A High-Resolution Investigation of mid-Miocene Global Cooling

Details A High-Resolution Investigation of mid-Miocene Global Cooling A High-Resolution Investigation of mid-Miocene Global Cooling

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmpp13a1023m&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #PP13A-1023

Astronomy and Astrophysics

Astrophysics

4806 Carbon Cycling (0428), 4870 Stable Isotopes (0454, 1041), 4900 Paleoceanography (0473, 3344)

Scientific paper

The mid-Miocene encompasses one of the four major cooling steps that formed part of the long-term decrease in global temperature during the Cenozoic. Four distinct mechanisms have been suggested to explain this mid- Miocene (c. 14 Ma) climatic change: drawdown of CO¬2 by burial of large amounts of marine organic matter (the "Monterey Hypothesis"); drawdown of CO¬2 by an increase in chemical weathering of fresh silicate rocks resulting from uplift of the Himalaya; changing ocean circulation patterns redistributing ocean-atmosphere heat; and the congruence of orbital parameters. However, the duration, timing and causative mechanisms of mid-Miocene cooling remain poorly constrained, largely due to incomplete sedimentary successions spanning this time period. In an attempt to resolve these issues we have conducted a high-resolution study of the Monterey Formation near Santa Barbara. Our detailed graphic logging has enabled us to identify eleven mudrock facies that are distributed cyclically (length-scales ranging from c. 0.5 to 20 m) throughout the section. Biostratigraphic dating of this section is based on diatom (this study) and calcareous nannofossil (Föllmi et al., 2005, GSA Bulletin, 117: 589-619) datums. Utilising these biostratigraphic age constraints and Fourier analysis of geochemical and physical parameters, we have produced an astronomical timescale that we have correlated with the Laskar orbital solution (Laskar et al. 2004, Astronomy and Astrophysics, 428: 261-285). High-resolution scanning electron microscopy has shown that several benthic foraminifera species are well preserved in these sedimentary deposits. The exceptional preservation of this sedimentary succession allows us to use a suite of geochemical proxies to investigate this global cooling event. The mid-Miocene carbon isotope record is characterised by a positive excursion, which commenced during global warming and ended after major expansion of the Antarctic ice sheet. We present an astronomically tuned high-resolution record (sampling on a sub-millenial timescale) of total organic carbon abundance and carbon isotope data that provides a detailed record of changes in the carbon cycle in the mid-Miocene. This new high- resolution carbon isotope record can be correlated with the distinctive carbon isotope maxima events CM5 and CM6 that have been identified previously in deep-sea sections. Our high-resolution data from the Monterey Formation provides us with a robust framework within which we can examine the dynamics and mechanisms of mid-Miocene global cooling.

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