ocean waters. Climatic modeling and analysis of oxygen isotopes cannot yet produce a consensus as to whether the tropics were also warmer at that time or whether they were cooler than today, generating gentler latitudinal temperature gradients. In the near future, fossil alkenones may yield a general temperature map for the 50-million-year-old global ocean.

At any time in earth history, deep ocean water is generated from the densest water masses that develop at shallower levels and have access to the major ocean basins. The high density of these waters may result from either low-temperature or high salinity. So when deep waters are relatively warm, they probably are saline, after descending from marginal basins with high evaporation rates. When relatively warm waters occupy the deep ocean basins, thermal gradients are weak, deep-sea circulation is sluggish, and bottom waters become depleted of oxygen. In contrast, at times such as the present the dynamic descent of cool polar waters constantly replenishes the oxygen, which maintains biological respiration and the oxidation of organic and inorganic compounds. In addition, the spread of cold water masses into low latitudes scours out areas of the deep seafloor. Cores of deep-sea sediment display features that reflect these conditions. At some levels, cores contain a substantial amount of red oxidized sediment or evidence of depositional hiatuses. These particular anomalies represent intervals when cold currents from polar regions plowed through the deep seafloor, supplying oxygen or eroding sediments respectively.

A particularly interesting interval extended from about 110 million to 90-million-years ago, when huge concentrations of hydrocarbons accumulated. The organic matter that finds its way into marine sediments, and is the source of most petroleum, derives ultimately from phytoplankton. As primary producers, phytoplankton utilize solar energy to synthesize inorganic carbon sources into organic material. Their biomass fuels the marine food web, in which energy required by other organisms is produced by the metabolic oxidation of organic material. If the food web operated with perfect efficiency, all organic material would be oxidized and the underlying sediments would contain no organic carbon. Where high amounts of organic carbon are found in sediments, the food web must have operated at low efficiency, allowing organic material to escape oxidation. There are two possible explanations for the vast reservoirs of organic carbon remaining in the 100-million-year-old deposits. Either the conditions of the water column or sediments were not favorable for the growth of efficient recyclers, or the production rates were so high that the food web was unable to utilize all of the supply. Close study of the sediments from that interval favors the first alternative, although the second might have been important in places. Even though large amounts of organic carbon were preserved, production rates were low in comparison to present values.

Because the organisms most efficient at reoxidizing organic carbon require free oxygen, this combination of high preservation despite low production could characterize a deep ocean containing little or no dissolved oxygen. Sediments from this period show fine laminations that are completely undisturbed by trails or burrows, indicating that oxygen-dependent animals were unable to colonize the deep environments despite an abundance of available food. On the basis of this interpretation, episodes of global deposition of organic-carbon-rich sediments have been termed oceanic anoxic events.

The climate prevailing 100 million and 90-million-years ago was warmer than today, and seawater would thus have held less oxygen. Moreover, the positions of the continents were different at that time. North and South America were in the early stages of separation from Europe and Africa; the Atlantic Ocean already had some deep narrow basins but widened as the Mid-Atlantic Ridge created new seafloor and moved the continents apart. At the Mid-Atlantic Ridge and similar active spreading centers, free oxygen reacted with sulfides and other oxidizable materials that were being introduced at unusually high rates.

That warm world may have been completely free of ice caps; the north polar region is known to have been covered with lush forests. The inventory of water was almost entirely in liquid form, so sea level was elevated. Two additional factors made the high stand even more pronounced. First, water expands when it is warmed, and the average temperature of seawater was much higher than at present. Second, the volume of the ocean basins decreased because the rapid extrusion of new hot crust at spreading centers meant that large areas of ocean floor rose upward. The resulting elevation of the sea level produced maximal flooding of the continents. In some localities even the oxygen-poor deeper waters extended onto continents, depositing marine sediments rich in organic carbon. When such sediments became deeply buried and were heated to temperatures of 90° to 120°C, they became important sources of petroleum. In summary, a powerful combination of global conditions discouraged the availability of oxygen in deep ocean wa



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement