The Uplift-Weathering Hypothesis
In 1988, Bill Ruddiman, Flip Froelich and I published
the first of an eventual series of papers (listed below) proposing that the late Cenozoic
cooling of climate, the Ice Age, was caused by enhanced chemical weathering and
consumption of atmospheric CO2 in the mountainous
regions of the world, in particular the Himalayas. About 40 to 50 million years
ago, the Indian subcontinent collided with the underside of Asia. This collision,
which continues to this day, resulted in the uplift of the Himalayas and the formation
of the Tibetan Plateau, the largest geographic feature on the Earth's surface (above the waves).
The uplift of the plateau would likely have intensified the Asian monsoon (see a series of papers by Ruddiman and Kutzbach) and this
rainfall, combined with the steep relief and high mechanical erosion rates in the Himalayas, may have resulted in dramatically higher
chemical weathering rates in the region.
It is these chemical weathering reactions that, over 40 million years, would have consumed
atmospheric CO2 thus weakening the global greenhouse
effect and leading to the growth of continent-spanning ice sheets at both poles.
Raymo et al. (1988) originally proposed that the marine strontium isotope record showed that chemical weathering rates
had increased over the Cenozoic, but by 1992 this record was shown to be ambiguous and hence not suitable for testing the
hypothesis. A major criticism of the hypothesis was that chemical weathering rates could not increase in the
absence of enhanced metamorphic delivery of CO2 to the atmosphere, otherwise CO2 would be completely stripped from
the atmosphere within a few hundred thousand years and the Earth would become a frozen "snowball" planet. We agreed that a negative feedback was needed to stabilize atmospheric
CO2 levels and argued
that this feedback may have operated through the organic carbon subcycle.
paleo-CO2 and silicate weathering proxies now being developed, along with high resolution geochemical
records of the last 60 Ma, will help unravel the secrets of Cenozoic climate change. Did silicate weathering rates increase or decrease since the Eocene?!
A book "Tectonic
Uplift and Climate Change", edited by Bill Ruddiman, is aimed at scientists and graduate students and contains many interesting and useful
PBS NOVA made a documentary about our work. If you'd like a DVD copy of "Cracking the Ice Age" send me an email.
Some articles in the popular press:
Paterson, D. (1993) "Did Tibet cool the world?" New Scientist. v. 139 No. 1880, pp 29-33.
Wilson, J. (1999) "The Big Chill Solved: the mystery of the first ice age", Popular Mechanics.
Watson, T. (1997) "What Causes Ice Ages?", U.S. News and World Report, August 18.
Our papers on uplift-weathering hypothesis:
Raymo, M.E., W.F. Ruddiman, and P.N. Froelich (1988) Influence of late Cenozoic mountain building on ocean geochemical cycles. Geology, v. 16, p. 649-653.
Raymo, M.E. (1991) Geochemical evidence supporting T.C. Chamberlin's theory of glaciation. Geology, v. 19, p. 344-347.
Raymo, M.E. and W.F. Ruddiman (1992) Tectonic forcing of late Cenozoic climate. Nature, v. 359, p. 117-122.
Raymo, M.E. and W.F. Ruddiman (1993) Cooling in the late Cenozoic-Scientific Correspondence. Nature, v. 361, p. 123-124.
Raymo, M.E. (1994) The initiation of Northern Hemisphere glaciation. Annual Reviews of Earth and Planetary Science, v. 22, p. 353-383.
Raymo, M.E. (1994) The Himalayas, organic carbon burial, and climate in the Miocene. Paleoceanography, v. 9, p. 399-404.
Raymo, M.E. (1997) Carbon cycle models: how strong are the constraints? In: Global Tectonics and Climate Change (eds. W.F. Ruddiman and W. Prell), Plenum Press, p. 368-382.
Ruddiman, W.F., M.E. Raymo, W. Prell, and J.E. Kutzbach (1997) The uplift-climate connection: a synthesis. In: Global Tectonics and Climate Change (eds. W.F. Ruddiman and W. Prell), Plenum Press, p. 471-515.
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