Re: paleosol=nonsense?

Posted by Les McFadden 16 Jun 1999 03:40:52

Dear Colleagues;

Concerning the Ongoing Debate Concerning "Paleosols" and "Geosols":

As part of my earliest education in the area of Quaternary Studies and
geomorphologic research, I was introduced to the concepts of soil-forming
intervals as they related to terms such as paleosol, geosol, and the like.
As I understood it, the utility and need for these terms were at least
partly justified on the basis of the central claim that researchers at the
time *actually understood* how environmental changes associated with
climate shifts influenced rates, processes and magnitude of soil
development. Perhaps this was so for some soils--but I in the course of my
dissertation research and the subsequent 18 years, I have determined
(relying of course not only on my research but that of many other
colleagues) that many, if not most of the arguments made regarding the
impacts of climate change on the formation of desert soils of arid and
semiarid regions as well as noncalcic soils of the more humid regions of
southwestern North America, were typically wrong. This is largely because
the "models" used as a basis for understanding the key characteristics of
these soils were flawed.
Since the late 1970's, the results of numerical modeling of soil
development (e.g., forward models of carbonate accumulation, constitutive
mass balance analysis, stable and radiogenic isotopic analysis of
authigenic soil materials) combined with much better soil age information
and improved analytical "tools" have shown us that the concept of the
"soil-forming interval" (insofar as it was described then, and as far as I
can tell, currently) is not applicable to soils associated with the
landscapes and deposits of the aforementioned and areally extensive
regions. Moreover, the timing and nature of the climate changes (and
associated biotic communities) themselves has changed considerably.
Consequently, many of my colleagues and I have developed what I believe to
be a heathy skepticism of the claims made concerning the origin and
climatostratigraphic significance of so-called Geosols (and paleosols). In
other words, if the key arguments (and in retrospect, these were little but
assertions) used as the original basis for these terms are (were) wrong (
at least in many circumstances), then let's just say it puts the whole
issue of "Geosols", etc. and the utility of such terms in soil-geomorphic
research on a rather bad footing, so to speak. Consequently, we simply
abandoned use of such terms, given their inapplicability to the landscapes,
deposits and research problems with with we were and are concerned.

Cheers,

Les McFadden

>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 that- 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 physica!
> l 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.

Leslie D. McFadden
Department of Earth and Planetary Sciences
University of New Mexico
Albuquerque, NM 87131
505 277 6121