Paleosols – a key to past environments

A paleosol is a soil that formed on the landscape of the past. Soils directly and indirectly interact with the surrounding environment and form chemical and physical characteristics in response to atmospheric composition and climate. The study of paleosols means that it is possible to infer ancient atmospheric composition by examining certain characteristics, such as the behaviour of certain elements e.g. iron, manganese and rare earth elements.

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I found a sequence of pennsylvanian aged sandstone, paleosols, and shale today. The sandstone overlies the paleosol layer in the first photo. The sphere thing pictured below is one of many such rocks I found embedded in the paleosol. You can see where I found the one I took in the bottom left part of the first photo. I’m not sure what it is, but it is very fissile, and not very well lithified (though not loose like compacted dirt). The grains are very fine, and the the sandstone that overlies the paleosol layer it was found in was deposited by a river. I was thinking a concretion, but do concretions sometimes split like this? Part of me thinks that it is a hunk of shale that was weathered by a river. I’m not sure at all though, have any of yall ever seen anything like it? 


You’ve heard of the atmosphere, the biosphere, and if you frequent posts at TES, you’ve certainly heard of the lithosphere. The Pedosphere is … the “sphere” that hosts all the soils of the planet and is, essentially, the cradle of terrestrial life. Depending on local climate conditions, it may take several thousand years to generate a mature topsoil from an exposed rock surface – with the help of more interactions of water, atmosphere, plants, microorganisms, minerals, exposure, topography, and – well, more factors than can be handled even by a super computer in today’s world. Soil scientists, such as Hans Jenny, the Charles Darwin of the field, simplify these interactions into elegant equations that can never be totally resolved. Soils tend to assume an equilibrium composition with their hosting climate, geology, flora and fauna. 

A paleosol is more than just a very old soil. It is as much a fossil of the earth as is any fossilized living organism. As fossils go, paleosols are rare “creatures.” The position of the pedosphere on the earth’s surface exposes it to surficial weathering and erosion: the components of a potential paleosoil are more commonly incorporated into the sediment load of a paleo-river than being left alone long enough to peacefully lithify. For soils to be preserved as part of the geologic record requires that they be covered and protected by rapid geologic /climatic induced phenomena such as the lavas or ash of volcanic eruptions, rapidly deposited flood sediments, or windstorms that carry glacial loess or desert sands.

Paleosols have been found that date to the Archaean of ~2.2 billion years ago: these are reported to show evidence of the rise of oxygen in the atmosphere. Paleozoic soils hosted the first land plants encroaching on the terrestrial world. Mesozoic soils, potentially fertilized by dinosaur dung, lead to a picture of the precise landscape where now extinct ecosystems thrived. Paleosols of the ice ages are a riotous mix of rapid climate fluctuations. Quaternary paleosoils show the advent of humans into the pedosphere and have been used to demonstrate the archeology of farming. 

Modern classifications of soils are in large part based on the climatic conditions prevailing during soil formation. In the case of the study of paleosols, the nature of their parent climate is largely unknown, and one goal of their study is to work backwards to determine just what the paleo-climate was. Atmospheric levels of CO2 in equilibrium with ancient soils is estimated by analyses of carbonate material that precipitated as the soils were originally formed. CO2 was high during the late Silurian then declined throughout the rest of the Paleozoic, coinciding with the evolution and spread of land plants and the late Paleozoic ice ages. Following the Permian/Triassic extinction even, CO2 levels deduced from paleosols rose throughout the Mesozoic, then fell after the K/T boundary and continued falling during the Tertiary to today. 

Topsoils are treasures of the soil world, but not always respected by careless agro-business or “deep-ripping” farmers. An odd outcome of this in my local world is, following the destruction of modern topsoils by overgrazing, farmers are now exploiting the shallow deposits of Pleistocene paleosol horizons. Possibly, our local wheat fields are in part fertile due to the prevalence of manure left by mastodons of the past. 

Annie R
Photo: Plio-Pleistocene paleosol sequence capped by loess deposits, over which are younger soil horizons. Diagram reprinted from Mora et al, 1996.

Further reading:
Bryant, W B, 2007 . Dirt: the ecstatic skin of the Earth. W W Norton & Co., 224 pp. – reprint of the 1995 “cult” classic! Recommended to all folk who are as ecstatic about the Earth as we are.
Jenny, Hans. Factors of Soil Formation, 2011. Dover Publications, 320 pp. Original edition 1941. 
Cerling, T.E., 1991. Carbon dioxide in the atmosphere: evidence from Cenozoic and Mesozoic paleosols. Am. J. Sci. 291, 377–400.
Mora, C.I., Driese, S.G., Colarusso, L.A., 1996. Middle to late Paleozoic atmospheric CO2 levels from soil carbonate and organic matter. Science 271, 1105–1107.