Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmgc24a..06b&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #GC24A-06
Mathematics
Logic
4910 Astronomical Forcing, 4946 Milankovitch Theory
Scientific paper
The prelude to the second oceanic-anoxic event of the Cretaceous (OAE 2), as recorded in the classic sections of the Scaglia Bianca from the Umbria-Marches region of Italy, is characterized by an oscillation between black to gray, organic-rich cherts, representing times of high productivity and associated anoxia, and white pelagic limestones representing times of lower productivity and oxygenated bottom waters. Using image-analysis software, we converted these lithologic variations into a quantitative, high-resolution optical densitometry (grayscale) data string and then carried out a spectral analysis to investigate the cyclicity. The relative values of the significant spectral peaks are remarkably similar to the orbital parameters calculated by Laskar et al. (2004); furthermore, the correlation between these peaks and the orbital periods results in a sedimentation rate very close to that predicted from the existing geochronology (Sageman et al., 2006). Band- pass filtering of our data shows that the 400-90 kyr signal is the envelope of the 25-18 signal, as expected if these components represent eccentricity and precession. The strong power in the eccentricity and obliquity bands at this low latitude site suggests an important high- latitude influence on global climate during this time period. A comparison between the presumed eccentricity signal and the theoretical eccentricity of Laskar et al. (2004) leads to an astrochronological framework for this time period, including an age for the base of the Bonarelli Level (onset of OAE 2) of 94.18 Ma, which is in relatively close agreement with a recent date of 94.08 Ma (Sageman et al., 2006) from the correlative level in North America. The strong astronomical influence on the timing of the organic-rich precursors of OAE 2 supports the idea that the timing of OAE 2 is similarly controlled by astronomical variations, specifically, a time period where weak obliquity and eccentricity variation coincide to create an unusual node in the theoretical insolation curve.
Bice D.
Christianson Knut
Cleaveland L.
Mitchell Richard
Montanari Andrea
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