Article

Saline Indian Ocean waters invaded the South Atlantic thermocline during glacial termination II

Abstract and Figures

Salty and warm Indian Ocean waters enter the South Atlanticvia the Agulhas leakage, south of Africa. Model simulations andproxy evidence of Agulhas leakage strengthening during glacial terminationsled to the hypothesis that it was an important modulator ofthe Atlantic Ocean circulation. Yet, the fate of the leakage salinity andtemperature anomalies remains undocumented beyond the southerntip of Africa. Downstream of the leakage, new paleoceanographicevidence from the central Walvis Ridge (southeast Atlantic) showsthat salinity increased at the thermocline, and less so at the surface,during glacial termination II. Thermocline salinity change coincidedwith higher frequency of Agulhas rings passage at the core locationand with salinity maxima in the Agulhas leakage area, suggesting thatleakage waters were incorporated in the Atlantic circulation throughthe thermocline. Hydrographic changes at the Walvis Ridge and inthe leakage area display a distinct two-step structure, with a reversalat ca. 134 ka. This matched a wet interlude within the East Asia weakmonsoon interval of termination II, and a short-lived North Atlanticwarming. Such concurrence points to a Bølling-Allerød-like recoveryof the Atlantic circulation amidst termination II, with a northwardshift of the Intertropical Convergence Zone and Southern Hemispherewesterlies, and attendant curtailment of the interocean connectionsouth of Africa.
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GEOLOGY
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Volume 43
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Number 2
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www.gsapubs.org 1
Saline Indian Ocean waters invaded the South Atlantic thermocline
during glacial termination II
Paolo Scussolini1*, Gianluca Marino2,3, Geert-Jan A. Brummer1,4, and Frank J.C. Peeters1
1Earth and Climate Group, Vrije Universiteit, Amsterdam 1081HV, Netherlands
2Institut de Ciència i Tecnologia Ambientals, Univesitat Autònoma de Barcelona, Bellaterra 08193, Spain
3Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia
4
Department of Geology and Chemical Oceanography, NIOZ Royal Netherlands Institute for Sea Research, Den Burg NL-1790AB,
Netherlands
ABSTRACT
Salty and warm Indian Ocean waters enter the South Atlan-
tic via the Agulhas leakage, south of Africa. Model simulations and
proxy evidence of Agulhas leakage strengthening during glacial ter-
minations led to the hypothesis that it was an important modulator of
the Atlantic Ocean circulation. Yet, the fate of the leakage salinity and
temperature anomalies remains undocumented beyond the southern
tip of Africa. Downstream of the leakage, new paleoceanographic
evidence from the central Walvis Ridge (southeast Atlantic) shows
that salinity increased at the thermocline, and less so at the surface,
during glacial termination II. Thermocline salinity change coincided
with higher frequency of Agulhas rings passage at the core location
and with salinity maxima in the Agulhas leakage area, suggesting that
leakage waters were incorporated in the Atlantic circulation through
the thermocline. Hydrographic changes at the Walvis Ridge and in
the leakage area display a distinct two-step structure, with a reversal
at ca. 134 ka. This matched a wet interlude within the East Asia weak
monsoon interval of termination II, and a short-lived North Atlantic
warming. Such concurrence points to a Bølling-Allerød–like recovery
of the Atlantic circulation amidst termination II, with a northward
shift of the Intertropical Convergence Zone and Southern Hemi-
sphere westerlies, and attendant curtailment of the interocean con-
nection south of Africa.
INTRODUCTION
The South Atlantic subtropical gyre is a main component of the
Atlantic meridional overturning circulation (AMOC), acting as a con-
duit for the warm and saline upper waters that flow northward across the
equator (Gordon, 1986; Garzoli and Matano, 2011). Most of those waters
originate in the subtropical Indian Ocean and are transported by the Agul-
has Current to the southern tip of Africa, where they leak into the South
Atlantic (Agulhas leakage, AL; Fig. 1A), increasing its heat and salinity
(Gordon et al., 1992; Biastoch et al., 2009; Beal et al., 2011). Numerical
models point to a correlation of AL intensity with North Atlantic salinity
and with AMOC strength (Biastoch et al., 2009; Biastoch and Böning,
2013). Advancing on indications by Gordon et al. (1992), others suggested
a role for the AL in Pleistocene glacial terminations (e.g., Weijer et al.,
2002; Knorr and Lohmann, 2007). Paleoceanographic reconstructions
reveal that AL maxima were a persistent feature of terminations (Peeters
et al., 2004), whereby AL intensifications resulted from deglacial reor-
ganizations of the Southern Hemisphere westerlies (Toggweiler et al.,
2006), due either to their southern shift (Sijp and England, 2009; Bias-
toch et al., 2009; Biastoch and Böning, 2013), or to the redistribution of
momentum associated with their intensification (Durgadoo et al., 2013).
The so-called AL hypothesis contends that enhanced salt input from the
AL into the upper branch of the AMOC may have helped to overcome the
glacial density stratification that impeded deep convection in the northern
North Atlantic (Knorr and Lohmann, 2007), thereby nudging the AMOC
*Current address: IVM (Institute for Environmental Studies), Vrije Univer-
siteit, Amsterdam 1081HV, Netherlands.
GEOLOGY, February 2015; v. 43; no. 2; p. 1–4; Data Repository item 2015054
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© 2014 Geological Society of America. For permission to copy, contact editing@geosociety.org.
Sea-surface height anomaly (cm)
-0.5
0
0.5
1
18O
sw-ivc
95 105 115 125 135 145 155 165
Age (ka)
1
1.5
2
2.5
18O
sw-ivc
6
7
8
9
10
11
Temp(Mg/Ca) (°C)
18
19
20
21
22
Temp
(Mg/Ca)
(°C)
2.5
2
1.5
1
0.5
0
18
G. ruber s.l.
G. truncatulinoides sin.
MIS 6MIS 5
C
D
T-II
MIS 5e
B
A
64PE-174P13
MD02-2588
MD96-2080
Ocean Data View
Figure 1. A: Map of the Agulhas system, with the location of Walvis
Ridge core 64PE-174P13 and other cores discussed in the text. Con-
toured sea-surface height anomalies (seasonal climatological mean;
www.aviso.altimetry.fr) indicate the transmission of Agulhas rings
into the Atlantic Ocean. B–D: Geochemical results from core 64PE-
174P13 for Globigerinoides ruber (s.l.—sensu lato) and Globorotalia
truncatulinoides (sin—sinistral). MIS—marine isotope stage. B: Cal-
cite d18O (VPDB—Vienna Peedee belemnite). C: Mg/Ca-derived tem-
peratures (Temp). D: Ice-volume corrected seawater d18O (d18Osw-ivc).
Vertical error bars are propagated. VSMOW—Vienna standard mean
ocean water.
as doi:10.1130/G36238.1Geology, published online on 5 January 2015
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GEOLOGY
from a shallower and potentially slower glacial mode to a stronger inter-
glacial one. During terminations, however, the AMOC underwent major
reductions (McManus et al., 2004; Denton et al., 2010). This appears to
contradict the link between AL strengthening and AMOC intensification
as derived from modern climate models (Biastoch and Böning, 2013), and
as formulated in the AL hypothesis. Lack of data tracking the Atlantic
propagation of salinity anomalies coupled with past AL intensifications
(Marino et al., 2013) casts further doubts on the AL hypothesis.
Here we document the incorporation of AL heat and salt anoma-
lies into the South Atlantic subtropical gyre during glacial termination II
(T-II), which featured the most prominent intensification in AL volume of
the past 640 k.y. (Caley et al., 2014).
METHODS
Core 64PE-174P13 (29°45.7S, 2°24.1E, 2912 m water depth) is
located on the central Walvis Ridge, in the eastern sector of the South
Atlantic gyre (Fig. 1A), directly in the path of present and past Agulhas
rings (Gordon et al., 1992; Scussolini et al., 2013). We generated stable
isotope (d18O) and trace element (Mg/Ca) time series from two planktic
foraminiferal species, Globigerinoides ruber (sensu lato), which dwells
close to the surface (within the upper ~100 m), and Globorotalia trun-
catulinoides (sinistral), which dwells in the subsurface (within the per-
manent thermocline at ~500 m, herein referred to as the thermocline) (cf.
Scussolini and Peeters, 2013). The d18O measurements were conducted as
reported in Scussolini and Peeters (2013). Mg/Ca ratios were analyzed by
inductively coupled plasma–optical emission spectrometry at the VU Uni-
versity Amsterdam (Netherlands) on cleaned foraminiferal samples (cf.
Barker et al., 2003). We converted Mg/Ca to calcification temperatures
using the calibration of Anand et al. (2003) for G. ruber and by Regen-
berg et al. (2009) for G. truncatulinoides. Paired Mg/Ca-d18O and coeval
sea-level records yield ice-volume corrected seawater d18O (d18Osw-ivc), a
proxy for local salinity changes (e.g., Marino et al., 2013). The chronol-
ogy adopted here for the 165–95 ka interval is consistent with that of the
230Th-dated speleothems (Cheng et al., 2009; Barker et al., 2011). See the
GSA Data Repository1 for details about the methods.
RESULTS
The d18O values of G. ruber and G. truncatulinoides from core
64PE-174P13 display distinct decreases during T-II that are synchronous
but different in magnitude: ~1.5‰ in G. truncatulinoides and ~1.1‰ in
G. ruber (Fig. 1B; Table DR4 in the Data Repository). The Mg/Ca tem-
peratures feature millennial-scale variations superimposed upon a longer
term glacial-interglacial warming (Fig. 1C). Surface temperatures exhibit
suborbital oscillations of ~1 °C during glacial marine isotope stage 6
and two distinct warming shifts of up to 2.5 °C between ca. 142 ka and
126 ka, interspersed with an ~3 k.y. return to near-glacial values. Glacial
thermocline temperatures show larger fluctuations (to 3 °C) and a two-
phase warming (2–4 °C) at ca. 142 ka and 130 ka. The d18Osw-ivc (Fig. 1D)
increased at both surface (~0.7‰) and thermocline (~1.2‰) from ca. 140
ka to 142 ka, decreased gradually from 125 ka, and remained at relatively
low levels afterward.
DISCUSSION
Agulhas Leakage Control on the Walvis Ridge Salinity
Several lines of evidence indicate that multimillennial changes,
starting ca. 142 ka, in upper ocean temperature and d18Osw-ivc (hence
salinity) in the central Walvis Ridge reflected the hydrographic response
of the South Atlantic subtropical gyre to the AL strengthening of T-II
(Figs. 2B–2F). First, the thermocline d18Osw-ivc anomaly, which is ~70%
larger than the simultaneous d18Osw-ivc change at surface, correlates with
the enhanced propagation of Agulhas rings to the Walvis Ridge, as
inferred from a proxy registered in the same core and foraminiferal spe-
cies (G. truncatulinoides) (Fig. 2C; Table DR2; Scussolini et al., 2013).
An increase in the advection of Indian Ocean saline waters transported
by anticyclonic Agulhas rings would depress the South Atlantic isoha-
lines (van Aken et al., 2003) and increase the salt content of the sub-
100110 120 130 140 150 160
Age (ka)
NE Brazil speleothem
growth Phases
-6
-8
-10
East Asia
speleothems
18O
18
19
20
21
22
Sea Surface
Temp. (°C)
0
0.5
1
1.5
2
18
Osw-ivc
-0.5
0
0.5
1
18Osw-ivc
0
0.5
1
1.5
2
18Osw-ivc
100110 120130 140 150 160
1.5
2
2.5
18Osw-ivc
-0.5
0
0.5
1
18Osw-ivc
0
0.1
0.2
0.3
0.4
0.5
Agulhas rings proxy
10
20
30
40
Agulhas leakage (%)
-8
-4
0
4
Antarctic temp. (°C)
MIS 6
MIS 5
A
C
T-II / WMI-II
Cape Basin record
Agulhas Bank
Surface
Walvis Ridge
Surface
Walvis Ridge
Thermocl.
Agulhas Bank
surface
Walvis Ridge
surface
B
SAR-II
D
Agulhas Bank
Surface
E
Walvis Ridge
Thermocl.
F
G
1GSA Data Repository item 2015054, method details; details on the com-
parison of the South Atlantic reversals with records from East Asia, Brazil and
the North Atlantic; and datasets, is available online at www.geosociety.org/pubs
/ft2015.htm, or on request from editing@geosociety.org or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
Figure 2. South Atlantic records of upper ocean hydrography com-
pared with Antarctic temperatures and time series of tropical pre-
cipitation across glacial termination II (T-II). MIS—marine isotope
stage; VSMOW—Vienna standard mean ocean water; WMI—weak
monsoon interval. A: Antarctic temperature (temp.) from European
Project for Ice Coring in Antarctica (EPICA) Dome C (Jouzel et al.,
2007) on the speleothem (230Th) chronology of Barker et al. (2011).
B: Agulhas leakage (AL) fauna record, proxy for AL intensity, from
the Cape Basin (core GeoB3603–2; Peeters et al., 2004; the age
model was adapted, see the Data Repository [see footnote 1]). C:
Thermocline (Thermocl.) d18Osw-ivc (sea water, ice-volume corrected;
proxy for salinity) from Walvis Ridge core 64PE-174P13 (this study),
compared to the proxy for Agulhas rings in the same core (Scusso-
lini et al., 2013). D, E: Surface and thermocline d18Osw-ivc from Walvis
Ridge (this study), compared to surface d18Osw-ivc from Agulhas Bank
(core MD96–2080; Marino et al., 2013). F: Surface temperatures from
Walvis Ridge (this study) and from Agulhas Bank (MD96–2080; Ma-
rino et al., 2013). G: East Asia speleothems from the Sanbao cave,
China (Wang et al., 2008; Cheng et al., 2009), and growth phases
from northeastern Brazil speleothems (Wang et al., 2004; 1s error
bars). White bar indicates South Atlantic reversals (SAR) during T-II.
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surface, thus explaining the prominent salinification of the thermocline
during T-II. Second, the thermocline waters at the Walvis Ridge, con-
trary to their counterpart from the Agulhas Plateau (core MD02–2588;
Ziegler et al., 2013), lacked the carbon isotope minimum (Fig. DR2)
that fingerprinted deglacial Subantarctic Mode Water (Spero and Lea,
2002). This indicates that the Walvis Ridge subsurface hydrography was
not controlled by fresh and low-d13C Subantarctic Mode Water, but by
saltier and more nutrient-depleted AL waters. Third, both surface and
thermocline d18Osw-ivc from the Walvis Ridge covaried with their surface
counterpart from the Agulhas Bank (Marino et al., 2013), where warm
and saline Indian Ocean waters leak into the South Atlantic (Beal et
al., 2011) (Figs. 2D–2F; Table DR2). Lastly, the upper ocean d18Osw-ivc
maxima of T-II at the Walvis Ridge coincided, within chronological
uncertainties (Table DR1), with maximum transport of AL planktic fora-
minifera to the South Atlantic (Peeters et al., 2004) (Fig. 2B).
Marino et al. (2013) found that an increase of saline waters trans-
port via the AL during T-II coincided with the Heinrich event 11 in the
North Atlantic and with the profound interhemispheric reorganization of
the ocean-atmosphere system coupled with a weakening of the AMOC
(Stocker and Johnsen, 2003; Lee et al., 2011). Likewise the temperature
relationship between bipolar ice cores across the last glacial cycle (Stocker
and Johnsen, 2003), the Agulhas salt leakage developed in response to
North Atlantic cooling and peaked prior to (or in step with) North Atlantic
warming (Marino et al., 2013). Our Walvis Ridge thermocline d18Osw-ivc
record indicates a strong similarity in timing and magnitude with the
Agulhas Bank surface d18Osw-ivc over the entire 160–100 ka interval. One
example is the glacial salinification at 152–147 ka, also tied to a North
Atlantic cold phase (Marino et al., 2013). This suggests that the efficient
transmission of the AL salt to the thermocline was a persistent millennial-
scale feature of the South Atlantic gyre circulation and the interhemi-
spheric climate links.
Although salinification of the wider South Atlantic may have
stemmed from salt retention in this sector of the basin, due to deglacial
AMOC slowdown (Lohmann, 2003), the surface d18Osw-ivc anomaly at
the Walvis Ridge during T-II is smaller in amplitude than its counterpart
from the Agulhas Bank (Figs. 2D and 2E). Consequently, enhanced AL
was likely a more important driver of the salinity increase at the Walvis
Ridge, at least during Heinrich event 11. Accordingly, the surface salinity
anomaly associated with the AL would attenuate downstream in the South
Atlantic, as observed in the data.
Modern AL and rings propagating in the South Atlantic are character-
ized by elevated heat and salinity at the thermocline (Arhan et al., 2011).
Still, at the Walvis Ridge, thermocline temperature mirrors the proxy for
Agulhas rings (Scussolini et al., 2013) less closely than the d18Osw-ivc proxy,
indicating that salinity was a more persistent property of the AL waters.
This, along with the lower correlation between the surface temperature
profiles at the Walvis Ridge and Agulhas Bank (Fig. 2F), suggests that
the AL heat dissipated faster than the salinity anomaly in the Cape Basin,
probably due to convection and air-sea interaction (Arhan et al., 2011;
Beal et al., 2011). Hence, cooling would reduce buoyancy, and sink the
AL waters in the South Atlantic to the thermocline.
South Atlantic Evidence for a Climate Reversal During T-II
Our new Walvis Ridge records reveal a brief, mid-termination return
to low surface temperature and d18Osw-ivc values (Figs. 2D and 2F) inter-
jecting the long-term temperature and salinity increases of T-II. This mil-
lennial-scale, relatively cold and fresh episode corresponded, within age
uncertainties (Table DR1), to analogous temperature and d18Osw-ivc drops
centered ca. 134 ka in the AL area (Marino et al., 2013). Although the
uncertainties in the South Atlantic chronology are relatively large (see
the Data Repository), such timing appears to agree with the brief inter-
ruption of the weak monsoon interval of T-II in East Asia (Fig. 2G; see
also Fig. DR3, Table DR3; see the Data Repository), and with a distinct
warm phase in the wider North Atlantic region (Barker et al., 2011; Mar-
trat et al., 2014) (Figs. DR3F and DR3G). It has been proposed that both
the South Atlantic (Barker et al., 2009) and the AL (Marino et al., 2013)
responded to interhemispheric reorganizations of the ocean-atmosphere
system on a millennial scale. The South Atlantic reversal punctuating T-II
likely reflected the effect of a short-lived reduction in Indian-to-Atlantic
heat and salt transport. This reversal may have arisen from: (1) a north-
ward shift of the tropical and subtropical wind fields and weakening of the
subtropical wind fields. This could have affected the mesoscale processes
at the oceanic fronts south of Africa (Biastoch et al., 2009; Durgadoo et
al. 2013), in turn reducing the AL (Peeters et al., 2004), and is further con-
sistent with the more northern position of the Intertropical Convergence
Zone inferred from Chinese speleothems (Wang et al., 2008; Cheng et al.,
2009). (2) A transient resumption of the AMOC may have restored the
piracy of heat from the upper layers of the South Atlantic (Crowley, 1992)
and contributed to the decrease in upper ocean salinity.
As the AMOC and the latitudinal arrangement of atmospheric cir-
culation appear mutually dependent (Sijp and England, 2009; Toggwei-
ler, 2009; Frierson et al., 2013), both mechanisms together may account
for the South Atlantic reversals of T-II. This reinforces the analogy with
events amid termination I (Martrat et al., 2014), when cold conditions in
the South Atlantic corresponded to the Bølling-Allerød interval (Barker
et al., 2009).
CONCLUSIONS
New paleoceanographic data from the Walvis Ridge document that
enhanced Agulhas leakage raised the salinity and heat content of surface
and, more prominently, of thermocline layers of the South Atlantic gyre
during T-II. AL waters seem to have sunk upon entrance in the South
Atlantic. The salinification of the South Atlantic did not seem to entail
the resumption of the AMOC envisaged by the AL hypothesis. Recon-
structions of thermocline salinity further downstream should determine
whether the salinity anomalies also impacted the North Atlantic, or were
retained in the South Atlantic, possibly by the severing of the oceanic bot-
tleneck of the North Brazil Current, produced by the deglacial southern
shift in the easterlies’ position (Wilson et al., 2011).
The T-II increase of surface temperature and salinity at the Walvis
Ridge and in the AL area occurred in two steps, with a return to cold and
fresh conditions ca. 134 ka. Comparison with records of Asian monsoon
and of North Atlantic climate ascribes the reversal to a transient resump-
tion of the AMOC, and attendant reduction of heat and salt transfer via the
AL, potentially mediated by rearrangements of the Southern Hemisphere
westerlies and associated oceanic fronts.
ACKNOWLEDGMENTS
This work was funded by the European Commission FP7 Marie-Curie ITN
grant agreement 238512, GATEWAYS project. Marino acknowledges support
from the Universitat Autonoma de Barcelona (postdoctoral research grant PS-688-
01/08) and from the Australian Laureate Fellowship project FL120100050 (to
E.J. Rohling). Core 64PE-174P13 was taken as part of the MARE-program
funded by the Netherlands Organisation of Scientific Research. We thank Sanne
Vogels and Laura Rodríguez-Sanz for advice on Mg/Ca cleaning, and John Visser
for assistance with the inductively coupled plasma–optical emission spectrometry
at the Vrije Universiteit Amsterdam. Comments from J.R. Toggweiler and two
reviewers greatly improved this paper.
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Manuscript received 2 September 2014
Revised manuscript received 22 November 2014
Manuscript accepted 25 November 2014
Printed in USA
as doi:10.1130/G36238.1Geology, published online on 5 January 2015
Geology
doi: 10.1130/G36238.1
published online 5 January 2015;Geology
Paolo Scussolini, Gianluca Marino, Geert-Jan A. Brummer and Frank J.C. Peeters
glacial termination II
Saline Indian Ocean waters invaded the South Atlantic thermocline during
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as doi:10.1130/G36238.1Geology, published online on 5 January 2015
... For example, during the transition from the penultimate glacial (i.e., Marine Isotope Stage, MIS 6) to the Last Interglacial (MIS 5e), records from the eastern South Atlantic on Mg/Ca-derived sea surface temperature (SST) and ice-volume-corrected seawater δ 18 O (δ 18 O IVC-SW , as a proxy for relative changes in ocean salinity) suggest the AL intensified from as early as ca. 142 ka BP (Scussolini et al., 2015). However, some deep-water convection in high latitudes of the North Atlantic apparently did not strengthen until late into glacial Termination II (TII), ca. ...
... NBC/NBUC: North Brazil Current/Undercurrent; BC: Brazil Current; MC: Malvinas Current; BMC: Brazil-Malvinas Confluence; SAC: South Atlantic Current; AL: Agulhas Leakage. The yellow dots mark the positions of core GL-1090 (this study) and other relevant records discussed in the text: 64PE-174P13 (Scussolini et al., 2015), ODP Site 1063 and 983 (Deaney et al., 2017), ODP Site 1058 (Bahr et al., 2013), MD03-2664(Irvalı et al., 2016, NEAP-18K (Hall et al., 1998), and IODP Site U1304 (Hodell et al., 2009). mid-Atlantic ridge along the South Atlantic Current (Garzoli & Matano, 2011). ...
... To reconstruct propagation of AL signal across the South Atlantic subtropical gyre via its salt signature, we compared the surface (Santos et al., 2017b) and thermocline (this study) δ 18 O IVC-SW records of core GL-1090 with surface and thermocline records from core 64PE-174P13 of the Walvis Ridge (eastern South Atlantic) (Scussolini et al., 2015) (Figure 4). Figure 4A and B). During the period between 160-142 ka BP, when the AL was weaker (Scussolini et al., 2015), similarity of surface δ 18 O IVC-SW records between both sites is low, suggesting that the surface salinities of Santos Basin and Walvis Ridge evolved independently, so they were likely controlled by independent processes prior to 142 ka BP. ...
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... However, the IOSTUW may play an important role in modulating the sea level change in the southeastern Indian Ocean, as has been suggested by Llovel and Lee (2015) and Huang et al. (2020) who found that that this region experiences a large halosteric contribution to sea level change. Additionally, the IOSTUW may invade the South Atlantic Ocean via Agulhas leakage (Scussolini et al. 2015) and play an important role in interoceanic salinity exchange. ...
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... The δ 18 O sw-ivc increases by ∼1.2 during Termination II (Fig. 5e). This salinity effect was possibly boosted by an increase in South Atlantic thermocline salinity at the beginning of Termination II (Fig. 5e, f) (Ballalai et al., 2019), probably due to increases in the advection of saline waters from the Indian Ocean to the South Atlantic thermocline (Scussolini et al., 2015). A decoupling between δ 18 O and temperature is also noticed during Termination I and the Holocene (Fig. 2c), which could possibly indicate a recurring feature of glacial terminations, though a further investigation would be required. ...
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The upper ocean circulation in the western tropical Atlantic (WTA) is responsible for the northward cross-equatorial heat transport as part of the Atlantic Meridional Overturning Circulation (AMOC). This cross-equatorial transport is influenced by the thermocline circulation and stratification. Although seasonal thermocline stratification in the WTA is precession-driven, the existence of an orbital pacemaker of changes in the entire WTA upper ocean stratification, which comprises the main thermocline, remains elusive. Here, we present a 300 ka-long record of the WTA upper ocean stratification and main thermocline temperature based on oxygen isotopes (δ 18 O) and Mg/Ca of planktonic foraminifera. Our δ 18 O record between Globigerinoides ruber and Globorotalia truncatulinoides, representing upper ocean stratification, shows a robust precession pacing, where strong stratification was linked to high summer insolation in the Northern Hemisphere (precession minima). Mg/Ca-based temperatures support that stratification is dominated by changes in thermocline temperature. We present a new mechanism to explain changes in WTA stratification, where during the Northern Hemisphere summer insolation maxima, the Intertropical Convergence Zone shifts northward, developing a negative wind stress curl anomaly in the tropical Atlantic. This, in turn, pulls the main thermocline up and pushes the South Atlantic Subtropical Gyre southwards, increasing the stratification to the north of the gyre. This mechanism is supported by experiments performed with the Community Earth System Model (CESM1.2). Finally, we hypothesize that the precession-driven WTA stratification may affect the cross-equatorial flow into the North Atlantic.
... The interest of the palaeoclimate community in Agulhas leakage arose from the finding that peak Agulhas leakage occurred during glacial terminations 8 and plausibly aided the AMOC to shift to its full-strength interglacial mode 9,10 . This hypothesis builds on a variety of records from within the Agulhas leakage pathway, which inferred fluctuations in the strength of Agulhas leakage over the late Pleistocene epoch based on a variety of faunal and geochemical proxy reconstructions 8,[11][12][13][14][15][16][17][18] . These findings are further reinforced by a numerical model study 19 indicating that the strength of the Agulhas leakage varied by ~10 Sv between glacial and interglacial periods. ...
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... The forward modelling exercise therefore indicates that bioturbation is a possible alternative mechanism for the variance peak, but also indicates that the conclusions are sensitive to the parameterization. Forward modelling cannot disprove enhanced Agulhas leakage as the source of increased IFA variance across the MIS 5-6 transition (Marine Isotope Stage), and there is other evidence for increased leakage such as the tight coupling between the Agulhas rings proxy and the δ 18 O of G. truncatulinoides Scussolini et al. (2015). However, given that bioturbation depths as low as 3 cm still produce a quite visible variance peak, we argue that bioturbation is at least a plausible mechanism behind some of the change in variance over the MIS 5-6 transition. ...
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A maximum in the strength of Agulhas leakage has been registered at the interface between the Indian and South Atlantic oceans during glacial Termination II (T-II). This presumably transported the salt and heat necessary for maintaining the Atlantic circulation at rates similar to the present day. However, it was never shown whether these waters were effectively incorporated into the South Atlantic gyre, or whether they retroflected into the Indian and/or Southern oceans. To resolve this question, we investigate the presence of paleo Agulhas rings from a sediment core on the central Walvis Ridge, almost 1800 km farther into the Atlantic Basin than previously studied. Analysis of a 60 yr data set from the global-nested INALT01 model allows us to relate density perturbations at the depth of the thermocline to the passage of individual rings over the core site. Using this relation from the numerical model as the basis for a proxy, we generate a time series of variability of individual Globorotalia truncatulinoides δ18O. We reveal high levels of pycnocline depth variability at the site, suggesting enhanced numbers of Agulhas rings moving into the South Atlantic Gyre around T-II. Our record closely follows the published quantifications of Agulhas leakage from the east of the Cape Basin, and thus shows that Indian Ocean waters entered the South Atlantic circulation. This provides crucial support for the view of a prominent role of the Agulhas leakage in the shift from a glacial to an interglacial mode of the Atlantic circulation.
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