Altitudinal Latitudinal and Diurnal Temperature Variations on a Very Warm Archean Early Proterozoic Earth Surface and Possible Implications to History of Weathering

The lapse rate, the change in air temperature with elevation, will determine the air temperature on mountains (as previously mentioned, the ground temperature may be considerably higher, depending on the local surface albedo). If we assume that the maximum height of mountains is 10 km (Everest is 9.1 km), then for an average lapse rate of 6.5°C/km (Henderson-Sellers and McGuffie 1987) the lowest temperature on the Earth's surface for a sea level temperature of 70°C is 5°C; for 50°C, — 15°C. Note that the lapse rate varies from 4°C/km (moist) to 10°C/km (dry).

Latitudinal differences in temperature decrease as global mean temperature increases, judging from the paleoclimate record of the Cenozoic (Hoff-ert and Covey 1992). While the processes that equalize temperature are not well understood, latitudinal differences in temperature are predicted to vanish for a global mean temperature of >30°C (Hoffert, personal communication). Likewise, diurnal variations would be expected to significantly diminish as the mean temperature increases, as a result of greater heat storage in the ground and troposphere, which is radiated at night.

An Archean/early Proterozoic Earth likely rotated at a faster rate than now, with a day some 10 hours shorter, based on the progressive slowing of the rotation rate from lunar tides (Walker and Zahnle 1986; Zahnle and Walker 1987). Based on a computer simulation of the Precambrian Earth, taking into account the effects of a faster rotating Earth, and assuming a near present day temperature, Jenkins et al. (1993) concluded there would have been a modest temperature elevation of a few degrees from this effect, as well as an increase in the latitudinal temperature gradient from the weakening of the tropical Hadley circulation, which carries heat from the equator poleward. However, much higher mean temperatures would have likely obliterated the latter tendency.

If the onset of latitudinal differences in temperature occurs at or below a mean global temperature of about 30°C, then the preferred temperature history for the Earth (figure 8-3) predicts the commencement of seasonality, as well as bigger fluctuations in diurnal temperature starting at about 1.5 Ga. Furthermore, as temperatures decrease, an expansion of mountain area subject to freeze/thaw cycling is expected. All of these factors may have contributed to a significant increase in physical weathering via ice wedging and thermal cracking (insolation weathering), generating silt size mineral and rock particles (Moss et al. 1981; Dove 1995; Fahey 1983; Summerfield 1991). The production and delivery of chemically immature mineral and rock particles to lowland flood plains might well have significantly increased the chemical weathering rate (Edmond 1992); thus, if confirmed, the long-term biotically mediated cooling in the Archean/Proterozoic set up the conditions for the onset of frost wedging as an important mechanism of physical weathering, itself leading to further cooling by its synergy with chemical weathering. The proposed influence of the onset of seasonality and intensified frost wedging in mountains at a given surface temperature and steady state chemical weathering flux (at T, W) on chemical weathering intensity (W = (V/V)(Ao/A), i.e., the silicate C sink (flux/unit land area) is shown in figure 8-4. Note that a transient increase in Wis expected, which since the Archean would not last more than 107 years, given the likely pCO2 level in the atmosphere/ocean system (Sunquist 1991). A new steady-state pCO2 and temperature (T) is reached at W, if A and V do not change significantly in this interval. A similar pattern might be expected with sudden events in biotic evolution, such as the emergence of higher plants, but their spread over the land surface probably took more than 107 years.

If the weathering intensity W is plotted versus surface temperature, then an activation energy can be computed for the temperature range of 70 to 60°C (corresponding to arange of about 3.5-2.6 Ga). Amodel of (V and A) variation and a ratio ofbiotic enhancement ofweathering factors for the two times (i.e., B35Ga/B26Ga) is assumed (figure 8-5; note that the temperature decrease from B to A [now], with a relatively modest decrease in weathering intensity, has been previously explained as a consequence of the increase in biotic enhancement of weathering). A marked decrease in W from 3.5 to 2.6 Ga is consistent with sedimentologic evidence already cited. For the preferred model "b" (see Appendix for definition), with B35 Ga/B26Ga = 0.5 (note that precipitation is assumed to saturate at about 55°C), the computed E = 58 kJ/mole, similar to the watershed value of White and Blum (1995).

FIGURE 8-4.

The effect of decreasing global surface temperature on the chemical weathering intensity (W). Frost wedging/climatic seasonality occurs at T, generating a transient change in W with steady state reestablished at W.

FIGURE 8-4.

The effect of decreasing global surface temperature on the chemical weathering intensity (W). Frost wedging/climatic seasonality occurs at T, generating a transient change in W with steady state reestablished at W.

I claim here no uniqueness in the model result, only that plausible activation energies of weathering can be computed. Obviously, weathering intensities and other factors need to be better established. One possible approach to inferring weathering intensities might be via studies of clastic sediments. For example, Heins (1995) has claimed that the stability of mineral interfaces in clastic sediments is sensitive to temperature and precipitation.

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