the continents through the repeated partial melting processes that fractionate the mantle. 68Sr is a stable nuclide that remains more strongly linked to the mantle. The 87Sr/86Sr ratio of seawater has varied with time (Figure 3.9) over the past 500-million-years. The variation can be attributed to changes in the relative influence of erosion from the continents, which promotes concentration of the rubidium daughter 87Sr in seawater, and of volcanism beneath the sea, which samples the mantle and promotes the concentration of 86Sr. With the fall of sea level over the past 100-million-years, the continental contribution has generally been rising. A serendipitous application of the change in 87Sr/86Sr of marine shells is that stratigraphers are using the ratio to date sedimentary rocks.
A more familiar example involves the cycling of carbon and oxygen. The concentration of CO2 in the atmospheric reservoir has risen rapidly in recent decades. These concentrations are usually in chemical equilibrium with dissolved CO2 and bicarbonate ions in the ocean waters and with calcium carbonate in the oceanic sediment reservoir. The cyclical transfer of CO2 through these three reservoirs appears to be considerably perturbed by the rapid rise in the atmospheric component; model simulations indicate that it could take hundreds of years to restore equilibrium to this subsystem.
The geological record offers information about a past scenario that involved disequilibrium among these same three reservoirs. About 100-million-years ago there was about twice as much underwater volcanism as there is at present because seafloor spreading was more rapid and the total volume of the oceanic ridges was about twice what it is now. This condition tended to perturb the CO2 cycle in two ways. The extra volcanism added CO2 to the ocean and the extra volume of young hot rock on the ocean floor displaced the oceanic waters so that they flooded the continents to an exceptional extent, thus reducing the area of rock available for the weathering that extracts CO2 from the atmosphere. There is a strong likelihood that the CO2 content of the atmospheric reservoir rose in response to these perturbations. The evidence indicates that climatic conditions were warmer than today, as the greenhouse principle would suggest.
A comforting general observation is that feedback mechanisms will come into play to ameliorate any extreme consequences of perturbations to cyclical processes. The shallow-waters of the flooded continents (some 100-million-years ago) were an ideal environment for the deposition of limestone, and the process of limestone deposition pulls CO2 out of the atmosphere and processes it through the oceanic waters into the rock reservoir. This feedback system would have brought the cycle back to a more normal state.
The geochemistry of carbon is uniquely exciting, primarily because of carbon's role in life. Carbon also forms economically important resources, in-