Paleosols

Paleosols - also called fossil soils - are buried soils or soil horizons of the geologic past. ^[1] They are the preserved remnants of ancient weathering and soil-forming processes, locked into the stratigraphic record rather than having been stripped away by erosion. Because soils form at the interface between the atmosphere, the biosphere, and the lithosphere, a preserved paleosol is one of the most direct records available of past surface conditions - the climate, vegetation, drainage, and chemical environment of the landscape at a specific moment in geologic time.

Most soil horizons that developed in the past on elevated landscapes were eventually destroyed as erosion lowered the landscape. Nonetheless, some soils - formed mainly in low-lying areas - escaped erosion to become part of the stratigraphic record. ^{[1] [1]} The survival bias toward low-lying positions is important: paleosols preserved in the rock record are not a random sample of all ancient soils, but predominantly those that formed in depositional or topographically protected settings where burial rather than erosion was the dominant fate of the weathering profile.

Soils that have not been buried are called relict soils. Many buried soils of Quaternary and much older age are also known. ^[1]

Occurrence in the Stratigraphic Record

Quaternary soils formed on glacial or fluvial deposits are the most common paleosols encountered. ^[1] At older ages, paleosols occur in the stratigraphic record at major unconformities, including unconformities in Precambrian rocks, where their presence may reflect the combined processes of soil formation, erosional landscape lowering, reorganisation of preexisting soil horizons, and changing flow of groundwater. ^[1] The presence of a paleosol at an unconformity is therefore not simply a passive marker of a time gap - it is an active record of what was happening at the surface during that gap.

Paleosols also occur as interbeds within sedimentary successions, particularly in alluvial successions, and are known from rocks at least as old as the Ordovician. ^[1] In alluvial systems, interbedded paleosols form when floodplain or fluvial deposition temporarily slows or stops, allowing the exposed sediment surface to develop soil characteristics before the next depositional event buries it. A vertical sequence of alternating fluvial sands and paleosol horizons therefore encodes a history of episodic deposition punctuated by intervals of landscape stability.

Why Paleosols Are Often Missed

Because interbedded paleosols in sedimentary successions superficially resemble sediments or sedimentary rocks, many paleosols have gone unrecognised in the past. Many have been simply identified as gray, red, or green mudstones. ^[1] Awareness of paleosols has grown significantly, and as a result, more and more paleosols are being recognised in sections that were once dismissed as featureless mudstone intervals. The key is knowing what to look for - a set of diagnostic features that distinguish paleosols from ordinary sedimentary deposits.

Three Principal Diagnostic Features

Three principal kinds of diagnostic characteristics help distinguish paleosols from sedimentary rocks: traces of life, soil horizons, and soil structure. ^[1]

Root Traces

Root traces are the most important traces of life preserved in paleosols. ^[1] Their significance is direct: they are evidence that the rock was exposed to the atmosphere and colonised by plants, which means it functioned as a soil. The top of a paleosol is the surface from which root traces emanate. ^[1]

Root traces mostly taper and branch downward, which helps distinguish them from burrows - a critical distinction because both are tubular structures that penetrate sediment. ^[1] However, some root traces spread laterally over hardpans within soils, and some kinds branch upward and out of the soil, so the downward tapering rule is not universal and judgment is required. ^[1] Root traces are most easily recognised when their original organic matter is preserved, which occurs mostly in paleosols formed in waterlogged, anoxic lowland environments where oxidative decomposition of organic matter was inhibited. ^[1] In red, oxidised paleosols - where organic matter has been destroyed - root traces appear mainly as tubular features filled with material different in composition or grain size from the surrounding paleosol matrix. ^[1]

Soil Horizons

The presence of soil horizons is a second key diagnostic feature. The top of the uppermost horizon of a paleosol is commonly sharply truncated by an erosional surface, but soil horizons typically show gradational changes in texture, colour, or mineral content downward into the parent material - a pattern the opposite of normal graded bedding. ^[1] Differences in grain size, colour, reaction with weak hydrochloric acid (used to test for the presence of carbonates), and the nature of the horizon boundaries must all be examined to identify soil horizons in outcrop. ^[1] Comparison with modern soil horizons aids significantly in recognition.

Soil Structures

Bioturbation by plants and animals, wetting and drying, and other soil-forming processes cause paleosols to develop characteristic soil structures at the expense of the original bedding and structures in the parent rock. ^[1] The destruction of primary sedimentary bedding by soil-forming processes is itself a useful indicator: if a mudstone has been thoroughly homogenised and shows no relict lamination, but retains root traces and a gradational lower boundary, it has more likely been through a soil-forming stage than it has been simply deposited as a massive mud.

One of the characteristic kinds of soil structure is a network of irregular planes called cutans, which bound more stable aggregates of soil material called peds. ^[1] Peds give the soil a hackly appearance and occur in a variety of sizes and shapes - platy, prismatic, columnar, angular blocky, subangular blocky, granular, and crumb types are all recognised, each with characteristic size classes ranging from sub-millimetre to over 10 cm depending on the type. ^[1] The clay skins that form around peds - also called cutans - are one of the most reliable field indicators of a ped-bearing soil structure, and their presence in an ancient rock signals that the material was once organised into stable soil aggregates rather than simply deposited and compacted.

Other characteristic soil structures include concentrations of specific minerals that form hard, distinct, calcareous, ferruginous, or sideritic lumps called glaebules - a general term that encompasses nodules and concretions. ^[1] More diffuse, irregular, or weakly mineralised concentrations are called mottles. ^[1] Glaebules are harder and more sharply bounded than mottles; both reflect localised redistribution of material within the soil profile driven by percolating groundwater chemistry, redox conditions, and the distribution of organic matter.

Paleopedological Classification

Paleosols can be recognised as having characteristics similar to those of modern soils, and U.S. Soil Taxonomy names such as aridosol and ultisol can be applied to paleosols. ^[1] Aridosols suggest formation under desert conditions, whereas ultisols reflect weathering under warm, moist conditions. ^[1] This connection between soil type and climate is what makes the U.S. Soil Taxonomy framework useful in paleoclimate work: identifying the correct soil order in the rock record is equivalent to making a direct statement about the temperature and moisture regime at the time of soil formation.

Paleosols can have a variety of colours and properties, not just the red coloration that is sometimes assumed to be characteristic. ^[1] Red colours signal iron oxidation (haematite formation), grey or greenish tones indicate reducing conditions, and dark brown to black tones reflect preserved organic carbon. Matching the observed colour pattern to its probable chemical origin is a routine part of interpreting a paleosol profile and contributes to the palaeoclimatic inference.

Significance

Geologists are increasingly interested in paleosols as indicators of paleoenvironments and ancient climatic conditions. ^[1] The type of paleosol - whether it is an iron-rich laterite, a carbonate-rich calcisol, or an organic-rich histosol - reflects the climate and drainage conditions under which it formed. A thick, kaolinite-dominated paleosol implies prolonged tropical weathering under warm, humid conditions; a carbonate-rich paleosol implies a semiarid climate with episodic wetting. Reading this palaeoclimatic signal requires integrating the mineralogy, geochemistry, and morphology of the profile, but the interpretive potential is substantial, particularly for Precambrian intervals where body fossils are absent and paleosols may be among the best available surface-condition proxies.

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