Solar Radiation Cosmic Rays and Weathering

As emphasized in the previous section, the rate of mineral dissolution during weathering is a function of the temperature of the land. Besides the atmospheric greenhouse effect, there is also the effect of changes in solar radiation on both global and land surface temperature. It is well documented by solar physicists (Endal and Sofia, 1981; Gough, 1981) that the gradual evolution of the sun over geologic time has resulted in increasing levels of radiation reaching the earth. On a geologic time scale the effect has been dramatic. At 4 Ga the level of radiation is estimated to have been 30% less than today, which means that the oceans should have been completely frozen. However, the presence of water-lain sediments at that time indicates that some other warming process must have been present to avoid global oceanic freezing. The generally agreed upon culprit is a very strong atmospheric greenhouse effect due either to very high levels of CO2 (e.g., Kasting and Ackerman, 1980) or of CH4 (e.g., Pavlov et al., 2000).

Over the Phanerozoic solar evolution has continued to increase linearly, starting at a level of about 0% less than now at the start of the Cambrian. This is still a dramatic effect. The level of elevated atmospheric CO2 necessary to counter this reduced radiation, in order to attain a global mean surface temperature the same as at present, can be calculated simply via a modification of equation (2.8):

where Ws expresses the effect on temperature of the linear increase in solar radiation with time. Using the values for Ws (7.4) and r (3.3°C)

fBg(C02)

10 12 14 16 18 20

10 12 14 16 18 20

rco2

Figure 2.5. Plot of fBg(CO2) versus RCO2. The variable fBg(CO2) is the nondimensional factor expressing the effect of temperature, as controlled by the atmospheric greenhouse effect, on silicate weathering. RCO2 is the ratio of CO2 concentration at some past time to that at present. The relation between the two parameters is given by equation (2.23) with Z = 0.09, Ws = 7.4, and r = 4°C. The reduction in slope of the curve with increasing RCO2 reflects the fact that with increasing CO2 levels the atmospheric greenhouse effect, per unit change in CO2, diminishes.

for the Cambrian, based on the NCAR CCM-3 GCM model (Berner and Kothavala, 2001) indicates that to maintain the same global mean surface temperature at the beginning of the Cambrian (550 Ma) as at present would require a value of RCO2 of 8.7, or about nine times higher than today. This illustrates the importance of including variations in solar radiation in carbon cycle modeling over long geologic times. A plot of RCO2 versus time necessary for the temperature to be the same as at present is shown in figure 2.6.

Recently it has been suggested that the passage of the solar system through arms of the Milky Way galaxy results in increased fluxes of cosmic rays toward the earth because of a greater density of supernovas in the spiral arms (Shaviv, 2002). Reactions of atmospheric gases with cosmic rays produce ions that can serve as cloud condensation nuclei. Thus, an increase in cosmic ray bombardment of the earth could lead to greater cloudiness and global cooling. Shaviv states that the major glaciations of the Phanerozoic can be explained as occurring during periods of spiral arm passages (although his predicted large Jurassic glaciation does not exist). If this hypothesis has merit and (it needs more verification), then an additional mechanism can be called upon to affect the rate of silicate weathering. However, greater cloudiness, while leading to cooling and slower mineral weathering, could also lead to rco2

Solar Radiation With Time

Time my

Figure 2.6. RCO2 versus time for the situation where the increase of solar radiation with time is balanced by a diminishing CO2 greenhouse effect to maintain mean annual surface temperature at all times to be the same as at present. Deviations from a smooth curve are due to changes in the value of the greenhouse coefficient r over time.

Time my

Figure 2.6. RCO2 versus time for the situation where the increase of solar radiation with time is balanced by a diminishing CO2 greenhouse effect to maintain mean annual surface temperature at all times to be the same as at present. Deviations from a smooth curve are due to changes in the value of the greenhouse coefficient r over time.

greater rainfall and faster weathering. Thus, it is not clear what the effect of changes in cosmic ray flux could have had on weathering rate.

Organic Gardening

Organic Gardening

Gardening is also a great way to provide healthy food for you and your loved ones. When you buy produce from the store, it just isnt the same as presenting a salad to your family that came exclusively from your garden worked by your own two hands.

Get My Free Ebook


Responses

  • marcus
    What is the weathering and radiation?
    6 years ago

Post a comment