Reconnaissance of Field Sites for the Study of Chemical Weathering on the Guayana Shield, South America

Steefell, C. I. ; Viani, B. E. ; Ramirez, A. ; Lee, K.

Lawrence Livermore National Laboratory

关键词: Chemical Reactions;    58;    Himalayas;    Scale Models;    Carbon Cycle;   

DOI  :  10.2172/15004042
RP-ID  :  UCRL-ID-151708
RP-ID  :  W-7405-ENG-48
RP-ID  :  15004042

美国|英语

来源: UNT Digital Library

【 摘 要 】

Despite the fact that chemical weathering of silicate rocks plays an important role in the draw-down of CO{sub 2} over geologic time scales (Berner and Berner, 1996), the overall controls on the rate of chemical weathering are still not completely understood. Lacking a mechanistic understanding of these controls, it remains difficult to evaluate a hypothesis such as that presented by Raymo and Ruddiman (1992), who suggested that enhanced weathering and CO{sub 2} draw-down resulting from the uplift of the Himalayas contributed to global cooling during the Cenozoic. At an even more fundamental level, the three to four order of magnitude discrepancy between laboratory and field weathering rates is still unresolved (White et al., 1996). There is as yet no comprehensive, mechanistic model for silicate chemical weathering that considers the coupled effects of precipitation, vadose zone flow, and chemical reactions. The absence of robust process models for silicate weathering and the failure to resolve some of these important questions may in fact be related-the controls on the overall rates of weathering cannot be understood without considering the weathering environment as one in which multiple, time-dependent chemical and physical processes are coupled (Malmstrom, 2000). Once chemical weathering is understood at a mechanistic process level, the important controls on chemical weathering (physical erosion, temperature, precipitation) can be folded into larger scale models tracking the global carbon cycle. Our goal in this study was to carry out the preliminary work needed to establish a field research site for chemical weathering om the Cuayana Shield in South America. The Guayana Shield is a Precambrian province greater than 1.5 billion years old covering portions of Venezuela, Guyana (the country), Surinam, French Guiana, and Brazil (Figure 1). More important than the age of the rocks themselves, however, is the age of the erosion surface developed on the Shield, with estimates ranging as old as 65 million years. Preserved mostly in highlands, this very old erosion surface represents an end-member site where physical erosion has been significantly slower than the rate of chemical weathering. Much of the Shield is also noteworthy for the fact that chemical weathering is still occurring today, thus offering the chance to study a system in which a present day weathering regime is accompanied by an integrated weathering record over millions of years (Soler and Lasaga, 2000). If rates of chemical weathering can be determined for this very old weathering system where physical erosion is minor, they can then be compared with rates determined from sites with similar annual temperatures and rainfall, but much higher physical erosion rates. Comparative studies of this kind can provide a parameterization of chemical weathering rates as a function of physical erosion and tectonic uplift that can be used in global models for the carbon cycle.