Carbon Dioxide and Polar Cooling in the Miocene: The Monterey Hypothesis
- E.T. Sundquist and
- W.S. Broecker
Published Online: 18 MAR 2013
Copyright 1985 by the American Geophysical Union.
The Carbon Cycle and Atmospheric CO: Natural Variations Archean to Present
How to Cite
Vincent, E. and Berger, W. H. (1985) Carbon Dioxide and Polar Cooling in the Miocene: The Monterey Hypothesis, in The Carbon Cycle and Atmospheric CO: Natural Variations Archean to Present (eds E.T. Sundquist and W.S. Broecker), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM032p0455
- Published Online: 18 MAR 2013
- Published Print: 1 JAN 1985
Print ISBN: 9780875900605
Online ISBN: 9781118664322
- Carbon cycle (Biogeochemistry)—Congresses;
- Atmospheric carbon dioxide—Congresses;
- Geological time—Congresses;
- Geology, Stratigraphic—Congresses
A pronounced shift in the δ13C of foraminifera in the latest early Miocene has been proposed by various authors. Our data in the tropical Indian Ocean show an excursion of δ13C signals toward heavier values, lasting for about 4 million years. The excursion is documented for benthic foraminifera as well as for deep-living and for shallow-dwelling planktonic species. The initial δ13c shift occurs within Magnetic Chron 16, at about 17.5 Ma. It represents a change toward heavier δ13C values by about 10/00 in surface and bottom waters. The excursion terminates at approximately 13.5 Ma. The Chron 16 Carbon shift coincides with the cessation of an early Miocene warming trend, seen in the δ18O signals. The mid-Miocene cooling step (presumably associated with Antarctic ice buildup, near 15 Ma) is centered on the carbon isotope excursion. We propose that the initial carbon shift was caused by rapid extraction of organic carbon from the ocean-atmosphere system. Subsequently, the excursion toward heavy values was maintained by continued extraction of organic carbon, into ocean-margin deposits. Beginning at the end of the early Miocene, fine-grained diatomaceous sediments rich in organic matter were deposited all around the margins of the northern Pacific. In California, these sediments are known as the Monterey Formation. This formation is the result of coastal upwelling, which arose because of the development of strong zonal winds and a strong permanent thermocline.
Zonal winds and thermocline evolution, in turn, depended on increasing temperature contrast between high and low latitudes. We hypothesize that a feedback loop was established, such that an initial increase in the planetary temperature gradient started thermocline development which led to organic carbon extraction at the ocean margins which resulted in a drop in atmospheric carbon dioxide concentration. Concomitant cooling (reverse greenhouse effect) strengthened thermocline development, leading to further cooling. The loop was broken when available nutrients were used up. The total amount of excess carbon buildup, according to the hypothesis, is between 40 and 80 atmospheric carbon masses for the duration of the Monterey carbon isotope excursion. This amount corresponds to that present in the ocean, that is, one ocean carbon mass.