THE T’ORA GEOSOL(?) (MAIN ETHIOPIAN RIFT, ETHIOPIA): PROBLEMS IN DEFINING PALAEOSOLS

Abstract

The Lake Region (Main Ethiopian Rift, MER), is object of major palaeoenvironmental research since the seventies. An integrated soil
and Late Quaternary stratigraphy study in this area revealed the presence, across a wide range of present soil-forming factors, of a
group of soils sharing major common features, including:
- intense smectite neoformation from clay-free parent materials
- a strongly developed petrocalcic horizon
- a fairly uniform tephra addition, making them pedocomplexes with two well defined compartments
In the MER lower elevation areas, these soils, found on Lake Ziway terrace V, are incompatible with the present semiarid climate. A
major pedogenetical hiatus separates them from those developed on the next oldest terrace, IV, last flooded about 5 14C ky BP.
These soils were chronologically placed with respect to the known palaeoclimatic history of the MER, with the help of a newly described
and dated stratigraphic section. There is multiple evidence that they developed during the Holocene climatic optimum, between 10 and
5 C14 ky BP, when proxy records show for the area a climate moister than present.
The wide spread of these soils, their easy field recognition and the importance of early Holocene in the palaeoenvironmental history of
the MER prompted their use as a key level. As such, they showed to be extremely useful in the interpretation of the geomorphic and
sedimentary evolution of the Lake Region. Specifically, they were used in reconstructing phases of landscape stability and instability
throughout terminal Pleistocene and Holocene.
Such importance and usefulness suggested their possible establishment as a pedostratigraphic unit, a Geosol, giving birth to the T’ora
geosol definition.
However, according to current standards, existing or in discussion, basic problems exist. The T’ora geosol cannot yet be formalized as,
in its present reference stratigraphy, it is not buried. A suitable stratotype has been located but not yet described. Furthermore, the
T’ora geosol is mostly relic, and rarely exhumed, in its surface occurrences.
According to the present version of the INQUA Palaeopedology Glossary, the T’ora geosol cannot be defined a palaeosol, as it formed
in Holocene. Consensus palaeoclimatic and chronological frameworks for many tropical areas, however, consider Holocene climatic
variations to be of a similar order of magnitude of those registered throughout Late Pleistocene. It appears then rather arbitrary, at least for intertropical areas, to limit the Palaeosol concept to the Pleistocene when, as shown by our findings, Holocene soils may be as useful, or more useful, than older soils in palaeoenvironmental reconstructions.