Geoscience Reference
In-Depth Information
As for the link to Heinrich events, considerable attention has been paid to “binge-
purge” behavior of the Laurentide Ice Sheet. The idea is that during cold conditions,
the ice sheet built up to a critical point where basal melting occurred (melting at the
base of the ice sheet). Basal melting lubricated the base of the ice sheet and initiated
a rapid discharge of icebergs via the Hudson Strait Ice Stream (the Hudson Strait
lies between northern Labrador and southern Baffin Island). The iceberg discharge
then ceased and the ice sheet rebuilt, followed by another surge episode (MacAyel,
1993
). More recent work (Marcott et al.,
2011
) points to how the floating ice shelf
fronting the Hudson Strait Ice Stream could be destabilized by basal melting linked
to subsurface ocean warming, with the subsurface warming occurring at the same
time that the AMOC became weak. J. Fluckiger, R. Knutti, and J. White (
2006
) in
turn argue that collapse of the AMOC, along with increased oceanic heat uptake
after the AMOC collapse, could lead to rises in sea level, destabilizing Northern
Hemisphere ice shelves and ice sheets and triggering ice surges. Alley et al. (
2006
),
by contrast, point to the potential for outburst flooding from subglacial reservoirs
following climatic cooling.
10.4.2
Processes
Although D-O cycles and Heinrich events are related phenomena, a full account-
ing of their causes and linkages has remained elusive. As to how the North Atlantic
could be freshened, one of the first ideas that was advanced is the “salt oscillator'
(Rooth,
1982
; Broecker et al.,
1990
). There were periods when the AMOC and
NADW production were relatively strong (albeit weaker than at present), resulting
in enhanced discharge of meltwater and icebergs (the warm part of a D-O cycle).
This resulted in freshening in the GIN seas. However, the surface freshening then
reduced NADW production, causing widespread cooling of the atmosphere (the
transition to the cold part of the D-O cycle). This reduced the meltwater and iceberg
discharge. Ocean salinities then rebuilt to a critical point at which NADW produc-
tion recovered, causing warming (the transition from the cold to warm part of a D-O
cycle). The process then repeated itself. The salt oscillator as originally conceived
stresses interactions with the Fennoscandian Ice Sheet. However, interactions with
the Greenland Ice Sheet and Laurentide Ice Sheet could also be involved. The salt-
oscillator (associated with “on” and “off” modes of NADW production) finds some
support in S. Lehman and L. Keigwin's (
1992
) study of climate variability during
the last deglaciation.
Other ideas involve “halocline catastrophes.” For example, P. Clark et al. (
2001
)
proposed that when the Laurentide Ice Sheet margin was between about 43 and 49
o
N,
fluctuations in the location of the margin allowed rerouting of the main runoff from
the Mississippi River drainage into the North Atlantic via the St. Lawrence River
or the Hudson Strait. The massive freshwater inputs to the northern North Atlantic
capped the convection and caused widespread cooling. Another possible forcing
is the discharge of water into the North Atlantic from the catastrophic drainage of
