PALEOCLIMATIC IMPLICATIONS OF VERTISOLS WITHIN THE KOOBI FORA FORMATION, TURKANA BASIN , NORTHERN KENYA (original) (raw)

PALEOCLIMATIC IMPLICATIONS OF VERTISOLS WITHIN THE KOOBI FORA FORMATION, TURKANA BASIN, NORTHERN KENYA

JONATHAN G. WYNN and CRAIG S. FEIBEL
Department of Geology and Geophysics, University of Utah

ABSTRACT

The paleosols of the hominid-bearing Plio-Pleistocene Koobi Fora Formation provide valuable paleoenvironmental information which adds to the understanding of the fossil record of early man. Models of modern vertisol genesis are used to interpret the record provided by the Koobi Fora paleosols. The widespread nature of fossil vertisols within the formation indicates the continuity of seasonal, semi-arid to arid climate during Koobi Fora time. A very calcareous vertisol at approximately 1.65 Ma. may indicate a period of increased aridity .

INTRODUCTION

The Plio-Pleistocene sediments of the Turkana Basin are of crucial interest in understanding the evolution of fossil man and his environment (c.f. Coppens et al. 1976;

Bishop 1976; Leakey and Leakey 1978; Harris 1983; Brown et al. 1985; Leakey and Walker, 1985. Walker et al. 1986; Harris et al. 1988; Harris 1991; Walker and Leakey, 1993). The Plio-Pleistocene sediments of the Omo Group in the Turkana Basin record the paleogeographic, paleoclimatologic and paleoecologic history of the basin during the interval of early man's occupation of the basin. The Koobi Fora Formation of the Omo Group is exposed along the eastern shores of Lake Turkana at the junction of the Gregory Rift and the Main Ethiopian Rift (Figure 1). The Koobi Fora Formation preserves a record of fluvial, lacustrine, deltaic and pedogenic environments present during the period from 4.2 to 0.6 Ma. (million years ago; Feibel 1988). Paleoenvironmental studies of the hominid-bearing Koobi Fora Formation have set the stage for interpreting critical events in the evolution of man.

Previous work on the paleoenvironments of the Turkana basin include multidisciplinary paleogeographic studies (Feibel 1988; Brown and Feibel 1988; Behrensmeyer and Cooke, 1985; Johnson and Raynolds 1976), paleopalynological studies (Bonnefille 1976) as well as isotopic studies of soil carbonate (Cerling et al. 1988).

Paleosols provide a valuable record of the conditions present at the earth's surface during periods of non-deposition. During these depositional lapses, soil forming factors such as climate, topography and organisms modify the sediment, or parent material, during a given period of time, preserving traces of these conditions in the rock record (Jenny 1941).

The features of the Koobi Fora paleosols preserve a record of the environmental conditions such as climate, topography and organisms present during early man's occupation of the Turkana Region. The Koobi Fora paleosols are crucial in the interpretation of the Vertisols paleoenvironments of hominid evolution because pedogenesis represents the dominant portion of Koobi Fora time, exceeding the time recorded by the depositional events overprinted with paleosol formation.

METHODS

Paleoenvironmental information is interpreted from paleosol profiles by a pedotype approach defined by Retallack (1994). The major types of paleosols within the Koobi Fora Formation are grouped into categories designated as 'pedotypes' which are based on classification systems used in the survey of modern soils (Soil Survey Staff 1975; 1992).

The soil forming conditions of the paleosol may then be compared to those represented by a similar modern soil taxonomic group. Decimeter scale samples spaced throughout the paleosol profiles were subjected to textural, chemical, petrographic and x-ray diffraction analyses defined by standard methods of soil science (Mehra and Jackson 1960; Hesse 1971;

Soil Survey Staff 1975; 1992; Head 1992).

MODELS OF VERTIC PEDOGENESIS

Vertisols are defined by the periodic inversion or churning of the soil profile (Buol et al. 1980). This process begins when drying of the soil produces large cracks at the soil surface. These cracks may extend a meter in depth and may be as wide as 10 cm. While the cracks are open at the surface, portions of the surface soil may fall into the open cracks.

At some time after the accumulation of surface soil within the cracks, the soil is reclosed by swelling due to periodic rainfall events. However, the extra material added to the cracks requires an increased volume and thus causes differential stresses within the soil mass. The pressure created by the expansion of the extra material is accommodated by shear failure, which allows large masses of soil to slide past one another. Shear failure is accompanied by the formation of diagnostic slickensided surfaces. After several wet / dry cycles, the soil develops typical vertic morphological features such as: an angular, blocky ped structure, gilgai microtopography, mottles and vosepic and skelsepic microfabrics (terminology defined by Buol et al. 1980). This process also leads to the homogenization of the soil profile, destroying A-B-C horizonation typical of other non-vertic soil types.

Young (1976) provides a model describing four soil forming conditions necessary for the genesis of vertisols, both modern and ancient. The factors necessary for vertic soil genesis are. (1) a topographic slope of less than 3ø, (2) fine clayey soil parent material, (3) precipitation between 250 and 1500 mm/yr and (4) a dry season of four months or more.

These genetic models of vertisol genesis can be used to interpret the conditions present during the formation of the Koobi Fora paleosols.

KOOBI FORA PALEOSOLS

Although a great variety of paleosols exist within the Koobi Fora Formation, most can be attributed to the class of vertisols. Two of the more significant vertisol pedotypes are Vertisols discussed here, although additional, more detailed treatments of other Koobi Fora pedotypes are in progress. The Elomaliya Pedotype is found throughout the formation, but is defined by a type section exposed along the ephemeral stream, Kolom Elomaliya in the Ileret Region.

This paleosol may be classified as a typic calciustert based on the USDA classification system (Soil Survey Staff 1990). The Kimere Pedotype is defined by a two stratigraphically correlative profiles exposed at the FxJj17 archaeological site and is also classified as a typic calciustert.

Elomaliya Pedotype

The typical Elomaliya profile is 1-2 m thick, with A1-A2-A2k-IICk horizons from top to bottom (Figure 2A). The soil is typically brown (10YR 5/3 see Munsell 1990 for color scheme) although no distinct color horizonation exists. Distinct, coarse mottles with diffuse boundaries are very common on slickensided surfaces. Mottle colors range from reddish brown (5YR 4/3) to gray (5Y 5/2). The clayey texture is homogenous among horizons and typically shows a bimodal distribution composed of clay with significant admixture of sand (4-20%). Vosepic and skelsepic plasmic fabrics (terminology of Brewer 1964) are commonly observed in thin section. The A1 horizon is composed of small (5-20 mm) granular to blocky peds often separated by isolated pockets and veins of clean sand. Gilgai microtopography and polygonal networks of cracks are occasionally preserved at the horizon surface. The A1 grades into the A2 horizon which is composed of very large (5-50 cm) angular, blocky wedge-shaped peds with structural surfaces bound by slickensided fractures.

The fracture planes are at a low angle to the horizontal and many fractures can be traced up to 2 meters. The A2 horizon usually comprises the bulk of the profile but typically grades into a structurally similar A2k horizon, which contains nodular carbonate. Carbonate generally occurs as discrete nodules (1-10 mm diameter) but also occurs as 1 cm-thick linings on slickensided fracture surfaces. The discrete nodules generally occur at depths between 30 and 65 cm below the apparent paleosurface and carbonate slickenside linings commonly extend throughout the profile. The A1 and A2(k) horizons rest on the IICk horizon which is generally much sandier.

Kimere Pedotype

The Kimere Pedotype is similar to the Elomaliya in structure, horizonation, and the homogeneity of soil color and texture. The soil color is similar although mottling is much more distinct and covers a wider range of colors from gray (5Y 6/1} to brown (7.5YR 5/2). The type profile consists of A1-A2k-IIC horizons (Figure 2B). The A1 horizon is uniformly 10 cm thick and is composed of small angular peds lacking carbonate. The Kimere paleosol is distinguished from the Elomaliya Pedotype by the massive accumulation of carbonate in the A2k horizon. The carbonate is concentrated in large elongate nodules roughly 3 by 10 cm. The A2k horizon is underlain by a sharp contact with the tuff pebble conglomerate of the IIC horizon.

PALEOCLIMATIC CONCLUSIONS

Comparison of the Koobi Fora paleosols with the models of vertic pedogenesis described above allows interpretation of Koobi Fora paleoenvironments. Because modern vertisols are known to form under the conditions described by the models of vertic pedogenesis, one can conclude that the Koobi Fora vertisols formed under similar conditions.

It follows that the Elomaliya pedotypes represent soils formed on gentle topographic slopes of clayey parent material. The floodplain settings of the ancestral Omo and Turkana Rivers (Feibel 1988; Brown and Feibel 1988; 1991) provide the hydrographic and geographic environment necessary for such vertic pedogenesis.

According to Young's model of vertic pedogenesis defined above, the Koobi Fora vertisols must also have formed in annual moisture flux conditions of 250-1500 mm/yr with a pronounced seasonality of moisture regimes. The seasonality of the moisture regime is defined by a short wet season during which the soil cracks are closed separated by a dry season of at least 4 months which leads to the formation of the soil cracks and associated vertic morphological features.

The widespread nature of vertic paleosols within the Koobi Fora Formation (see Feibel 1988, figure 31) has significant implications for the paleoenvironments of fossil man within the Turkana Basin. If vertisols are pervasive throughout the formation and can be correlated with arid to semi-arid, seasonal climatic conditions, this implies the constancy of such conditions during the entire interval of Koobi Fora time. The aridity may be quantified based on a model relating annual moisture to carbonate depth (Arkley 1963; Retallack 1990 and references cited therein) . This model provides an empirical relationship of.

P = 6.13 (D + 1.86) ,

where P is the annual precipitation (mm/yr) and D is the observed depth to carbonate (cm). Use of this equation for the Elomaliya paleosols provides annual moisture estimates ranging between 135 and 410 mm/yr, which fall at the lower limit of that required for vertic pedogenesis .

The interpretation of consistent paleoclimates provided by the Koobi Fora paleovertisols supports those of Brown (1981) and Brown and Feibel (1991) which describe fairly constant climatic conditions in the Turkana Basin during the past 4 million years. The climatic implications of the Koobi Fora paleovertisols contrast with other paleoclimatic views which describe an increase in aridity with time and an associated change from lush forest environments to grasslands (Bonnefille 1976; Cerling et al. 1988).

The distinguishing characteristics of the Kimere Pedotype also carry significant implications for the climate during Koobi Fora time. The abundance of carbonate and its proximity to the surface are indicative of drier climatic conditions than those of the Elomaliya Pedotype. Use of the equation relating depth to carbonate and net annual moisture yields only 80 mm/yr of average annual moisture. Although this value falls below that presumed necessary for vertic pedogenesis, it may not take into account some moisture received from sporadic flooding events. This may indicate that the time of formation of the Kimere paleosol (about 1.65 Ma.) represents an arid deviation from the prevailing climatic regime. This may extend the upper boundary of a period of aridity between 1.3 and 1.6 Ma. described by Feibel (1988). However, because these conditions hold for only one locality, the apparent increase in aridity may be due to other non-climatic soil forming conditions such as channel migration away from the soil locality.

GLOSSARY OF SOIL SCIENCE TERMS (Jenny 1941; Buol et al. 1980)

Gilgai- surface microtopography of small mounds and depressions.

Horizon- a layer of soil parallel to the soil surface formed by pedogenic processes.

Mottle - diversified spots or blotches of soil color .

Paleosol - a soil once present at the earth's surface but now preserved in the rock record.

Ped- individual structural aggregates of soil particle.

Plasmic fabric - the arrangement of very fine particles of soil as viewed in thin section.

Profile- a vertical exposure of a soil.

Slickenside- a surface polished by shear failure.

Soil- a natural body of mineral and organic matter that responds to conditions such as climate, topography, organisms, parent matter and time.

Texture - the relative proportions of sand, silt and clay within a soil.

Vertisol - a soil typified by the inversion or churning of the profile. Vertic- of or pertaining to a vertisol.

ACKNOWLEDGMENTS

I would like to thank Dr. Craig Feibel for his inspiration and continued encouragement. The field and laboratory studies were conducted with the aid of funding from the National Science Foundation and the L.S.B. Leakey Foundation. The National Museums of Kenya, the University of Utah Department of Geology and Geophysics and the Koobi Fora Field School provided field and laboratory logistical support. The faculty, staff and student members of the Field School deserve particular thanks, notably Dr. Harry Merrick, Dr. Craig Feibel, Dr. Henry Bunn, John Kimengich, Dr. Margaret Schoeninger, Mike Rogers, Marsha Smith and Tomas Muthoka Kivingo.

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Figure 1. A. Location map of the Turkana Basin within the Horn of Africa Region. B.

Detailed view of the Turkana Basin, showing location of exposures of Koobi Fora Formation and other geographic features. Adapted from Feibel ( 1988).

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