Geology Reference
In-Depth Information
Tuning of a data series means simply that the peaks or troughs for a given
cycle in the data are adjusted in their spacing to match either the theoretical
insolation for that cycle (precession, obliquity, or eccentricity) calculated
from Laskar et al. (2004, 2011) or regularly spaced intervals set to a cycle
expected to be regular throughout geologic time (the 405 kyr long eccen-
tricity cycle, for example). Analyseries can generate theoretical insolation
series based on Laskar et al. (2004). The Appendix offers a way of calculating
theoretical precession and obliquity insolation variations based on Laskar et
al. (2011). Analyseries's Linage function can be used to tune a filtered data
series (distorted series) to the theoretical insolation series (reference series).
A pointer series can also be generated to adjust the unfiltered data series to
its new “tuned” spacing using Analyseries's Age Scale function. In MATLAB,
picktune.m
(Appendix) has now been developed as an alternative to the
Linage function.
7.6 Identifying Astronomically Forced
Climate Cycles
Of course, the whole point of a rock magnetic cyclostratigraphy study is the
identification of astronomically forced climate cycles in the rock magnetic
record. These cycles have well-defined durations and when properly
identified will provide a continuous, high-resolution chronostratigraphy
throughout the sedimentary sequence. The most desirable way to acquire a
high-resolution chronostratigraphy is to be able to tune the rock magnetic
cyclostratigraphy to theoretical variations in insolation for eccentricity,
obliquity, and precession. Laskar et al. (2011) give the most recent theoret-
ical calculations for astronomically forced insolation variations. They indi-
cate that the solutions are stable back to ~50 Ma. For times before that, Laskar
et al. (2011) indicate that the 405 kyr, long eccentricity cycle has been stable,
at least back to 250 Ma, but probably even further back in geologic time.
Therefore, the best way to identify astronomically forced climate cycles is to
use absolute time control at a coarse scale to be able to identify a long-period
astronomically forced cycle, usually the 405 kyr long eccentricity cycle. Once
that cycle is observed, the rock magnetic cyclostratigraphy data series is tied to
time, by tuning it to that period. If the sedimentary rocks are younger than
50 Ma, the data series can be band pass filtered at the possible 405 kyr period
and the filtered record can be tied to theoretical insolation at the 405 kyr
period. For time periods before 50 Ma, the 405 kyr filtered record can simply
be tuned by evenly spacing the possible long eccentricity periods at 405 kyr
intervals. After the rock magnetic cyclostratigraphy record is tied to time in
one of these manners, time series analysis of the unfiltered data series will
determine whether statistically significant spectral peaks emerge in the power
spectrum. This approach was used successfully for the Eocene Arguis
Formation (Kodama et al. 2010). Short eccentricity and precession, for
