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Evaluation of Amino Acid Composition As a Geochronometer in Buried Soils on Mount Kenya, East Africa Article "Evaluation of Amino Acid Composition as a Geochronometer in Buried Soils on Mount Kenya, East Africa" William C. Mahaney, Michael G. Boyer et Nathaniel W. Rutter Géographie physique et Quaternaire, vol. 40, n° 2, 1986, p. 171-183. Pour citer cet article, utiliser l'information suivante : URI: http://id.erudit.org/iderudit/032637ar DOI: 10.7202/032637ar Note : les règles d'écriture des références bibliographiques peuvent varier selon les différents domaines du savoir. Ce document est protégé par la loi sur le droit d'auteur. L'utilisation des services d'Érudit (y compris la reproduction) est assujettie à sa politique d'utilisation que vous pouvez consulter à l'URI https://apropos.erudit.org/fr/usagers/politique-dutilisation/ Érudit est un consortium interuniversitaire sans but lucratif composé de l'Université de Montréal, l'Université Laval et l'Université du Québec à Montréal. Il a pour mission la promotion et la valorisation de la recherche. Érudit offre des services d'édition numérique de documents scientifiques depuis 1998. Pour communiquer avec les responsables d'Érudit : [email protected] Document téléchargé le 12 février 2017 06:16 Géographie physique et Quaternaire, 1986, vol. XL, n° 2, p. 171-183, 4 fig., 2 tabl., 11 app. EVALUATION OF AMINO ACID COMPOSITION AS A GEOCHRONOMETER IN BURIED SOILS ON MOUNT KENYA, EAST AFRICA* William C. MAHANEY, Michael G. BOYER and Nathaniel W. RUTTER, respectively: Geography Department, Atkinson College, York University, North York, Ontario M3J 1P3; Biology Department, York University, North York, Ontario M3J 1P3; Geology Department, University of Alberta, Edmonton, Alberta T6G 2E3. ABSTRACT A sequence of surface and RÉSUMÉ Détermination de la composition ZUSAMMENFASSUNG Bestimmung der buried paleosols from the slopes of Mount de l'acide aminé pour le géochronologie des Amino-Sàure als Geochronometer der be- Kenya, East Africa, has been identified and sols enfouis du mont Kenya, Afrique de l'Est. grabenen Bôden auf dem Mount Kenya, Ost- dated by radiocarbon and amino acid dating On a identifié et daté, au radiocarbone et à Afrika. Eine Série von an der Oberflâche Ne- techniques in order to elucidate the Quater­ l'aide de techniques de datations à l'acide genden und begrabenen Palàobôden von den nary history of the area. Buried paleosols aminé, des paléosols enfouis et de surface Hàngen des Mount Kenya, Ost-Afrika, wurde vary in radiocarbon age from 900 to > 40,000 afin de comprendre l'évolution quaternaire mittels Radiokarbon- und Aminosâuredatie- yrs BP. They have developed in glacial and de la région. Les datations au radiocarbone rungs-techniken identifiziert und datiert, um periglacial deposits of variable texture, con­ des paléosols enfouis varient de 900 à plus die Geschichte dieses Gebiets im Quaternàr sisting of a high percentage of clasts of phon- de 40 000 BP. Les sols se sont développés zu erhellen. Das durch Radiokarbon bes- olite, basalt and syenite. All but two paleosols dans des dépôts glaciaires et périglaciaires timmte Alter der begrabenen Palàobôden va- are located in the Afroalpine zone (above de différentes textures, constitués de frag­ riiert von 900 bis > 40,000 Jahren v.u.Z. Dièse 3200 m). D/L ratios of amino acids in Ab ho­ ments de roches détritiques, de phonolite, Bôden haben sich in glazialen und perigla- rizons were determined in order to establish de basalte et de syénite. Tous les paléosols, zialen Ablagerungen verschiedener Bes- their reliability for relative age dating. Alanine, sauf deux, sont situés dans la zone afroalpine chaffenheit entwickelt, welche zu einem hohen aspartic acid, glutamic acid, leucine, valine, (au-dessus de 3200 m). On a déterminé par Prozentsatz aus Trùmmern von Phonolith, and phenylalanine were routinely analyzed. racémisation les rapports D/L des acides Basait und Syenit bestehen. Au|Ser zweien Aspartic acid, as in other cases, proved re­ aminés dans les horizons Ab en vue d'évaluer befinden sich aile Palàobôden in der afroal- liable yielding remarkably consistent results, leur fiabilité pour la datation des âges relatifs. pinen Zone (oberhalb 3200 m). Die D/L Anteile with higher ratios corresponding to increasing On a fait l'analyse de l'alaline, de l'acide as- der Aminosâuren in den Ab-Horizonten wur- age. Other acids analyzed showed distinct partique, de l'acide glutamique, de la leucine, den bestimmt, um ihre Verlâpiichkeit bei der trends, although not as convincing as aspartic de la valine et du phénylalaline. L'acide as- relativen Altersbestimmung festzustellen. acid. In most cases, the aspartic acid ratio/ partique, comme dans d'autres cas, a donné Alamin, aspartische Sàure, Glutamin-Sàure, age relationships were supported by radi­ des résultats particulièrement satisfaisants, Leuzin, Valin und Phenylalanin wurden lau- ocarbon dates. D/L ratios of aspartic acid les quotients plus élevés correspondant aux fend analysiert. Wie in anderen Fallen erwies varied from approximately 0.07 for modern âges les plus anciens. D'autres acides mon­ sich die aspartische Sàure als verlàpiich, in- samples, to approximately 0.45 in samples traient des tendances bien distinctes, mais dem sie bemerkenswert bestàndige Ergeb- > 40,000 years old. moins convaincantes que dans le cas de nisse ergab, bei denen die hôheren l'acide aspartique. Ainsi, dans la plupart des Quotienten dem hôheren Alter entsprachen. cas, les relations quotients/âges de l'acide Andere analysierte Sàuren zeigten ausge- aspartique étaient corroborées par les da­ pràgte Trends, wenn auch nicht so ùberzeu- tations au radiocarbone. Les rapports D/L de gend wie die aspartische Sàure. In den l'acide aspartique variaient d'environ 0,07 pour meisten Fallen wurden die Beziehungen les échantillons modernes à environ 0,45 pour Quotient/Alter der aspartischen Sàure durch les échantillons de plus de 40 000 ans. Radiokarbondatierungen gestùtzt. Die D/L Anteile der aspartischen Sàure variierten von ungefâhr 0.07 fur moderne Proben bis un- gefâhr 0.45 in Proben, die > 40,000 Jahre ait sind. * Contribution du premier symposium de la CANQUA, sous la direction de René W. Barendregt 172 W. C. MAHANEY, M. G. BOYER and N. W. RUTTER INTRODUCTION a convenient field setting in which to test the reliability of using amino acid racemization for age determination, and The fate of amino acids in buried paleosols is of considerable where the deposits can be calibrated by radiocarbon. Moreover, interest to Quaternary geologists, particularly since the ra- time stratigraphy refinement of amino acid racemization assists cemization of aspartic acid and others may provide a means in increasing our understanding of the timing of geologic events of dating organic-rich horizons. Dated paleosols ranging from before, during, and following the last (Liki) glaciation on Mount 960 to > 42,000 yrs BP have been described on Mount Kenya Kenya (Fig, 2). (Fig. 1a and 1b; 0°10'S; 37°15'E) in several drainage basins between 2990 and 4050 m elevation (MAHANEY, 1979, 1981, Many varieties of fossils are now routinely analyzed to 1982a, 1982b, 1983, 1984a, 1984b and 1985). Most soils determine the D/L ratios of amino acids for relative and absolute have developed in transported regolith with different lithologie age dating of sediments. The method has been particularly compositions. We consider this paleosol sequence to provide useful when fossils have experienced similar temperature FIGURE 1. Paleosols sites on the western flank (a) and eastern flank (b) of Mount Kenya, East Africa. Sites des paléosols sur les versants ouest (a) et est (b) du mont Kenya, Afrique de l'Est. EVALUATION OF AMINO ACID COMPOSITION 173 1974 ; H ARE, 1974 ; HO, 1965,1967; LEE ef a/., 1976; RUTTER and CRAWFORD, 1984). In addition, investigations involving amino acid racemization age dating have been carried out with marine sediments (BADA ef a/., 1970; KVENOLDEN ef a/., 1970), foraminifera (KING, 1980; WEHMILLER and HARE, 1971) coral (WEHMILLER ef a/., 1976, WEHMILLER and HARE, 1970) and with soils (LIMMER and WILSON, 1980). Racemization (or epimerization) of amino acids in geological specimens is a complex process involving a number of factors. The rates of interconversion processes are dependent upon the species of amino acid, ambient temperatures, types of materials in which the amino acids are embedded (shells, bones, wood, etc.), and the state in which the amino acids occur, i.e. whether they are terminally bound, internally bound, or free amino acids. In addition, other external diagenetic factors such as moisture content, acidity, and oxidation-re­ duction conditions can play a role. The accuracy of D/L de­ terminations rests with the care of preparation and the reliability of the analytical methods employed. These factors have con­ tributed to the apparent lack of interest in amino acid D/L ratio determinations of paleosols. The complex nature of soil formation and diagenetic changes that take place at the surface and upon burial compound the problems already inherent in the method. It will be seen, however, that in the present work there is a trend for D/L ratios, of aspartic acid, to increase with age of the paleosol. FIELD AREA A number of major deposits of Pleistocene and Holocene age are found in the Afroalpine area of Mount Kenya (Fig. 1 a and 1 b for location; Fig. 2 for stratigraphy). These glacial and nonglacial deposits have been assigned ages on the basis of topographic position (BAKER, 1967), as well as weathering features, degree of soil expression, and radiocarbon (MA- FIGURE 2. Correlation diagram for Pleistocene and Holocene de­ HANEY, 1979, 1980, 1981, 1982a, 1982b). posits on Mount Kenya. Unit boundaries are based on maximum and minimum dates for glacial and nonglacial sediments. Dashes denote CLIMATE boundaries for which there is no radiometric control. Arrows indicate directions in which boundaries are likely to be shifted when additional The Afroalpine area rises from ~ 3200 m to the summit dates become available.
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