Historical Geology/Soils and paleosols

In this article we shall discuss soils, their formation, and their preservation as fossil soils (paleosols). It will be useful for the reader to be familiar with the article on chemical weathering before reading further.

Soil: what is it?

The reader will of course have seen plenty of soil. But what exactly is it?

Sediments lying on the surface of the land will undergo chemical weathering as the result of rain falling on them. As this sediment is on the surface it will not have been compacted and lithified, so it will be highly porous as compared to a sedimentary rock, and the rainwater will easily be able to seep through it. This will cause the sort of changes in the composition of the sediment that you would expect if you have read the article on chemical weathering: feldspar minerals, for example, will be converted into clays; soluble minerals will be dissolved, and, depending on the climate and drainage of the local environment, can be deposited lower down in the soil; residual minerals will also be transported.

The other thing that will happen to such sediment is, of course, that plants will grow in it and organisms will live in it. This has various effects: first, it means that decaying organic material will be deposited in the upper layer of the sediment. Second, the actions of plant roots and of burrowing worms and so forth will keep the soil loose and porous. Third, the decaying plant material will release organic acids into the soil, which increases the rate of chemical weathering. In fact, a rock will actually undergo chemical weathering faster buried in soil than it will exposed to the open air.

These processes, known as soil formation or pedogenesis cause the sediment to become soil. The reader will notice from this discussion that a soil is defined rather differently from the other sediments that we've discussed so far in this textbook. Other sediments are defined chiefly by their origin and mode of deposition. Soil, on the other hand, is defined by what happens to it after deposition: a soil is a sediment that has undergone pedogenesis, and this can happen to all sorts of sediments, from volcanic ash to glacial outwash.

Cross-section through soil of the type known as a podzol.

The photograph to the right shows you the results of pedogenetic processes on one particular soil. You will observe a number of distinct layers (horizons) in the soil. On top, just under the grass cover, is a black horizon which gets its color by being rich in organic material. The white-ish horizon below that has had minerals leached out of it by chemical weathering of the soil. And the pinkish horizon below that takes its color from the iron oxides that have been deposited in it by the rainwater seeping through the soil.

This one example is just an illustration of the sort of thing that can happen in soil formation: the horizons will differ from soil type to soil type, and, indeed, identifying the different horizons is the first step towards identifying a soil type. We shall not here discuss all the different types of soil, because this would be something of a digression from the main thrust of this article.

Many determining factors combine to influence how a soil develops and so what type of soil it becomes. These include:

• The original sedimentary material. Obviously, for example, something that starts off with no iron-bearing minerals in it is not going to end up with a horizon rich in iron oxides.

• The climate. Chemical weathering acts much more vigorously in warm climates with plenty of rainfall.

• Drainage. For example, in waterlogged soil the decay of organic material is retarded by the fact of being waterlogged, causing the accumulation of such material.

• The vegetation type. For example, pine forests produce particularly acidic leaf litter, accelerating chemical weathering.

• Time. Since pedogensis takes time, it is clearly going to be the case that sediments of recent origin will not be so well developed as older soils.

• Human activities, such as the addition of manure to fields.

Paleosols: definition

In geology, a paleosol is a fossilized soil. Note that this does not necessarily mean that it has been lithified, merely that it has been preserved by burial, perhaps by volcanic ash, or a lava flow, or aolian sediment, or whatever.

We should note that in pedology (soil science) the word paleosol has a different meaning: it that context it means a soil which developed under a set of conditions that are no longer present, for example a soil which develops under tropical conditions in a country that later acquires an arid climate. In the remainder of this article, we shall be using the word "paleosol" exclusively in the first sense.

Paleosols: how do we know?

As usual in this series of articles, we ask: how can geologists recognize paleosols when they see them?

Paleosol, Mexican Hat, Utah.

The identification of rocks as paleosols is not really challenging. For one thing, paleosols, though necessarily buried, are not necessarily lithified. So some paleosols can be recognized as once having been soil because they still are soil. When they have been lithified, they often retain a superficially soily appearance. The photograph to the right, for example, shows a paleosol found near Mexican Hat, Utah: note its distinctly soil-ish appearance.

Whether or not a paleosol has been lithified, it will retain the mineralogical changes caused by pedogenesis, and this allows geologists not just to recognize the fact that the paleosol was once a soil, but also to identify the soil type, and so to come to conclusions about the climate at the time it formed.

Paleosols will also typically show distinctive signs of biological activity, such as animal burrows and casts, and roots or root casts (these are the white features in the photograph to the right): sometimes one can even find tree-stumps rooted in paleosols, leaving one in no doubt that they were once fertile soil. The exception to this is one that proves the rule: obviously we are not going to find such signs of biological activity in soil which was buried before life evolved to live in it: so when we find paleosols that are dated from the Cambrian (for example) we find no such signs of life, just as one would expect.

The stratigraphy of paleosols is also consistent with geologists' theories of their origin. It would be peculiar to the point of inexplicable to find that a paleosol had been buried by (for example) a distinctively deep-water marine sediment such as a turbidite, because what would soil, which forms on land, be doing getting itself buried under a turbidite? What we expect to find, and do find, is that the sorts of rocks that geologists identify as paleosols are found buried under volcanic ash, or lava flows, or coal, or wind-borne sediment, or sediment deposited by rivers; or such sediments as have undergone pedogenesis themselves and become soil.