cluding oil, coal, and diamonds. And 14C, a radioactive isotope with a half-life of 5,000 years, is the most important means for timing environmental events over the past 40,000 years. The existence of two readily fractionated stable isotopes and a single short-lived radioactive isotope, along with the preservation of carbon from a wide range of environments throughout the geological record, means that interpretation of the geochemical cycles of carbon is particularly informative. The resulting understanding of the rates of past changes allows researchers to assess ongoing changes.
The carbon cycle has played a major role in the development of the global environment. In any body of water, dead organic matter settles to the bottom where animals and bacteria have a chance to oxidize the contained carbon. But mud and the minerals produced by organisms also settle to the bottom. Their accumulation may be rapid enough to trap and bury organic material before it can be consumed by oxidation, sometimes preserving it to become fossil fuel. The abundance of fossil fuels and other organic debris in the sedimentary shell is considerable, and every atom of that organic carbon, as it was buried, left behind a molecule of O2 that was released into the surface environment.
Balancing the accounts should be possible. From the inventory of elements in sedimentary rocks, we should be able to calculate the amount of oxidizing power that the buried accumulation of organic carbon left behind at the surface—and also the timing, or history, of the accumulation. There are two problems. We cannot collect samples of all buried rocks for analysis of carbon content, and even if we could take a perfect inventory of all existing sedimentary rocks we could not account for rocks destroyed by erosion, altered by metamorphism, or subducted into the mantle.
There is another way to approach the accounting. The problem can be restated usefully by asking: What fraction of the carbon passing through the system has been buried in the form of organic material? This turns out to be a question that we can answer with the help of the two stable carbon isotopes, 12C and 13C. The 12C isotope is more abundant, amounting to 98.895 percent of all terrestrial carbon; most of the remainder is the 13C isotope. Because both isotopes are stable, their abundances have not changed throughout earth history. At any time, the isotopic composition of the carbon entering the surface part of the system—the atmosphere, biosphere, and hydrosphere—is given by the terrestrial average, but the two processes of biomass synthesis and carbonate precipitation tend to slightly separate the carbon isotopes. At present, for example, carbonate forming in the ocean contains 1.113 percent 13C, and, on average, organic material being buried in sediments contains 1.086 percent 13C.
Measuring isotopic abundances at that level of precision is not simple, but it is incomparably easier than constructing a global inventory of carbonates and organic material, and it provides a way to monitor the behavior of the carbon cycle. By calculating the abundances of buried organic and inorganic carbon from the total carbon and 13C mass balances, indications are that at present about 30 percent of the carbon passing through the hydrosphere, atmosphere, and biosphere is being buried. Characteristics of ancient carbon cycles can be similarly determined. For every interval it is necessary only to obtain globally representative carbon isotopic abundances for carbonate sediments and organic carbon.
Recognition of changes in variables such as the chemistry of the oceans, the global sea level, the configuration of ocean basins, the three-dimensional thermal structure of the ocean, and the history of marine organisms permits the description of ancient conditions, which even during the past 18,000 years have undergone remarkable transformations. On a broader scale of time, changes have been even more profound.
About 70-million-years ago, shortly before mammals inherited the Earth from dinosaurs, the oceans supported a huge population of calcareous nannoplankton. They were so abundant that their minute skeletal remains rained down on the seafloor to produce thick deposits of chalk that stand now as the White Cliffs of Dover in England and the cliffs of the Selma Chalk in Alabama. Today, photosynthesizing calcareous nannoplankton survive as very important producers in the marine food chain but have never again generated such widespread deposits of chalk; they suffered severe losses at the same time the dinosaurs met their end. Probably part of the explanation is that they never rediversified fully because other taxa took their place. Certainly another important factor is that relatively cool climatic regimes, which do not favor calcareous nannoplankton, have prevailed during the past 60-million-years. On the other hand, diatoms—silica-precipitating organisms that thrive in cold water—expanded greatly during that time. The deep-sea,