Today’s tourist attractions …..
….are yesterdays soils.
Soils of the Past Types How to identify Properties effects of diagenesis What do paleosols tell us about the past? Origin of land plants Evolution of grasslands Past CO2 of atmosphere Past temperature/circulation patterns
Paleosols: “soils of the past” Types Buried soil Previously at surface Relict soil Soil exposed to multiple combinations of soil forming factors Exhumed soil Previously buried soil
Identification of paleosols in geologic record Root traces Diffuse horizon boundaries Structure
Root traces
Diffuse boundaries
Soil vs. sedimentary structure…
Diagenesis, or alteration, of soils after burial Compaction Organic matter/histosol to coal Vertisol to shale Psamment to sandstone Cementation Carbonate oxides Loss of organic matter A horizon character is lost Color changes (due to mineralogy) Loss/alteration of minerals/fossils
Mineral/fossil preservation depends on burial environment….
Application of soil taxonomy to paleosols
Paleosol properties used to decipher earth history Presence/absence of root traces to determine when land plants evolved Examples from Pennsylvania Fe content and mineralogy used to learn about the O2 concentration of early earth (Precambrian) Calcium Carbonate (the multipurpose mineral) Depth = f (moisture balance) C isotopes = f (plant type, atmospheric CO2 levels) O isotopes = f (temperature, circulation patterns)
Where/why have paleosols been studied? East Africa: environmental context to human evolution South Asia and beyond: evolution of grasslands Around world: atmospheric CO2 levels Wyoming: to date landscapes and determine changes in circulation
Example 1: Paleosols as guide to land plant evolution Pennsylvania during the Paleozoic……
How do we know this? From paleosol evidence for example…
Road cuts reveal series of paleosols interspersed with marine sediment Paleosols have: Root traces in silurian Carbonates Bk horizons Slickensides Evidence of Vertisols
Contact between overlying marine sediment and paleosol Marine is less oxidized Paleosol (Vertisol) highly oxidized Paleosol shows soil structure and diffuse boundaries
Devonian root traces
More root traces
Slickensides: evidence of shrink/swell
Carbonates (nodules): evidence of semi-aridity
Carbonate morphology related to parent material type: gravelly vs Carbonate morphology related to parent material type: gravelly vs. fine grained (like Vertisols) Gravels accumulate carbonate on bottom (or top) in pore space Fine grained soils tend to form discrete concretions of carbonate Trend with time in both cases is the infilling of porosity and plugging of soil with carbonate