mirrored file at http://SaturnianCosmology.Org/ 
For complete access to all the files of this collection
	see http://SaturnianCosmology.org/search.php 
==========================================================
<BASE HREF="file://localhost/www/saturnian/files/ice/node6.html">
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">

------------------------------------------------------------------------
next <node7.html> up <mayews01.html> previous <node5.html>
* Next:* The Last 110 <node7.html> *Up:* The GISP2 ice core
<mayews01.html> * Previous:* The Little Ice <node5.html>
------------------------------------------------------------------------

The Younger Dryas

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 130070 years duration that terminated abruptly, as evidenced
by an 7C 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].

------------------------------------------------------------------------
next <node7.html> up <mayews01.html> previous <node5.html>
* Next:* The Last 110 <node7.html> *Up:* The GISP2 ice core
<mayews01.html> * Previous:* The Little Ice <node5.html>
------------------------------------------------------------------------

------------------------------------------------------------------------

/U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33 Suppl., © 1995 American Geophysical Union/