IV.

Dating Techniques

Vast quantities of microscopic plants and animals live near the ocean surface. When they die, their remains drift down to the ocean floor and accumulate in thick layers of sediment. When studied in sedimentary core samples, which can represent many millions of years of deposits, they provide a detailed and continuous history of the earth’s environmental changes. The record is particularly informative for the most recent 2 million to 5 million years, during which major fluctuations in global climate have occurred. Successive ice ages can be traced by the relative scarcity or abundance of the shells of warm-water and cold-water diatoms in various layers of a sedimentary core, as the organisms migrated to more hospitable habitats. Geochemical records of these same fluctuations are revealed by determining the ratios of two isotopes of oxygen, oxygen-16 and oxygen-18, in the shells of foraminifera. The ratio of the two isotopes is proportional to the temperature of the water in which the organism grew; hence, a temperature record is preserved when each organism dies and its shell drifts to the ocean floor. The records of climatic fluctuation found in ocean-floor sediments are much more continuous than similar records on land; they also lend themselves to worldwide correlation. The absolute ages of climatic changes can be determined by correlating the evidence of temperature changes with radioactive-dating techniques (see Chronology; Dating Methods; Radioactivity). Thorium-230 dating is applicable to samples younger than 300,000 years, potassium-argon dating to samples in the range of 75,000 years, and carbon-14 dating to samples younger than 40,000 years. Several other radioactive dating techniques are available for samples of very recent age. A geophysical dating method is also commonly used; it determines the magnetic orientation of sediment particles, since it is now known that the earth’s magnetic field has reversed its orientation several times in the past few million years (see Earth: The Core and Earth's Magnetism). Such dating techniques indicate that the ocean basins are no older than 200 million years.