Geology Reference
In-Depth Information
5.3
Insolation
The astronomical parameters affect changes in the intensity and timing of
the incoming solar radiation, or insolation, at all points on the Earth. The
insolation variations are the well-known Milankovitch cycles. Geographical
location, time of year, and time of day together determine the relative con-
tributions of the orbital parameters to the interannual insolation (e.g.,
Berger et al. 1993, 2010). Climate integrates insolation over certain times
of the year and collectively over specific geographic areas, possibly over
different areas at different times. This “climatic filtering” alters the relative
contributions of the orbital parameters to the total output climate response,
this even prior to internal climate system responses to the insolation. Thus,
it is left to the discretion of the paleoclimatologist to determine which
time(s) of the year and at which location(s) a prevailing climate has
responded to insolation.
A still largely unrecognized aspect of insolation is the interannual phasing
of the astronomical parameters as a function of time of year. For example, at
35° North (Figure 5.4), the obliquity is in phase with insolation during the
summer months, but antiphased during the winter months. Precession-forced
insolation continuously changes phase with respect to the standard precession
index over the course of a year. Eccentricity forcing, while extremely low in
power, is in phase with insolation throughout the year, i.e., higher eccentricity
corresponds to higher insolation.
5.4
Astronomical Tuning and Timescales
Tuning stratigraphic sequences can have different meanings, and it is always
controversial in one way or another, particularly “astronomical tuning.” In
this section, a detailed example of traditional astronomical tuning is
presented, with discussion of the limitations imposed by the tuning in terms
of interpretation. This is followed by a discussion of a new class of tuning
methods that are coming online, based on searches for the sedimentation
rate that statistically produces the best fit to an astronomical model.
Concepts and procedures are demonstrated on the Eocene Arguis Formation
(Kodama et al. 2010) (Figure 5.5).
5.4.1
The Initial Timescale
Assigning an initial timescale is a required step in determining whether
Milankovitch cycles are present in cyclostratigraphy. The usual tools involve
some combination of magnetostratigraphy, biostratigraphy, chemostratigra-
phy, and radioisotope dating, depending on what is available. In the case of
the Arguis Formation, biostratigraphy indicates that the formation spans
nannoplankton (NP) zones NP16-NP17 and planktonic foraminifera zones
