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

1. Root traces

2. Diffuse horizon boundaries

3. 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. 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