Magnitude of the 8.2 ka event freshwater forcing based on stable isotope modelling and comparison to future Greenland melting

Abstract

The northern hemisphere experienced an abrupt cold event ~ 8200 years ago (the 8.2 ka event) that was triggered by the release of meltwater into the Labrador Sea, and resulting in a weakening of the poleward oceanic heat transport. Although this event has been considered a possible analogue for future ocean circulation changes due to the projected Greenland Ice Sheet (GIS) melting, large uncertainties in the amount and rate of freshwater released during the 8.2 ka event make such a comparison difficult. In this study, we compare sea surface temperatures and oxygen isotope ratios from 28 isotope-enabled model simulations with 35 paleoproxy records to constrain the meltwater released during the 8.2 ka event. Our results suggest that a combination of 5.3 m of meltwater in sea level rise equivalent (SLR) released over a thousand years, with a short intensification over ~ 130 years (an additional 2.2 m of equivalent SLR) due to routing of the Canadian river discharge, best reproduces the proxy anomalies. Our estimate is of the same order of magnitude as projected future GIS melting rates under the high emission scenario RCP8.5.

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Magnitude of the 8.2 ka event
freshwater forcing based on stable
isotope modelling and comparison
to future Greenland melting
Wilton Aguiar1*, Katrin J. Meissner2, Alvaro Montenegro3, Luciana Prado4,6, Ilana Wainer4,
Anders E. Carlson5 & Mauricio M. Mata1
The northern hemisphere experienced an abrupt cold event ~ 8200 years ago (the 8.2 ka event) that
was triggered by the release of meltwater into the Labrador Sea, and resulting in a weakening of
the poleward oceanic heat transport. Although this event has been considered a possible analogue
for future ocean circulation changes due to the projected Greenland Ice Sheet (GIS) melting, large
uncertainties in the amount and rate of freshwater released during the 8.2 ka event make such a
comparison dicult. In this study, we compare sea surface temperatures and oxygen isotope ratios
from 28 isotope-enabled model simulations with 35 paleoproxy records to constrain the meltwater
released during the 8.2 ka event. Our results suggest that a combination of 5.3 m of meltwater in sea
level rise equivalent (SLR) released over a thousand years, with a short intensication over ~ 130 years
(an additional 2.2 m of equivalent SLR) due to routing of the Canadian river discharge, best reproduces
the proxy anomalies. Our estimate is of the same order of magnitude as projected future GIS melting
rates under the high emission scenario RCP8.5.
Greenland ice-sheet melting is one of the major responses to the rising atmospheric greenhouse gas concentra-
tions and global mean temperature1–3. e addition of ice-sheet meltwater to the North Atlantic will potentially
have a destabilizing eect on the Atlantic Meridional Overturning Circulation (AMOC), which could weaken
by more than 70% within the next few centuries4–6. Past meltwater-driven AMOC slowdowns have repeatedly
led to millennia-long cold events in the Northern Hemisphere: for example, the Oldest and Younger Dryas
(~ 19 to 14.7 kiloyears and 12.9 to 11.7 kiloyears before the present, respectively)7–9. However, the cold event 8.2
kiloyears before present (8.2ka event hereaer) diers from previous cold events due to its short, century-long
duration10,11. e 8.2ka event also took place in the current interglacial period under boundary conditions that
were closer to pre-industrial conditions than earlier cold events12.
Several freshwater forcing hypotheses involving the Laurentide Ice Sheet (LIS) have been suggested for the
8.2ka event. ese scenarios include three freshwater sources: the drainage of Lake Agassiz13, the change in
North American continental freshwater routing from LIS retreat14,15, and the on-going retreat of the LIS and its
associated meltwater production16–18. e rst two sources have relatively well-constrained discharge rates and
volumes19,20 when compared to the direct ice-sheet meltwater source16,17,19,20. Even though the outburst of Lake
Agassiz is commonly considered the main trigger for the 8.2ka event13, recent studies have found that both the
LIS retreat and change in the routing of continental discharge might have had a signicant role in causing the
climate event’s anomalies15,16,21, thus raising uncertainties on the role of each of the three meltwater sources in
triggering the 8.2ka event.
e range of estimates of the magnitude of total freshwater release during the 8.2ka event is also large10,11,
ranging from 1.5 to 9m in equivalent sea-level rise (SLR)16,22. Some of these scenarios were previously used
to simulate the cold event with numerical climate models in an attempt to estimate the climatic impacts of the
freshwater discharge23–25, and simulation skill was evaluated by comparison with sea surface temperature (SST)
OPEN
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reconstructions. Such a large range of meltwater volume is enough to create scenarios ranging from a small
change in circulation to a total collapse of the AMOC26. Furthermore, due to a model-dependent stability of
ocean overturning, location of deep convection sites and meridional heat transport, the simulated SST response
to freshwater forcing varies signicantly between distinct simulations27,28. is model dependency makes it dif-
cult to test which freshwater source played the dominant role in triggering the 8.2ka based on the comparison
between the simulated SST response and reconstructed SST changes. However, these uncertainties can be reduced
by using a combination of active and non-active tracers, such as oxygen isotopes as well as SSTs.
Constraining the amount of freshwater involved in the 8.2ka event, and the role of each freshwater source
in creating the climate anomalies of the event, will enhance our understanding of the sensitivity of the climate
system to freshwater uxes, which is of obvious importance for future scenarios given the observed recent
acceleration of Greenland ice-sheet (GIS) mass loss1,2,6. In this study, we aim to constrain the magnitude and
length of freshwater ux that caused the 8.2ka event. We explore this question by using numerical simulations
that calculate both oxygen isotopes, in seawater, in carbonates, and in ice cores, and SSTs prognostically and by
comparing the simulations with paleoclimate records of the same variables.
Simulations based on earlier reconstructions
e simulations presented in this section are based on dierent freshwater release processes that have been
suggested in previous studies15,18,22,30. FWpe simulates a scenario where the estimated LIS melting is released
exclusively into the Labrador Sea29; FWca represents a scenario where the Canadian continental runo discharges
into the Labrador Sea15; FWul simulates the melting of the remaining LIS aer the collapse of the Hudson Bay’s
ice saddle22, and FWli simulates a fast rise in sea level surrounding the 8.2ka event due to prolonged drainage
from Lake Agassiz18 (see “Methods”, “Freshwater forcing for the simulations based on earlier reconstructions”
section). SSTs and δ18O anomalies from these simulations are then compared with proxy data from 27 locations,
at the model’s grid cell closest to the geographical coordinates of each core (see “Methods”).
Linear regression slopes and RMSEs for simulated tracers (Fig.1a) show that FWpe and FWca yield the best
estimate of δ18Osw and δ18Oice (
αpe
sw
=
0.89
,
αca
sw
=0.99, α
pe
ice
=
0.86
and
αca
ice
=
0.85
). However, FWpe overes-
timates SST anomalies and estimates a decrease in δ18Oc while proxy records point to an increase during this
period (
αpe
sst
=
1.43
,
αpe
c
=−
0.4
). e FWpe simulation represents the total amount of LIS melting during this
period of time, however this ow did not go entirely into the Labrador Sea30. us, the overestimation of the
SST response could be a result of an overestimation of the total freshwater forcing. Since the model calculates
δ18Osw prognostically, and obtains δ18Oc using a SST-based transfer function, the misrepresentation of δ18Oc is
likely due to the SST overestimation. In turn, FWca yields the lowest RMSEs and best slopes across most trac-
ers, with the exception of δ18Osw. FWli and FWul have the lowest regression slopes (
αul
all
<
0.3
,
αli
all
<
0.5
) and
highest RMSEs of the four simulations.
Figure1. Comparison between time series of proxies and simulations for δ18Osw, δ18Oice, δ18Oc and SST for
RAPiD (c,g), Gardar Dri (d,h), Florida Strait (e,i), GISP and Gulf cores (b,j), and slopes and RMSEs (a) in all
simulations (locations in Fig.S1 and TableS1). Black dashed and full lines are core values and 2-point moving
averages, respectively. Green, blue, magenta and red lines are time series for FWpe, FWli, FWul and FWca. e
pink horizontal crosses are the dating ( ) and dating errors ( ) for the proxies. In (a) RMSE values are plotted in
the center of the image while the colors of the squares indicate the values of the slopes. From (f–j), top series are
for δ18Oc, while bottom series are for δ18Osw.
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e time series of the tracers conrm that FWpe overestimates both the long-term decrease in δ18O prior
to 8ka (Fig.1b,f–i) and the cold SST anomalies (Fig.1c–e). Analysis of the δ18O time series for the remaining
simulations show the eect of each discharge in the early-Holocene proxy signal. e routing event in FWca
reproduces most of the early-Holocene anomalies recorded between 8.5 and 8.3ka, especially in the Labrador
Sea SST and δ18Osw (Fig.1d,j), Gardar dri δ18Oc (Fig.1h) and RAPiD subsurface (Fig.1f). e magnitude of the
short negative excursion in δ18Oice at 8.2ka in the GISP2 record is also best reproduced by FWca when compared
to the other simulations (Fig.1b). FWli also reproduces a sharp decrease in δ18Oice at 8.2ka (Fig.1b), and in SST
and δ18Osw at Gardar dri (Fig.1d,h), although it underestimates the magnitude of δ18Oice anomalies at 8.2ka.
Finally, the remaining melting of LIS aer its collapse simulated in FWul reproduces the stable low δ18Oc values
at subsurface in the RAPiD core (Fig.1f).
us, simulated δ18O shows that FWpe, FWca, FWli, and FWul reproduce dierent parts of the early Holocene
signal. is suggests that a realistic freshwater ow for prompting the 8.2ka event anomalies requires a combina-
tion of a long-term meltwater ux with a short-term ux intensication, possibly due to a change in routing of
continental runo and draining of Lake Agassis21.
Hybrid simulations
e simulations analyzed in this section (called hybrid hereaer) follow more complex freshwater release sce-
narios, testing the range of uncertainties in freshwater ux magnitude and duration as well as changes in fresh-
water forcing over time. In one set of the hybrid simulations (Table1, Part A), the freshwater forcing is separated
into two components: one lasts longer (1000years background ux) with relatively low magnitudes (0.086Sv,
0.066Sv, and 0.046Sv), while the other is shorter (130year-long ux intensication) with relatively high mag-
nitudes (0.13Sv, 0.07Sv, 0.19Sv and 0.26Sv). A comparison between the short uxes in the Part A simulations
(Fig.2a–d) shows that a ux intensication of 0.19Sv achieves the lowest RMSEs in δ18Osw, δ18Oc and δ18Oice
when compared with simulations with the same background ux but dierent short uxes (TableS2). In turn,
when comparing the long uxes in the Part A simulations (Fig.2a–d, columns), it is noticeable that the simula-
tions with 0.066Sv of background ux have the lowest RMSEs and slopes closest to 1 for SST, δ18Oc and δ18Oice.
us, the comparisons based on RMSEs and slopes suggest that a background ux of 0.066Sv and a short ux
of 0.19Sv best represent the tracer anomalies (simulation FW06—TableS2).
e Part B experiments were aimed at evaluating model sensitivity to the duration of the freshwater forcing.
is was accomplished by adopting freshwater ux magnitudes from FW06, the experiment that best represented
8.2ka event anomalies in Part A, and varying the durations of the individual phases of freshwater addition. e
length of the shorter ux in this set of simulations varies from 50 to 300years, while the longer ux varies from
200 to 1000years (Table1).
Simulations FW61 and FW63 show the best match with proxy data with slopes closest to 1 and consistently
low RMSEs (Fig.2e–h, TableS3). Further testing the similarity between the simulated and core time series of
δ18O in a Taylor diagram allows for a more detailed comparison between simulations. All correlation values in
Table 1. Details of simulations used in this study. e reconstructions table (I) describes the meltwater
volumes, uxes and durations for the homogeneous forcing experiments described in “Simulations based on
earlier reconstructions” section. Experiments with hybrid freshwater forcing are separated into Part A and B
(II and III). e long meltwater ux in the hybrid experiments in Part A have a xed duration of 1000years
(9–8ka), and the short ux is xed at 130years (8.31–8.18ka). FW06 is the simulation in best agreement with
proxy data in Part A, so the ux magnitudes of FW06 were used in Part B to test ux duration of the short ux.
Note that FW06 is the same simulation as FW61. Volume (a) is in 105 km3.
(I) Reconstructions
Experiment VolumeaDuration Flow (Sv) References
FWp e 27.1 9–8ka 0.086 Peltier30
FWc a 8.2 8.5–8.2ka 0.13 Carlson etal.15
FWli 5.3 8.31–8.18ka 0.13 Li etal.22
FWu l 9.5 8.2–7.6ka 0.05 Ullmann etal.18
Duration (II) Flux magnitude (Part A)
9–8ka Sv 0.046 0.066 0.086
8.31–8.18ka
0.26 FW10 FW11 FW12
0.19 FW09 FW06 FW03
0.13 FW07 FW04 FW01
0.07 FW08 FW05 FW02
Flux (III) Flux duration (Part B)
0.066Sv Duration 200 years 600 years 1000 years
0.19Sv
300 years FW610 FW611 FW612
130 years FW67 FW64 FW61
90 years FW68 FW65 FW62
50 years FW69 FW66 FW63
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the Taylor diagram are statistically signicant (p < 0.05, n > 1000—Fig.2i,j). e highest correlations for δ18Osw
are for the Labrador Sea cores and simulations FW61 (0.84), FW62 (0.88), and FW63 (0.83). In the Gardar Dri
time series of δ18Osw, FW61 and FW63 have the strongest correlation with the core (both 0.75), but the lowest
RMSE value is achieved by FW66 (0.76‰). FW61 and FW63 also have the highest positive correlations for
δ18Osw at the RAPiD core based on G. bulloides (0.47 and 0.48 respectively), for G. bulloides δ18Oc in the RAPiD
core (0.54 for FW61). According to the Diebold-Mariano test, RMSEs for FW61, FW63 and FW66 in δ18Oc
and δ18Osw are signicantly dierent with condence varying from 85 to 99%. e exceptions are δ18Oc errors
between FW61 and FW63, which are equal with 90% condence. ese results suggest that errors in δ18Osw for
simulations FW61, FW63 and FW66 are statistically dierent and possibly not random. Simulations FW61,
FW63 and FW66 are the ones that best reproduce δ18O mean anomalies in most proxies and locations and have
the best correlations and RMSEs for the whole time series. As a last step, and in an eort to determine the most
Figure2. Analysis of simulations in Table1—Parts A and B. (a) to (h) are the slope and RMSE values for
each experiment. Plots (a–d) are for experiments in part A, while plots (e–h) are for experiments in part B.
e colour of the squares represents the slopes according to the color bar, and RMSE values are indicated in
the center of each cell. (i) Taylor diagram for comparison between proxy and simulated time series of δ18O
anomalies: GISP ice core (lled square), Rapid Core δ18Oc in G. inata (lled rhombus) and G. bulloides (plus),
Gardar Dri core δ18Oc for G. bulloides (lled circle), Florida Strait core δ18Oc for G. ruber (X), Gulf core δ18Oc
for G. ruber (star), Rapid Core δ18Osw in G. inata (asterisk) and G. bulloides (lled triangle), Gardar Dri core
δ18Osw (lled inverted triangle), Florida Strait core δ18Osw (open circle), and δ18Osw in Labrador Sea core (open
square). e colors represent dierent simulations. Taylor diagram (j) is the same as (i), but zoomed in closer to
the 0. Standard deviations are normalized by the core value, while RMSE is centered.
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realistic freshwater forcing, we will compare the time series produced by the three best tting simulations with
the proxy reconstructions at the six locations with high resolution data (Fig.3).
Simulated δ18Osw and δ18Oc for FW61/62/63 at the location of the RAPiD core now capture the magnitude
of proxy anomalies for G. bulloides (Fig.3f). FW61 and FW63 also reproduce the magnitude of the anomalies at
the Gardar Dri core in the G. bulloides time series of δ18Oc and δ18Osw (Fig.3g), and the SST time series at the
RAPiD location (Fig.3b-top), and Gardar Dri (Fig.3c-top).
GISP2 δ18Oice is best simulated by FW61, which reproduces both the long-term oxygen isotope decrease and
the timing of the short-lived decrease at 8.2ka (Fig.3a-top). Neither simulated δ18Oc in the Gulf of Mexico or
SSTs in the Strait of Florida show signicant variability (Fig.3d,h,i-top). Taking into account that FW61 exhibits
the best match to SST and δ18O in the proxy record, while also reproducing the GISP2 δ18Oice, we conclude that
this simulation is the best representation of the 8.2ka event in our study.
Discussion
Our rst set of simulations evaluate how well dierent freshwater sources to the North Atlantic reproduce ocean
anomalies associated with the 8.2ka event. Although FWca represents only one of these sources, i.e., a runo
routing event15, it yields the lowest RMSEs, best slopes and the best representation of most cores time series. is
points to the routing event being one of the main contributors to the changes captured by the proxies during the
early Holocene. Given that the regression slopes for FWul are signicantly lower than for FWca, melting of the
remaining LIS aer its collapse (FWul) likely only played a background role in creating the climate anomalies at
the 8.2ka, while the routing event (FWca) had much more impact.
We then conducted several additional meltwater ux experiments in order to answer the following question:
What magnitudes and rates of freshwater uxes are most consistent with the 8.2ka event proxy anomalies? e
short 8.2ka event anomalies recorded in δ18O climate archives are best reproduced with a simulation forced by
a freshwater ux intensication of 0.19Sv lasting for 130years. is is in line with earlier simulations performed
with the Community Climate System Model version 3, which reproduced the 8.2ka SST anomalies with 0.13Sv
of freshwater discharge for 99 years24. Here, we show that a higher discharge estimate of 0.19Sv embedded in a
background ux of 0.066Sv is able to reproduce δ18O anomalies in addition to SST anomalies.
Based on 35 δ18O and SST records from 27 dierent locations, we consider that our FW61 simulation was
able to accurately reproduce the major trends and anomalies recorded in the proxy records for the 8.2ka event
and early-Holocene (Fig.3). e FW61 simulation suggests that anomalies similar to those associated with the
event could have been caused by a total meltwater addition of 7.5m in SLR equivalent between 9–8ka, with a
short period of intensied ooding, equivalent to a SLR of 2.2m (included in the 7.5m estimate), between 8.31
and 8.18ka (Figs.4a,b, S2). is short intensication of the freshwater ux in FW61 has similar magnitude as the
relative SLR in Southwest Scotland (1.45m within 300 and 500 years)31, but the absolute value for our estimate
is 0.75m higher. is discrepancy could be explained by either local land upli due to glacial isostatic adjust-
ment over Scotland32, or by a combination of LIS melting and Canadian basin routing, since the routing would
not contribute to eustatic SLR. e intensication in freshwater input of 2.2m also matches previous eustatic
Figure3. Comparison of simulated and reconstructed δ18Osw, δ18Oc, δ18Oice and SST time series for the three
best tting hybrid models: RAPiD (b,e,f), Gardar Dri (c,g), Florida Strait (d,h), GISP δ18Oice (a), Gulf Strait
δ18Oc (i-top) and Labrador Sea δ18Osw (i-bottom) and SST (c-bottom). Black dashed and full lines are core values
and 2-points moving average. Green, cyan, and magenta lines show FW61, FW63, and FW66 simulations,
respectively. e pink horizontal crosses are the dating () and dating errors ( ) for the proxies.
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SLR estimates from the Netherlands (3 ± 1.2m within 200 years)33 and Mississippi delta (0.8–2.2m within 130
years)22. Estimates of SLR rates on longer time-scales for the early-Holocene however dier considerably from
ours. Rates of 17.9mm year−1 (8600–7100 BP), and 24mm year−1 (extending up to 8948–8206 BP) are recorded
on the coast of Germany34 and Norway35, much higher than our 7.5mm year−1 estimate. Because the melting
of Antarctic Ice Sheet contributed to less than 3cm of SLR in early-Holocene51, this dierence in meltwater
uxes likely derives from LIS additional melting. is is expected since the meltwater volume in this study is an
estimate of the meltwater that was added to the Labrador Sea, part of which was then advected to deep water
formation sites, thus aecting large-scale ocean circulation and climate. Additionally, meltwater from the LIS
in the early-Holocene was discharged into wide regions in the Arctic and North Atlantic and thus account for a
total volume higher than the one we nd here30. erefore, our estimate does not represent the total LIS melting
and corresponding SLR for this time span. Neither FW61 nor the proxy records show a clear 8.2ka response in
the Florida Strait (SST). e 8.2ka event might therefore not have had a signicant and far-reaching impact on
the Florida Strait region, causing a climate response within model or proxy data background variability. A simu-
lation of the 8.2ka event with the Hadley Centre Coupled Model, version 3 (HadCM3) also did not reproduce
any measurable SST anomalies in the Gulf of Mexico36, thus suggesting that the core SST signal in that location
is likely not due to meltwater forcings involved in the 8.2ka event. e AMOC response to the 8.2ka freshwater
forcing is still debated in the scientic community. In our best-tting FW61 simulation, AMOC weakens by
62% (13Sv, Fig.4b) without collapsing, supporting earlier evidence of substantial AMOC weakening without a
collapse during the 8.2ka event37,38. Matero etal.36 nd that AMOC weakens by 55% of its initial overturning,
similar to our estimate, based on simulations with the HadCM3.
Implications
e magnitude of the simulated climate change during the 8.2ka event oers a pertinent reference point for
future climate trends12. e Greenland Ice Sheet is undergoing considerable melting and this is likely to continue
well into the future1,29. Greenland melting scenarios for the next millennium project SLR of 7.28m for the RCP8.5
scenario of the Intergovernmental Panel on Climate Change39. Current meltwater uxes from the Greenland Ice
Figure4. Climate impacts for the hybrid simulation FW61. (a) Proxy and model SST anomalies for the FW61
simulation. e color of the circles is plotted according to the anomaly value of the reconstructed SSTs. (b)
Simulated maximum overturning streamfunction for the North Atlantic as a measurement for the Atlantic
Meridional Overturning Circulation (right, blue line), and meltwater added in the FW61 experiment, in Sea
Level Rise equivalent (SLRe, green line). Map (a) drawn by Wilton Aguiar on Python v2.7 (https ://www.pytho
n.org/downl oad/relea ses/2.7/).
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Sheet are estimated to be ~ 0.005 Sv29. Even though this ux is considerably smaller than the ones used in our
experiments, projections of freshwater ux intensication for the next centuries are similar to the FW61 baseline
ows. For example, Golledge etal.6 found an increase in freshwater ux from Greenland ice-sheet melting of
0.015Sv by 2100 in the RCP8.5 scenario. Lenaertes etal.40 project an acceleration of the meltwater ux from
Greenland up to 0.08 ± 0.003Sv by 2200, while the maximum melting scenario of Aschwanden etal.39 projects a
ux exceeding 0.17Sv by 2300 (15mm of SLR year−1). Bakker etal.4 found a median discharge higher than 0.08Sv
by the year 2300. e projected input of freshwater into the North Atlantic associated with the RCP8.5 scenario
is therefore of the same magnitude as those in the FW61 simulation in terms of total SLR contribution (7.5m),
duration (1000years) and ux magnitude (0.066 SV to 0.19 SV). However, it is important to highlight that future
emission scenarios also include intensive surface radiative warming, which will add to the stratication eect
and thus intensify the future overturning weakening28. Additionally, future GIS meltwater will likely ow into
the coastal areas surrounding the ice sheet, instead of exclusively into the Labrador Sea39, and thus, its impact
on the ocean overturning will potentially dier from the focused meltwater injection in the Labrador during the
8.2ka event. Moreover, the climate response to an increase in meltwater will be in addition to the much greater
warming response due to increasing greenhouse gas concentrations, as well as changes due to topography and
albedo changes over Greenland. Nevertheless, the estimated meltwater ux from the GIS in the not too distant
future is comparable to the uxes we nd as the forcing behind the 8.2ka event.
Methods
Model and data. Simulations were performed using the University of Victoria Earth System Climate Model
version 2.9 (UVic Model)41, with the addition of oxygen isotopes42,43. Water in the ocean, atmosphere, sea-ice,
and on land is compartmentalized into 18O and 16O to allow the estimation of δ18O distribution42–46. A detailed
description of the experimental setup is given in the Supporting Information. We compared simulated δ18O and
SSTs to paleoclimate record mean anomalies for the 8.2ka event (averaged between 7.9 and 8.5ka—Supple-
mentary) and time series from six locations (Fig.S1-stars). Mean anomalies of oxygen isotope ratios in seawater
(δ18Osw), carbonate (δ18Oc), ice (δ18Oice), and SSTs, were taken from Morrill etal.47. ese proxy anomalies are
based on data from 27 cores (Fig.S1), some recording more than one paleoclimate proxy. Overall, our analysis
includes ten SST records, ten δ18Osw records, seven δ18Oc records and eight δ18Oice records. e mean anomalies
in the simulations are calculated following the methodology by Morrill etal.47. ey are dened as the dierence
between SST (or δ18O) values averaged between 7.9 and 8.5ka and their climatological mean, only for values
above (or below) the mean plus (minus) two standard deviations. e climatological mean is dened as the aver-
age between 9 and 7ka, excluding the period between 7.9 and 8.5ka.
For most records, the simulated values were taken at the model’s grid cell closest to the geographical coordi-
nates of each core, at the surface level of the ocean model (17.5m). e tracers reconstructed from Globorotalia
inata were compared to the simulated ocean tracers averaged between 82.5 and 177.5m, due to the wide range
of vertical migration inherent to this species. us, time series for the RAPiD core based on Globigerina bulloides
reect surface changes, while those based on G. inata reect changes in the upper thermocline. e UVic model
does not simulate isotopic fractionation during foraminiferal calcication. us, model δ18Oc was estimated by
an SST-based transfer function48,49.
In order to evaluate the simulations’ skill in reproducing the reconstructed δ18O, the linear regression’s slope
(⍺) and Root Mean Square Errors (RMSE) were calculated for the model anomalies using proxy anomalies as
reference. Equality between model and proxy happens when ⍺ = 1. For the time series, centered RMSE, normal-
ized standard deviations and Pearson’s correlations were compared in a Taylor diagram in order to evaluate the
performance of the simulations in reproducing the proxy time series. To assure that the dierence of the RMSEs
for the time series of δ18Osw and δ18Oc are signicant, we performed a Diebold-Mariano test50,51 between each
of the experiments in Part B. We then report the Diebold-Mariano test results and its signicance level for the
simulations with the best performances. All remaining values of the Diebold-Mariano test and its critical con-
dence percentages are presented in the Supplementary Information (Supplementary S2). More information on
the experimental setup and core data can be found in the Supplementary Material.
Freshwater forcing for the simulations based on earlier reconstructions. ere are four main
estimates of freshwater input into the North Atlantic close to the time of the 8.2ka event. A glacial isostatic
adjustment model by Peltier30 estimates that 27.1 × 105 km3 of freshwater were added to the North Atlantic from
LIS retreat from 9 to 8ka. e meltwater from LIS estimated by Peltier30 did not ow entirely into the Labrador
Sea, so this estimate can be used as an upper constraint for total meltwater discharged in the Labrador Sea in
the period. Carlson etal.15 estimate a 0.13Sv ± 0.03Sv increase in the inow of freshwater into Labrador Sea
aer the collapse of Hudson Bay that ended ~ 8.2 ka18 due to the routing of the western Canadian Plains runo
(8.2 × 105 km3 in volume). Although the routing event does not contribute to SLR, it would still alter the oxygen
isotope ratios and surface water buoyancy in the Labrador Sea, thus potentially aecting deepwater formation
rates. Li etal.22 found a 1.5 ± 0.7m of eustatic SLR between 8.31ka and 8.18ka (5.3 × 105 km3 in volume) from a
SLR reconstruction, which includes the freshwater release from the lake outburst. Ullman etal.18 estimate that
additional melting of the LIS aer its collapse contributed to 3.6 ± 0.4m of SLR that began ~ 8.2ka and ended
7.6 ± 0.6ka (~ 9.5 × 105 km3 in volume). e estimated Antarctic Ice Sheet contribution to SLR during the early-
Holocene is lower than 3cm, i.e. substantially smaller than LIS51, so no meltwater was added in the Southern
Hemisphere in the simulations. Using these estimates, we derived four main freshwater release experiments
running from 9ka until 7ka (Table1, (I) Reconstructions).
It is important to highlight that the four freshwater release estimates refer to dierent processes, and thus
each simulation will represent the eect of a specic process in creating proxy anomalies of the 8.2ka event:
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FWca represents the Canadian Plains routing event, FWul represents the eect of meltwater from the remaining
LIS aer its collapse, FWli represents the eect of the total freshwater addition to the ocean surrounding the
8.2ka event (not accounting for routing events), and FWpe represents the total early-Holocene meltwater from
the LIS. By simulating these separately, we estimate the signature of each process on the δ18O and SST records.
In our simulations, all freshwater was added to the Labrador Sea (50°N–65°N; 70°W–35°W). Meltwater
from the LIS and Lake Agassiz are estimated to have had a δ18O varying from − 24 to − 25‰ during the early-
Holocene52,53; we therefore added freshwater with a δ18O of − 25‰. Overturning in FWpe collapsed aer 8ka;
to restart the North Atlantic deep convection smoothly a virtual salt ux decreasing from −0.2 to −0.05Sv (8ka
until 7.5ka) with no isotopic signature was added.
Freshwater forcing of the hybrid scenarios. In addition to this rst set of simulations, which are based
on earlier geological reconstructions and described in “Freshwater forcing for the simulations based on earlier
reconstructions” section, we also integrated additional sensitivity simulations. Twenty-four experiments were
performed based on the uncertainty ranges of the Peltier30, Li etal.22, and Carlson etal.15 estimates (Table1).
e 7.5m in SLR equivalent estimated by Peltier30 was not fully released into the Labrador Sea. In turn, Li
etal.30 estimated the date of the meltwater outburst within 8.245 ± 0.065ka and their ux estimate has a 0.06Sv
uncertainty. Additionally, the Canadian continental basin routing event from Carlson etal.15 likely contributed
to an enhancement of freshwater ow to the Labrador Sea of 0.13Sv lasting up to 300years. Together, these
result in potential freshwater uxes varying between 0.046 and 0.26Sv and lasting between 200 and 1000years.
With these experiments, called “hybrid”, we test a more complex meltwater ux scenario, based on a background
freshwater forcing over a longer time period, a rerouting event and a shorter pulse, more intensive, drainage
event. Both the magnitude of the meltwater uxes (Part A), and their duration (Part B) are tested. Finally, a 2.5
SV freshwater ow to the Labrador Sea was added at year 8.47ka in all simulations in order to simulate the Lake
Agassiz outburst19. e exact date of the Lake Agassiz collapse is uncertain due to uncertainties on reservoir ages
of marine cores, which precludes further exploration of the date of the collapse in the simulations in this study.
Data availability
Simulated data for this research is available in the Zenodo database (https ://doi.org/10.5281/zenod o.42825 63)
and by contact to the rst author. e core data used is available in these in-text data citation references: Mor-
rill etal.10, Peltier29, Carlson etal.14, Li etal.21, Ullmann etal.17. Remaining data not present in these sources are
available in the supplementary material.
Received: 20 November 2020; Accepted: 17 February 2021
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Acknowledgements
is work is a part of the activities from the Brazilian High Latitudes Oceanography Group (GOAL) and the
Brazilian National Institute of Science and Technology of the Cryosphere (INCT-CRIOSFERA; 573720/2008-
8, 465680/2014-3, FAPERGS 17/2551-0000518-0). e GOAL is currently funded by the Brazilian Antarctic
Program (PROANTAR) through the Brazilian Ministry of the Environment (MMA), the Brazilian Ministry of
Science, Technology, Innovation and Communication (MCTIC), and the Council for Research and Scientic
Development of Brazil (CNPq;442628/2018-8, CAPES AUXPE 1995/2014). W. Aguiar acknowledges the nancial
support from the CAPES Foundation, and the Fulbright association for promoting the scientic interchange
required by this work. M. M. Mata acknowledges CNPq grant nos. 306896/2015-0. K. J. Meissner acknowledges
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funding from the Australian Research Council (DP180100048, DP18012357). L. Prado acknowledges the
INCT-CRIOSFERA (88887.495715/2020-00), and I. Wainer acknowledges the São Paulo Research Foundation
(2018/14789-9; 2019/08247-1) and CNPq (300970/2018-8). All authors acknowledge Dr. James Scourse for the
valuable review.
Author contributions
e respective contribution of each author to the manuscript is listed below. e descriptions here depicted are
accurate and agreed upon by all authors. All authors approved the submitted version of this manuscript. W.A.:
conceptualization, methodology, validation, formal analysis, investigation, data curation, writing—original dra,
writing—review and editing, visualization. L.F.P.: methodology, soware, validation, formal analysis, investiga-
tion, writing—review and editing, visualization. I.W.: methodology, validation, formal analysis, writing—review
and editing, visualization, resources, supervision. A.E.C., A.M., K.J.M.: conceptualization, methodology, valida-
tion, formal analysis, writing—review and editing, visualization, resources, supervision. M.M.M.: conceptualiza-
tion, methodology, validation, formal analysis, writing—review and editing, visualization, resources, supervision,
project administration, funding acquisition.
Competing interests
e authors declare no competing interests.
Additional information
Supplementary Information e online version contains supplementary material available at https ://doi.
org/10.1038/s4159 8-021-84709 -5.
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