Geology Reference
In-Depth Information
5
Milankovitch Forcing Theory
Abstract:
This chapter reviews the basic theory of astronomical forcing of paleo-
climate that was originally described by Milutin Milankovitch, with a summary
of the frequencies of Earth's orbital eccentricity, obliquity (tilt), and precession
index. The astronomically forced insolation is explained and analyzed with a
focus on the seasonal aspects of the forcing. The chapter concludes with a descrip-
tion of practices for developing cyclostratigraphic timescales, with the traditional
tuning of an Eocene section.
5.1
Introduction
Once time series analysis (Chapter 4) has identified the dominant frequencies
in a cyclostratigraphic sequence, the real challenge is determining if any are
Milankovitch cycles in scale and in origin. This chapter reviews the
Milankovitch forcing theory of climate (Milankovitch 1941) and how
Milankovitch cycles are identified through time series analysis. Tuning tech-
niques and approaches are also covered here. Developing a refined timescale
is accomplished through application of tuning, identifying likely
Milankovitch frequencies, and adjusting errors in the frequencies that are
not attenuated by the initial timescale.
5.2
Astronomical Parameters
Variations in the Earth's orbit and tilt relative to the Sun force changes in
seasonal, latitudinal, and total insolation, leading to global climate cycles at
10
4
-10
6
year timescales. There are two categories of variations, those arising
from solar system dynamics (the orbital elements) and those arising from
Earth-Moon dynamics (Earth rotation and shape, obliquity of the ecliptic, and
precession rate) (Figure 5.1).
