Raymo - uplift hypothesis

Ph.D., Columbia University
Research Professor, Dept. of Earth Sciences, Boston University
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The Uplift-Weathering Hypothesis

Portal Dawn

       This page is for students seeking more information about the uplift-weathering hypothesis. In 1988, Maureen Raymo, Flip Froelich and Bill Ruddiman published the first of a series of papers (listed below) proposing, and then examining, the hypothesis 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. The uplift of the plateau also intensified the Asian monsoon and the large amount of rainfall combined with the steep relief and high mechanical erosion rates has resulted in some of the highest chemical weathering rates observed for any region. It is these chemical weathering reactions that, over 40 million years, have consumed atmospheric CO₂, thus weakening the global "greenhouse" effect and causing the growth of continent-spanning ice sheets at both poles.

       This idea has proven influential across a range of research disciplines although it is by no means proven. It has been examined and discussed by scientists working not only with deep sea sediments, but by those studying tectonics, geomorphology, river chemistry, weathering reactions, and climate and carbon cycle modeling. For over a decade this hypothesis has remained controversial and continues to be the subject of debate within the literature, primarily because the data needed to definitively test the hypothesis are not available (a paper published in 1997 by McCauley and DePaolo in a volume dedicated to this topic "Tectonic Uplift and Climate Change" provides an excellent review of the geochemical aspects of this scientific controversy).

       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 recent paper by Bickle et al. [2005] nicely updates the Himalayan Sr/silicate weathering story). A major criticism of the uplift 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 and our critics agreed that a negative feedback which stabilized atmospheric CO₂ levels was needed. We argued that a negative feedback may have operated through the organic carbon subcycle. Others argued for the temperature-weathering feedback of Berner et al. (1983). In that model (the BLAG model), global chemical weathering rates would always be higher during times of warmer global temperatures (the Eocene) and would have decreased, not increased, toward the present.

       This 15-year stalemate has recently been made more interesting by a paper published posthumously by John Edmond of MIT and Youngsook Huh of Northwestern University (Edmond and Huh, 2003). They argue that the input of metamorphic CO₂ to the atmosphere and the removal of CO₂ by weathering are completely decoupled in time and space---no negative feedback modulates the operation of the long-term carbon cycle. The removal of atmospheric CO₂ is indeed controlled by tectonically-driven weathering reactions and CO₂ is presently at its lowest possible limit (a "kinetic minimum"). They argue that at present, the Himalayan-Tethyan mountain range has an almost infinite capacity to absorb (by weathering reactions) any CO₂ released from the mantle or by organic carbon oxidation.

       Edmond and Huh's ideas make some interesting predictions for Cenozoic records of atmospheric CO₂ and chemical weathering rates. Once the removal rate of CO₂ exceeded the input rate from subduction zones (possibly in the late-Eocene to early Oligocene), CO₂ would have rapidly fallen to this "kinetic lower limit" and remained there for the last 30-40 million years. E.g. Miocene CO₂ levels may not have been significantly higher than today, a possibility that was suggested by the paleo-pCO₂ study of Pagani et al. (1999). Chemical weathering rates, on the other hand, could have been higher or lower than today over the last 40 million years being dependent on the amount of mantle outgassing/organic carbon oxidation per unit time. In fact, it seems possible that the highest weathering rates of the Cenozoic could have occurred in the early to middle Miocene when extensive movement along faults in the Himalayas could have resulted in enhanced metamorphic release of CO₂ simultaneously with the generation of extreme topographic relief.

       Only with the development of accurate paleo-PCO₂ and silicate weathering proxies will these various possibilities be sorted out.

Uplift-related links:

A companion website for the NOVA film put together by WGBH. It contains basic information about long-term climate change and has a list of links to other climate-related sites. http://www.pbs.org/wgbh/nova/ice/

A book dedicated to the uplift-climate hypothesis, "Tectonic Uplift and Climate Change", edited by Bill Ruddiman, is aimed at scientists and graduate students and contains many interesting and useful papers. If you are a graduate student thinking about this topic this is a great place to start.

If you'd like to obtain a DVD copy of NOVA "Cracking the Ice Age" send me an email.

Articles about the Uplift-Weathering Hypothesis 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.

References mentioned above:

Bickle, M.J., H.J. Chapman, J. Bunbury, N. Harris, I.J. Fairchild, T. Ahmad, and C. Pomies, 2005, Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges, Geochimica et Cosmochimica Acta, v. 69, p. 2221-2240.

Edmond, J.M. and Youngsook Huh, 2003, Non-steady state carbonate recycling and implications for the evolution of atmospheric PCO₂, Earth Planet. Sci. Lett., 216, 125-139.

McCauley, S.E. and D. DePaolo, 1997, The marine ⁸⁷Sr/⁸⁶Sr and d¹⁸O records, Himalayan alkalinity fluxes and Cenozoic climate models. In: Global Tectonics and Climate Change (eds. W.F. Ruddiman and W. Prell), Plenum Press, p. 428-465.

Pagani, M., M. Arthur, and K. Freeman, 1999, Miocene evolution of atmospheric carbon dioxide, Paleoceanography, v. 14, p. 273-292.

Papers on Uplift-Weathering Hypothesis by M. E. Raymo:

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.

Ruddiman, W.F. and M.E. Raymo (1988) Northern hemisphere climate regimes during the past 3 Ma: possible tectonic connections. In: The Past Three Million Years: Evolution of Climatic Variability in the North Atlantic Region, edited by N.J. Shackleton, R.G. West, and D.Q. Bowen, pp. 227-234, University Press, Cambridge.

Ruddiman, W.F., W.L. Prell, and M.E. Raymo (1989) History of late Cenozoic uplift on Southeast Asia and the American Southwest: rationale for general circulation modeling experiments. Journal of Geophysical Research, v. 94, p. 18379-18391.

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.