The global carbon cycle: a test of our knowledge of earth as a system.

Motivated by the rapid increase in atmospheric CO2 due to human activities since the Industrial Revolution, several international scientific research programs have analyzed the role of individual components of the Earth system in the global carbon cycle. Our knowledge of the carbon cycle within the oceans, terrestrial ecosystems, and the atmosphere is sufficiently extensive to permit us to conclude that although natural processes can potentially slow the rate of increase in atmospheric CO2, there is no natural "savior" waiting to assimilate all the anthropogenically produced CO2 in the coming century. Our knowledge is insufficient to describe the interactions between the components of the Earth system and the relationship between the carbon cycle and other biogeochemical and climatological processes. Overcoming this limitation requires a systems approach.

Aluminum in seawater: control by biological activity.

The distribution and concentration of dissolved aluminum in a vertical hydrographic profile in the Mediterranean Sea near Corsica are controlled by biological activity. The concentrations of dissolved silica and aluminum covary in the profile and exhibit minima coincident with the seasonal thermocline, a nitrate minimum, and an oxygen maximum. These observations support the hypothesis that the silicon and aluminum cycles in the oceans are linked through the activity of diatoms.

Quartz: synthesis at Earth-surface conditions.

Quartz has been crystallized directly from seawater at room temperature. This is the first time that identifiable quartz has been synthesized in aqueous solution at earth-surface conditions without the aging of an original amorphous precipitate. The concentration of dissolved silica in equilibrium with quartz at 20 degrees C and 1 atmosphere is 4.4+/-0.3 parts per million, a value in agreement with the theoretical value obtained by a constant heat capacity fit of higher temperature equilibrium data. This experiment confirms the results of petrologic investigations which suggest that quartz precipitates directly from aqueous solution during chemical weathering and early diagenesis.

Sedimentary rock types: relative proportions as a function of geological time.

Proportions of sedimentary rock types remaining today differ from period to period. These differences may be chiefly the result of differential rates of deposition and erosion of the various components of the rocks. Lower percentages of limestones and evaporites in Precambrian rocks than in post-Precambrian rocks probably represent selective loss of these more easily removable components from the original deposits.

Silica in sea water: control by silica minerals.

Silicate minerals typical of those carried in the suspended load of streams release silica to silica-deficient sea water and abstract silica from silicaenriched sea water. Experimental rates of release and uptake permit the conclusion that the suspended solids carried into the oceans by streams are a major control of the concentration of silica in the ocean.