Geology Reference
In-Depth Information
Eccentricity: e
2
405
0-40 Ma
1
95
125
99
2400
131
960
0
0
0.005
0.01
0.015
Cycles/kyr
Obliquity:
ε
39.8
30-40 Ma
0.04
0.02
51.7
38.5
29.2
28.3
0.04
0
Figure 5.3
−2π prolate
multitapered power spectra
of the La2004 astronomical
parameters over the past
40 million years. Spectra for
0-10 Ma and 30-40 Ma are
calculated for the obliquity
and precession index to
demonstrate the shift to
higher frequencies (shorter
periodicities) due to the
tidal dissipation effect.
Source: Hinnov 2013.
Reproduced with permission
of Geological Society of
America.
40.8
0-10 Ma
0.02
39.5
53.3
29.7
28.8
0
0.01
0.015
0.02
0.025
0.03
0.035
0.04
Cycles/kyr
Precession Index: esin
ω
×10
-5
23.3
4
30-40 Ma
22.0
18.7
2
18.8
×10
-5
0
23.6
4
0-10 Ma
22.3
18.9
2
19.1
0
0.03
0.04
0.05
0.06
0.07
Cycles/kyr
eccentricity has varied between ~0 and 0.06, with major periodicities in the
~100 kyr band and at 405 kyr. The
obliquity
has ranged between 22.5° and
24.5°. Faster rotation rates from tidal dissipation shifts variations to shorter
periods in earlier time. The
precession index
tracks the drift of the seasons and
relative Earth-Sun distance around the Earth's orbit relative to perihelion due
to the Earth's precession. This drift, along with the
precession of the orbital
perihelion
(Π) in the opposite direction plus changes in orbital eccentricity,
produces the modulated precession index (esinω).
The terms “orbital forcing” and “astronomical forcing” have become synony-
mous in the literature. In fact, “orbital” is not technically correct, in that the
obliquity and precession index do not have strict orbital origins but also involve
geophysical phenomena (Hinnov 2013). The terms “precessional parameter,”
“
climatic precession
,” “astroclimatic parameter,” and “
precession-eccentricity
syndrome
” are all synonyms for the precession index. ETP (Figure 5.2) is often
adopted in cyclostratigraphic-astronomical forcing studies where details of the
forcing, e.g., the magnitude and/or phase response of the forcing, are not
understood.
