Mineral duEs To PAsT cLIMATE

Like tree rings, ice cores, oxygen isotopes, and coral, minerals are another proxy used to reconstruct past climates. They can leave specific clues about not only the environment they were formed in but the ancient climate as well. Minerals react with both water and the atmosphere by being shaped and physically and chemically weathered. The most useful are those that exist within specific environmental settings, such as arid areas, tropical areas, and polar areas.

According to Dr. Vivien Gornitz of NASA, "Minerals furnish important clues about ancient climates. The most useful are those deposited under relatively narrow climatic ranges or within specific environmental settings." When geologists identify specific mineral deposits, the required conditions for their formation can tell a lot about the climate at the time of the formation. In arid areas, for instance, evaporite minerals are common. They form by the evaporation of sea-water; evaporation of lakes in narrow, closed basins; and under very hot and dry conditions. If the minerals are in an enclosed basin (with no rivers flowing out), then as the water slowly evaporates, salts will begin to precipitate out in a certain order.

Beginning with carbonates, they precipitate to sulfates and end with chloride salts. By identifying the stage to which they are precipitated, assumptions can be made about the climate of the area. Common evap-orite minerals include halite (rock salt), gypsum, borax, anhydrite, and nitrates. An example of a major evaporite deposit is the Bonneville Salt Flats located in the Great Basin province in western Utah. Located in a closed basin, the world-famous salt flats are a remnant of an ancient lake, Lake Bonneville, that existed during the last ice age. Gypsum and anhydrite are common, usually found in desert playas and arid coastal tidal pools.

Ancient sand dunes are other evidence of past arid climates. Fossil desert sands can contain information about wind direction and severity because sometimes wind ripples and cross-bedding evidence is preserved in fossil sand dunes.

Scientists at NASA also look for clues to past climates in soils and sediments. For example, the specific type of clay and its relative abundance is related to climate. In warm, humid climates in which chemical weathering is significant, silica is leeched out of the soil profile, leaving the soils with high concentrations of alumina.

Chlorite and illite, two other types of clay, are found in soils subjected to mechanical weathering processes. They can occur in both hot and dry climates and colder climates. By analyzing oxygen isotopes in clays sampled from ocean sediment cores, glaciation episodes can be studied.

When evidence of chemical weathering is present in soils and bauxite is the predominant mineral, it is evident that the climate at the time of formation was humid, wet, and tropical. These areas also have concentrated amounts of aluminum minerals such as iron oxides, kaolinite, and gibbsite.

NASA has also uncovered information about the Earth's ancient atmosphere. The clues lie in the fact that iron can occur in several oxidation states. Under the conditions of the Earth's present atmosphere, pyrite (fool's gold) oxidizes rapidly because of the abundance of oxygen. In ancient times, however, the atmosphere was not so oxygen rich, which caused pyrite to form well-rounded grains. These deposits have been found in sediments dated at 3 billion years old in the Wit-watersrand basin in South Africa. This tells scientists that the Earth's atmosphere during the Archean eon (more than 2.5 billion years ago) had much lower oxygen levels than it has today.

Mineral deposits also reveal past cold climates. The presence of ikaite, a mineral that occurs at temperatures up to 45°F (7°C), indicates an environment in which near-freezing water temperatures existed at one time.

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