Systematic changes in the amount of
incoming solar radiation, caused by variations in Earth’s
orbital parameters around the Sun. These changes can affect
many Earth systems, causing glaciations, global warming,
and changes in the patterns of climate and sedimentation.
Astronomical effects influence the amount of incoming
solar radiation; minor variations in the path of the Earth in
its orbit around the Sun, and the inclination or tilt of its axis
cause variations in the amount of solar energy reaching the
top of the atmosphere. These variations are thought to be
responsible for the advance and retreat of the Northern and
Southern Hemisphere ice sheets in the past few million years.
In the past 2 million years alone, the Earth has seen the ice
sheets advance and retreat approximately 20 times. The climate
record, as deduced from ice-core records from Greenland
and isotopic tracer studies from deep ocean, lake, and
cave sediments, suggests that the ice builds up gradually over
periods of about 100,000 years, then retreats rapidly over a
period of decades to a few thousand years. These patterns
result from the cumulative effects of different astronomical
phenomena.
Several movements are involved in changing the amount
of incoming solar radiation. The Earth rotates around the Sun
following an elliptical orbit, and the shape of this elliptical
orbit is known as its eccentricity. The eccentricity changes
cyclically with time with a period of 100,000 years, alternately
bringing the Earth closer to and farther from the Sun in summer
and winter. This 100,000-year cycle is about the same as
the general pattern of glaciers advancing and retreating every
100,000 years in the past 2 million years, suggesting that this
is the main cause of variations within the present-day ice age.
The Earth’s axis is presently tilting by 23.5°N/S away
from the orbital plane, and the tilt varies between 21.5°N/S
and 24.5°N/S. The tilt changes by plus or minus 1.5°N/S
from a tilt of 23°N/S every 41,000 years. When the tilt is
greater, there is greater seasonal variation in temperature.
Wobble of the rotation axis describes a motion much
like a top rapidly spinning and rotating with a wobbling
motion, such that the direction of tilt toward or away from
the Sun changes, even though the tilt amount stays the same.
This wobbling phenomenon is known as precession of the
equinoxes, and it has the effect of placing different hemispheres
closest to the Sun in different seasons. Presently the
precession of the equinoxes is such that the Earth is closest to
the Sun during the Northern Hemisphere winter. This precession
changes with a double cycle, with periodicities of 23,000
years and 19,000 years.
Because each of these astronomical factors act on different
timescales, they interact in a complicated way, known as
Milankovitch cycles, after a Yugoslavian (Milutin Milankovitch)
who first analyzed them in the 1920s. Using the power
of understanding these cycles, we can make predictions of
where the Earth’s climate is heading, whether we are heading
into a warming or cooling period, and whether we need to
plan for sea-level rise, desertification, glaciation, sea-level
drops, floods, or droughts.
Milankovitch cycles have been invoked to explain the
rhythmic repetitions of layers in some sedimentary rock
sequences. The cyclical orbital variations cause cyclical climate
variations, which in turn are reflected in the cyclical
deposition of specific types of sedimentary layers in sensitive
environments. There are numerous examples of sedimentary
sequences where stratigraphic and age control are sufficient to
be able to detect cyclical variation in the timescales of
Milankovitch cycles, and studies of these layers have proven
consistent with a control of sedimentation by the planet’s
orbital variations. Some examples of Milankovitch-forced sedimentation
have been documented from the Dolomite Mountains
of Italy, the Proterozoic Rocknest Formation of northern
Canada, and from numerous coral reef environments.
See also STRATIGRAPHY; SEQUENCE STRATIGRAPHY.
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