paleosol=nonsense?

Posted by Howard Hobbs 16 Jun 1999 00:03:56

It's difficult to reply to such a bald statement. I'm tempted to ask:"What part of paleosol don't you understand?" There is a distinction between buried soil and paleosol, even though the two overlap considerably. The "paleo" (old) element is a necessary part of the paleosol concept, but not of the buried soil. For example, since it became known that I was interested in paleosols, I have had numerous people wanting to drag me out and show me the buried soil they have discovered. Invariably, these turned out to be just thatŠ buried soils. In other words, modern soils buried by construction debris, quarry and gravel pit spoil, etc.
On the other hand, pre-Holocene paleosols in my area (southeastern Minnesota) don't look like buried soils, and are never recognized as such except by specialists. With only one exception* they are stripped of the most recognizable layer, the organic-rich A horizon; in some older Holocene paleosols, the organic material has oxidized, leaving only a faint lumpy-bumpy structure to show that it ever was a soil horizon.

So if we discard the simple-minded notion that every buried soil is a paleosol, we need to consider how long does a soil have to be buried in order for it to become a paleosol? This is a legitimate subject for debate, and different people are going to have different answers, depending on their particular problems. I would suggest that when there is observable soil development in the material that buries the soil, then you can think of the buried soil as a paleosol. By this criterion, the pre-settlement alluvial soils of southeastern Minnesota are borderline paleosols, where they have been buried by sediments eroded from plowed fields. This enhanced erosion began about 150 years ago; erosion has been somewhat reduced in the last 50 years, as conservation practices have become common, but probably has still not returned to pre-settlement rates. The surface soils on these recent alluvial sediments have A/C profiles (Arenzville, Chaseburg and Radford silt loams), and the physical characteristics of the A horizons are not much different from the C horizons, but at least A horizons are recognized (Poch and others, 1980).

Another way to look at the question is to determine if the buried soil is in or out of equilibrium with the current environment. Being buried automatically puts it out of equilibrium, of course, but a related question would be: can you learn anything about the past environment from this soil that you can't get from adjacent unburied soil profiles? The answer is no for modern soils buried by spoil, but a qualified yes for these buried alluvial soils. In the agricultural landscape, most soils that are not buried are partly eroded; and the soil-forming environment is different from the pre-settlement one, even though the climate is similar.

I will restrict this comment to the distinction between buried soils and buried paleosols. Others can explore the differences between unburied soils and unburied paleosols (exhumed or relict).

*Lively, R.S., Bettis, E.A.lll, Hallberg, G.R., Hobbs, H.C., 1987, An exposure of the Sangamon soil in southeastern Minnesota; Proceedings Iowa Academy of Science 94 (4): p.111-115.

Poch, G.A., Carlson, C., Cummins, J., Harms, G., Aldeen, D., McCormick, J., Hundley, S., Saari, C., and Lueth, R., 1980, Soil survey of Olmsted County, Minnesota; United States Department of Agriculture, Soil Conservation Service, and Minnesota Agricultural Experiment Station.