Sediment cores
Sediment cores MD09-3257 (04∘14.69′ S, 36∘21.18′ W; 2344 m
water depth) and MD09-3256Q (03∘32.81′ S, 35∘23.11′ W;
3537 m water depth) were recovered from the Brazilian margin during R/V
Marion Dufresne cruise MD173/RETRO3 (Fig. 1). Improved recovery of deep-sea
sediments with no or little deformation of sediment layers was achieved
during this coring cruise thanks to the systematic use of the CINEMA
software (Bourillet et al., 2007; Woerther and Bourillet, 2005). This
software computes the amplitude and duration of the elastic recoil of the
aramid cable and the piston displacement throughout the coring phase,
accounting for the length of the cable (water depth) and total weight of the
coring system. The length of the coring cable is indeed of primary
importance regarding the deformation rate of the Calypso long piston cores
(Bourillet et al., 2007; Skinner and McCave, 2003).
Core GeoB3910 (04∘14.7′ S, 36∘20.7′ W; 2362 m water
depth; Jaeschke et al., 2007) was recovered from approximately the same
position and depth as core MD09-3257, during the Meteor cruise M34/4.
Hereafter, we refer to both GeoB3910 and MD09-3257 as intermediate equatorial cores and
to MD09-3256Q as the deep equatorial core. At present, the Brazilian margin
at these depths is bathed by the North Atlantic deep water (NADW; Fig. 1).
Because the Brazilian margin is affected by western boundary currents (Rhein
et al., 1995), these sediment cores are ideally located to observe changes
in the strength and extent of the intermediate and deep AMOC water masses
(Schott, 2003).
Sediment core SU90-03 (40∘30.3′ N, 32∘3.198′ W; 2475 m water depth)
was recovered from the northern margin of the subtropical gyre (Chapman et
al., 2000). Its location in the midlatitude North Atlantic provides
information on changes in NADW production rates that could not be deduced
from the sole equatorial depth transect.
We compare these Pa / Th records with published records from other Atlantic
cores that span the 20–50 ka period: ODP Leg 172 site 1063 (33∘41′ N, 57∘37′ W; 4584 m water depth; hereafter, ODP1063) (Böhm et al.,
2015); MD02-2594 (34∘43′ S, 17∘20′ E; 2440 m water depth) (Negre et
al., 2010) and V29-172 (33∘42′ N, 29∘22.98′ W; 3457 m water depth) (Bradtmiller et al., 2014) (Fig. 1, Table S1 in the Supplement).
Benthic δ13C
The stable carbon isotopic composition (δ13C) of the epifaunal
benthic foraminifer Cibicides wuellerstorfi has been shown to record the δ13C of
bottom-water dissolved inorganic carbon (DIC) with minor isotopic
fractionation (Duplessy et al., 1984; Zahn et al., 1986). Initial DIC
isotopic concentration is acquired by a water mass in its formation region
by surface productivity (which consumes 12C therefore increasing
dissolved δ13C) and temperature-dependent air–sea interactions
(Lynch-Stieglitz et al., 1995; Rohling and Cooke, 2003). DIC δ13C then evolves as deep water ages, because the constant export of
12C-enriched biogenic material that is remineralized at depth leads to
the decrease of the DIC δ13C along the flow path of the water
mass. As DIC δ13C largely follows water mass structure and
circulation in the modern ocean, C. wuellerstorfi δ13C has been used to trace
water masses, with a decrease in C. wuellerstorfi δ13C being interpreted as a
decrease in bottom water ventilation, and conversely (e.g., Duplessy et al.,
1988). However, the information on bottom water ventilation embedded in C. wuellerstorfi
δ13C is complicated by the impact of changes in surface water
δ13C, marine biological productivity and continental biomass
changes.
Because LGM δ13C values are higher in northern-sourced waters
(1.5 ‰) than in southern-sourced waters (< -0.2 ‰; Curry and Oppo, 2005), we interpret a decrease in
C. wuellerstorfi δ13C values at the equatorial sites as an increase in the time
elapsed since the water mass was last in contact with the atmosphere or as
an increased influence of nutrient-rich southern-sourced deep waters. Note
that because we lack information on past marine productivity changes, we do
not account for their potential impact on benthic δ13C in the
present study.
Core MD09-3256Q benthic foraminifer C. wuellerstorfi were handpicked in the size fraction
higher than 250 µm, washed with methanol in an ultrasonic bath and
then roasted in glass vials at 380 ∘C under vacuum for 45 min.
C. wuellerstorfi δ13C (expressed in ‰ VPDB) was measured
at LSCE (Gif-sur-Yvette) using an Elementar Isoprime mass spectrometer. VPDB
is defined with respect to NBS-19 calcite standard (δ18O = -2.20 and δ13C =+1.95 ‰; Coplen, 1988). The mean external reproducibility
(1σ) of carbonate standards is ±0.05
for δ18O and ±0.03 ‰ for δ13C. Measured NBS-18 δ18O is -23.2 ± 0.2
VPDB and δ13C is -5.0 ± 0.1 ‰ VPDB. δ13C measurements were done at the
highest possible resolution, depending on the availability of
C. wuellerstorfi (usually every 1 to 2 cm).
Sedimentary Pa / Th
In contrast to C. wuellerstorfi δ13C, which reflects the nutrient content of
bottom waters, sedimentary Pa / Th is a relatively recent tracer that can be
used to estimate the renewal rate of water masses occupying the first
∼ 1000 m above the seafloor (Thomas et al., 2006; Luo et al.,
2010). This tracer has been successfully used to reconstruct past changes in
deep Atlantic circulation intensity (Böhm et al., 2015; Gherardi et al.,
2005, 2009; Guihou et al., 2010, 2011; Hall et al., 2006; Jonkers et al.,
2015; Lippold et al., 2011, 2012; McManus et al., 2004; Negre et al., 2012;
Yu et al., 1996).
231Pa and 230Th are produced at a constant Pa / Th activity ratio of
0.093 by dissolved uranium, which is homogeneously distributed in the
oceans. 230Th is however much more particle reactive than 231Pa,
as reflected by their respective residence time in the ocean (30–40 years for
230Th, 200 years for 231Pa; Francois, 2007). 230Th is therefore
rapidly removed from the water column to the underlying sediment, while
231Pa can be advected by oceanic currents. High (low) rates of
overturning therefore result in high (low) 231Pa export and hence low
(high) sedimentary Pa / Th ratio in the Atlantic. However, affinities of
231Pa and 230Th for settling particles depend on the particle type
(Chase et al., 2002). For instance, 231Pa has a high affinity for opal,
so that high opal fluxes can result in high sedimentary Pa / Th values even in
the presence of lateral advection (Chase et al., 2002). The origin of
sedimentary Pa / Th variability therefore needs to be carefully assessed.
Pa / Th measurements on core MD09-3256Q were performed by isotopic dilution
mass spectrometry on a Thermo Finnigan MC-ICP-MS Neptune, following the
method of Guihou et al. (2010).
Core SU90-03 sedimentary Pa / Th was measured by isotopic dilution on a single
collector, sector field ICP-MS (Element2) at the University of British
Columbia, following the procedure described by Choi et al. (2001).
For both cores, Pa and Th are corrected from radioactive decay since the
time of sediment deposition and from authigenic and lithogenic components
using a 238U / 232Th ratio of 0.5 ± 0.1 (Fig. S1 in the Supplement; Guihou et
al., 2010).
Simulated sedimentary Pa / Th values (left) in response to different
stream functions (right): (a, b) Holocene, (c, d) off-mode, (e, f) shallow
overturning. White dots indicate the position of the studied sediment cores
(see Fig. 1). Data gridding was achieved using the Ocean Data View software
(Schlitzer, 2015).
Age model
Over the period 0–34 ka, core MD09-3256Q age model is based on 11 14C
dates measured on planktic foraminifer G. ruber white and converted to calendar age
using the Marine13 curve with no additional reservoir age correction (Burckel et al., 2016; Reimer
et al., 2013; Fig. S2). During the last glacial, Heinrich stadials were
recorded in marine sediment cores from the Brazilian margin as Ti / Ca peaks
resulting from increased terrigenous input during periods of increased
precipitation associated with southward shifts in the position of the intertropical convergence zone (ITCZ; Jaeschke et al., 2007). Ti / Ca peaks are
therefore good stratigraphic markers for correlating sediment cores with
neighboring well-dated cores. Therefore, from 34 to 50 ka, core
MD09-3256Q was dated by correlation of its Ti / Ca record with that of core
GeoB3910 using two tie points corresponding to the Ti / Ca peaks associated
with HS4 and HS5. The GeoB3910 age model over the 34 to 50 ka period is based
on one 14C date calibrated using the Marine13 curve and on four
speleothem tie points at the onset and end of HS4 and HS5 (Burckel et al.,
2015). Core MD09-3256Q age model and sedimentation rates are given in Table
S2 and Fig. S3. The age model of core SU90-03 is based on 17 14C dates
measured on various species of planktic foraminifera (Chapman et al., 2000)
that were converted to calendar ages using Marine13 calibration curve with
no additional reservoir age correction.
14C dates of all the published Atlantic cores used in this study were
converted into calendar ages using the same method (Table S1).
Circulation and Pa / Th model
Description of the models
In order to assess the vertical layout and renewal rate of the water masses
constituting the AMOC during the last glacial period, the sedimentary Pa / Th
data of the studied sediment cores were compared to Pa / Th values simulated
with a simple 2-D box model (Luo et al., 2010) forced by different
stream functions (Fig. 2b, d, f). Stream functions were generated using the
iLOVECLIM coupled climate model, comprising atmosphere, ocean and vegetation
components (Roche et al., 2014). A LGM equilibrium state computed using the
PMIP-2 protocol was used as background climate (see Roche et al., 2007) for
details). Stream functions characterized by a shallow (< 2500 m)
northern-sourced overturning cell (shallow overturning stream function) and
by a complete shutdown of the AMOC (off-mode stream function) were generated
by imposing a 0.16 and 0.35 Sv freshwater forcing in the Labrador Sea,
respectively (Roche et al., 2010, 2014). The freshwater input is added
during 300 years on the LGM background state. The stream functions are taken
as the mean over the 100-year period of lowest deep-water formation in the
North Atlantic, during or right after the period of freshwater forcing. A
freshwater input of 0.16 Sv allows the presence of a shallow circulation
cell in the Atlantic Ocean, while a freshwater forcing of 0.35 Sv leads to
an almost complete shutdown of the AMOC (Roche et al., 2014). Note that the
freshwater input values needed to modify the AMOC are strongly model
dependent and the important information carried by the model in the present
context is the state of the AMOC rather than the freshwater input value.
Contrary to shallow overturning and off-mode stream functions, the Holocene
stream function was computed using data-based geostrophic velocity estimates
(Talley et al., 2003).
Dissolved Pa and Th concentrations in the 2-D box model are controlled by (1) production from U decay in the water column, (2) adsorption and desorption
on settling particles and (3) advection by oceanic circulation. Particulate
Pa and Th concentrations are controlled by (1) adsorption and desorption
from the dissolved pool and (2) removal of sedimentary particles to the
seafloor (Luo et al., 2010).
Modeled sedimentary meridional Pa / Th sections generated with the different
stream functions are shown in Fig. 2a, c, e. Different water mass circulation
intensities and geometries result in different simulated sedimentary Pa / Th
(Luo et al., 2010). In the deep Atlantic, increasing circulation intensity
above a specific location causes Pa / Th to decrease at that water depth
because of the increased Pa export and conversely. Increasing water depth
without modifying circulation intensity also causes sedimentary Pa / Th ratio
to decrease in the model because of the increased residence time of Pa and
resulting higher Pa export, and conversely. Finally, the sedimentary Pa / Th
ratio increases along the flow path of any newly formed water mass as low
dissolved Pa concentrations of newly formed water masses increase by
desorption of Pa from Pa-concentrated settling particles equilibrating with
ambient waters (Francois, 2007). Adsorption and desorption rate constants
also impact the simulated Pa / Th ratio. These constants were adjusted to
reflect the opal belt in the Southern Ocean (Luo et al., 2010). For the
Holocene, these constants were also changed to reflect preferential
scavenging of Pa by biogenic opal in the northern North Atlantic (Lippold et
al., 2012).
Comparison between sedimentary Pa / Th and benthic δ13C
data from the Brazilian margin, Bermuda Rise, midlatitude North Atlantic Ocean and South
Atlantic Ocean and Greenland temperatures. (a) MD09-3256Q (this study),
SU90-03 (this study), MD09-3257 (Burckel et al., 2015), ODP1063 (Böhm et
al., 2015), MD02-2594 (Negre et al., 2010) and V29-172 (Bradtmiller et al.,
2014) Pa / Th; (b) MD09-3256Q (this study), SU90-03 (Chapman et al., 2000) and
GeoB3910 (Burckel et al., 2015) C. wuellerstorfi δ13C; and (c) NGRIP temperature
record on the GICC05 timescale (Kindler et al., 2014). In (a) lines pass
through average Pa / Th values in case of replicates, while diamonds and
squares (MD09-3257) correspond to individual Pa / Th measurements. V29-172
Pa / Th values are represented by two purple diamonds. White squares indicate
Pa / Th values not considered in core MD09-3257, as they might not be
influenced by oceanic circulation only (Burckel et al., 2015). The red and
blue arrows indicate the late Holocene Pa / Th values in cores MD09-3257
(0.065 ± 0.004, Burckel et al., 2015) and MD09-3256Q (0.043 ± 0.002), respectively. Error bars on Pa / Th measurements are given in Fig. S1
and Tables S7 and S8. In (b) thick lines are three-point running averages of the
C. wuellerstorfi δ13C records; the black arrow indicates present-day NADW
δ13C value (∼ 1.36 ‰;
the Supplement). δ13C values are given in Table S9.
In (c) numbers indicate the GIs. Red vertical bands represent the GI-3, GI-8
and GI-10 time slices and the blue vertical band the HS2 time slice.
Limits of the models
The absence of margins in the simple 2-D Pa / Th model (Luo et al., 2010)
prevents it from simulating boundary scavenging, which is the lateral
advection of dissolved Pa from open ocean regions characterized by high Pa
concentrations to coastal regions of low Pa concentration such as in
upwelling zones (Christl et al., 2010). However, as described in the results
section below, we verified that our Pa / Th signal is mainly driven by oceanic
circulation changes and the importance of diffusive transport is therefore
likely negligible here. This simple 2-D Pa / Th model therefore appears
adequate for comparison with our Pa / Th data. The vertical resolution of the
iLOVECLIM model is depth dependent, with higher resolution (10 to 100 m) in
the upper water column than below 1000 m (500 to 700 m). Hence, the
uncertainty in the position of the water mass transitions in the
stream functions below 1000 m is of 500 to 700 m. However, because
sedimentary Pa / Th likely reflects the Pa export in the bottom 1000 m of the
water column (Thomas et al., 2006), the model vertical resolution is
sufficient to properly simulate sedimentary Pa / Th values. Hence, the
relatively low vertical resolution of the iLOVECLIM model in the deep ocean
does not affect our conclusions.
Time slice definition
We define three interstadial time slices and one Heinrich stadial time slice (Fig. 3)
to compare Pa / Th data measured in Atlantic cores to Pa / Th values simulated
with the different stream functions. We focus on HS2, the preceding GI and
the GIs bracketing HS4. We did not include HS4 in our study because core
MD09-3257 HS4 Pa / Th data are affected by boundary scavenging (see Sect. 3.1,
Burckel et al., 2015) and we therefore lack information from an important
location of the Atlantic Ocean.
GI-3, GI-8 and GI-10 time slices are defined as the periods of stable
sedimentary Pa / Th values in core MD09-3257 associated with the NGRIP GI time
intervals (Fig. 3). More specifically, we used as a reference MD09-3257
Pa / Th values bracketing the middle of NGRIP GI time intervals in the GICC05
age scale (Rasmussen et al., 2014). Contiguous Pa / Th values within
1σ
uncertainty of the Pa / Th reference value form a plateau of stable Pa / Th
values that was used to define the GI time slices. With this definition, GI
time slices represent the periods of stable oceanic circulation associated
with each GI. The HS2 time slice was defined in core MD09-3257 as the period
of maximum sedimentary Pa / Th after the abrupt rise associated with the onset
of HS2.
Sedimentary Pa / Th values associated with each time slice and core are given
in Table S3. We computed uncertainties on the Pa / Th values associated with
each time slice accounting for the uncertainty on individual Pa / Th
measurements and uncertainties on the age model (see the Supplement).
Both GI-8 and GI-10 time slices are associated with high temperatures recorded
in Greenland ice cores. The GI-3 time slice includes both the period of high
Greenland temperatures associated with GI-3 and periods of low temperatures
associated with GS-4 and the beginning of GS-3. Given the low Pa / Th and high
δ13C values in the intermediate equatorial core at that time,
we consider that the GI-3 time slice mainly reflects interstadial
conditions. However, because temporal resolution of the marine records is
too low to clearly distinguish between GI-3 and GS-4, information about the
state of the AMOC during GIs derived from this time slice should be
considered with caution.