Mount Cameroon: a natural laboratory for reconstructing soil history

The mechanisms behind rock-weathering processes can provide vital clues for understanding and reconstructing the history of ancient environments and visualizing the physical conditions in which they were formed, especially climatic situations. Thick ancient coverings of weathered material such as laterites are still the most intensively studied to date. However, little is known about the initial stages of weathering, owing to the rare occurrence of well-preserved examples.

As a contribution to the PEGI-PROSE (1) programme, scientists from the IRD, the CNRS and the University of Strasburg (2) are conducting investigations on Mount Cameroon. They have identified some of the mechanisms that operate during the first stages of basalt rock weathering and have postulated a particular weathering rate. For that they have studied interactions between water and rock by analysing the chemical compositions of spring waters arising from rainwater percolation through the rock. This rock undergoes changes in response to the various factors (such as temperature, precipitation rate, vegetation) that prevail. The processes give rise to the formation of secondary minerals such as hydrated silicates.

Mount Cameroon, an active volcano which is also the country’s highest peak (4095 m), was chosen for its geological, geographical and climatic characteristics. It has become a reference site for the study of basaltic rocks in a humid tropical climate. Its volcanic nature, massive form and location on the Atlantic coast offers some extreme and varied conditions of temperature and rainfall (3). The parent-rock consists mainly of basaltic lava flows and pyroclasts (pumice, ash). This type weathers 10 to 100 times more rapidly than the other continental rocks owing to its high glass content and the porosity of the pyroclasts, which makes it possible to study the initial stages of weathering. The recent activity of Mount Cameroon has moreover been well mapped (4) and the oldest lavas date from 11 million years B.P. (Upper Miocene).

The chemical composition of the spring waters arising from the volcano, fed by rainwater ion-enriched during infiltration, depends on the dissolution reactions of primary minerals and oxidation-reduction equilibrium reactions which occur in the surface layer of organic matter, where this exists. At low elevation, vegetation is extensive, soils are more developed and the waters are heavily loaded with ions, whereas at high elevation they have low ion content. This mineral impoverishment can be explained by a what is still only a slight interaction between the water and the rock, characteristic of the start of the weathering process. That result was confirmed by the small size of differences in strontium isotope ratios obtained at high elevation between the waters and the rock and similar to those of fresh lavas. The speed of the process, expressed as weathering rate and calculated as a function of the water input from rainfall and losses by evapotranspiration, was estimated at a maximum of 20 mm per 1000 years at the volcano summit, as against 100 mm at sea level. Comparison of these results in 14 C-dated soil profiles has brought into relief a rise in rainfall on Mount Cameroon over the past 5 400 years.

Another factor characteristic of erosion processes is the uranium isotype ratio. This is highly variable here, except on the high ground and slopes where the rock is devoid of plant cover and organic material. At this elevation the bare rock does not undergo only climatic weathering. Consequently, at low elevation, other mechanisms, different in character and intensity (mechanical weathering, interaction with organic matter) come into play. Rainfall and temperature conditions favour the development of dense vegetation and the accumulation of organic matter. Plants accentuate the erosion process by their mechanical action. They also contribute to silica entrapment on the surface organic horizons of the soil (5), where bacteria can also use it, partly explaining the weakness of concentrations found in the solutions. Other behaviours characteristic of early stages of erosion become involved in this loss: the sequestration of silica by iron hydroxides and its co-precipitation with aluminium to form poorly-crystallized intermediate components in underlying horizons.

These first results emphasize the influence exerted by climatic conditions and of vegetation on the intensity and rate of the weathering and pedogenesis. Further research conducted at microscopic scale should reveal clues about the role micro-organisms play in these processes and facilitate study of the transformation of primary phases, like glass, into crystallized phases, or genesis of secondary minerals, indicators of pedoclimates (soil climates), in the context of Mount Cameroon. Variations in the carbon cycle in the soil horizons and secondary mineral signatures together will be useful for estimating changes in vegetation and pedoclimate and, therefore, of climate changes.

Marie Guillaume – IRD
Translation : Nicholas Flay

(1) PEGI: Programme d’étude de la géosphère intertropicale (Programme for the Study of the Intertropical Geosphere); PROSE: Programme sol et érosion (Soil and Erosion Programme). These programmes were initiated by the IRD and the CNRS in 1996.
(2) Laboratoire de Géochimie et Métallogénie (Laboratory of Geochemistry and Metallogeny), Laboratoire de Minéralogie-Cristallographie (Laboratory of Mineralogy-Crystallography) and Laboratoire de Magmatologie et Géochimie (Laboratory of Magmatology and Geochemistry (CNRS Paris)), Geosciences (CNRS Rennes), Centre de Géochimie de surface Centre for Surface Geochemistry (University of Strasburg), IRD.
(3) The south-west slope, exposed to the ocean, has an average annual rainfall of 5 m/year (upto 10m/year on the lava-flows situated on the coast) as against 1.5 to 2 m/year for the south-east slope, facing inland. The temperatures range from 26 to 29°C at sea level to 0°C at 4 000 m.
(4) The eruptions of 1815, 1909, 1922, 1925, 1959, 1982, 1999 and 2000.
(5) Formation of phytoliths (silicification of plant fragments).