Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmpp33d..08c&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #PP33D-08
Astronomy and Astrophysics
Astronomy
1041 Stable Isotope Geochemistry (0454, 4870), 4901 Abrupt/Rapid Climate Change (1605), 4910 Astronomical Forcing, 9604 Cenozoic
Scientific paper
The mid-Miocene represents one of the four major cooling steps of the last c. 50 Myr of Earth history but the driving mechanism for this climate transition is contentious. Mechanisms proposed for the cooling include: the large-scale burial of organic rich-mudrocks (The Monterey Hypothesis); the uplift and weathering of the Himalaya leading to an increase in the rate of chemical weathering and enhanced drawdown of carbon dioxide; changes in ocean circulation; and the congruence of orbital parameters. This interval of time is represented in the Circum-Pacific area by the deposition of organic-rich mudrocks and in some sections the amount of organic matter increases during the pronounced cooling. In order to quantify the stages of this climatic transition and to understand the underlying driving mechanism, we have accurately documented the sedimentary and geochemical characteristics of the Monterey Formation near Santa Barbara, California through high-resolution graphic logging and geochemical analysis. Eleven organic-rich mudrock facies have been defined. These facies are distributed cyclically on length-scales ranging from c. 0.5 to 20 m and Fourier analysis of geochemical and physical parameters reveals regular cycles. Using new diatom ages (this study) and nannofossil ages (Follmi et al. 2005), the regular cycles have been correlated to the Laskar orbital solution (Laskar et al. 2004) for about 16 to 13 Myr to produce an accurate astronomical timescale. The distribution of organic-rich facies changes on timescales that range from the sub-Milankovitch to the million- year and these changes are clearly linked to climate. Our new high-resolution C-isotope data can be tied to the distinctive CM events from deep sea sections. The record from the Monterey Formation shows a number of very rapid large C-isotope shifts which may relate to either methane hydrate release or to changes in the organic-C source. Our combined high resolution observations will allow us to discriminate between the different mechanisms proposed for mid-Miocene cooling.
Coe Angela L.
Cohen Seth A.
Maddison E. J.
Weedon Graham P.
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