Original paper
Bioturbation responsible for the concentration of stonelines?
Lichte, Martin
Zeitschrift für Geomorphologie Volume 44 Issue 4 (2000), p. 533 - 534
5 references
published: Dec 21, 2000
ArtNo. ESP022004404008, Price: 29.00 €
Abstract
The contribution to the discussion on the origin of stonelines entitled “Inadequate pedogeomorphic evidence for dry and cold climatic conditions in southeastern Brazil”, written by Humphreys and Adamson, makes clear that the authors see bioturbation as the main factor in the concentration of the coarser particles in one clearly defined stoneline (or layer), while the mantle atop the stoneline contains only the remaining very fine material. We were criticized for not paying enough attention to the question of bioturbation. Indeed, we only mentioned that Cailleux (1966) saw bioturbation as a possible explanation of the phenomenon in question, but did not add the subsequent discussion in the Brazilian literature on this topic, which utterly rejected this idea for various reasons. This discussion may e. g. be found in various articles in Bigarella and Becker (1975) and the main gist of it is that (i) the fossil landscape covered by the stoneline reminds of the microrelief which is today found in the semi-arid regions of NE-Brazil, showing more irregular lines and “more powerful” slopes than the modern soil cover, (ii) the pebbles that form the stonelines usually show clear signs of transport, which cannot be ascribed to bioturbation (also supported by Semmel 1977), and (iii) especially in more southern parts of Brazil soil profiles are found which show up to three stonelines, which are covered by the fine mantle, a feature which certainly cannot be explained with any process of bioturbation. In our paper we pointed out that we found in the mountainous areas in SE-Brazil the formation of modern stonelines (cf the photo Fig. 9 in our paper). Humphreys and Adamson criticize that we do not find evidence of this in lower altitude sites. To us this is completely clear: only in the higher altitude areas we find at present the climatic and geomorphic conditions necessary for the formation of stonelines. The lower altitude areas show at present a fossil landscape under conditons that do not allow the formation of the feature in question. Especially the palynologic evidence we cite finally clarifies that the climatic conditions at the time of the LGM were considerably drier and 5-7 °C colder than today. These conditions are the ideal ones for the formation of stonelines as explained in our paper, and as found in sites of the modern formation of stonelines. Of course, bioturbation is an important factor in soil-development in general, and this applies especially strongly to tropical soils. Most certainly zoogeneous activity destroys old layer-structures, which makes it difficult to prove aeolian (as well as any other kind of) sedimentation, which in our opinion is at least partly present, though other forms of sedimentation have influenced the modern landscape as well. Bioturbation also mixes bedrock minerals into the fine mantle atop the stonelines, no doubt. Still, in our study areas as well as in other areas described in the relevant Brazilian literature, the landscape covered by this system of stoneline and mantle shows all aspects of a clear fossil landscape of semi-arid origin, which agrees with our paleo-climatological findings. Humphreys and Adamson put the finger on the one weak point of our paper: as yet we really cannot define clearly the origin of the material of the mantle atop the stoneline. More work has still to be done on this aspect. To be sure: Stonelines obviously can be formed through various different processes, as especially Stocking (1978) stresses. We do not doubt that there may also be stonelines formed through bioturbational processes as mentioned by Johnson (1990). That is not the case concerning the stonelines in our study areas in SE Brazil, though.
Keywords
stoneline • microrelief • bioturbation • fossil landscape • semi-arid • soil-development • Brazil