September 2025 | Oceanography
17
FRESHWATER MECHANISMS FOR DRIVING
ABRUPT CHANGES IN THE AMOC
Given that geologic reconstructions suggest that HS1 and the YD
were not times of accelerated melting of Northern Hemisphere
ice sheets and elevated freshwater fluxes to the North Atlantic
(Figure 3a), alternative mechanisms for AMOC weakening at
these times must be sought. The mechanisms driving AMOC
reduction at HS1 and the YD need not have been the same, and
paleoceanographic data are consistent with different magnitudes
of AMOC change at each event, with HS1 thought to be the larger
and longer reduction of the two (Ng et al., 2018).
Heinrich events were associated with massive iceberg dis-
charges from the Laurentide Ice Sheet (Ruddiman, 1977; Heinrich,
1988; Broecker, 1994; Hemming, 2004), so it is possible that fresh
water from melting icebergs played an important role. Unlike the
deglacial meltwater that enters the ocean directly in liquid form,
icebergs can travel much farther from the coast before they dis-
integrate (Fendrock et al., 2022). The paths of large icebergs orig-
inating from terrestrial ice sheets can be tracked by ice-rafted
debris (IRD), which consists of coarse grains of continental origin
that are embedded in the icebergs and deposited on the seafloor
as the icebergs melt. In general, IRD is found most prominently
in marine sediment cores collected from between ~40°N and
50°N in the Atlantic Ocean (Ruddiman, 1977); however, smaller
amounts of IRD have been found much farther north, including
in the Nordic Seas and south of Iceland (e.g., Elliot et al., 2001;
Thornalley et al., 2010), and to the south on the Bermuda Rise
(Keigwin and Boyle, 1999). Therefore, the supply of fresh water
from melting icebergs could be a mechanism for reducing the
AMOC (Broecker, 1994). This hypothesis is supported by low
δ18O values measured in fossil planktonic foraminifera (indica-
tive of low salinity) from Heinrich layers within the main IRD
belt proposed by Ruddiman (Bond et al., 1992; Hemming 2004).
However, as yet, clear evidence of low salinity farther north has
not been found.
The Younger Dryas is another period of IRD deposition in the
North Atlantic (e.g., Zhou and McManus, 2024) and the Arctic
Ocean (e.g., Hillaire-Marcel et al., 2013; Lakeman et al., 2018),
although IRD fluxes at this time appear to have been smaller than
at HS1 (e.g., Zhou and McManus, 2024). While the YD is not asso-
ciated with a time of widespread ice sheet melting according to
sea level data and ice sheet models (Tarasov and Peltier, 2002;
Tarasov et al., 2012; Lambeck et al., 2014; Briggs et al., 2014), it
is notably associated with the abrupt draining of Lake Agassiz, a
proglacial lake formed by the melting Laurentide Ice Sheet that
sat at the boundary of Minnesota, North Dakota, Ontario, and
Manitoba (Broecker et al., 1989; Teller et al., 2002). It was initially
thought that Lake Agassiz drained east at the YD, directly into the
North Atlantic via the St. Lawrence River (Broecker et al., 1989;
Clark et al., 2001), but direct evidence for this has been elusive,
and more recent studies suggest that the lake instead drained north
into the Arctic via the Mackenzie River (Tarasov and Peltier, 2005;
Murton et al., 2010; Keigwin et al., 2018; Süfke et al., 2022). This
result is supported by model simulations, which show that fresh
water discharged into the ocean from the St. Lawrence River does
not immediately spread offshore but is instead transported away
from the subpolar North Atlantic in boundary currents, into the
subtropical gyre. Meltwater from the Mackenzie Valley into the
Arctic Ocean is more likely to reach deep-water formation regions
directly, regardless of whether the Canadian Arctic Archipelago is
ice-covered or open (Condron and Winsor, 2012).
The focus has often been on deep convection regions when it
comes to deglacial freshwater-driven perturbations of the AMOC,
whether the fresh water is delivered by icebergs or directly in liq-
uid form; however, recent physical oceanographic observations
and modeling indicate that capping water mass transformation
along the boundary currents or reducing the zonal density gradi-
ent across the mid-latitude North Atlantic (Buckley and Marshall,
2016; Yeager et al., 2021; Chafik et al., 2023; Frajka-Williams
et al., 2023) may be more important for disrupting the AMOC.
During the deglaciation, meltwater introduced to the western sub-
polar North Atlantic could have been entrained offshore along
the northern flank of the western boundary currents that consti-
tute the upper limb of the AMOC, including the Gulf Stream and
the North Atlantic Current (the eastward extension of the Gulf
Stream). This entrained meltwater could significantly alter the
density gradients across these powerful currents and hence reduce
their strength and associated heat transport (e.g., Yeager et al.,
2021; Madan et al. 2024).
0.0
0.2
0.4
0.6
0.8
Freshwater Flux (Sv)
Younger
Dryas
Heinrich
Stadial 1
GLAC-1D
TraCE-21k
10
15
20
25
AMOC (Sv)
-10
-12
-14
-16
-18
-20
Age (ka)
iLOVECLIM
TraCE-21k
B/A
FIGURE 3. Transient model simulations of AMOC across the deglaciation.
(a) Freshwater flux from ice sheet model simulation GLAC-1D with fresh-
water flux time series in the TraCE-21k model (Liu et al., 2009; Bouttes
et al., 2023). (b) AMOC strength calculated as the maximum streamfunction
between 20°N and 50°N below 500 m from TraCE-20k (Liu et al., 2009) and
iLOVECLIM (Bouttes et al., 2023).