Geology Reference
In-Depth Information
cycles are probably short eccentricity and precession, while 16 and 1.1 m
peaks are most likely long eccentricity and obliquity suggesting the G/H
magnetic parameter may be a viable way of detecting ancient climate cycles
magnetically. However, it is probably incorrect to interpret the goethite to be
depositional in the Rainstorm Member, and hence a measure of moisture in
its Neoproterozoic source area. Thermal demagnetization suggests that the
goethite, removed at low temperatures in thermal demagnetization, carries
a present-day field direction indicating that it probably formed during
recent surface weathering. Furthermore, conodont alteration indices of 4.5
in Ordovician rocks stratigraphically above the Johnnie Formation (Gillett
1982) indicate that the Johnnie Formation was heated at least to tempera-
tures of 200-300°C which would have been hot enough to invert any ancient
goethite to hematite (deFaria & Lopes 2007). It is more likely that pres-
ent-day weathering converted Fe-rich silicates or clays to goethite. Therefore,
log
10
(G/H) measures subtle variations in Fe-rich clays in the rocks, and
hence fluctuations in the input of terrestrial sediment into the carbonate.
The encoding of the astronomically forced climate cycles by the mag-
netics is similar to that suggested for the Triassic Daye Formation from
China (Wu et al. 2012) and the Eocene Arguis Formation marine marls
(Kodama et al. 2010). The magnetite concentration probably records
variations in terrigenous sediment input into marine carbonate being
produced at a relatively constant rate. This interpretation suggests that
magnetic methods can be a sensitive way of detecting variations in
the amount of continental sedimentation into a near shore marine
environment.
6.9
Encoding of Orbitally Forced Climate Signals
The case studies presented here indicate that one of the main mechanisms for
encoding climate variations by magnetic mineral concentration is the input
of varying amounts of terrestrial sediment into a relatively constant
background of marine carbonate production. This was the explanation for
the encoding of Milankovitch climate cycles by magnetite concentration in
the Eocene Arguis Formation (Kodama et al. 2010), the Triassic Daye
Formation (Wu et al. 2012), the Neoproterozoic Johnnie Formation, and
at least part of the astronomically forced climate signal (precession and
eccentricity) detected in the Plio-Pleistocene Stirone River section
(Gunderson et al. 2012). The Arguis Formation interpretation is backed
up by an interesting cross-correlation analysis that suggests that ARM
maxima coincide in phase with autumn insolation and, therefore, the rainy
season for low latitude, monsoonal climate. Of course, it is also possible that
carbonate production could have varied due to astronomically forced climate
change. There is no way of determining whether magnetic mineral con-
centration variations in the rocks studied are due to changes in terrestrial
