Geoscience Reference
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proglacial lakes (Siegert,
2001
). These are large lakes found ahead of the ice sheet
margins, such as Lake Agassiz. Drainage of Lake Agassiz is a contender to explain
the Younger Dryas cold event (
Section 10.6.2
).
These ideas provide some explanation as to why the climate of the Holocene has
been relatively stable. As land ice has been much more limited during the Holocene
(Greenland and Antarctica excepted), it is harder to get the requisite massive fresh-
water discharges. NADW production under present conditions, although still some-
what variable, hence stays in the “on” mode. In
Chapter 7
, we examined the Great
Salinity Anomaly of the late 1960s and early 1970s and noted an apparent associa-
tion with reduced NADW production. The Great Salinity Anomaly could be viewed
as a small-scale modern example of a NADW-freshwater link, although this event
appears to have been associated with freshwater discharge through the Fram Strait.
A problem with the ideas of drainage rerouting or the rapid drainage of progla-
cial lakes is that, although D-O events are quasi-periodic, catastrophes, by their
nature, tend to have a large random component. Regarding the salt oscillator, there
is a problem in timing, questioning cause and effect. Whereas one might consider
the freshwater input from icebergs associated with IRD events (and attendant direct
inputs of meltwater from the ice sheets) as drivers of D-O cycles, as noted, the IRD
events occurred during the cold phases of D-O events, not before them. Indeed, five
of the six Heinrich events (
Figure 10.4
) occurred at the culminations of progressive
cooling and were then followed by sharp warming to almost interglacial conditions
(Bond and Lotti,
1995
).
IRD from at least two widely separated sources is present at the same time in
the same eastern North Atlantic cores examined by G. Bond and R. Lotti (
1995
).
This indicates nearly synchronous rates of iceberg calving between these distant
areas. One explanation for the synchronous iceberg discharge is that different ice
sheets exhibited nearly identical “binge-purge” behavior. However, another expla-
nation is that the synchronous calving represents an external climate forcing. In
other words, freshwater input did not force the cooling phases of the D-O cycles, but
rather followed the cooling. The freshwater input may have subsequently triggered
further cooling. However, Bond and Lotti (
1995
) acknowledge that recurring binge-
purge processes in another ice sheet may have discharged enough ice each time to
alter NADW production, with the resulting ocean cooling triggering the iceberg
discharges they identified.
Bond and Lotti (
1995
) found that each discharge of detrital carbonate-bearing ice
from the Hudson Strait (associated with the Heinrich events) seems to lag slightly
behind the discharge from the other two sources. G. Farmer, D. Barber, and J.
Andrews (
2003
) also find that IRD fluxes into the Nordic Seas appear to have been
nearly synchronous with Heinrich events recorded elsewhere in the North Atlantic.
It may be that the sudden coolings within the D-O cycles triggered the ice dis-
charges in the Hudson Strait at nearly the same time as discharges from the other
two sources. An alternative view is that the trigger could have been sea level rises
that seem to accompany each 2,000-3,000-year flood of icebergs to the ocean.
