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In-Depth Information
freshening beginning about fifty years after the onset of the YD and initial discharge
of stored meltwater from the so-called Baltic Ice Lake. This may have prolonged
and intensified suppression of NADW production during the YD. They find another
interval of freshening after about 10.4 ka
14
C (~12.6 ka calendar) associated with
final discharge of the Baltic Ice Lake. The major point is that warming and associ-
ated enhanced meltwater input promoted weakening of the THC and cooling, fol-
lowed by recovery. The intensity of the YD event may have been in part fostered by
additional freshwater inputs provided by drainage of the Baltic Ice Lake.
However, as outlined by Lehman and Keigwin (
1992
), a complete focus on the
Fennoscandian ice sheet seems insufficient - the Laurentide Ice Sheet must also
be involved. It appears that while the melting rates of both the Fennoscandian and
Laurentide ice sheets may have been influenced by changes in the THC, their indi-
vidual meltwater contributions may have had very different effects on the THC.
Meltwater from the Laurentide Ice Sheet (e.g., from the drainage of Lake Agassiz)
must travel farther to reach areas of convection in the GIN seas, so a greater or pro-
longed melting of the Laurentide Ice Sheet may have been needed for an equivalent
damping effect on the circulation. It may be that the Laurentide Ice Sheet meltwater
built up in the Atlantic, leading to a non-linear response of NADW production to ice
sheet melting. This may account for the increasingly severe reductions in the THC
during the deglaciation period that culminated in the millennial-scale YD.
Another idea involves sea ice. During the LGM, when sea level was much lower,
the Arctic Ocean was very isolated, and the Fram Strait was narrower, restricting
sea ice transport into the North Atlantic. Very thick sea ice would have built up in
the Arctic Ocean, remaining largely immobile until rising sea level, breakup of the
Barents Sea Ice Sheet and the return of warmer Atlantic waters to the Arctic Ocean
allowed for massive discharge of sea ice into the North Atlantic. A final trigger may
have come when sea level rose sufficiently such that the Bering land bridge was
covered, allowing throughflow from the Pacific to the Atlantic, flushing out the ice
(Bradley,
1999
). Dating of terrestrial peats on the Chukchi shelf place this crucial
event at about 13 ka, just before the YD (Elias et al.,
1996
). The processes are hence
reminiscent of those that led to the Great Salinity Anomaly.
A hypothesized link between the YD and a large meteorite impact, while initially
gaining considerable attention, has not been supported by subsequent studies. A
link with volcanic activity remains controversial.
10.7
The Holocene
10.7.1
A Return to Warm Conditions
The Holocene is generally considered to begin at the close of the YD episode.
Around 10 ka the perihelion occurred in July as opposed to January at present
(and during the LGM). This made Northern Hemisphere summers warmer than
today and the winters colder. The tilt of the earth was about 1 degree greater, fur-
ther adding to increased seasonality and greater summer radiation for the Northern
