Continental Growth

The continental crust is especially diverse and heterogeneous, and its formation is less understood than that of the geologically relatively simple oceanic crust. In contrast to the seafloor which is primarily made ofbasalts it is granitic in composition. Two different hypotheses have been suggested to explain the evolution of the continental crust. The first proposes that the present continental crust formed very early in the Earth's history and has been recycled through the mantle in steadily decreasing fashion such that new additions are balanced by losses, resulting in a steady-state system. The return of the continental material to the mantle and its replacement by new younger additions reduces its mean age, both of which keep the mass of the continents constant. The second hypothesis proposes crustal growth throughout geological time without recycling into the mantle.

Figure 3 summarizes the different classes of continental growth scenarios. The continental area can be assumed to be fixed at its present value, can be grown linearly with a constant growth rate, or linearly with a delay. Geological investigations of the best-studied regions, North America and Europe, which formed a single land mass for most of the Proterozoic suggests that the continental crust grows episodically, and it is concluded that at least 60% of the crust has been replaced by the late Archaean. This data-based description is clearly more realistic than the theoretical models. The continental area is directly related to the weathering process. A larger continental area leads to proportionally higher weathering rates.

(C)

(b)//-'/

- / / / {a)

(d)/ /

-4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Time (Gyr from present)

Figure 3 Normalized continental area as a consequence of the following continental-growth scenarios: (a) delayed growth, (b) linear growth, (c) constant area, (d) growth derived from geological investigations. From Condie KC (1990) Growth and accretion of continental crust: Inferences based on Laurentia. Chemical Geology 83: 183-194.

Atmospheric Carbon Dioxide and Climate

The climate of the Earth is governed by the energy-balance equation between incoming and outgoing radiation and depends on the concentration of greenhouse gases and the solar luminosity. CO2 and H2O are the most abundant greenhouse gases in the atmosphere. The partial pressure of H2O, pHlO, can be expressed as a function of temperature and relative humidity using the Clausius-Clapeyron equation. Therefore, the global mean temperature of the Earth depends primarily on the CO2 concentration in the atmosphere and the solar luminosity. During the Earth's history the luminosity of the Sun has increased at a rate of about 10% per Gyr and will increase in the future (up to the next 5 Gyr) at approximately the same rate.

The efficiency of weathering processes and the biosphere productivity strongly depend on the partial pressure of atmospheric CO2, pCO. On geological time-scales atmospheric carbon content is always in chemical equilibrium with the carbon content in the ocean. In the case of a constant density profile of carbon dioxide in the atmosphere the distribution of carbon can be calculated from the condition of equal partial pressures of CO2 at the interface between atmosphere and ocean.

Biotic Enhancement of Weathering

The rate of weathering is greatly amplified by a range of biological processes that respond to photosynthetic productivity. First, there is an increase of soil CO2 partial pressure due to respiration of soil organisms and due to the respiration from the roots of vascular plants. Furthermore litter is decomposed by microorganisms, through providing organic matter for the formation of humic acids, and through mycorrhizal and fUngal digestion. This can be expressed by a direct dependence of weathering on biological productivity by a factor ß mediating the weathering rate, Fweath:

where aH+,s denotes the activity of fresh soil water aH+ is the corresponding present-day values. The activity aH+,s itself is a function of the surface temperature and the CO2 partial pressure in the soil, psoil, where the equilibrium constants for the chemical activities of the carbon and sulfur systems have been taken into account. psoil depends on the terrestrial biological productivity n, the atmospheric CO2 partial pressure, pCO, and their corresponding (long-term mean pre-industrial) present values pioib n*, pC o2:

PcoA | PCO2

It is assumed that p*oil = 10pCo2. The parametrization of weathering that considers only the variation of soil carbon dioxide levels gives a biotic amplification of weathering of 1.56 for the present state, which is a significant underestimate. Furthermore vascular land plants increasing the partial pressure of CO2 in the soil appeared on Earth only 0.35 Gyr ago. The total weathering amplification due to land life is at least a factor of 10. This indicates that much of the observed biotic amplification of weathering is due to processes other than increased soil pCO/ This is considered by the prefactor ft that reflects the biotic enhancement of weathering by the biosphere types, i:

The factor fti denotes the specific biotic amplification of weathering, ni the specific biological productivity, and n* the respective present-day value of biosphere type i. Biotic enhancement of weathering is only affected by complex multicellular life (ft 1 = ft2 = 1, ft3 > 1). Complex multicellular life contributes about 10-100 times more to the biotic enhancement of weathering than primitive life.

Biological Productivity

The biological productivity n is the amount of biomass that is produced by photosynthesis per unit time. In reality, n is a function of various parameters as water supply, photosynthetically active radiation (PAR), nutrients (N, P, etc.), atmospheric CO2 content and surface temperature:

where nmaX;/- is the maximum productivity of biosphere type i. For simplification biological productivity should depend only on the mean global surface temperature, Ts, and on the CO2 partial pressure of the atmosphere, pco2-Both variables are affected by the global carbon cycle. The qualitative dependence on CO2 partial pressure and temperature is shown in Figure 4. The function for the temperature dependence, fTsi, can described by a parabola and the function for the pco2 dependence is an increasing function with a saturation level. A minimum CO2 atmospheric partial pressure, pmin/ allowing photosynthesis is necessary for all biosphere types. A biosphere based on C3 photosynthesis has a minimum value of 150 x 10"6 bar, while C4 photosynthesis results in a value of 10"5 bar. The interval [Tmini ...Tmaxi] is the temperature tolerance window for the biosphere. If the global surface temperature is inside this window a global abundance of biosphere type i is possible. It must be emphasized that this window is related to the mean global surface temperature. Latitudinal differences in temperature decrease as global mean temperature increases and might vanish for T>30°C. Table 1 contains estimated

Pmin

PCO2(ppm)

Figure 4 (a) The dependence of biosphere productivity on CO2 partial pressure in the atmosphere. (b) The dependence of biosphere productivity on global surface temperature for prokaryotes, eukaryotes, and complex multicellular life.

Surface temperature (°C)

Figure 4 (a) The dependence of biosphere productivity on CO2 partial pressure in the atmosphere. (b) The dependence of biosphere productivity on global surface temperature for prokaryotes, eukaryotes, and complex multicellular life.

parameter ranges for the prokaryotic, eukaryotic, and complex multicellular biosphere, respectively.

Carbonate Precipitation

Weathering products are transported to the ocean and, depending on the solubility product, precipitated to the ocean floor. Because there exists a calcium carbonate compensation depth level in the present ocean, carbonates can precipitate only in the shallower regions such as around the mid-ocean ridges and the continental shelves. A total of 8% of the Earth's area is covered with ocean less shallow than 103 m. The change of equilibrium concentrations of Ca and Mg in water results in a change of solubility of carbonates in ocean water. Furthermore, oceanic photosynthesis provides an additional way to sequester carbon on the seafloor.

Hydrothermal Reactions

Due to hydrothermal reactions CO2 dissolved in the oceans reacts with fresh mid-ocean basalts and precipitates in the form of carbonates to the ocean floor. Therefore it is an additional sink in the atmosphere-ocean reservoir. The hydrothermal flux is proportional to the production of fresh basalt at mid-ocean ridges, which in turn is proportional to the areal spreading rate. The area around the spreading centers is likely to be one of the most habitable environments for a subsurface biosphere. It is porous and characterized by extensive hydrothermal circulation. Such hydrothermal systems provided a site for the rapid emergence of life through a sequence of abiotic synthesis.

Kerogen

Kerogen comprehends the dispersed, insoluble, organic carbon in rock including coal and mineral oil deposits. It is probably the least important reservoir from the point of view of carbon cycling because it is relatively inert. However, there are processes of kerogen weathering and kerogen formation. Kerogen is formed from ^0.1% of the dead biomass that is not returned to the atmosphere through litter decomposition. The present size of the kerogen reservoir of 10-20% of the surface reservoirs is obviously the net result of these processes. The main constraint for the reservoir size results from isotopic geochemistry. Since kerogen is isotopically light due to its biological origin it sequesters preferentially 12C, while the continental carbon reservoir must get enriched in the heavier isotope 13C. The isotopic signature is measured as a difference to a standard sample:

3C/12C)sample

Table 1 Parameter estimates for the three different life forms (prokaryotes, eukaryotes, complex multicellular life)

Life form Prokaryotes Eukaryotes Complex multicellular

Table 1 Parameter estimates for the three different life forms (prokaryotes, eukaryotes, complex multicellular life)

Life form Prokaryotes Eukaryotes Complex multicellular

Tmin (0C)

2

5

0

Tmax (0 C)

100-130

45-60

30-45

Pmin (10~6 bar)

10a-150b

10a-150b

10a-150b

ß

1

1

4-20

aFor C4 plants. bFor C3 plants.

aFor C4 plants. bFor C3 plants.

The kerogen has a £13C value of 20%o. The iso-topic composition of the two carbon reservoirs kerogen and continental crust might have been constant over the last 3.5 Gyr. The ratio of kerogen carbon to continental carbon would also have been constant at a value of 1:4 taking into account the isotopic signature of the mantle carbon of 613C--5%o.

Atmospheric Oxygen

The evolution of the atmospheric partial pressure of oxygen, pOcan be derived from the evolution of the kerogen pool Cker, that is, the long-term deposition of reduced organic carbon. Between about 2.2 and 2.0 Gyr ago there was a global oxidation event in which atmospheric po2 rose from <0.0008 to >0.002 bar. Under the assumption that before 2.2 Gyr all oxygen had been chemically bound it is possible to make the following simple estimate:

where pO2 is the present atmospheric O2 level and Cker is the size of the present kerogen pool.

Coevolution of the Biosphere-Geosphere System

The feedback between the biosphere and the surface reservoirs of carbon leads to several bifurcation points in Earth's history. In particular the evolution of the climate is affected by the change in CO2 concentration in the atmosphere. Atmospheric CO2 concentration is regulated by biologically mediated weathering processes and is driven by an increase in solar luminosity, continental growth, and lowering mantle temperatures. The decline of mantle temperature is causing a decrease in the spreading rate with lower outgassing at mid-ocean ridges.

Evolution of the Climate

Figure 5 a shows the results for the evolution of the mean global surface temperature (solid line). The figure has been derived from a coupled model of the global carbon

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Time (Gyr from present)

Figure 5 (a) Evolution of global surface temperature (solid line). The dashed line denotes a second possible evolution path triggered by a temperature perturbation in the Neoproterozoic era. (b) Evolution of the cumulative biosphere pools for prokaryotes, eukaryotes, and complex multicellular life. From Von Bloh W, Bounama C, and Franck S (2003) Cambrian explosion triggered by geosphere-biosphere feedbacks. Geophysical Research Letters 30(18): 1963-1967.

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 Time (Gyr from present)

Figure 5 (a) Evolution of global surface temperature (solid line). The dashed line denotes a second possible evolution path triggered by a temperature perturbation in the Neoproterozoic era. (b) Evolution of the cumulative biosphere pools for prokaryotes, eukaryotes, and complex multicellular life. From Von Bloh W, Bounama C, and Franck S (2003) Cambrian explosion triggered by geosphere-biosphere feedbacks. Geophysical Research Letters 30(18): 1963-1967.

cycle including the biosphere. The modeled surface temperature curve is in good agreement with the 18O chert thermometer. According to these data, the ocean surface water has cooled from 70 ° C (±15 ° C) in the Archaean to the present value. This is caused by the growth of the continental area increasing the weathering processes and decreasing spreading rates lowering CO2 outgassing at mid-ocean ridges. There was a drop in temperature 0.54 Gyr ago due to an increase in weathering rates caused by the first occurrence of complex life. After that event temperatures have roughly stabilized around the optimum growth temperatures for complex life. In the future the global surface temperature will rise because the increase in solar luminosity cannot be balanced by intensified weathering rates.

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