Paleosols

Paleosols are ancient soils that have been buried. Although natural acids (largely dissolved carbon dioxide) are almost entirely supplied by the atmosphere during modern soil formation, that does not seem to have been the case with all paleosols, particularly the most ancient paleosols and those that are associated with ore deposits. Paleosols that were produced by ancient atmospheric gases may record the environmental conditions of the ancient earth.

Ancient Soils

Soil is one of the best-known, yet least precisely defined, geologic entities. Many definitions would not exclude beds of graded sediment; other definitions would not exclude sediment that has experienced only mild physical and chemical alteration. Additionally, many soil science textbooks restrict “soil” to that lying within the depth of plant roots, but such a definition is inappropriate for Precambrian and Early Paleozoic soils, which formed before the evolution of rooted plants.

The rock record of the earth covers the past 4.28 billion years (out of a total earth history of 4.6 billion years). Throughout the known rock record, soil formation (weathering) has involved more energy than other geologic processes, such as mountain building. Mountain building is driven by the internal energy of the earth (for example, by natural radioactivity), whereas soil formation is driven by solar energy and by chemical reactions between acidic atmospheric gases and exposed rock. Solar energy fuels photosynthetic plants, which concentrate the most abundant atmospheric acid (carbon dioxide); subsequent decay of these plants releases concentrated carbon dioxide into soil waters, which thereby become acidic and dissolve minerals. The downward solar flux of energy, which enhances weathering, currently is about 7,500 times greater than the upward flux of internal energy. The internal energy of the earth has progressively decreased with time as radioactive elements decay; solar radiation has progressively increased. In addition, the input of solar energy has exceeded the output of internal energy by at least a factor of 1,000 since the beginning of the rock record.

The collective volume of soils produced throughout earth's history may have been comparable to the present volume of the continental crust, but only an insignificant volume of these soils has become preserved by burial within the crust. Paleosols now constitute a smaller proportion of continental crust than most other well-known rock types. The proportion of paleosols appears to be particularly small in the oldest rocks. In these rocks, the preserved portion is so small and the conditions for preservation of paleosols appear to have been so peculiar that it is dangerous to make sweeping interpretations of ancient earth environments based on paleosols.

Paleosols and Sedimentary Rock

Virtually all old soils have been eroded to become sediment rather than buried to become paleosols. Intense weathering causes a high proportion of rock to dissolve, and the remnant soil becomes rich in insoluble elements such as aluminum. Erosion of this soil produces aluminum-rich sediment, which usually accumulates in the shallow ocean as a clay-rich rock (mudrock), such as shale. Sedimentary rocks, therefore, may be environmental indicators, and the vastly greater volume of sedimentary rock has resulted in most interpretations of ancient environments coming from study of sedimentary rock rather than from study of paleosols. Environmental studies of paleosols are less constrained than those of sedimentary rock, however, because the unweathered parent of the weathered material (soil) is observable beneath a paleosol, whereas the parent for a weathered sediment generally cannot be deduced with certainty since the parent rock for any given sediment may lie thousands of miles from where the sediment accumulates—for example, the Andean Mountain parent for much of the Amazon deltaic sediment.

Paleosols are known from all major divisions of earth's history, from the Archean eon to the present epoch. The proportion of paleosols to other rock types roughly increases with time, consistent with the theory of continental growth through earth's history; an increase in the area of exposed continents would lead to an increase in the volume of soil. Although the area of exposed continents probably was smallest in the Archean eon, well-preserved Archean paleosols are known from Canada and from South Africa, where Archean continental crust never has been so deeply buried that regional metamorphism could destroy evidence of the weathering processes that produced the paleosols. The area of exposed continents apparently increased dramatically from the Archean to the Proterozoic, so Proterozoic paleosols are correspondingly more abundant.

The role that plant life may have played in the weathering of these Proterozoic soils is unclear. Plants currently concentrate so much carbon in the upper portion of a typical soil that oxidation of this carbon provides more carbon dioxide to the underlying soil than does diffusion of carbon dioxide directly from the atmosphere. Much of this carbon dioxide comes from roots, but roots did not evolve until after the Proterozoic. Plant life in Precambrian (Archean plus Proterozoic) soils was limited to bacteria. Mats of photosynthesizing cyanobacteria (also called blue-green algae) lay at the surface, and other bacteria occurred deeper in the soil. Modern soil contains abundant bacteria below the surface, but the soil surface generally is occupied by complex photosynthesizing plants—for example, trees—instead of by cyanobacteria. A partial image of Precambrian soils may be obtained by studying cyanobacterial mats that grow in environments too harsh for more complex plants—for example, on salt flats and around geysers. These environments are not only harsh but also highly variable. The chemical composition of water on a modern salt flat may vary from hypersaline to nonsaline following a thunderstorm. The water temperature around a geyser may vary by several tens of degrees within a few minutes. The ability of cyanobacteria to withstand such variations may indicate that the environmental conditions of Precambrian soils either were consistently harsh or were more variable than those of more recent soils.

Geographic Variation

Ancient paleosols are generally too scarce to study the geographic variation of soil types. Geographic variations in rainfall and temperature are the two prime variables that control the distribution of modern soils because the atmosphere mixes so rapidly that any local variation in the partial pressure of oxygen or carbon dioxide rapidly becomes globally homogenized. Molecular oxygen and carbon dioxide would also have been evenly distributed in ancient atmospheres, but their proportions of the atmosphere may have varied throughout earth history. The oldest paleosols, therefore, are studied to examine variation in the earth's atmosphere through earth history, whereas the youngest paleosols are studied to learn about geographic variation in climate. Paleosols and other indicators record dramatic variations in climates on the earth during the past 2 million years, related to the growth and melting of enormous continental glaciers.

Tropical climates produce distinct soils and, thus, distinct paleosols. Both wet and dry tropical climates characteristically produce yellow-to-red soils because of oxidation and retention of iron in the soil. Wet tropical conditions can produce soils that are so rich in aluminum that these soils may be mined profitably. In such soils, the more soluble elements have been leached away by groundwater draining lush vegetation. Decay of the vegetation produces acids that attack even quartz, leaving only aluminum-rich minerals. Aluminum-rich paleosols are among the oldest (Archean) paleosols on earth, so wet tropical conditions appear to have existed long ago, despite the fact that solar radiation should have been much smaller during the Archean eon.

Warm, dry climates may produce little soil of any kind. For example, more characteristic of the Sahara than the “sand seas” that are commonly illustrated in documentaries are vast stretches of bare rock. Caliche is a characteristic soil and paleosol produced in such a climate. Caliche generally forms by precipitation of calcium carbonate from upwardly moving groundwater as the groundwater approaches the earth's surface. The upward decrease in pressure may allow carbon dioxide to be released, just as carbon dioxide is released upon opening a bottle of soda pop. Release of carbon dioxide favors precipitation of calcium carbonate; this precipitation may be aided by evaporation of the groundwater—which points to a dry climate for the formation of an ancient caliche. A caliche paleosol generally may be interpreted to record an ancient dry climate, even if the precipitation of calcium carbonate was not related to evaporation within soil but was simply the result of release of carbon dioxide. The original excess of carbon dioxide could have been provided by the escape of gases to the earth's surface during metamorphism of carbon-bearing sedimentary rocks at great depth. The abundant infiltration of rainwater in a wet climate would dissolve calcium carbonate from soil, whatever its origin, so the preservation of calcium carbonate in a caliche paleosol generally records a dry climate, even if the precipitation of calcium carbonate were induced by deep crustal processes independent of climate.

Elemental Composition and

Interpretation of the elemental composition and mineralogy of a paleosol generally is controversial, especially for paleosols older than 544 million years (Precambrian paleosols), because paleosols potentially have experienced substantial modification (diagenesis) after burial. One of the most consistent chemical peculiarities of Precambrian paleosols is that they contain extreme ratios of potassium to sodium, unlike modern clayey soils. No known weathering process could fractionate sodium from potassium so severely. Precambrian paleosols commonly contain more than ten times as much potassium as sodium, whereas these two elements generally behave similarly under modern weathering conditions. This potassium in Precambrian paleosols mostly occurs in fine-grained, aluminum-rich mica. The potassium either is a record of pervasive diagenetic alteration of Precambrian paleosols or indicates that, unlike modern soils, they did not form as a result of atmospheric acid-forming gases. The majority of Precambrian paleosols could represent alterations on the ancient land resulting from exhalation of acid-containing mud from deep in the earth. Although paleosols that are older than 544 million years generally have the greatest ratios of potassium to sodium, paleosols that are 245 to 544 million years old (Paleozoic paleosols) also are more potassium-rich than are modern soils. In these Paleozoic paleosols, the potassium-bearing mineral typically is a clay mineral called illite. In illite-bearing paleosols, some investigators attribute the high potassium content to peculiar weathering conditions, whereas others attribute it to the precipitation of potassium from through-flowing groundwater long after burial of the soil.

Principal Terms

caliche: a type of soil or paleosol that contains a high proportion of calcium carbonate, calcium sulfate, or both

clay mineral: a type of mineral that is the most common product of soil formation; it is composed of silicon, oxygen, hydrogen, usually aluminum, and possibly other elements

partial pressure: the proportion of a gas mixture (for example, the atmosphere) that a particular type of molecule (for example, carbon dioxide) represents

soil: all material that has been substantially altered at the earth's surface by interaction with the atmosphere, living things, or both, and that has not been laterally displaced subsequent to that alteration

soil horizon: a distinct layer in a soil

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