Arctic Paleoclimates - The Arctic Climate System - page 334

Figure 10.8. Inferred changes in Artemisia , sea surface temperature and sea ice cover
in the North Sea during deglaciation (from Rochon et al., 1998 , by permission of
Elsevier).
concentrations in the GRIP ice core decreased by a factor of 3 within a twenty-year
period indicating that storminess rapidly decreased.
Like D-O events, the cause of theYD is still uncertain, but evidence again implicates
changes in NADW production. One theory (Broecker et al., 1989 ) involves a large
influx of melt water through the St. Lawrence Seaway and into the North Atlantic.
This resulted from the drainage of Lake Agassiz, a proglacial lake that formed at
the foot of the retreating Laurentide ice sheet. The lake had been draining into the
Mississippi basin. However, it is argued that just before the start of the YD, there was
a catastrophic rerouting into the St. Lawrence seaway, associated with a 40 m drop
in the lake level. Drainage was then routed back to the Mississippi ( Figure 10.9 ). As
reviewed by Bradley ( 1999 ), a number of problems became evident with this view,
including indications that the YD occurred at a time when discharge to the world
ocean was less than either the preceding or following 1,000 years, and that the melt-
water flux from the St. Lawrence was actually reduced during the YD episode. In
a subsequent study, Broecker ( 1990 ) argued that the period of rapid sea level rise
prior to the YD lowered salinity in the surface layer so much that the North Atlantic
was already predisposed for a shut down or major cessation in NADW production.
A. Fanning and A, Weaver ( 1997 ) offer modeling support for this view. An ongoing
challenge to the flood hypothesis has been the lack of clear geomorphic evidence for
the flood pathway (Broecker, 2006 ). However, J. Murton et al. ( 2010 ) identify a path
along the Mackenzie River to the Arctic Ocean that may resolve this problem.
Lehman and Keigwin ( 1992 ) give a different perspective. Their study addressed
the YD as part of a focus on sudden changes in the North Atlantic circulation dur-
ing the deglaciation phase. They show that prior to the YD, episodes of freshen-
ing, associated with meltwater inputs from the decaying Fennoscandian Ice Sheet,