7-Introduction

There were four eons in Earth’s history (Figure 7-1). The Hadean Eon experienced periodic asteroid bombardment although some periods were probably habitable for microorganisms. The Archaean Eon was anoxic and was suitable for the evolution of archaebacteria and prokaryote algae even though there was an orange haze atmosphere. Although the sun was 20% cooler, there was liquid water on Earth. The skies cleared when algae finally oxidized the atmosphere at the beginning of the Proterozoic Eon. There is controversy about the climate at the beginning and end of the Proterozoic. Why were there equatorial glaciations? Oxygen rose dramatically at the end of the Proterozoic, and the climate became habitable for animals approximately 580 Ma, which led to the Avalon and Cambrian explosions of animal life. The Phanerozoic Eon was the age of animals and had three eras: Paleozoic (invertebrates and fish), Mesozoic (dinosaurs), and Cenozoic (mammals). The 550 million years of the Phanerozoic Eon had a relatively constant temperature, breathable atmosphere, moderate seasons, and a pleasant diurnal cycle.

Nicolaus Steno introduced the concept of geologic time in 1669. He realized that rock layers above other layers were younger than the deeper layers and that the bones discovered in rock layers were the remains of former living organisms. This might seem obvious to us but it was revolutionary at the time. Many people thought that the bones somehow appeared in the rocks but were not the remains of living organisms. Steno thought that the rock layers provided a history of previous events although he erroneously believed that the Earth was only thousands of years old, as with everyone else at the time.

An Italian geologist, Giovanni Arduino (1714 - 1795), defined periods in Earth's history based on layers of rock in the Italian Alps. He classified geologic formations as Primary (volcanic and metamorphic rock at base of Alps), Secondary (hard sedimentary rocks), and Tertiary (softer alluvium). The modern geologic time scale in Figure 7‑1 replaced Arduino’s classification scheme.

In 1819, William Smith realized that fossils could date geologic layers. This provided a universal geologic time scale. He found a great geologic time clock in England in the tipped geologic formations, which exposed eons of geologic strata in England (Figure 7‑2). Based on the fossils, he drew a geologic map of England, which was the first of its kind.

Section 7-2 describes the climate of the Archaean Eon. Archaebacteria and algae thrived in its anoxic orange haze atmosphere. Algae blooms in sea caused periodic surges in oxygen and left behind banded iron formations. One of the major problems in climatology is the Faint Young Sun paradox. The Sun was 20% dimmer in the early Archaean, and there should only have been frozen water; however, there was liquid water, which is the paradox. Section 7-2 also describes basic principles of the atmospheric energy balance and global warming.

Section 7-3 describes the Proterozoic Eon, which began with the Great Oxygenation Event, 2.45 Ga. After this event, the skies were clear, and there was a small amount of oxygen in the atmosphere, which led to the development of glaciers. A major problem in climatology is the fact that the geologic evidence for glaciers at that time was at the equator. Some have attributed the equatorial glaciers to a global layer of ice that covered the entire Earth. This is called the snowball Earth hypothesis. Others attribute the equatorial glaciers to a high angle of obliquity of earth’s axis, meaning that earth’s axis was pointing toward the sun, the HOLIST hypothesis. This section describes the arguments for and against these two hypotheses.

Section 7-4 describes the Phanerozoic Eon. Although there were sporadic mass extinctions due to asteroids, magmatic events, and alignment of continents, the 550 million years of our present eon has generally been hospitable to animal and plant life and the development of the complex ecosystems. Section 7-4 focuses on the perturbations of our climate by solar and orbital cycles, as well as greenhouse gases.

Section 7-5, which is the optional Excursus, assesses Moses’ description of the fourth day of creation and compares it to a possible shift in the angle of obliquity of Earth's axis.