 1.6 % and 10.4 % and
0 % and 7.6 %, respectively, with higher abundance during the postglacial
interval (mean of 7.5 % and 6.9 %, respectively).
Relative abundance of planktonic foraminifera species and
reconstructed subsurface temperature (100 m water depth) from core JPC-17
over the last 70 kyr. (a) Relative abundance of Globigerina bulloides, (b)Globorotalia inflata,
(c)Globigerinoides ruber, (d)Globigerinoides sacculifer and (e)Globigerinella siphonifera.
(e) Temperature at a water depth of 100 m. The black dashed line in
(f) indicates the 20 ∘C isotherm which defines the modern maximum
temperature of South Atlantic Central Water (Castelao et al., 2004), the
water mass entering the photic zone in the upwelling sites of the southern
Brazilian margin. The vertical yellow bar marks the interval with reversed
radiocarbon ages listed in Table 1.
The temperature at a water depth of 100 m derived from MAT ranged from 16
to 21.3 ∘C over the last 70 kyr with lower temperatures recorded during
the glacial (16–20.3 ∘C) in comparison with the Holocene
(∼21∘C) (Fig. 3f). A pronounced warming trend is
observed after 30 ka cal BP toward the Holocene.
Discussion
The planktonic foraminifera G. bulloides is a non-spinose surface-dwelling species generally
inhabiting cold regions with high phytoplankton biomass that are typically
associated with upwelling zones (Sautter and Thunell, 1991; Mohtadi et al., 2007; Lessa
et al., 2014). Thus, high
abundances of G. bulloides in marine sediments from regions potentially affected by
upwelling have been widely used as an upwelling indicator
(Peeters et al., 2002; Godad et al., 2011), including the SBM upwelling zones
(Portilho-Ramos et al., 2015; Lessa et al., 2017). The last glacial abundance of
G. bulloides in core JPC-17 (8 %–18 %; Fig. 3a) closely matches abundances found in surface
sediments from the Cabo Frio upwelling (10 %–20 %) (Lessa et al., 2014),
suggesting the occurrence of a sustained upwelling off Cape Santa Marta in
the SBM. It also closely matches glacial records from cores collected
further north at the SBM such as GL-75 (21∘83′ S, 40∘01′ W; Portilho-Ramos et al., 2015), GL-77 (21∘12′ S,
40∘02′ W; Petró et al., 2016) and
SAN 76 (24∘26′ S, 42∘17′ W; Toledo et al., 2007), where G. bulloides ranged between
8% and 17%, suggesting widespread cooling and elevated productivity at
the SBM during the last glacial period relative to the modern oligotrophic
conditions (Fig. 4a, b). During the last glacial period, the reduced
abundance of non-upwelling species G. ruber (25 %–50 %) and other warm and
oligotrophic symbiont-bearing species such as G. sacculifer (2.1 %–10.4 %) and G. siphonifera
(0.3 %–4.3 %) (Fig. 3) support the occurrence of cold, productive
conditions promoted by upwelling. Simultaneously, lower temperatures at a water depth of 100 m (≤20∘C) suggest that SACW may have been frequently
located in the photic zone. The 20 ∘C isotherm is used to track the
boundary between tropical water and SACW (Castelao et al., 2004), and has
been used as a proxy for the presence of SACW in the photic zone in the past
(Portilho-Ramos et al., 2015; Lessa et al., 2017). It should be highlighted
that a relative warming of thermocline waters observed after 30 ka cal BP may
be related to heat accumulation in the western South Atlantic associated with
the glacial reduced mode of the Atlantic meridional overturning circulation
(Santos et al., 2017) as well as increased transport of heat from Indian
Ocean into South Atlantic via Agulhas leakage
(Martínez-Méndez et al., 2010).
The mechanism behind the changes in productivity in the
southern Brazilian margin over the last 70 kyr. Abundance of upwelling
indicator species Globigerina bulloides from (a) core GL-75 (21∘ S; Portilho-Ramos et
al., 2015) and (b) core JPC-17 (27∘ S; this study).
(c) Eccentricity of the Earth's orbit (Berger
and Loutre, 1991). (d) The abundance of dinoflagellate cysts from core
GeoB2107-3 (27∘ S) representing the influence of the Plata Plume
Water in the southern Brazilian margin (Gu et al., 2017) and austral winter
(June) insolation at 60∘ S (Berger and Loutre, 1991). (e) Antarctic sea
ice presence (SIP) in the Atlantic sector core TN057-13-PC4 (53∘20′ S, 5∘10′ W; Shemesh et al., 2002) and Indian sector core
SO136-111 (56∘40′ S, 160∘14′ W; Crosta et al., 2004)
of the Southern Ocean derived from diatoms assemblage. (f) Opal content in the
equatorial Atlantic upwelling off northwestern Africa from core RC14-01 (0.55∘ N, 13∘65′ W) and core RC24-07
(1∘33′ S, 11∘92′ W) (Bradtmiller et al., 2007)
as well as (g) in the Atlantic sector of the Southern Ocean from core RC13-254
(48∘34′ S, 5∘34′ E) and core
RC13-259 (53∘53′ S, 4∘56′ W) (Mortlock et al., 1991) as proxies for silicic acid transport toward low latitudes. Core RC13-254 is
located to the north of the Antarctic Polar Zone (APZ), whereas core RC13-254
is located to the south of the APZ. Note the inverted (RC13-259) axis in (f).
Enhanced glacial productivity in the SBM was recently reported by a 74 kyr long record of dinoflagellate cysts from adjacent core GeoB2107-3
(27∘17′ S, 46∘45'′ W; Gu et al., 2017). The authors
provided evidence of increased eutrophic conditions associated with more
frequent northward intrusions of the PPW (Fig. 4d)
(Gu et al., 2017).
However, G. bulloides is virtually absent in surface sediments deposited under the
influence of the PPW to the north of the Brazil–Malvinas Confluence
(Chiessi et al., 2007). Thus, more frequent northward
penetrations of the PPW in our study site are unlikely to explain the
enhanced glacial abundance of G. bulloides. In addition, the abundance of the eutrophic
environmental dinocysts and G. bulloides show different behavior during the last glacial
period (Fig. 4a, d). The dinocysts increased in abundance between 54 and 74 and 14 and 40 ka cal BP, suggesting increased productivity related to
frequent northward intrusions of the PPW, whereas the opposite is observed
between 40 and 54 ka cal BP (Fig. 4d), suggesting the reduced influence of the PPW
and relatively low productivity (Gu et al., 2017). In contrast, the
abundance of G. bulloides remained relatively high during the entire glacial with a
decreasing trend after 30 ka cal BP toward the Holocene (Figs. 3a, 4b).
Furthermore, the enhanced abundance of G. bulloides during the last glacial period is also
observed as far north as 21∘ S (i.e., core GL-75) (Fig. 4a)
(Portilho-Ramos et al., 2015), which is unlikely to be explained by the continuous
presence of the PPW (Möller et al., 2008).
Modern and past seasonal productivity processes in the SBM
Modern surface productivity in the SBM is seasonally modulated by two
different processes that inject nutrients in the photic zone: (i) austral
summer coastal and shelf-break upwelling (Campos et al., 2013); and (ii) austral winter northward intrusions of the PPW
(Garcia and Garcia, 2008; Möller et
al., 2008). During austral summer upwelling events, diatoms are the dominant
group of the phytoplankton, accounting for 29 %–90 % of the phytoplankton
density and 31 %–90 % of the carbon biomass, while dinoflagellates are the
dominant group during the austral winter associated with northward intrusions
of the PPW (Brandini et al., 2014). We suggest that both taxa reveal
different seasonal conditions during the last glacial period, with G. bulloides
recording upwelling events during austral summer and dinoflagellates
recording northward intrusions of the PPW during austral winter.
Prolonged winter-like conditions of prevalent alongshore southwesterly winds and
frequent cold front passages during the last glacial period may have limited
the SBM upwelling systems to a short period of austral summer-like
conditions, as suggested by Portilho-Ramos et al. (2015). Furthermore,
increased continental runoff (i.e., Itajaí River, Plata River and
Patos/Mirim Lagoon complex) associated with enhanced precipitation over southeast
South America (Cruz et al., 2005; Wang et al.,
2007) as well as a vigorous alongshore southwesterly winds were favorable for the
northward penetration of the PPW. These conditions increased the abundance
of dinocysts characteristic of eutrophic conditions in the SBM during the
glacial (Gu et al., 2017). The lower sea level
(Waelbroeck et al., 2002) may have caused the offshore
displacement of the PPW to our core site location
(Lantzsch et al., 2014).
Importantly, our reconstructed temperature at a water depth of 100 m indicates
that SACW may have reached the photic zone during the last glacial period
(Fig. 3f). Modern hydrographic data and model simulations show that
shelf-break upwelling in the SBM induced by the interaction of the BC with
bottom topography occurs all year long but is modulated by the seasonal
alongshore-wind direction (Campos et al., 2013; Brandini et al.,
2018). Thus, a prolonged presence of the low-salinity
PPW in the region may have increased the upper water stratification and
suppressed the surfacing of SACW, favoring the proliferation of eutrophic
dinocysts. In contrast, during short austral summer periods, the
strengthening of both the alongshore northeasterly winds and the BC hampered the
northward PPW migration, inducing the upwelling of SACW and creating
favorable conditions for the proliferation of G. bulloides. Owing to the low resolution
of core JPC-17, we cannot rule out an antiphase between the G. bulloides (i.e., core
JPC-17) and the dinocyst (i.e., core GeoB2107-3) records that would assign
the southwesterly winds and the associated northward penetration of PPW a key role in
controlling the upwelling zones in the SBM during the last glacial period.
Orbital forcing of SBM upwelling systems
A recent study proposed orbitally forced changes in insolation
(eccentricity) as a major mechanism modulating the
intensification/deintensification and the expansion/contraction
dynamics of the SBM upwelling zones (Lessa et al., 2017). In accordance with
Lessa et al. (2017), an eccentricity maximum (≥0.03) during MIS5 would have
altered the seasonality of the wind regime by controlling the amplitude of
austral summer and winter insolation and the South Atlantic subtropical
high-pressure position. Thus, prolonged northeasterly winds during austral summer
promoted intensification and expansion (from 24 to 28∘ S) of the
southeastern Brazil upwelling systems during MIS5. Conversely, an eccentricity
minimum (≤0.02) during the Holocene would result in weak northeasterly winds and
a contraction and decrease in the intensity of these upwelling systems in comparison with MIS5
(Lessa et al., 2017).
However, the orbital mechanism proposed by Lessa et al. (2017) does not
explain the intensified/expanded upwelling in the SBM during the last
glacial period, as eccentricity was ≤0.02 during the entire interval
(Fig. 4c). Interestingly, the dinocyst record from core GeoB2107-3 matches
austral winter (June) insolation at 60∘ S very well over the last 70 kyr
(Fig. 4d), highlighting the close connection between dinocyst abundance and
winter conditions through vigorous alongshore southwesterly winds and the increased
presence of the PPW at our core site. Periods of increased austral winter
insolation at 65∘ S may have steeped the thermal gradient between the
high- and mid-latitudes in the Atlantic sector of the Southern Ocean,
intensifying the alongshore southwesterly wind system and the northward incursion of
PPW, thereby boosting the eutrophic environmental dinocyst productivity in
the SBM.
The silicic acid leakage hypothesis (SALH)
In Sect. 5.1 and 5.2, we showed that G. bulloides and dinocysts record different
seasonal productivity processes in the SMB. While austral winter
productivity events were triggered by the more frequent northward intrusions
of the PPW, it is not clear what could explain the occurrence of austral
summer productivity events related to upwelling in the SBM.
We suggest that, rather than being driven by changes in upwelling intensity
as observed during interglacial MIS5
(Portilho-Ramos et al., 2015; Lessa et al., 2017), the increased productivity may
have been a result of the increased silicic acid (Si(OH)4) content supplied
by the glacial SACW. Several paleorecords and model experiments addressed
the hypothesis that the increased export of dissolved Si(OH)4 preformed in
the Southern Ocean fueled primary diatom productivity in low-latitude upwelling zones and continental margins during the last glacial
period, the so called “silicic acid leakage hypothesis” (SALH)
(DeMaster, 2002; Sarmiento et al.,
2004; Bradtmiller et al., 2007; Matsumoto et al., 2014). The SALH postulates that during glacial periods, imposed sea ice
around Antarctica displaced the zone of high diatom production to the north
of the Antarctic Polar Front (APF), where thermocline waters (i.e.,
Subantarctic Mode Water, a precursor of SACW and Antarctic Intermediate
Water – AAIW) is formed (Sarmiento et al., 2004; Bradtmiller et al., 2007; Abelmann
et al., 2015). Thus,
(unused) high-Si waters were exported from the Southern Ocean to the low-latitude world ocean, where diatom production increased at the expense of
other types of phytoplankton (Sarmiento et al., 2004; Bradtmiller
et al., 2007; Griffiths et al., 2013). In the
Brazilian margin, the silicon isotopic composition (δ30Si) of
sponge spicules from nearby core GeoB2107-3 indicates no substantial
difference in the Si(OH)4 content of AAIW between the Last Glacial
Maximum and the Holocene, which would contradict the SALH
(Hendry et al., 2012). However, high-Si(OH)4 pulses that occurred during the Younger Dryas and Heinrich stadials
(Hendry et al., 2012) make the Si(OH)4 content of the average last
glacial AAIW higher than that of the average Holocene AAIW. A significant
increment of Si(OH)4 transported by AAIW was also observed in the
western equatorial Atlantic at the onset of the last glacial (40–80 ka cal BP), which was not followed by enhanced surface productivity; this indicated that
AAIW Si(OH)4 did not reach the photic zone (Griffiths et
al., 2013). However, equatorial Atlantic upwelling zones are fed by SACW, and
high glacial opal burial as a consequence of enhanced surface diatoms
production due to intense upwelling is considered to be direct evidence of the
SALH (Bradtmiller et al., 2007). Indeed, SACW is the major conduit for sub-Antarctic
thermocline waters involved in the SALH (Sarmiento et al., 2004) and have
great potential to boost primary production in the SBM (Campos et al.,
2000).
Currently, diatoms dominate the phytoplankton in the SBM during austral
summer SACW upwelling (Brandini et al., 2014) and are an important component
of the diet of the symbiotic-barren G. bulloides, which can alternatively feed on
zooplankton (i.e., copepods) (Sautter and
Thunell, 1991; Thunell and Sautter, 1992; Schiebel and Hemleben, 2017).
Within age model uncertainties (also including the radiocarbon reversals in
our core) and considering the different temporal resolution of the records,
the glacial high abundance of G. bulloides in core JPC-17 matches well with the high
biogenic opal percentage in cores RC24-01 and RC24-07 from the equatorial upwelling
off northwestern Africa (Bradtmiller et al., 2007) as well as core RC13-254, to the
north of the APF Front (Atlantic sector) (Mortlock et al., 1991) (Fig. 4b, f, g). It is noteworthy that SACW also receives contributions from the
Indian Ocean via Agulhas leakage (warm water route;
Donners and Drijfhout, 2004), where
the production of opal (north of the APF) and export of silicic acid
remained high over the entire last glaciation (Dezileau
et al., 2003). We suggest that G. bulloides in the SBM may have benefited the silicic
acid-induced diatom blooms directly by feeding diatoms and/or indirectly by
preying on other zooplankton that also feed on diatoms. Thus, the increased
abundance of G. bulloides in the SBM during the last glacial period was related to
upwelling-driven high productivity during short austral summer periods, as
previously suggested in Portilho-Ramos et al. (2015). In contrast, prolonged
austral winter conditions with vigorous alongshore southwesterly winds as well as
increased precipitation over southeastern South America
(Cruz et al., 2005; Wang et al., 2007)
increased the northward penetration of the PPW leading to enhanced eutrophic
environmental dinocyst productivity during the last glaciation (Gu et al.,
2017). In combination, both processes may have boosted biological primary
productivity throughout the year during the last glacial period. This is
supported by the enhanced abundance of the deep-dwelling herbivorous planktonic
foraminifera species Globorotalia inflata (Schiebel and Hemleben, 2017) that calcifies between water depths of
200 and 400 m (Chiessi et al., 2007) and would have benefited from
grazing the increased amount of sinking organic particles.
Post-glacial conditions
After the Last Glacial Maximum, the abundance of G. bulloides and eutrophic dinocysts
decrease until the onset of the Holocene, suggesting decreased regional
productivity and more oligotrophic conditions in comparison with the last
glacial period (Fig. 4b, d). The presence of oligotrophic conditions is
supported by the increased abundance of the tropical symbiont-bearing species G. ruber
(37 %–52 %), G. sacculifer (6 %–8.8 %) and G. siphonifera (6.3 %–7.5 %) (Fig. 3). The low abundance
of G. inflata also suggests oligotrophic conditions during the Holocene (Fig. 3b).
Despite the favorable conditions for upwelling in the SBM during the
Holocene (i.e., occurrence of alongshore northeasterly winds and a strong BC;
Chiessi et al., 2014; Portilho-Ramos et al., 2015; Lessa et al., 2017), upwelling
productivity may have been hampered by the reduced export of preformed
silicic acid through SACW (Fig. 4f, g). This is supported by the substantial
decrease of biogenic opal in equatorial upwelling cores RC24-01 and RC24-07
after 15 ka cal BP (Fig. 4f; Bradtmiller et al., 2007). Indeed, the
retraction of Antarctic sea ice displaced the zone of enhanced biogenic opal
production to the south of the APF, retaining the excess of silicic acid and
opal burial in the Southern Ocean
(Sarmiento et al., 2004; Bradtmiller et al., 2007) as evidenced by increased Holocene
biogenic opal to the south of the APF at core RC13-259 (Fig. 4g; Mortlock et
al., 1991). In addition, low austral winter insolation at 65∘ S and
reduced sea ice may have decrease the thermal gradient between the high- and
mid-latitudes in the Atlantic sector of the Southern Ocean and consequently
weakened the alongshore southwesterly winds in the SBM, inhibiting the northward
intrusions of the PPW. Simultaneously, the high sea level stand modified the
SBM morphology increasing the width of the southern Brazilian shelf and
displacing the core of the upwelling zone to the inner shelf off Cape Santa
Marta, where it is controlled by local factors such as the coastal wind system
(Möller et al., 2008; Campos et al., 2013). The high temperatures at a water depth of 100 m from core JPC-17 (Fig. 3f)
support this hypothesis, suggesting that the SACW was not frequently in the
photic zone at the core location.
Conclusions
In this study we used planktonic foraminifera assemblage information and associated temperatures at a water depth of 100 m to discuss changes in productivity on the
southern Brazilian margin over the last 70 kyr. The enhanced abundance of the
upwelling indicator Globigerina bulloides (12 %–16 %) in addition to the reduced abundance of
oligotrophic species and subsurface temperatures lower than 20 ∘C
suggest the occurrence of upwelling off Cape Santa Marta during the last
glacial period. We suggest that rather than being driven by changes in
upwelling intensity, the increased productivity may have been a result of
increased silicic acid export from the Southern Ocean via South Atlantic
Central Water. Our results show that orbital forcing did not have a major
influence on changes in upwelling during the last glacial period. We further
show that more frequent northward intrusions of Plata Plume Water modulated
by austral winter insolation at 60∘ S through enhanced alongshore
southwesterly winds boosted austral winter productivity at the SBM. Thus, a productive
upwelling during short austral summer events and the prolonged presence of
Plata Plume Water during austral winter enhanced the biological productivity
year-round in the SBM during the last glacial period relative to modern
conditions. After the Last Glacial Maximum, the low silicic acid content in
thermocline waters decreased the productivity of the upwelling, while lower
austral winter insolation at 60∘ S and associated weakened southwesterly winds
reduced the presence of the Plata Plume Water in the SBM. In addition, last
deglaciation sea level rise may have modified the geomorphology of the SBM
limiting the upwelling system to the coast, south to Cape Santa Marta.
Data availability
The data reported here will be archived in the World
Data Center PANGAEA (https://doi.pangaea.de/10.1594/PANGAEA.902040) (last access: 24 May 2019) (Portilho-Ramos et al., 2019).
Author contributions
RdCPR and CMC designed the study. RdCPR and TMLP analyzed the planktonic foraminifera assemblage. RdCPR and CMC wrote the
paper. RdCPR performed the modern analogue technique. RdCPR and
CFB performed age modeling. All authors contributed to the interpretation
of the data.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank the two anonymous referees for their constructive
comments. We are grateful to Delia Oppo and William B. Curry from Woods Hole
Oceanographic Institute, United States, for providing the oxygen isotope
data from core JPC-17. Rodrigo da Costa Portilho-Ramos is thankful for a PNPD (Programa Nacional de Pós-doutorado) scholarship from CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior).
Tainã Marcos Lima Pinho is grateful for a PIBIC (Programa Institucional de Bolsas de Iniciação Científica) scholarship from CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico; 2017-482). Cristiano Mazur Chiessi
acknowledges financial support from FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo; grant no. 2012/17517-3), CAPES
(grant nos. 1976/2014 and 564/2015) and CNPq (grant nos. 302607/2016-1 and
422255/2016-5).
Financial support
The article processing charges for this open-access publication were covered by the University of Bremen.
Review statement
This paper was edited by Arne Winguth and reviewed by two anonymous referees.
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