Geology Reference
In-Depth Information
each sample. Interestingly, even for these weakly magnetized carbonates,
the ARM could be measured with a spinner magnetometer (Agico JR6), a
relatively inexpensive instrument to purchase and maintain compared to
the superconducting rock magnetometers often used for remanence
measurements.
Wu et al. (2012) used the log of ARM to stabilize the variance of the ARM
data series. They prewhitened their data and used both MTM spectral anal-
ysis and wavelet analysis to search for any periodicities. In order to identify
astronomically forced cycles, they determined the average sediment
accumulation rate for the Daye Formation from an estimate of the duration
of the Induan stage in the Triassic to be between 1.0 and 1.4 million years.
The upper and lower boundaries of the Induan stage were identified by
conodont biostratigraphy.
Many cycles were observed in the ARM and susceptibility power
spectra, with long and short eccentricity (13-16 m and 3-4 m), obliquity
(1.3-1.7 m), precession (0.69-0.78 m) identified based on the average
sediment accumulation rate, as well as many sub-Milankovitch frequency
cycles (Figure 6.10).
To further nail down the identification of astronomically forced cycles in
the Daye Formation, Wu et al. (2012) conducted amplitude modulation
(AM) analysis, similar to that conducted by Hinnov et al. (2013) for the
Cretaceous Cupido Formation. Wu et al. (2012) filtered the data series at the
precessional scale with a Gaussian band-pass filter and then ran MTM
spectral analysis of the envelope of the filtered precession. They observed
eccentricity peaks for the power spectrum of the envelope showing that
eccentricity modulated precession, exactly what would be expected if the
astronomically forced cycles were correctly identified (Figure 6.11). Wu
et al. (2012) also performed this analysis on sub-Milankovitch cycles,
filtering in the sub-Milankovitch frequency (4-5 kyr periods) and detecting
Milankovitch frequencies in the MTM spectrum of the envelope (precession,
obliquity, and eccentricity), suggesting that orbital frequencies modulate the
sub-Milankovitch frequencies.
Wu et al. (2012) suggest an encoding mechanism for the Daye
Formation similar to that envisioned for the Eocene Arguis Formation
(Kodama et al. 2010). The variations in the concentration of fine-
grained, low-coercivity magnetite that carries the orbital and suborbital
frequency periodicities are due to the fluctuations of siliciclastic terres-
trial input into the platform carbonates of the Daye Formation. Wu et al.
(2012) suggest that these fluctuations are due to changes in the strength
of the monsoon at the low paleolatitudes of the Daye Formation. The
strong modulation of precession by eccentricity in the ARM and MS
records is the primary basis for this interpretation since is it recognized
that at low latitudes precession controls the strength of the monsoon.
Hence, according to Wu et al. (2012), eccentricity maximum and pre-
cession minima caused higher continental runoff and, consequently,
greater magnetite concentration.
