Geoscience Reference
In-Depth Information
YD, from about 12.8-11.5 ka, which represented a return to near glacial conditions.
The YD is labeled in
Figure 10.4
. It corresponds clearly to a δ
18
O minimum in the
GRIP record as well as a high percentage of sinistral
N. Pachyderma
(implying low
sea surface temperatures) at the VEMA 23-81 ocean core site. The YD had variable
regional expressions.
The Holocene (starting at MIS 1, 11.5 ka), generally considered to start at the end
of the YD, has been a generally warm period, and although more stable than the last
glacial, still exhibited strong temperature variability. Reconstructions from many
different sources indicate that, during the Holocene Thermal Maximum or Climatic
Optimum, global average temperatures were considerably higher than those of the
twentieth century. The period at which maximum warmth occurred varied greatly
by region. The Holocene Thermal Maximum was followed by general cooling start-
ing around 5 ka. The most notable cooling since the YD is represented by the LIA.
Details of the LGM, deglaciation and the Holocene are examined in
Sections 10.5
through
10.7
.
10.3.2
Causes of the Quaternary Glacial Cycles
It is widely accepted that the major Quaternary glacial cycles, in considerable part,
can be related to cyclic variations in earth orbital geometry that cause variations in
the amount of radiation received at different parts of the year over different parts of
the earth's surface. These Milankovitch cycles are named after astronomer Milutin
Milankovitch, who made the first detailed calculations of their effects on the dis-
tribution of solar radiation (Milankovitch,
1941
). They involve three factors. The
first, eccentricity, varies over an approximate 100,000-year cycle. The earth's orbit
is not circular, but elliptical. As such, earth-sun distance varies throughout the year.
Radiation receipts are greatest at the time of minimum earth-sun distance (perihe-
lion) and least at maximum earth-sun distance (aphelion). Today, perihelion and
aphelion are in January and July, respectively, associated with about a 6 percent
difference in the amount of total solar radiation received at the surface. When the
earth's orbit becomes more elliptical (greater eccentricity), the amount of energy
received varies more strongly. The second factor is the obliquity (or tilt) of the
earth's axis relative to its orbital plane about the sun. Obliquity varies over a 41,000-
year cycle. The earth's axial tilt is presently 23.5
o
but varies between about 22
o
and
24
o
. The tilt affects the summer-winter radiation contrast and is especially impor-
tant in high latitudes. The third factor is the timing of the perihelion with respect to
the seasons (i.e., the precession of the equinoxes), which changes on a 23,000-year
cycle owing to wobbling of the earth on its axis.
Using ocean core records, J. Hayes, J. Imbrie, and N. Shackleton (
1976
) pro-
vided some of the first convincing evidence of a climate link with the Milankovitch
cycles. This is considered to be a landmark paper. Conventional paradigm identifies
global ice volume being tuned to June extraterrestrial solar radiation at 60-65
o
N.
Glacial conditions seem to be favored by small tilt and perihelion in the Northern
Hemisphere winter. These conditions lead to a diminished seasonal contrast with
