A paleosol is an ancient soil preserved within a sedimentary sequence. Cretaceous paleosols can retain root traces, soil horizons, clay minerals and carbonate nodules. Together, these features help researchers reconstruct conditions at a particular place and time. They do not provide a single global picture of Cretaceous vegetation or climate.
Soils form where sediment meets water, air and living organisms. Their properties can respond to rainfall, temperature, drainage, topography and the plants growing nearby. A fossil soil is therefore an environmental archive, but its signal combines several causes that must be separated before a climate estimate is made.
What survives in an ancient soil?
Root traces and soil structures show that sediment was exposed long enough for plants and soil processes to alter it. Iron-rich colours can reflect oxidation and drainage. Clay minerals and chemical weathering profiles can constrain how strongly rocks broke down. Carbonate nodules may preserve isotopic information about soil water and carbon dioxide in the soil zone.
Researchers use these observations with other evidence from the same region, including fossil leaves, river deposits and marine records. For example, paleosols in Patagonia's Mata Amarilla Formation have been used to estimate a relatively warm, humid but seasonal environment at high southern palaeolatitudes during parts of the mid-Cretaceous. A separate Campanian study in the Western Interior combined carbonate clumped-isotope temperatures with other methods to examine summer warmth and annual temperature range. These are regional reconstructions, not direct measurements of a worldwide average.
How the estimates are made
Some paleosol methods relate soil chemistry or mineral composition to modern relationships between soils and climate. Stable isotopes can be measured in pedogenic carbonates or clays. Clumped-isotope thermometry uses the pairing of heavy isotopes within carbonate molecules to estimate the temperature at which the mineral formed. Researchers then combine that temperature with information about the water and the surrounding soil system.
Each method rests on assumptions. Carbonate may form mainly in a warm season, so its temperature can differ from the annual mean. Rainfall and evaporation influence isotope values, while burial fluids can alter the original mineral. A soil profile may also combine several periods of formation or be truncated by erosion before it was buried.
Why one soil cannot stand for the whole period
Cretaceous landscapes varied with latitude, elevation, distance from the sea and seasonal circulation. A well-drained soil in one basin cannot be compared directly with a waterlogged soil in another without accounting for those differences. Even neighbouring profiles can record different positions on a floodplain.
Paleosols are strongest when several independent clues agree. Their chemistry can test interpretations from plant fossils, while the sedimentary setting constrains where and how the soil formed. Read about regional Cretaceous climate evidence, the rocks and plants of Late Cretaceous wetlands and the changing Cretaceous plant record.
Frequently asked questions
What is a paleosol?
A paleosol is an ancient soil preserved in the rock record. It may retain horizons, root traces, minerals and chemical signatures formed before burial.
Can Cretaceous soil reveal the global climate?
It provides evidence for a specific locality and interval. Global patterns require many sites and comparison with other climate records.
What do carbonate nodules record?
Their chemistry and isotopes can constrain soil water, carbon and mineral-formation temperature, subject to seasonal and burial effects.
Why can paleosol climate estimates disagree?
Different sites, seasons, soil processes, analytical methods and later alteration can produce different signals or uncertainty ranges.