Newsgroup: it.discussioni.misteri
Titolo: Glaciazioni e orbite

Nel caso a qualcuno possa interessare, mi è arrivato con il notiziario di Le
Scienze:
Sconvolgimenti climatici
Fu una rara anomalia dell'orbita terrestre a causare una glaciazione 23
milioni di anni fa - 
http://www.lescienze.it/index.php3?id=2706
saluti,
Peltio
che forse non è un mistero. Non più almeno.

milioni di anni fa - 
http://www.lescienze.it/index.php3?id=2706
Ecco il comunicato stampa originale:
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University of California-Santa Cruz
Contact:
Tim Stephens, 
(831) 459-2495
; 
step...@cats.ucsc.edu
April 12, 2001
ANCIENT CLIMATE EXCURSION LINKED TO A RARE ANOMALY IN EARTH'S ORBIT
SANTA CRUZ, CA -- About 23 million years ago, a huge ice sheet spread
over Antarctica, temporarily reversing a general trend of global
warming and decreasing ice volume. Now a team of researchers has
discovered that this climatic blip at the boundary between the
Oligocene and Miocene epochs corresponded with a rare combination of
events in the pattern of Earth's orbit around the Sun.
In a paper published in the April 13 issue of the journal Science, the
researchers show that the transient glaciation and other climatic
variations during a period from about 20 to 25.5 million years ago
correspond with variations in Earth's orbit known as Milankovitch
cycles. Although the concept of such relationships is not new, some of
the results were surprising, said James Zachos, a professor of Earth
sciences at the University of California, Santa Cruz, and lead author
of the paper.
"When we began examining the temporal relationship of the orbital
oscillations relative to the oscillations in the climate record, we
never suspected that the transient glaciation at 23 million years ago
had anything to do with orbital anomalies," Zachos said.
The astrophysicist Milutin Milankovitch first proposed that cyclical
variations in certain elements of Earth-Sun geometry can cause major
changes in Earth's climate. The main variables are eccentricity,
obliquity, and precession. Eccentricity refers to the changing shape
of Earth's orbit around the Sun, which varies from nearly circular to
elliptical over a cycle of about 100,000 years. Obliquity refers to
the angle at which Earth's axis is tilted with respect to the plane of
its orbit, varying between 22.1 degrees and 24.5 degrees over a
41,000-year cycle. And precession is the gradual change in the
direction Earth's axis is pointing, which completes a cycle every
21,000 years.
"Because there are several components of orbital variability, each
with lower frequency components of amplitude modulation, there is the
potential for unusual interactions between them on long timescales of
tens of millions of years," Zachos said. "What we found at 23 million
years ago is a rare congruence of a low point in Earth's eccentricity
and a period of minimal variation in obliquity."
The result of this rare congruence was a period of about 200,000 years
when there was unusually low variability in the planet's climate, with
reduced extremes of seasonal warmth and coldness. Earth's orbit was
nearly circular, so its distance from the Sun stayed about the same
throughout the year. In addition, the tilt of Earth's axis, which
gives rise to the seasons, varied less than usual. In other words, the
tilt doesn't always vary between the same extremes in its 41,000-year
cycles; the obliquity cycle itself varies in amplitude over a longer
period of about 1.25 million years. Similarly, the eccentricity cycle
peaks every 400,000 years.
The combination of a low-amplitude "node" in the obliquity cycle and a
minimum in eccentricity would have caused only several degrees
difference in summer temperatures at the poles, but it was probably
enough to allow the Antarctic ice sheet to expand, Zachos said.
Zachos's collaborators on the paper were Nicholas Shackleton and Heiko
Pälike of Cambridge University, Justin Revenaugh of UC Santa Cruz, and
Benjamin Flower of the University of South Florida. The researchers
obtained detailed climate records for the late Oligocene and early
Miocene by analyzing sediment cores drilled out of the ocean floor.
Cutting through layers of sediments laid down over millions of years,
such cores contain a chronological record of past climates written in
the chemistry of fossilized shells left behind by tiny marine
organisms. Oxygen isotopes in the shells, for example, reflect ocean
water temperatures and the amount of ice trapped in glaciers.
In the 1970s, scientists using these techniques obtained the first
good evidence in support of Milankovitch's theory, almost 50 years
after he had proposed it. According to Zachos, researchers are still
trying to get a handle on the relationships between climate cycles and
orbital variations. Since most of the research has focused on the past
5 million years, the new paper is valuable because it looks at a more
distant window in time when conditions on the planet were different.
In the period they examined, the late Oligocene and early Miocene,
Zachos and his collaborators found evidence of several climate cycles
with frequencies corresponding to the Milankovitch cycles. But the
correspondence of the orbital anomaly with the transient glaciation
event at the boundary between the two epochs is especially
interesting, Zachos said. The climate system seems to have undergone a
fundamental shift at this boundary, which also marks a major break in
the paleontologic record.
"I'm not sure everyone will be convinced that the orbital anomaly
alone is responsible," Zachos said. "But the congruence of those
orbital cycles is a very rare event, and the fact that it exactly
corresponds with this rare climatic event is compelling."
Editor's note: Reporters may contact Zachos at 
(831) 459-4644
 or
jza...@es.ucsc.edu
 
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Saluti zetetici,
Roberto