Laterización: Génesis de Oxisoles, Ultisoles y las Termitas

El origen de los suelos profundamente alterados y maduros, típicos de los ambientes tropicales y subtropicales ha despertado numerosas hipótesis y especulaciones. En abundantes casos, los expertos apuntan la importancia de los organismos edáficos en la génesis de Oxisoles y Ultisoles (taxonomía americana de suelos –USDA-), también conocidos como Ferralsoles y Acrisoles (FAO), que son los tipos de edafotaxa que incluyen tales suelos. El otro día, casi por casualidad, encontré una página Web en donde Charles Weber, propone que la solubilización de la sílice y la acumulación de hidróxidos de hierro y/o aluminio, puede deberse a la acción de las termitas, en cuyo tracto digestivo se alcanzan pH superiores a 12. Tal ambiente hiperalcalino, de acuerdo al autor, emergió con vistas a que estos insectos sociales consumidores de humus (en muchas especies de tales insectos) pudieran disponer de fósforo asimilable. El resultado según el fue la aparición de bauxitas, lateritas y silicretas (Durisoles en muchos casos) a comienzos del cretácico. También comenta algunas repercusiones de este “posible” mecanismo en océanos y ciclos biogeoquímicos.

 Antes de seguir narrando el contenido de este post, tan solo llamar la atención sobre la catarsis que han sufrido las bitácoras más populares (mucho más que las restantes) del sistema mi+d, tras la migración al sistema WordPress. Siempre me toca a mi pagar las conscuencias, en mucha mayor medida que los demás (eso sí). Ya explicaré la razón en otra ocasión. Como resultado de tal “puñalada trapera” a penas entran jóvenes estudiantes de Latinoamérica a leer los contenidos de los post más divulgativos y básicos del blog. Mi lógica reacción será poner freno a estos últimos, a favor de los escritos para adultos. Por tanto, no traduciré con tanto esmero o empeño los textos originales del suahili que suelo dejar al final de los post. Estoy a punto de cansarme y cerrar el kiosco, empero debemos dar tiempo a ver como evoluciona tal cataclísmica crisis de audiencia.

 Perfil de un Ferralsol (Oxisol). Fuente: ISRIC (Holanda)

Weber sugiere también que el lavado de la sílice en las bauxitas (material rico en aluminio, pero pobre en hierro) podría ser generada por la actividad de las termitas a la hora de disponer de materiales geomecánicamente adecuados sobre los que construir sus ciudades enterradas. Seguidamente, este autor conjetura que los gruesos bancos de sílice y la explosión de la abundancia y diversidad de las diatomeas en los fondos marinos (emergidos o no a la superficie terrestre en la actualidad) bien pudieran ser el producto del surgimiento de los comentados insectos sociales y sus repercusiones aludidas sobre el medio edáfico, en los inicios del Cretácico. En este sentido debemos recordar que, como señala el autor, efectivamente estos tipos de suelos parecieron ocupar grandes extensiones durante tal periodo geológico, si bien otros expertos consideran que la razón devino de un enorme periodo de escasa actividad tectónica a escala global (mayor tiempo para la el desarrollo edafogenético).

Sin embargo, Weber va más allá, postulando que la acción de otros organismos de la fauna del suelo pudo propiciar otros alguunos rasgos ya detectados ya en el Pérmico, como las formaciones bandeadas de hierro de los fondos marinos (por ejemplo, el ambiente anaeróbico de las termitas pudo inducir a la solubilización de ingentes cantidades de hierro y su ulterior deposición de los fondos marinos). En cualquier caso, Weber defiende que las termitas ya habían surgido en tal periodo geológico, así como que los Oxisoles (Ferralsoles) también abundaron por aquel entonces, aunque en menor medida que en el Cretácico.

Debe quedar claro que entramos dentro del ámbito de las elucubraciones científicas, que no de los hechos aceptados como tales por la comunidad científica. Sin embargo, las argumentaciones de Weber se me antojan sugerentes (lo cual ni añade y/o resta plausibilidad a sus hipótesis) debido a las siguientes  razones:

– Génesis de edafotaxa o tipos de suelos acoplados a la evolución biológica, como bien pudieron surgir los Mollisoles (Chernozems) tras la dispersión de las gramíneas en los comienzos del Cenozoico.

– Vinculación entre la geoquímica terrestre y la oceánica, de tal modo que los suelos contribuyeran a cambiar tanto los ciclos biogeoquímicos y climas, como la evolución ulterior de la vida.

En cualquier caso, reitero que no soy experto en el tema y tan solo expongo esta sugerente hipótesis.        

Juan José Ibáñez   

DID THE ALKALINE GUT OF TERMITES CREATE LATOSOLS (ULTISOLS and OXISOLS), BAUXITES, AND LATERITES?

by Charles Weber, MS

Abstract

It is proposed here that the laterization of tropical soils, whereby silica is leached out to leave iron and aluminum hydroxides behind, is caused by a high alkalinity past the mid gut of soil eating termites. It is suggested that this alkalinity first arose primarily in order to make phosphate available to the termites and it resulted in laterites, bauxites, glauconite, and diatom rise especially in early Cretaceous,

Discusion

The humus eating termites, which make up over half of the termite species [Brune & Kuhl, 1996], must have to solve the problem of binding of phosphorus by iron and aluminum in soil. They may have done this by creating an alkaline medium in their gut. The humus eating termites create an alkaline pH of 11 to 12.5 just past the mid gut-hindgut junction (the P1 segment) [Bignell, 1998 (there are diagrams of the various gut designs of termites here)]. It is said that this is the highest pH in the biological world. It was obtained using micro electrodes, so is probably accurate [Brune & Kuhl, 1996]. This is 100,000 times as many hydroxyl ions as in a neutral solution, the equivalent of a solution of potassium lye. To put this in perspective, a tenth normal solution of potassium carbonate has a pH of 11.6, a tenth normal solution of potassium silicate has a pH of 12.6, saturated lime (calcium hydroxide) has a pH of 12.4, and a one hundredth normal solution of potassium hydroxide (potassium lye) has a pH of 12. A 200 mM (one fifth normal) solution of potassium carbonate mimics the pH of the gut’s P1 region [Kappler].

They must be doing it by removing all the carbonate and plant acid anions and leaving behind the potassium ions or by removing potassium from rectum of the hind gut, which has a pH of 5.0, and transferring it to the P-1 segment. Potassium is the dominant cation in the gut fluid of Zootermopsis. Such a high pH would tend to displace phosphate from the iron [Dixon p414] and aluminum and make it available to be absorbed. Brune and Kuhl (1996) suggest that the reason for the high pH is to enable termites to digest soil bacteria and/or to make polyphenolic compounds soluble and unable to bind peptide nitrogen compounds. This is plausible since wood eating termites have a high pH also and Lepidoptera and Diptera larvae that eat leaves and detritus have a high pH also [Brune and Kuhl, 1996]. Apparently termites can degrade lignin somewhat [Brune, Miambi, & Breznak, ]. This may add to the desirability of a high intestinal pH to termites, but I suggest the main imperative is to make phosphate soluble in eaters of soil humus. Sodium hydroxide is the best extracting medium of phosphate from soil [Cade-Menum] and the hydroxide ion is a strong competitor for aluminum compared to others even phosphate and fluoride [Dixon p366]. Phosphate adsorbed on goethite (Fe OOH) has a steep decline after a pH of 8.0 to almost zero [Dixon p414].

Like the phosphate, the silicate of the soils would also tend to be displaced to form sodium silicate. The fore gut and rectum are acid. So the silicate must become the hydroxide and become very small colloids before it is excreted. Silicon hydroxide formed should be much more soluble than parent materials, and may even be able to move down through the soil in the form of small colloids, which are probably formed when the potassium silicate reaches the hind gut, especially if protected by organic anions. This may be the reason why tropical soils have the silica leached out of them. It could also explain the formation of silicretes and opals deep in Australian Tertiary savanna soils, silica for which arise from dissolution at the top [Thiry p733].

Weathering and leaching proceeds to greater depths when rainfall is concentrated in a short period [Dixon p361]. This, then, would explain the laterization of tropical soils, and the large deposits of bauxite (aluminum hydroxide) characteristic of some of them from the appropriate parent materials. Bauxite tends to be low in iron when subjected to constant rainfall [Dixon p361]. Bauxite probably remains intact in the middle soil profile because it is friable and therefore not suitable for cementing together sand particles. The termites bore down past the bauxite to the lower part of the profile to where the kaolin clay is in order to build their nests and runways in Australia, probably because the bauxite is friable and does not make strong construction. So it appears that some species move silicon up to the surface of the soil. These soils used to be called latisols, but are called oxisols and ultisols by the USDA and ferralsols or acrisols by the UN these days. There would be plenty of time for the leaching to take place because humivorus (eat humus) termites do not usually make above ground runways [Kooyman] as compared to the short time that erosion susceptible Amitermitinae (now called Termitinae) soils probably linger. It could also account for the source of the silicate to form the deposits of marine glauconite (green sand or iron silicate) laid down starting around early Cretaceous. and account for the rise in diatom diversity that commenced then [Miller 2005, diagram on p1294]. Diatoms are algae with a silica skeleton. The sediments in the North American interior sea way averaged 81% silica in the (Campanian) [Young]. Most of it was thought to be from diatoms, but some from sponges and radiolarians. It is primarily in the form of cristabolite (64%). It must be that the water from that seaway is what provided the silica for the diatoms in the Arctic ocean [Davies]. It may be that that southern water flowed up there to take the place of the cold water that I proposed caused the thermal shock that created the Pacific Ocean trenches.

It is possible that some of the ancient soil eating roaches and wood roaches had the beginnings of such an attribute and thus account for the red beds which started in early Permian in South Africa, and in mid Permian in Europe, North America and Argentina [Veevers, et al]. In the Permian it was not necessarily humus eating wood roaches, but could have been humus eating cockroaches. The anaerobic reducing conditions in part of the termite gut may have assisted this process by reducing the iron in the minerals to the ferrous form [Vu], which oxidized to the ferric form upon reaching the soil, and could conceivably have contributed to red bed formation even with less of an alkaline gut. The soils of early Triassic were said to have no modern parallels, although some Madagascar soils may be fairly close. They were extremely low in organic matter and had no detritus. There was much less podzolization (or lacked podosols), although iron did migrate down to precipitate as iron carbonate [Retallack 1997]. Laterites or soils high in iron oxides are thought to have formed as early as the Triassic in north Australia [Twidale p170]. Since termites probably existed by that time [Emerson 1955 p476], ascribing laterites to termites is tentatively supported by paleontology. Laterites were widespread by the Cretaceous.

It has been proposed that the roots of plants remove silicon from the B horizon (soil below the top soil) of the soil and add it to the A horizon to form kaolin from the gibbsite clay [Lucas]. This is plausible, but I suspect that they were able to use more efficiently subsoil silicon because it had already been solubilized by the termites. It used to be proposed that tropical silica dissolved in the tropics because of high soil temperatures. However, silica solubility is independent of temperature between 0 and 200 degrees centigrade between a pH between 2 and 9.5 [Stever]. Do not feel that termites are not numerous enough to have had such an effect. Even in today’s world kept in check by ants they transpire 2% of the world’s carbon dioxide and 4% of the world’s atmospheric methane [Sanderson], even though they are largely confined to tropical areas, and they were almost certainly much more numerous in the past. They are responsible for eating almost as much vegetation in their areas as vertebrates [Wood & Sands p280] and are the chief consumers in southwest USA range lands [Whiteford]. There can be over a kilogram of termites in a square meter in some areas [Eggleton].

REFERENCES are below

LINKS to other effects of termites and roaches on soil and vegetation.

Did the Wood Roach Cause the Permian – Triassic Coal Hiatus?: Digestion of cellulose by the wood roach may have reduced soil litter and enabled the rise of conifers in the Permian.
The Battles of Termites with Ants: The ability of Amitermitinae to smother plants with phosphorus rich runways may have caused a phosphorus famine in the Cretaceous.

Cretaceous Termites and Soil Phosphorus: Removal of soil phosphorus by erosion of termite runways may have resulted in changes in vertebrate bone evolution and explain animal sizes and shapes in our world.
Evolution of Angiosperm Trees: Angiosperm trees migrated across Western Pacific atolls and were made additionally successful by termites.

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