The Younger Dryas (YD) was the most significant rapid climate change event
that occurred during the last deglaciation of the North Atlantic region.
Previous ice core studies have focused on the abrupt termination of this event
[ Dansgaard et al., 1989] because this transition marks the end of the last
major climate reorganization during the deglaciation. Most recently the YD has
been redated--using precision, subannually resolved, multivariate measurements
from the GISP2 core--as an event of 1300
70 years duration that
terminated abruptly, as evidenced by an 
7
C rise in temperature
and a twofold increase in accumulation rate, at 
11.64 kyr BP [ Alley
et al., 1993] (Figure 2). The transition into the Preboreal (PB), the PB/YD
transition, and the YD/Holocene transition were all remarkably fast, each
occurring over a period of a decade or so [ Alley et al., 1993].
Fluctuations in the electrical conductivity of GISP2 ice on the scale of
<5-20 years have been used to reveal rapid changes in the dust content of the
atmosphere during the same periods and throughout the last glacial [ Taylor
et al., 1993b]. These rapid changes appear to reflect a type of ``flickering''
between preferred states of the atmosphere [ Taylor et al., 1993b], which
provides a new view of climate change. Holocene climates are by comparison
stable and warm.
High resolution (mean: 3.48 years/sample), continuous measurements of GISP2
major anions (chloride, sulfate and nitrate) and cations (sodium, magnesium,
potassium, calcium and ammonium) were used to reconstruct the
paleoenvironment during the YD because these series record the history of the
major soluble constituents transported in the atmosphere and deposited over
central Greenland [ Mayewski et al., 1993c]. These multivariate
glaciochemical records provide a robust indication of changes in the
characteristics of the sources of these soluble components or changes in their
transport paths, in response to climate change. A dramatic example is provided
by the calcium series (Figure 2) covering the last 
10-18 kyr BP.
Prominent periods of increased dustiness have been observed in the record,
peaking approximately every 500 years (see figures in Mayewski et al.
[1993c]): during the early PB at 
11.4 kyr BP; throughout the YD at
11.81, 12.22 and 12.64 kyr BP; during the Bolling/Allerod (B/A) at
13.18, 13.65, and 14.02 kyr BP; and during much of the Glacial. Such
events have been attributed by Mayewski et al. [1993c] to changes in the
size of the polar atmospheric cell and in source regions (e.g., growth and decay
of continental biogenic and terrestrial source regions).
The climate change that accompanied the YD was not restricted to Greenland.
The record of variations in the CH
concentration of trapped gases in the
GRIP ice core [ Chappellaz et al., 1993] shows that tropical and subtropical
climates were colder and drier during the YD and also earlier cold events. The
major natural source region of CH
is low-latitude wetlands [
Chappellaz et al., 1993]; higher atmospheric concentrations are presumably due
to the greater areal extent of tropical and subtropical wetlands [
Chappellaz et al., 1993].
The ammonium flux record from GISP2 provides an estimate of continental
biogenic source strength [ Mayewski et al., 1993a] during the YD.
Although at the onset of the Bolling/Allerod ammonium flux levels and outliers
rose dramatically, during the YD ammonium flux levels dropped only minimally
and the number of ammonium outliers decreased slightly. Since ammonium
concentrations are highest near continents [ Logan, 1983] and decrease
with transport as a consequence of deposition, it appears that continental
sources close to Greenland (North America and Europe) were not as
dramatically affected during the YD as were low-latitude wetland regions, as
evidenced by the CH
record. This may indicate the continued importance
of ice sheets and permafrost in limiting the growth of vegetation at higher
latitudes until the end of the YD. Both low-latitude source CH
and
ammonium rise at the end of the YD [ Chappellaz et al., 1993;
Mayewski et al., 1993c].