been warmed so gently that much of the ocean would have frozen. But the early atmosphere was rich in carbon dioxide, a strong absorbent of the infrared wavelengths that conduct heat reflected from the surface. The abundant carbon dioxide would have created the effect of an enhanced greenhouse, sheltering the fragile surface of the planet from a complete freeze that would have been difficult to reverse.

Development of the modern atmosphere from those primeval conditions accompanied biological evolution. The oldest carbon compound deposits, stored in limestones of the rock reservoir, were produced by organic precipitation. Those early organisms consumed carbon dioxide in photosynthesis, released free oxygen to the atmosphere, and left their carbonaceous skeletons to accumulate as limestone on the ocean floor.

The record of change in the oxidation state of the atmosphere-ocean system is fragmentary, and interpretation is necessarily speculative. Even so, a case can be built on a collection of evidence that suggests a gradual buildup toward saturation. There are remnants of detrital grains that could not have resisted oxidation in an oxygen-rich atmosphere. These grains are preserved in sediments that are at least 2.0-billion-years old, and the grains themselves may be 2.5-billion-years old. Also, abundant iron ore deposits chemically precipitated occur in sediments deposited between 2.8 billion and 1.8-billion-years ago. These ores are richly oxidized compounds that when formed would have acted as an oxygen sink; that means newly freed oxygen would have had to react with such exposed minerals before any atmospheric buildup could have become available for biological functions. Furthermore, the oldest uncontestable evidence indicating biological use of oxygen dates to 2.8-billion-years ago. Finally, the first evidence of persistent oxygenation of surface environments occurs in rocks that formed 2.2-billion-years ago.

The interpretation of much of this evidence is disputable, and active research, including a search for stable carbon isotope variations, is attempting to clarify the picture. Recent isotopic investigations emphasize the role of the carbon cycle in modifying the environment. These investigations focus on another episode that witnessed the burial of large quantities of organic carbon in sediments between 900 million and 550-million-years ago—a period that preceded an explosion of multicellular organisms throughout the world's oceans. Complementary release of oxygen from carbon dioxide would have driven atmospheric oxygen concentrations up to present levels or higher. This interpretation supports those evolutionary biologists who speculate that multicellular life forms could not develop until the atmospheric partial pressure of oxygen became high enough to diffuse the element across multiple cell layers. Paleontological evidence of prolific multicellular life fills the stratigraphic record immediately after the isotopic evidence of increasing oxygen.

Projecting the carbon isotope record forward to 550-million-years ago produces evidence of numerous smaller cyclical changes. For example, significant variations in 13C abundances have been linked to complementary shifts in the abundances of sulfates and sulfides, the oxidized and reduced forms of sulfur. It seems likely that, when larger-than-average quantities of organic material have been buried and the carbon cycle frees excess oxygen, the excess is consumed by the oxidation of sulfur minerals exposed at the surface. The oxygen release during the carbon burial event 900 million to 550-million-years ago was apparently so large that oxidation of sulfur could, at best, attenuate it.

Record of Change Between 600 Million and 150 Million Years Ago

During this period organisms colonized vastly differing environments through a variety of physical adaptations. Among the plants, ferns and conifers evolved, and animal life developed from marine invertebrates to fish, insects, amphibians, reptiles, and mammals. Interpretation of change between 600 million and 150-million-years ago profits from the richness of the fossil record and the relatively accurate reconstructions of former continental positions based in part on paleomagnetic data (see Figure 3.8). Sedimentary particles containing iron tend to settle in magnetic alignment with the Earth's magnetic field, and, similarly, iron-bearing lavas become magnetized as they crystalize while cooling. A rock magnetized in one of these ways is, in effect, a paleocompass that reveals its own orientation to the magnetic pole at the time when it formed. Reliably preserved data come from the magnetic declination, which shows directional orientation, and from the magnetic inclination, which increases at higher latitudes. These relative magnetic deviations are set into a rock body when it forms, allowing geologists to determine the orientations of ancient continents as well as their latitudinal positions.

Rock and fossil distributions can also indicate possible paleogeographies, and maps of ancient



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