Figure 1. Changes in atmospheric carbon dioxide during the last 150,000 years recorded by the carbon dioxide content of air bubbles trapped in ice that accumulated on Antarctica. (Barnola, J.-M., D. Raynaud, Y. S. Korotkevitch, and C. Lorius (1987) Vostok ice core provides 160,000-year record of atmospheric CO2. Nature 329, 408-414.)
Figure 2. Ocean color measured by satellite. In the tropical open ocean, productivity is comparatively low due to permanent thermal stratification of surface waters that limits the supply of nutrients from deeper waters. The resulting flux of biogenic material to the deep sea is also low. In contrast, in the more productive regions of the ocean, abundant phytoplankton sustain much larger sinking fluxes of biogenic material. These regions are most prominent at the eastern margins of the oceans and at the equator, where divergence of wind-driven surface currents brings nutrient-rich water from intermediate depths to the surface. Seasonally high productivity can also be found at higher latitudes due to the seasonal breakdown of upper water stratification in winter and to deep convective mixing.
Figure 3. Uranium decay series: U-238 initiates a decay series that produces thorium 230, while U-235 produces protactinium 231.
Figure 4. Boundary scavenging. Thorium 230 is rapidly scavenged, that is, adsorbed onto settling particles and removed to the underlying sediments, shortly after its formation from the decay of U-234. Because of its short residence time in the water column, very little thorium 230 can be transported laterally before removal from seawater. Protactinium 231 is less rapidly scavenged from the water column and is laterally transported towards regions of higher productivity where particle settling flux and scavenging rates are higher. As a result, the ratio of protactinium 231 to thorium 230 in sediments underlying more productive regions is higher than in sediments underlying less productive regions.
Figure 5: The protactinium 231 to thorium 230 ratio in surface sediments in the eastern equatorial Pacific. Compare the color pattern with that of area A in Figure 2.
Figure 6: The protactinium 231 to thorium 230 ratio in recent sediments deposited in the Atlantic and Indian Ocean sectors of the southern ocean around Antarctica (compare the color pattern to area B in Fig. 2) and in sediments deposited during the last glacial period.
Figure 7: The protactinium 231 to thorium 230 ratio in Atlantic Ocean surface sediments