Page 260 - Earth's Climate Past and Future

236 PART IV • Deglacial Climate Changes

Because a distinctive Arctic plant called Dryas arrived Ice accumulation Ca in dust
in Europe during this episode, scientists call it the (m/yr) (ppb)
Younger Dryas event. Years ago 0.1 0.2 0 200 400
10,000
Later work on sediments in the North Atlantic
Ocean also detected a clear Younger Dryas imprint: a
rapid oscillation in the regional extent of icy polar water
(Figure 13–6A). At the glacial maximum, polar water 100
reached southward across the North Atlantic to 45°N. years 12,000
The southern margin of this cold water was defined by Younger
the polar front, a zone of rapid transition to the more Dryas
temperate waters to the south. Early in the deglaciation,
near 15,000 years ago, the polar front had shifted
toward the northwest as if hinged at a point near eastern
Canada. During this change, warm water began to flow 14,000
100
northward along the European coast and to moderate years
climate enough to permit trees to advance northward
from their full-glacial positions in far-southern Europe. Late
Near 13,000 years ago the polar front abruptly glacial
advanced back to the south, almost reaching its glacial 16,000
position. At the same time, with the North Atlantic
Ocean having again cooled, the Arctic vegetation (includ-
ing Dryas) returned to northern Europe (Figure
13–6B). Later, near 11,700 years ago, the polar front
abruptly retreated to the north, and forests began their
final advance into north-central Europe. FIGURE 13-7 Deglacial ice accumulation in Greenland
The Younger Dryas readvance of the polar front Rates of accumulation of ice in the Greenland ice sheet
represents a major reversal in circulation patterns. The abruptly decreased during the Younger Dryas cold event and
estimated sea surface cooling in the Atlantic Ocean west then increased when it ended, with the major changes occur-
of Ireland was at least 7°C, close to the difference ring within 100 years. Concentrations of windblown dust
between fully glacial and interglacial extremes. A simi- increased during the Younger Dryas and decreased afterward.
lar cooling has been estimated from changes in the fos- (Modified from R. B. Alley et al., “Abrupt Increase in Greenland
sil remains of temperature-sensitive insect populations Snow Accumulation at the End of the Younger Dryas Event,”
in England (Figure 13–6C). Nature 362 [1993]: 527–29).
Ice cores from Greenland contain a remarkably
detailed record of the Younger Dryas event (Figure What caused the Younger Dryas oscillation? The
13–7). During fully glacial climates, snow had been accu- geochemist Wally Broecker proposed that changes in
mulating slowly, but the rates increased abruptly near the path of meltwater flow from the North American
15,000 years ago when the North Atlantic Ocean ice sheet was the cause (Figure 13–8). He suggested that
warmed. Accumulation rates then slowed during the an abrupt diversion of the major meltwater route from
transition into the Younger Dryas event but again the Gulf of Mexico to the North Atlantic Ocean during
increased when it ended 11,700 years ago. Some of these the Younger Dryas delivered a pulse of low-salinity
transitions occurred in less than a century, with much of water that altered the circulation of the North Atlantic
the change concentrated in a single decade. Similar by lowering the density of surface waters enough to
changes occurred in ice-core concentrations of wind- prevent them from sinking and forming deep water.
blown dust, which peaked during the cold, dry, windy cli- Because ocean surface waters give off heat when deep
mate of the Younger Dryas and then abruptly decreased. water forms in the North Atlantic Ocean, cutting off
During the Younger Dryas event, the ice sheets in this process could have cooled climate in the North
Scandinavia stopped retreating and in some regions Atlantic and surrounding continents.
readvanced a few hundred kilometers (see Figure 13–6). At first this explanation looked promising. Before
These pauses or small advances are thought to have the Younger Dryas event, the major pathway of meltwa-
been a response to the large regional cooling. The ter had been down the Mississippi, into the Gulf of
deglacial sea level curve derived from coral reefs (see Mexico, and out into the Atlantic through the Gulf
Figure 13–4) shows that global ice volume continued to Stream. This early meltwater flow is recorded by the
18
shrink during the Younger Dryas, although at a much light-δ O signal in sediments from the Gulf of Mexico
slower rate than before or afterward. (see Figure 13–5). During the Younger Dryas event