The Story of Earth by Robert M. Hazen

Author:Robert M. Hazen [Robert M. Hazen]
Language: eng
Format: epub
Published: 0101-01-01T00:00:00+00:00

Testimony of the Rocks

Evidence for the Great Oxidation Event comes from a growing catalog of observations on rocks and minerals that date from a vast chunk of Earth’s history—roughly 3.5 to 2.0 billion years ago. On the one hand, many rocks older than 2.5 billion years contain minerals that are easily destroyed by the corrosive effects of oxygen, suggesting an oxygen-free environment prior to that time. Geologists find unweathered and rounded pebbles of pyrite (the iron sulfide also known as fool’s gold) and uraninite (the commonest uranium mineral) in ancient streambeds—places where such minerals would quickly corrode and break down in today’s oxygen-rich surface environment. Such ancient sandy layers also have a telltale chemistry—unusually concentrated in oxygen-avoiding elements like cerium, while strikingly deficient in others like iron compared with today’s soils. These chemical quirks further evince an atmosphere devoid of oxygen.

By contrast, rocks younger than 2.5 billion years contain many unambiguous signs of oxygen. Between 2.5 and 1.8 billion years ago, massive deposits of iron oxides called banded iron formations were deposited in staggering abundance. These distinctive, dense accumulations of alternating black and rusty red layers hold 90 percent of the world’s known iron ore reserves. Manganese oxides also suddenly appear, as thickly stratified deposits that now provide the world’s chief repositories of manganese ores. Hundreds of other new minerals—oxidized ores of copper, nickel, uranium, and more—also appear in the rock record for the first time after the Great Oxidation Event. Yet in spite of this expanded mineralogical repertoire, some scientists remained unconvinced that the Great Oxidation Event was really an event at all. Perhaps there was just a slow and steady increase in atmospheric oxygen. Perhaps the spotty, eroded rock record is incomplete and misleading.

The smoking gun of the Great Oxidation Event came from an unexpected source—remarkable recent data on the isotopes of the common element sulfur. The 1990s saw dramatic increases in the resolving power and sensitivity of mass spectrometers, which are the workhorse analytical instruments of the isotope world. The new generation of mass spectrometers allowed scientists to analyze smaller and smaller samples, even microscopic mineral grains or individual living cells, with higher and higher precision. Sulfur, one of life’s essential elements, proved a particularly tempting target for study, as there are four stable sulfur isotopes in nature: sulfur-32, -33, -34, and -36. All of these isotopes have sulfur’s requisite sixteen protons in the nucleus, but the number of neutrons varies from sixteen to twenty.

The distribution of sulfur isotopes is usually predictable on the simple basis of mass. All atoms wiggle, but less massive isotopes wiggle more. Consequently, in any chemical reaction light isotopes are more likely to jump about than heavy ones. This selective process, called isotope fractionation, occurs anytime a collection of sulfur atoms experiences a chemical reaction, whether in a solid rock or a living cell. In the case of sulfur, an isotope of mass 32 will typically fractionate more than an isotope of mass 34 or 36. What’s more, the fractionation ratio is

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