Geology Reference
In-Depth Information
P12-P15, constraining the magnetic reversal stratigraphy of the formation
to Chrons C16r-C18r. This provides an initial timescale to be inferred using
the GPTS (Geomagnetic Polarity Time Scale) provided in the Geologic
Time Scale 2004 (Gradstein et al. 2004), from 39,898.816 to 36,065.988 ka.
The anhysteretic remanent magnetization (ARM) series tuned to GPTS2004
is shown in Figure 5.5h; its power spectrum in Figure 5.5b.
5.4.2
Traditional Astronomical Tuning
Traditional astronomical tuning is a “user-intensive” procedure that involves
directly matching cyclostratigraphic variations to an astronomical model. This
is where much of the criticism arises, because the procedure can (i) erroneously
confine variability into the astronomical frequencies and (ii) lead to circular
reasoning. Therefore, careful calculations must be made to avoid or minimize
these problems and to judge what the final objectives are for the tuning.
For example, the Plio-Pleistocene sapropels of the Mediterranean were
tuned to a full astronomical model incorporated in a 65° North summer
insolation curve (Lourens et al. 1996, 2004; Hilgen et al. 2012). The objective
was to define an
astrochronology
for the Plio-Pleistocene epochs based on
the success of fit between the sapropels and insolation maxima, guided by
the magnetostratigraphic timescale of Cande and Kent (1992, 1995). This
procedure can be used safely to interpret the goodness of fit between
astronomical target and sapropel pattern; independently determined geo-
magnetic polarity reversal ages can support the outcome.
One solution is to tune only one frequency, or one parameter, a procedure
known as “minimal tuning” (Muller & MacDonald 2000). The results of
minimal tuning, to the 405-kyr eccentricity cycle, and then to the full eccen-
tricity solution (in two iterations), are shown in Figure 5.5c-e and h. Already
in the GPTS2004-tuned ARM series, a strong 400 kyr scale cyclicity is
evident through the series, and from this, a new 405 kyr-tuned ARM series
was constructed. The tuning could be stopped at this point (and for stratig-
raphy older than 50 Ma, it must; Laskar et al. 2011). However, uncorrected
sedimentation rate fluctuations likely remain that distort the timescale
within the 405 kyr tie points.
A more radical minimal tuning may be considered that matches the
405 kyr-tuned ARM series to the full orbital eccentricity solution. For the
ARM series, two iterations were undertaken: the first tuned ARM minima
to eccentricity minima; the second iteration tuned a low-pass version of
the ARM first iteration eccentricity time series to the maxima and minima
of the same orbital eccentricity solution. This procedure sharpens the peak
at 100 kyr and progressively suppresses the non-orbital peak at 250 kyr, to
a weaker peak at 178 kyr, and finally to a minor peak at 180 kyr. At this
point, the eccentricity-tuned ARM time series can still be somewhat safely
examined for the presence of obliquity and precession forcing (Kodama
et al. 2010; see also Chapter 6).
