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meltwater sources from the circum-North Atlantic ice sheets.
One record is especially important because it comes from
offshore Newfoundland, off the St. Lawrence River system
[Labeyrie et al., 1999]. In core CH69-K09, planktic
18
O
data on G. bulloides and N. pachyderma (s.) exhibit distinct
minima associated with Heinrich events 1, 2, and 4 (Figure 9).
Obbink et al. [2010] also present evidence from this region
for LIS meltwater during the H1 stadial and the Bølling-
Allerød. Overall, these
δ
findings on LIS melting during
AMOC reductions highlight the importance of circum-North
Atlantic meltwater supply, either by direct or indirect supply
to deep convection regions. However, re
ning the timing of
the different sources of meltwater to the North Atlantic
Ocean remains a continuing challenge.
8. THE
HYPOTHESIS
AND SEASONALITY IN THE NORTH ATLANTIC
“
MELTWATER CAPACITOR
”
Figure 10.
Conceptual model of the
“
meltwater capacitor
”
hypoth-
esis for enhanced seasonality during Heinrich stadials.
Our observations on LIS meltwater history are consistent
with the
sea ice increases the continentality of the region and pro-
motes hypercold winters. Just as an electronic capacitor
stores energy, persistent summer meltwater stores a low-
salinity signal to inhibit deep convection during subsequent
winters. Our hypothesis highlights the role of summer melt-
water in enhancing winter sea ice, as well as reducing
AMOC and thereby contributing to the enigmatic increased
seasonality observed during Heinrich stadials and the Mys-
tery Interval [Denton et al., 2005]. This proposed set of
mechanisms was not operating during the Bølling/Allerød
or major D-O interstadials despite continued meltwater input
because of greatly enhanced AMOC. Glacial meltwater ap-
parently was unable to weaken the AMOC at these times,
perhaps because of extensive salt storage in the low-latitude
Atlantic Ocean during long intervals of AMOC reduction
such as the H1 and H4 stadials [Vellinga and Wu, 2004;
Schmidt et al., 2004, 2006; Weldeab et al., 2006; Jaeschke
et al., 2007; Knorr and Lohmann, 2007]. Our hypothesis
highlights the need for continued data/model comparisons on
the role of Northern Hemisphere ice sheet meltwater in
regional seasonality changes and global ocean circulation
changes.
hypothesis [Denton et al.,
2005; Broecker, 2006b] in which Heinrich stadials and the
Younger Dryas are marked by greatly enhanced seasonality
in the North Atlantic region. Marshalling evidence from
Greenland ice core records, snowline records from east
Greenland, northern Europe and North America, and floral
and faunal assemblage records from northern Europe, Den-
ton et al. [2005] documented an important mismatch be-
tween inferred summer and mean annual temperatures. For
example, Greenland ice cores indicated a Younger Dryas
mean annual cooling of ~16°C [Severinghaus et al., 1998],
yet snowline records suggested only 4°C
-
6°C. The likely
resolution to this seeming mismatch is that snowlines re
“
deglacial seasonality
”
ect
summer conditions and that winters must have been ~26°C
-
28°C colder. The only viable way to produce such cold
winters is greatly expanded sea ice, which would limit heat
release from the upper limb of the AMOC and increase the
continentality of the region. Summer meltwater is one way to
enhance sea ice formation the following winter and may be
the link between extreme seasonality and AMOC reduction.
We propose a scenario to help explain enhanced seasonal-
ity during Heinrich stadials: the
hy-
pothesis (Figure 10). The hypothesis has the following four
components: (1) Northern Hemisphere ice sheets start decay-
ing early in the deglacial sequence, mainly during summers,
in response to increased summer insolation and atmospheric
CO
2
rise. (2) Ice sheet meltwater from the circum-North
Atlantic ice sheets decreases salinity in the North Atlantic
Ocean, whose signal is preserved during subsequent winters.
(3) Low-density meltwaters inhibit winter deep convection
in the high-latitude North Atlantic, resulting in reduction of
AMOC and associated heat transport. (4) Expanded winter
“
meltwater capacitor
”
9. CONCLUSIONS
LIS meltwater has long been invoked as the trigger of
AMOC reduction during the Younger Dryas, by re-routing
from the Gulf of Mexico to the North Atlantic. A review of
published and new data from three Orca Basin cores con-
flow coin-
cided with the onset of the Younger Dryas at about 11,000
14
C years before present (12.9 ka). In contrast, the timing of
LIS in
firms that the cessation of southward meltwater
ow during MIS 3 is inconsistent with a simple
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