HR: 08:15h
AN: H41H-02 INVITED     [Abstracts]
TI: Atmospheric CO$_{2}$ Consumption in Uplifting Mountain Ranges: New Insight From the New Zealand Southern Alps
AU: * Jacobson, A D
EM: adj@earth.northwestern.edu
AF: Northwestern University, Department of Geological Sciences 1850 Campus Dr., Evanston, IL 60208 United States
AU: Blum, J D
EM: jdblum@umich.edu
AF: University of Michigan, Department of Geological Sciences 2534 C.C. Little Bldg. 425 E. University Ave, Ann Arbor, MI 48109 United States
AU: Chamberlain, C P
EM: chamb@pangea.stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences 450 Serra Mall Bldg. 320, Stanford, CA 94305 United States
AB: Rates of physical erosion and chemical weathering in uplifting mountain ranges are generally higher than the rates observed in tectonically stable regions. This observation has led to the hypothesis that orogenic events lead to global cooling over geologic time scales by accelerating the rate of atmospheric CO$_{2}$ drawdown from silicate weathering. However, recent studies of rivers draining the rapidly uplifting Himalaya Mountains have demonstrated that much of the chemical weathering flux is dominated by carbonate dissolution, which does not influence long-term atmospheric CO$_{2}$ levels. To examine if carbonate weathering dominates in other orogenic environments, we have undertaken investigations of rivers draining the New Zealand Southern Alps, which present a largely unexplored setting for systematically examining tectonic controls on the carbon cycle. In particular, we quantified rates of physical erosion and both silicate and carbonate weathering across a gradient of variable uplift rates but constant bedrock composition. We also compared the findings to global mean values as well as to data for major world rivers in other tectonic and climatic settings. Rapid uplift in the western Southern Alps elevates mechanical erosion rates by a factor of ~13 relative to those on the tectonically stable eastern side. Similarly, the average chemical weathering rate is ~5 times higher on the western compared to eastern side of the mountain range. However, because the proportion of stream-water Ca$^{2+}$ and Mg$^{2+}$ from the weathering of trace hydrothermal calcite increases as the rate of mechanical erosion increases, the long-term atmospheric CO$_{2}$ consumption rate on the western side is only ~2 times higher than that on the eastern side and only ~1.5 times higher than the global mean value. These data demonstrate that tectonic uplift in the New Zealand Southern Alps accelerates physical erosion and chemical weathering rates but does not greatly enhance the rate of long-term atmospheric CO$_{2}$ consumption, because mechanical erosion effectively maintains low ratios of silicate to carbonate weathering. The highest ratios of silicate to carbonate weathering occur in the eastern Southern Alps, where landscape stabilization leads to the depletion of carbonate from developing soil profiles. Data for major world rivers (including Himalayan rivers) yield a consistent interpretation. We find that landscapes subject to either intense mechanical erosion or cool temperatures experience the lowest ratios of silicate to carbonate weathering. By comparison, landscapes experiencing modest mechanical erosion and warm temperatures have the highest ratios of silicate to carbonate weathering. The strong similarity between river chemistry in the Southern Alps and the Himalaya Mountains suggests that these results are generally applicable to understanding the relationship between uplift and chemical weathering. We therefore conclude that mountain building increases atmospheric CO$_{2}$ consumption rates by only a factor of ~2, which is much smaller than previous estimates. Because the area of uplifting mountain ranges is small relative to the total area of the continents, we suggest that stable landscapes with overall lower chemical weathering rates but high ratios of silicate to carbonate weathering may exert a more significant influence on long-term atmospheric CO$_{2}$ levels.
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
DE: 1045 Low-temperature geochemistry
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting