Image Not Available


Strontium isotope ratio as a function of age for ocean, derived from measurements on deep-sea carbonates. (Smoothed curve, after Edmond 1992.)

All ofthe preceding arguments have now been cogently challenged. First, it is not clear that the trend in 87Sr/86Sr ratio of the ocean in the last 100 million years is tracking an increase in continental weathering or a change in the continental source material to more radiogenic strontium (Berner 1995a). Derry and France-Lanord (1995) found that in the mid-Pliocene with reduced erosion of the Himalayas, the 87Sr/86Sr ratio of the ocean increased. Furthermore, most of the rock particles transported to the Bengal fan are not weathered chemically. Harris (1995) has explained much of the rise in the 87Sr/86Sr ratio as a result of weathering of radiogenic metasedimen-tary rocks exposed during uplift. Blum et al. (1998) found that the strontium flux and high 87Sr/86Sr ratio in Himalayan rivers is likely derived from trace amounts of calcite in silicate rocks and not from silicate weathering itself (only the latter is a net consumer of carbon dioxide from the atmosphere). Godderis and Francois (1995) argued from their modeling of the strontium and carbon cycles that Cenozoic cooling resulted mainly from the decrease in outgassing at the mid-oceanic ridges and changes in the chemical weathering rates in the rest of the world excluding the Himalayas.

Second, the timing ofthe uplift ofthe Tibetan plateau apparently does not coincide with the climate history (Caldeira and Searle 1997). The plateau's elevation was higher 8 million years ago than today; therefore, cooling in the last 8 million years, including the glacial episodes of the Pleistocene, cannot be simply explained by its uplift.

Evidence for enhanced silicate weathering from glacial erosion is also equivocal because it has not been proven that carbonates and ion exchange reactions do not dominate the measured fluxes of Ca2+ and HCO— (Berner 1995a). White and Blum (1995) found no correlation between chemical denudation in granitic terrains and the presence or absence of prior glaciation. Looking at the available data from some ten different glacier-covered catchments, Anderson et al. (1997) found that glacial water is enriched in potassium and calcium relative to other cations, but silica is low. They attribute the elevated calcium to dissolution of trace carbonate (not a carbon sink) and cation leaching from biotite (a potential sink if magnesium uptake by the biotite is less than calcium loss). However, they concluded that silicate weathering is inhibited by low temperatures and lack of vegetation. The grinding of minerals apparently produced a temporary (few years?) enhancement of dissolution that is analogous to the artifact effect produced by grinding powders in the laboratory (see discussion in chapter 5). This short-lived enhancement also may play a role in the continued exposure of reactive mineral surface by ice-induced cracking of rock in environments that are above freezing in the day, but below freezing at night (e.g., mountains, summers in Arctic and Antarctic). This effect may be at least partially responsible for the enhanced silicate weathering rates inferred along an Antarctic stream (Blum et al. 1997). Furthermore, although the authors believe weathering in this locale is abiotic, it is plausible that the presence of a microbial biota contributes to enhanced mineral dissolution in the hyporheic zone (sediment bordering stream), where the chemical weathering is argued to take place, given that liquid water in cold environments is hardly sterile. A recent study of glacial environments demonstrated the apparent presence of microbially mediated weathering (Skidmore et al. 1997).

A major problem with Raymo's hypothesis is that unless a concomitant source of carbon dioxide to the atmosphere is present that balances the proposed increase in the silicate weathering sink, the whole inventory of carbon dioxide in the atmosphere/ocean pool will be removed in a very short time (Volk 1993; Caldeira et al. 1993; Berner and Caldeira 1997). For example, a 10% excess of the sink today would remove all the carbon dioxide from this pool in 5 million years (Volk 1993). There is no evidence for such drastic reduction of carbon dioxide in the Cenozoic. To address this issue, Raymo (1994) proposed that a decrease in organic matter burial (an equivalent carbon dioxide source to the atmosphere/ocean) kept the atmospheric carbon dioxide level from declining. However, this proposed solution has its own problems; there must be near perfect variation in organic matter burial to keep the carbon dioxide level in the atmosphere in balance (Volk 1993), but unlike silicate weathering, there is no evidence of a feedback between organic matter burial and the level of atmospheric carbon dioxide (Berner and Caldeira 1997; Broecker and Sanyal 1998).

One other mechanism for maintaining a steady-state level ofcarbon dioxide has been proposed; it is regulated by aerial volcanism, which increases both the rates ofrelease ofcarbon dioxide to the atmosphere and its removal by silicate weathering ofthe more easily weathered volcanics, especially with higher levels of carbon dioxide in rain (Allegre et al. 1997). However, weathering under these conditions is likely to be strongly mediated by climate (see Louvat 1997), and there is little support for a strong component of volcanic weathering in global fluxes.

Finally, in an interesting twist to this story, Molnar and England (1990) have argued that late Cenozoic global climatic change itselfled to the continued uplift of the Tibetan Plateau. Global cooling would have increased latitudinal temperature differences, perhaps facilitating the monsoon and the increase in erosion rates and isostatic rebound. They suggest that climate change, weathering, erosion and isostatic rebound might be linked in a positive feedback, thereby enhancing a negative greenhouse effect. However, the carbonate-silicate stabilizer will kick in at time scales of 5 X 105 to 106 years, limiting any negative greenhouse effect.

On the other hand, Brozovic et al. (1997) argued the opposite influence of climate on uplift as proposed by Molnar and England (1990), that is, that warmer or drier climates raise peak elevations of mountains by elevating the erosive action of glaciers. They noted that both maximum and mean mountain elevations presently decrease with increasing latitude.

0 0

Post a comment