- Email: [email protected]
Paleopedological evidence of past climatic changes in Nigeria
26
Part II - Essays
hypothesis upon which this conclusion is based could be conclusively tested. So, do we attempt to modify, erect, or construct another improved paleosol paradigm? Of course there is merit in discussing this issue, if only because in the discussion, the revelation of the ultimate futility of the exercise is revealed. It is not clear to us, however, why we must continue to focus on the proper meaning, usage, definition, or paradigmatic aspects of the term 'paleosol', given the confusion and apparently endless debate engendered by the paradigm. One 'radical' solution: don't use the term. We find it sufficient and preferable to describe and recognize soils, utilizing the perfectly reasonable definition of 'soil' found in Birkeland (1984). Accordingly, we recognize buried soils, which in turn can be modified in various manner. Some soils are shallowly buried and continue to undergo some modification via pedogenic processes; some have been more deeply buried and have been modified by diagenetic processes related to burial phenomena. We assume nothin 9, a priori, regardin 9 the climatostratigraphic significance of buried (or surface) soils, although we recognize that some soils may indeed have such significance. What of paleopedology's disciplinary state? Ironically, successfully raising paleopedology to 'disciplinary' status may carry with it new (and maybe worse) problems: disciplinary status might actually increase 'insularity', that is, be conducive to construction of new walls that would actually discourage the accommodation of new insights into soils gained from studies by scientists working 'outside' the traditional limits of the discipline. Perhaps there is no really final 'solution' to the basic problems historically associated with paleopedologic research addressed at this symposium. Nevertheless, we do believe that paleopedology would be well served by the following: (1) solve the current problem of the soil science-earth science 'split' (at least
PALEOPEDOLOGICAL
EVIDENCE
as it exists in academic institutions in the United States); this might be done or at least the process initiated by simply broadening and diversifying program curricula; (2) encourage and nurture collaboration among scientists with pedologic, geologic, geomorphologic, paleoclimatologic, and biologic/ ecologic training, (note: we understand this may increase the proposal budget, but it also increases the probability of successful attainment of the research goals.) Does a paleosol paradigm exist, or differently stated, can a paradigm in paleopedology be 'erected'? Perhaps the answer to this depends on one's definition of a paradigm. We believe the emergence of a paradigm, insofar as we understand the meaning of the term, to be fundamentally unpredictable, and that the paleosol-oriented paradigm as thus far characterized is inherently and conceptually flawed. We think it most likely that the seemingly unspectacular strategies described immediately above might play a critical role in supporting research efforts that might profoundly change the way we understand soils, a sort of 'plate tectonics' of paleopedology. Such a paradigm would actually explain most of the complexity we observe in ancient soils, enhance our academic standing, and even foster a new age in paleopedologic research, rather than a curious paradigm that continues to engender serious debate over how to define its conceptual anchor: the paleosol. REFERENCES
Birkeland, P.W. (1984). Soils and Geomorphology. 372 pp. Oxford University Press, New York, NY. Catt, J.A. (1986). Soils and Quaternary Geology. 265 pp. Clarendon Press, Oxford. Tandarich, J.P., Darmody, R.G. and Follmer, L.R. (1987). Some connectionsin the historyof geologyand pedologyas exemplified by the evolutionof profileconcepts.GeologicalSocietyof America Abstracts, 19(7), 864.
OF PAST CLIMATIC
CHANGES
IN NIGERIA
T e m i t o p e A. O k u s a m i Soil Science Department, Obajemi Awolowo UniversiO', lle-lfe, Osun State, Nigeria
The tropical, sub-Saharan Africa lies outside the glaciated landscapes of the World. The landscapes and the residing regoliths have therefore been dominated by physico-chemical weathering processes in a warm environment that has been continuously undergoing wet-dry climatic changes. An attempt is made here to give preliminary information on possible paleosols and their relationships to geological formations as records or evidences of past climatic changes. Nigeria adequately represents a sub-Saharan country spanning latitudes 4°N to 14°N and with the
following representative biomes: tropical moist and dry forest; sub-humid tropical savanna, and semi-arid tropical savanna. These ecosystems are reflections of the present rainfall distribution with modifications due to perturbations from land use practices in some areas. They have parent rocks of variable texture/composition distributed throughout the country irrespective of the dominating ecosystem. The geology is dominated by pre-Cambrian and post-Paleozoic rock formations. The latter are made up of Cretaceous, Tertiary and Quaternary sediments.
Part II -- Essays The Cretaceous and Tertiary parent materials are mostly sandstones with some arenaceous and calcitic shales. In areas where soils in sandstone and shale lie side-by-side, as observed in some areas of southwest Nigeria, the mineralogy of the regolith formed in sandstone (kaolinite group) compared to that formed in shale (smectite group) can be variable and therefore differentiating. Those formed in shale can be considered as relicts, their smectite clay mineralogy suite is incompatible with the weathering processes associated with the present climatic regime. If this is so, are the morphologic and mineralo-chemical properties of the profiles formed in sandstone reflecting the present or are they relict? If relict, what are the relevant identifying criteria? In other parts of Nigeria, such as in the northeast region, shale parent materials that are also rich in smectite clay minerals are common. The Cretaceous/Tertiary sediments also occur in other parts of Nigeria, such as in the Benue-Niger trough where road cuts in deep regoliths exhibit different colors of sediments. What evidence exists that can be identified and possibly quantified in morphological, chemical or mineralogical terms that would differentiate soils in these sediments from soils of other geological ages and therefore serve as pointers to past climatic changes? Pediplanation is the retreat of scarps and is typically associated in Nigeria with the pre-Cambrian rocks. It can be taken as an extension of the erosion-deposition cycles of the wetter Tertiary period that has continued through the Quaternary. The ages of the development and differentiation of the present landscapes and by inference the initiation of the catena/toposequence soilscapes may be as old as Tertiary. It was a time of mild or gradual surficial and subsurface movement of sediments and colloids. What then is the evidence that indicates changes in climate from wetter, intensive erosion-deposition intervals to a not-so-wet but mildly erosive one? The following features are likely involved: (1) Presence of iron pans near the summits of mesas on sedimentary rocks (Cretaceous/Tertiary) or at slope breaks of landscapes on pre-Cambrian or basement complex formations. This may indicate a lowering (gradual?) of the regional water-table; plinthites represent later periods with continual fluctuations in regional water table. (2) Regoliths that are formed in multi-layered parent materials are sometimes lithologically separated by stonelines. (3) Buried soils (although in most cases shallow) based on identification of Ab horizons; they are usually
27
observed in valley-bottoms of landscapes; what about other buried soils without the easy-to-beidentified surface soil horizons?, e.g. the multilayered parent materials. (4) Hyperseasonal grasslands (hydromorphic savannas) typically found within the humid rainforest or the Guinea Savanna - - they are relatively lower in elevation compared to surrounding landforms. Some (as observed in Vertic Tropaquepts) do host very coarse, rounded/subrounded quartz Cobbles that sit on clay horizons formed from saprolite which suggests a past water activity but is now intermittently wet only during the rainy season. Is this equivalent to a soil type (Eutrustult) that appears conglomeratic but formed in parent material of quartzite/quartz schist? Are all these features Quaternary or evidence of the late Tertiary Period? If the Cretaceous and Tertiary were relatively wetter with a relatively higher regional water table, when did the water table start to drop and what is the pedological evidence? Some soil profiles, especially those that occur within the Guinea Savanna (e.g. Oxic or Ustoxic Dystrochrepts), are ferruginized and these possibly occurred at an earlier time than the iron pan located within soil regolith (Lithic Ruptic-Ultic Dystrochrepts) which is located at a lower elevation in the same land system. Does the former represent a transition period between a wetter Tertiary and a relatively moist/dry Quaternary? Paleopedology seems to be a relatively young science within the tropical world basically because of the paucity of evidence of buried soils in contrast to what has been witnessed in the glaciated regions of the world. Relict features may be the dominant characteristics to use in paleopedological studies of soils of the humid/subhumid/semi-arid tropics. But what type of data will adequately represent these relict features? Iron oxides must play a dominant role in identification and the distinction between different periods of regolith formation. More work is needed to focus on the following questions: (1) What pedological studies give insight to the iron mobilization/differentiation history, the pedologic meaning of lithologic discontinuities, and age control in tropical areas? (2) What are the landform features in relation to soils on pre-Quaternary and Quaternary sediments/ rocks? (3) What is the status of soil genesis and soil quality studies on different parent materials in the tropics?
Part II - Essays
hypothesis upon which this conclusion is based could be conclusively tested. So, do we attempt to modify, erect, or construct another improved paleosol paradigm? Of course there is merit in discussing this issue, if only because in the discussion, the revelation of the ultimate futility of the exercise is revealed. It is not clear to us, however, why we must continue to focus on the proper meaning, usage, definition, or paradigmatic aspects of the term 'paleosol', given the confusion and apparently endless debate engendered by the paradigm. One 'radical' solution: don't use the term. We find it sufficient and preferable to describe and recognize soils, utilizing the perfectly reasonable definition of 'soil' found in Birkeland (1984). Accordingly, we recognize buried soils, which in turn can be modified in various manner. Some soils are shallowly buried and continue to undergo some modification via pedogenic processes; some have been more deeply buried and have been modified by diagenetic processes related to burial phenomena. We assume nothin 9, a priori, regardin 9 the climatostratigraphic significance of buried (or surface) soils, although we recognize that some soils may indeed have such significance. What of paleopedology's disciplinary state? Ironically, successfully raising paleopedology to 'disciplinary' status may carry with it new (and maybe worse) problems: disciplinary status might actually increase 'insularity', that is, be conducive to construction of new walls that would actually discourage the accommodation of new insights into soils gained from studies by scientists working 'outside' the traditional limits of the discipline. Perhaps there is no really final 'solution' to the basic problems historically associated with paleopedologic research addressed at this symposium. Nevertheless, we do believe that paleopedology would be well served by the following: (1) solve the current problem of the soil science-earth science 'split' (at least
PALEOPEDOLOGICAL
EVIDENCE
as it exists in academic institutions in the United States); this might be done or at least the process initiated by simply broadening and diversifying program curricula; (2) encourage and nurture collaboration among scientists with pedologic, geologic, geomorphologic, paleoclimatologic, and biologic/ ecologic training, (note: we understand this may increase the proposal budget, but it also increases the probability of successful attainment of the research goals.) Does a paleosol paradigm exist, or differently stated, can a paradigm in paleopedology be 'erected'? Perhaps the answer to this depends on one's definition of a paradigm. We believe the emergence of a paradigm, insofar as we understand the meaning of the term, to be fundamentally unpredictable, and that the paleosol-oriented paradigm as thus far characterized is inherently and conceptually flawed. We think it most likely that the seemingly unspectacular strategies described immediately above might play a critical role in supporting research efforts that might profoundly change the way we understand soils, a sort of 'plate tectonics' of paleopedology. Such a paradigm would actually explain most of the complexity we observe in ancient soils, enhance our academic standing, and even foster a new age in paleopedologic research, rather than a curious paradigm that continues to engender serious debate over how to define its conceptual anchor: the paleosol. REFERENCES
Birkeland, P.W. (1984). Soils and Geomorphology. 372 pp. Oxford University Press, New York, NY. Catt, J.A. (1986). Soils and Quaternary Geology. 265 pp. Clarendon Press, Oxford. Tandarich, J.P., Darmody, R.G. and Follmer, L.R. (1987). Some connectionsin the historyof geologyand pedologyas exemplified by the evolutionof profileconcepts.GeologicalSocietyof America Abstracts, 19(7), 864.
OF PAST CLIMATIC
CHANGES
IN NIGERIA
T e m i t o p e A. O k u s a m i Soil Science Department, Obajemi Awolowo UniversiO', lle-lfe, Osun State, Nigeria
The tropical, sub-Saharan Africa lies outside the glaciated landscapes of the World. The landscapes and the residing regoliths have therefore been dominated by physico-chemical weathering processes in a warm environment that has been continuously undergoing wet-dry climatic changes. An attempt is made here to give preliminary information on possible paleosols and their relationships to geological formations as records or evidences of past climatic changes. Nigeria adequately represents a sub-Saharan country spanning latitudes 4°N to 14°N and with the
following representative biomes: tropical moist and dry forest; sub-humid tropical savanna, and semi-arid tropical savanna. These ecosystems are reflections of the present rainfall distribution with modifications due to perturbations from land use practices in some areas. They have parent rocks of variable texture/composition distributed throughout the country irrespective of the dominating ecosystem. The geology is dominated by pre-Cambrian and post-Paleozoic rock formations. The latter are made up of Cretaceous, Tertiary and Quaternary sediments.
Part II -- Essays The Cretaceous and Tertiary parent materials are mostly sandstones with some arenaceous and calcitic shales. In areas where soils in sandstone and shale lie side-by-side, as observed in some areas of southwest Nigeria, the mineralogy of the regolith formed in sandstone (kaolinite group) compared to that formed in shale (smectite group) can be variable and therefore differentiating. Those formed in shale can be considered as relicts, their smectite clay mineralogy suite is incompatible with the weathering processes associated with the present climatic regime. If this is so, are the morphologic and mineralo-chemical properties of the profiles formed in sandstone reflecting the present or are they relict? If relict, what are the relevant identifying criteria? In other parts of Nigeria, such as in the northeast region, shale parent materials that are also rich in smectite clay minerals are common. The Cretaceous/Tertiary sediments also occur in other parts of Nigeria, such as in the Benue-Niger trough where road cuts in deep regoliths exhibit different colors of sediments. What evidence exists that can be identified and possibly quantified in morphological, chemical or mineralogical terms that would differentiate soils in these sediments from soils of other geological ages and therefore serve as pointers to past climatic changes? Pediplanation is the retreat of scarps and is typically associated in Nigeria with the pre-Cambrian rocks. It can be taken as an extension of the erosion-deposition cycles of the wetter Tertiary period that has continued through the Quaternary. The ages of the development and differentiation of the present landscapes and by inference the initiation of the catena/toposequence soilscapes may be as old as Tertiary. It was a time of mild or gradual surficial and subsurface movement of sediments and colloids. What then is the evidence that indicates changes in climate from wetter, intensive erosion-deposition intervals to a not-so-wet but mildly erosive one? The following features are likely involved: (1) Presence of iron pans near the summits of mesas on sedimentary rocks (Cretaceous/Tertiary) or at slope breaks of landscapes on pre-Cambrian or basement complex formations. This may indicate a lowering (gradual?) of the regional water-table; plinthites represent later periods with continual fluctuations in regional water table. (2) Regoliths that are formed in multi-layered parent materials are sometimes lithologically separated by stonelines. (3) Buried soils (although in most cases shallow) based on identification of Ab horizons; they are usually
27
observed in valley-bottoms of landscapes; what about other buried soils without the easy-to-beidentified surface soil horizons?, e.g. the multilayered parent materials. (4) Hyperseasonal grasslands (hydromorphic savannas) typically found within the humid rainforest or the Guinea Savanna - - they are relatively lower in elevation compared to surrounding landforms. Some (as observed in Vertic Tropaquepts) do host very coarse, rounded/subrounded quartz Cobbles that sit on clay horizons formed from saprolite which suggests a past water activity but is now intermittently wet only during the rainy season. Is this equivalent to a soil type (Eutrustult) that appears conglomeratic but formed in parent material of quartzite/quartz schist? Are all these features Quaternary or evidence of the late Tertiary Period? If the Cretaceous and Tertiary were relatively wetter with a relatively higher regional water table, when did the water table start to drop and what is the pedological evidence? Some soil profiles, especially those that occur within the Guinea Savanna (e.g. Oxic or Ustoxic Dystrochrepts), are ferruginized and these possibly occurred at an earlier time than the iron pan located within soil regolith (Lithic Ruptic-Ultic Dystrochrepts) which is located at a lower elevation in the same land system. Does the former represent a transition period between a wetter Tertiary and a relatively moist/dry Quaternary? Paleopedology seems to be a relatively young science within the tropical world basically because of the paucity of evidence of buried soils in contrast to what has been witnessed in the glaciated regions of the world. Relict features may be the dominant characteristics to use in paleopedological studies of soils of the humid/subhumid/semi-arid tropics. But what type of data will adequately represent these relict features? Iron oxides must play a dominant role in identification and the distinction between different periods of regolith formation. More work is needed to focus on the following questions: (1) What pedological studies give insight to the iron mobilization/differentiation history, the pedologic meaning of lithologic discontinuities, and age control in tropical areas? (2) What are the landform features in relation to soils on pre-Quaternary and Quaternary sediments/ rocks? (3) What is the status of soil genesis and soil quality studies on different parent materials in the tropics?