September 2025 | Oceanography
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also alter isotopic compositions along deep-water flow paths, par-
ticularly in poorly ventilated basins, such as the deep Pacific and
Indian Oceans (Abbott et al., 2015; Du et al., 2018, 2020). Certain
types of sediment (particularly volcanic ash and ice-rafted debris)
can also be more reactive and prone to delivering non-conservative
additions of Nd to seawater (Wilson et al., 2013; Blaser et al., 2016;
Du et al., 2016).
The use of bulk sediment 231Pa/230Th as a circulation tracer relies
on the theoretical expectation that, while 231Pa and 230Th are pro-
duced at approximately uniform rates in the ocean (from the decay
of 235U and 234U, respectively), 231Pa is in general scavenged less
intensively by sinking particles than 230Th and hence is more sensi-
tive to circulation than 230Th (Henderson and Anderson, 2003). As
a result, the ratios of the two isotopes in sinking particles and sedi-
ment would be dependent on lateral transport of water (i.e., on the
AMOC), with stronger transport leading to lower 231Pa/230Th in
the underlying sediment. However, the 231Pa/230Th ratio of marine
particles in the water column has been found to vary with their
chemical compositions (e.g., Chase et al., 2002; Hayes et al., 2015)
and with sediment lateral redistribution (S.Y.-S. Chen et al., 2021),
complicating its use as an AMOC proxy.
One of the most widely cited reconstructions used as evidence
of AMOC change across the deglaciation is the 231Pa/230Th record
from the Bermuda Rise in the Northwest Atlantic (Figure 2e;
McManus et al., 2004). This record shows an abrupt increase in
231Pa/230Th to values close to the production ratio (which would
imply very little lateral flow out of the North Atlantic) during
HS1, and another smaller increase during the Younger Dryas. The
high 231Pa/230Th values during HS1 were attributed to a dramati-
cally weakened AMOC. Other 231Pa/230Th data from across the
North Atlantic broadly support this interpretation (Ng et al., 2018).
Compilations of benthic foraminifera δ13C from across the deep
Atlantic show low values during HS1 and an abrupt increase at the
start of the Bølling-Allerød (Figure 2g; Thiagarajan et al., 2014;
Lynch-Stieglitz et al., 2014; Lynch-Stieglitz, 2017), values that have
been interpreted as the resumption of a deep AMOC at the Bølling-
Allerød from a weaker state during HS1. Radiocarbon data from
the Northwest Atlantic also show an abrupt decrease in apparent
ventilation age at the start of the Bølling-Allerød from “older” val-
ues during HS1 and another pulse of old water at the YD (Figure 2f;
Robinson et al., 2005; Hines, 2017; Rafter et al., 2022). Compiled
εNd data are also consistent with a weakened AMOC during HS1
and the YD (Figure 2h; Pöppelmeier et al., 2019; Du et al., 2020),
although these data are less supportive of a fully collapsed AMOC.
The processes that might decouple variations in each proxy from
AMOC differ among proxies. Therefore, if these processes were the
dominant control on the deglacial variability in each record, we
would not expect them to correlate with one another. The finding
that many deglacial ocean circulation proxy records share com-
mon features at approximately the same times is apparent evidence
for changes in AMOC over the deglaciation. In other words, while
each proxy record could be explained by processes other than
circulation, the most parsimonious explanation for all the records
taken together would be that AMOC was abruptly reduced (or col-
lapsed) during HS1 and the YD.
This interpretation is also consistent with paleoclimate records
from terrestrial archives, including the oxygen isotopic composition
of Greenland ice cores (Figure 2a; North Greenland Ice Core Project
Members, 2004); the oxygen isotopic composition of Chinese spe-
leothems (Figure 2d; Wang et al. 2001; Cheng et al., 2009, 2016),
which records coeval shifts in atmospheric circulation patterns;
10
15
20
Age (ka)
HS 1
YD B/A
LGM
Holocene
-16
-14
-12
-10
εNd
-0.5
0.0
0.5
1.0
Benthic δ13C (‰)
1000
2000
3000
14C Ventilation Age (yr)
0.05
0.06
0.07
0.08
0.09
0.10
231Pa/230Th
-12
-10
-8
-6
Hulu cave δ18O (‰)
10
IRD (103 grains/g)
150
200
250
300
Atm. CO2 (ppm)
-50
-45
-40
-35
-30
NGRIP δ18O (‰)
FIGURE 2. Paleoclimate records across the deglaciation. (a) Northern
Hemisphere temperature from NGRIP δ18O of ice (North Greenland Ice Core
Project Members, 2004; Andersen et al., 2006; Rasmussen et al., 2014).
(b) Atmospheric CO2 from the West Antarctic Ice Sheet (Marcott et al., 2014).
(c) Ice-rafted debris concentration in the Northwest Atlantic at sites DY081-
GVY001 (solid) and EW9309-37JPC (dashed) (Zhou et al., 2021). (d) Hulu
cave δ18O (Cheng et al., 2016). (e) 231Pa/230Th from the Bermuda Rise (thin
lines: McManus et al., 2004; Lippold et al., 2009, 2019) and across the North
Atlantic (thick line: Ng et al., 2018). (f) Compiled deep Atlantic 14C venti-
lation age (Rafter et al., 2022). (g) Deep North Atlantic δ13C (as in Lynch-
Stieglitz et al., 2014; data from Hodell et al., 2008; Tjallingii et al., 2008;
Mulitza et al., 2008; Zarriess and Mackensen, 2011; Shackleton et al., 2000;
Skinner and Shackleton, 2004; Skinner et al., 2007). (h) εNd from the
Blake Bahama Outer Ridge (Pöppelmeier et al., 2019). YD = Younger Dryas.
B/A = Bølling-Allerød. HS 1 = Heinrich Stadial 1. LGM = Last Glacial Maximum.
IRD = Ice-rafted debris.