jeanflop/post_ocr_correction2 huggingface dataset:
2. Glaciolacustrine deposits; 3. Supraglacial till; 4. Subglacial till; 5. Paleozoic bedrock. Location of 14C samples in the glaciolacustrine unit. White dots correspond to radiocarbon dates presented by Vilaplana [15]: ≧ 34,000 yr BP (Ly-3045); ≧31,500 yr BP (Ly-3046); ≧33,000 yr BP (Ly-2942) and ≧ 33,000 yr BP (Ly-2943). Black dots correspond to radiocarbon dates presented in this paper: Llestui A, 21,650 ±900 yr BP (Gif-8778 bis); Llestui B, Not enough gas obtained (Gif-8779 bis) and Llestui C, 18,240 ±600 yr BP (Gif-8780 bis). Fig. 2. — Coupe géologique du complexe glacio-lacustre de Llestui. Légende: 1. Dépôts de cône de déjection; 2. Dépôts glacio-lacustres; 3. Till supra-glaciaire; 4. Till sous-glaciaire; 5. Substrat paléozoïque. Situation des échantillons datés dans l'unité glacio-lacustre. Les points blancs correspondent aux datations présentées par Vilaplana [15]: ≧ 34 000 ans BP (Ly-3045); ≧ 31 500 ans BP (Ly-3046); ≧ 33 000 ans BP (Ly-2942) et ≧ 33 000 ans BP (Ly-2943). Les points noirs correspondent aux nouvelles datations présentées dans cette note : Llestui A, 21 650± 900 ans BP (Gif-8778 bis); Llestui B, Insuffisante quantité de gaz (Gif-8779 bis) et Llestui C, 18 240 ±600 ans BP (Gif-8780 bis). The relative position of Llestui complex within the general context of the last glacial cycle in the Pyrenees was established after the geomorphological study of the whole Noguera Ribagorçana glacial basin. According to several authors ([6], [14], [15]), the sedimentation of several units took place during a period of glacial stabilization within the long deglaciation period which followed the last glacial maximum. However, the geometry of the glaciolacustrine unit shows an upstream migration of the glaciolacustrine environment as a result of a glacial progression (fig. 2). Thus, during the so-called Postmaximum stabilization phase, Llauset glacier was active and even showed a small pulsation or readvance. RADIOCARBON DATES. — Four radiocarbon dates of glaciolacustrine rhythmites from Llestui were presented by Vilaplana [15] being older or equal to 34,000 (Ly-3045), 31,500 (Ly-3046) and 33,000 years BP (Ly-2942 and Ly-2943). According to these dates, several ages have been proposed and discussed for the Llestui complex: between 65,000 and 31,500
within the general context of the last glacial cycle in the Pyrenees was established after the geomorphological study of the whole Noguera Ribagorçana glacial basin. According to several authors ([6], [14], [15]), the sedimentation of several units took place during a period of glacial stabilization within the long deglaciation period which followed the last glacial maximum. However, the geometry of the glaciolacustrine unit shows an upstream migration of the glaciolacustrine environment as a result of a glacial progression (fig. 2). Thus, during the so-called Postmaximum stabilization phase, Llauset glacier was active and even showed a small pulsation or readvance. RADIOCARBON DATES. — Four radiocarbon dates of glaciolacustrine rhythmites from Llestui were presented by Vilaplana [15] being older or equal to 34,000 (Ly-3045), 31,500 (Ly-3046) and 33,000 years BP (Ly-2942 and Ly-2943). According to these dates, several ages have been proposed and discussed for the Llestui complex: between 65,000 and 31,500 years BP [15], between 45,000 and 34,000 years BP [6]. Contamination of glaciolacustrine sediments by allochthonous material (e.g. dead carbon) and/or the hard water effect may cause an aging of the 14C dates obtained. Silurian shales containing graphite crop out extensively in Llauset basin and the presence of inactive carbon (graphite) in the glaciolacustrine rhythmites can significantly modify the age of the sediments [19]. In order to demonstrate the aging of the former dates from Llestui, three new samples corresponding to the bottom, middle and top of the glaciolacustrine sequence were taken for 14C dating (fig. 2). Combustion of the sample within a furnace, at low temperature under pure oxygen flow, would provide the CO2 gas deriving from organic matter and, therefore, the representative fraction to be dated. The ideal combustion temperature for soot carbon contained in aerosol collected on a glass fibre filter was determined by Cachier et al. [20] to be 340°C. However, we were obliged to perform the combustion at a temperature of 400°C, due to the nature of the sample itself and to the large amount (1 kg) needed, resulting in an abrupt temperature decrease towards the centre of the furnace. Thus, the 14C ages obtained
(graphite) in the glaciolacustrine rhythmites can significantly modify the age of the sediments [19]. In order to demonstrate the aging of the former dates from Llestui, three new samples corresponding to the bottom, middle and top of the glaciolacustrine sequence were taken for 14C dating (fig. 2). Combustion of the sample within a furnace, at low temperature under pure oxygen flow, would provide the CO2 gas deriving from organic matter and, therefore, the representative fraction to be dated. The ideal combustion temperature for soot carbon contained in aerosol collected on a glass fibre filter was determined by Cachier et al. [20] to be 340°C. However, we were obliged to perform the combustion at a temperature of 400°C, due to the nature of the sample itself and to the large amount (1 kg) needed, resulting in an abrupt temperature decrease towards the centre of the furnace. Thus, the 14C ages obtained are: Llestui A: 21,650±900 years BP; with δ13C : -20.42‰ (Gif-8778 bis). Llestui B: Not enough gas obtained (Gif-8779 bis). Llestui C : 18,240 ±600 years BP; with δ13C: -21.68‰ (Gif-8780 bis). The ages of the three samples (Llestui A, B and C) from the combustion of the residue at 900°C, are older than 40,000 years BP. These ages may reflect the aging due to the graphite content in the glaciolacustrine rhythmites of Llestui. However, age inversion between Llestui A and C has to be noted. If we agree with the fact that either contamination by modern carbon that might have occurred during sampling or aging due to slightly higher temperature combustion are unlikely, an explanation would be the large variations of atmospheric 14C content at that time, resulting in a variable 14C age drift ranging between 1,000 and 3,000 years [21]. According to these new 14C dates, the age of Llestui complex cannot be more accurately fixed than around 18,000-21,000 years BP. Former dates presented by Vilaplana [15] show an aging due to contamination of glaciolacustrine sediments by allochthonous material coming from the Silurian graphitic shales. CONCLUSIONS. — The juxtaglacial complex of Llestui was formed during the so-called Post-maximum
900°C, are older than 40,000 years BP. These ages may reflect the aging due to the graphite content in the glaciolacustrine rhythmites of Llestui. However, age inversion between Llestui A and C has to be noted. If we agree with the fact that either contamination by modern carbon that might have occurred during sampling or aging due to slightly higher temperature combustion are unlikely, an explanation would be the large variations of atmospheric 14C content at that time, resulting in a variable 14C age drift ranging between 1,000 and 3,000 years [21]. According to these new 14C dates, the age of Llestui complex cannot be more accurately fixed than around 18,000-21,000 years BP. Former dates presented by Vilaplana [15] show an aging due to contamination of glaciolacustrine sediments by allochthonous material coming from the Silurian graphitic shales. CONCLUSIONS. — The juxtaglacial complex of Llestui was formed during the so-called Post-maximum stabilization phase corresponding to the long deglaciation period which followed the last glacial maximum in the Pyrenees ([6], [14], [15]). This complex is placed in a margin of Llauset glacier, at a middle altitude (about 1,600 m a.s.l.) and far away from the maximum extent achieved by the Noguera Ribagorçana glacier (fig. 1). New 14C dates presented in this note allow us to date the Llestui complex at around 18,000-21,000 years BP. Former ages proposed for this complex ([6], [7], [15]) are to be considered as too old due to an aging of 14C dates resulting from contamination of glaciolacustrine sediments by Silurian graphite. The proposed age for the Llestui complex clearly shows that the last glacial maximum extent in the Pyrenees is previous to the Last Glacial Maximum. Surely, at around 18,000-20,000 years BP Pyrenean glaciers had significantly regressed with respect to their position during their phase of maximum development. Moreover, Jalut et al. 10] asserted that no advance of Pyrenean glaciers took place between 18,000 and 20,000 years BP. In any case, the age which has been proposed for the Llestui complex in this note suggests that during the Last Glacial Maximum, a major phase of deposition is
glacier (fig. 1). New 14C dates presented in this note allow us to date the Llestui complex at around 18,000-21,000 years BP. Former ages proposed for this complex ([6], [7], [15]) are to be considered as too old due to an aging of 14C dates resulting from contamination of glaciolacustrine sediments by Silurian graphite. The proposed age for the Llestui complex clearly shows that the last glacial maximum extent in the Pyrenees is previous to the Last Glacial Maximum. Surely, at around 18,000-20,000 years BP Pyrenean glaciers had significantly regressed with respect to their position during their phase of maximum development. Moreover, Jalut et al. 10] asserted that no advance of Pyrenean glaciers took place between 18,000 and 20,000 years BP. In any case, the age which has been proposed for the Llestui complex in this note suggests that during the Last Glacial Maximum, a major phase of deposition is recorded in the Pyrenees. The glacial dynamics of Llauset glacier was characterized by a pulsation which gave way to the formation of the juxtaglacial complex of Llestui. This glacial pulsation could be the local response in the Pyrenees to the global cooling recorded throughout the world during the Last Glacial Maximum. However, the narrowness of Pyrenean valleys has not allowed the preservation of terminal morainic complexes related to the glacial pulsation which took place during the Post-maximum stabilization phase, and only juxtaglacial complexes such as the Llestui complex are preserved in the Pyrenees. Financial support has been provided from the DGICYT Project PB88-0209. Note remise le 20 août 1992, acceptée après revision le 8 janvier 1993. REFERENCES [1] W. F. RUDDIMAN and A. McINTYRE, Palaeogeogr., Palaeoclimatol., Palaeoecol., 35, 1981, pp. 145-214. [2] D. G. MARTINSON, N. G. PISIAS, J. H. HAYS, J. IMBRIE, T. C. MOORE and N. J. SHACKLETON, Quaternary Research, 27, 1987, pp. 1-29. [3] G. SERET, E. DRICOT and G. WANSARD, Nature, 346, 1990, pp. 453-456. [4] J. GUIOT, A. PONS, J. L. DE BEAULIEU and M. REILLE, Nature, 338, 1989, pp. 309-313. [5] J. BRU, A. GÓMEZ, D. SERRAT, J. VENTURA and J. M. VILAPLANA, Actas
Maximum. However, the narrowness of Pyrenean valleys has not allowed the preservation of terminal morainic complexes related to the glacial pulsation which took place during the Post-maximum stabilization phase, and only juxtaglacial complexes such as the Llestui complex are preserved in the Pyrenees. Financial support has been provided from the DGICYT Project PB88-0209. Note remise le 20 août 1992, acceptée après revision le 8 janvier 1993. REFERENCES [1] W. F. RUDDIMAN and A. McINTYRE, Palaeogeogr., Palaeoclimatol., Palaeoecol., 35, 1981, pp. 145-214. [2] D. G. MARTINSON, N. G. PISIAS, J. H. HAYS, J. IMBRIE, T. C. MOORE and N. J. SHACKLETON, Quaternary Research, 27, 1987, pp. 1-29. [3] G. SERET, E. DRICOT and G. WANSARD, Nature, 346, 1990, pp. 453-456. [4] J. GUIOT, A. PONS, J. L. DE BEAULIEU and M. REILLE, Nature, 338, 1989, pp. 309-313. [5] J. BRU, A. GÓMEZ, D. SERRAT, J. VENTURA and J. M. VILAPLANA, Actas de la 1 Reunión del Cuaternario Ibérico, GETC-GTPEQ, 1985, pp. 165-183. [6] J. BORDONAU, Els complexos glacio-lacustres relacionats amb el darrer cicle glacial als Pirineus, Geoforma Ediciones, Logroño, 1992, 251 p. [7] J. BORDONAU, III Congreso Geológico de España y VIII Congreso Latinoamericano de Geología, Simposios, 2, 1992, pp. 48-53. [8] J. BORDONAU, D. SERRAT and J. M. VILAPLANA, in A. CEARRETA and F. M. UGARTE Eds., The Late Quaternary in the Western Pyrenean Region, Servicio Editorial Universidad del País Vasco, Bilbao, 1992, pp. 303-312. [9] V. ANDRIEU, J. HUBSCHMAN, G. JALUT and G. HÉRAIL, Bulletin de l'Association française pour l'étude du Quaternaire, 2/3, 1988, pp. 55-67. [10] G. JALUT, J. MONTSERRAT MARTI, M. FONTUGNE, G. DELIBRIS, J. M. VILAPLANA and R. JULIA, Quaternary Science Reviews, 11, 1992, pp. 449-480. [11] M. BAKALOWICZ, P. SORRIAUX and D. C. FORD, Norsk Geografisk Tidsskrift, 38, 1984, pp. 193-197. [12] J. MONTSERRAT MARTI, Monografías del Instituto Pirenaico de Ecología (CSIC), 6, 1992, 147 p. [13] J. BORDONAU, J. BRO, D. SERRAT and J. M. VILAPLANA, Abstracts XIIth INQUA International Congress, Ottawa, 1987, p. 133. [14] D. SERRAT, J. M. VILAPLANA and C. E. MARTI BONO, in E. B. EVENSON, Ch. SCHLOCHTER and J.
[8] J. BORDONAU, D. SERRAT and J. M. VILAPLANA, in A. CEARRETA and F. M. UGARTE Eds., The Late Quaternary in the Western Pyrenean Region, Servicio Editorial Universidad del País Vasco, Bilbao, 1992, pp. 303-312. [9] V. ANDRIEU, J. HUBSCHMAN, G. JALUT and G. HÉRAIL, Bulletin de l'Association française pour l'étude du Quaternaire, 2/3, 1988, pp. 55-67. [10] G. JALUT, J. MONTSERRAT MARTI, M. FONTUGNE, G. DELIBRIS, J. M. VILAPLANA and R. JULIA, Quaternary Science Reviews, 11, 1992, pp. 449-480. [11] M. BAKALOWICZ, P. SORRIAUX and D. C. FORD, Norsk Geografisk Tidsskrift, 38, 1984, pp. 193-197. [12] J. MONTSERRAT MARTI, Monografías del Instituto Pirenaico de Ecología (CSIC), 6, 1992, 147 p. [13] J. BORDONAU, J. BRO, D. SERRAT and J. M. VILAPLANA, Abstracts XIIth INQUA International Congress, Ottawa, 1987, p. 133. [14] D. SERRAT, J. M. VILAPLANA and C. E. MARTI BONO, in E. B. EVENSON, Ch. SCHLOCHTER and J. RABASSA Eds., Tills and related deposits, A. A. Balkema, Rotterdam, 1983, pp. 132-144. [15] J. M. VILAPLANA, Acta Geolólgica Hispanica, 18, 3/4, 1983, pp. 217-233. [16] J. VENTURA, Revista Catalana de Geografía, 1, No. 2, 1986, pp. 57-69. [17] J. BORDONAU, Quaternary International, 18, 1993 (in press). [18] J. M. VILAPLANA and J. BORDONAU, Bulletin de l'Association française pour l'étude du Quaternaire, 4, 1989, pp. 219-224. [19] J. M. VILAPLANA, J. MONTSERRAT MARTI and Ch. SCHLOCHTER, in J. ROSE and Ch. SCHLOCHTER Eds, Quaternary type sections, Imagination or Reality?, A. A. Balkema, Rotterdam, 1989, pp. 113-124. [20] H. CACHIER, M. P. BRÉMOND and P. BUAT-MÉNARD, Tellus, 41 B, 1989, pp. 379-390. [21] E. BARD, B. HAMELIN, R. FAIRBANKS and A. ZINDLER, Nature, 345, 1990, pp. 405-409. J. B. and J. M. V. : Departament de Geologia Dinàmica, Geofísica i Paleontologia, Universitat de Barcelona, Zona Universitària de Pedralbes, 08071 Barcelona, Spain; M. F. : Centre des Faibles Radioactivités, Laboratoire mixte CNRS-CEA, Domaine du CNRS, avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France. In Beni-Snassen, this phenomenon would be linked with the morphology of the basin (Boudchiche, 1986; Naciri, 1986; Ouahhabi, 1986), which results in unfavourable conditions for the development of benthic
VILAPLANA and J. BORDONAU, Bulletin de l'Association française pour l'étude du Quaternaire, 4, 1989, pp. 219-224. [19] J. M. VILAPLANA, J. MONTSERRAT MARTI and Ch. SCHLOCHTER, in J. ROSE and Ch. SCHLOCHTER Eds, Quaternary type sections, Imagination or Reality?, A. A. Balkema, Rotterdam, 1989, pp. 113-124. [20] H. CACHIER, M. P. BRÉMOND and P. BUAT-MÉNARD, Tellus, 41 B, 1989, pp. 379-390. [21] E. BARD, B. HAMELIN, R. FAIRBANKS and A. ZINDLER, Nature, 345, 1990, pp. 405-409. J. B. and J. M. V. : Departament de Geologia Dinàmica, Geofísica i Paleontologia, Universitat de Barcelona, Zona Universitària de Pedralbes, 08071 Barcelona, Spain; M. F. : Centre des Faibles Radioactivités, Laboratoire mixte CNRS-CEA, Domaine du CNRS, avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France. In Beni-Snassen, this phenomenon would be linked with the morphology of the basin (Boudchiche, 1986; Naciri, 1986; Ouahhabi, 1986), which results in unfavourable conditions for the development of benthic life, as is seen in Valdorbia in the Apennines (Elmi, 1981). In this study, we concentrated on foraminiferal reactions in these environmental conditions. The study of quantitative and qualitative variations leads us to distinguish three faunal groups, each group corresponding to environmental conditions which are more or less oxygenated. The first group, characterized by the Lenticulina dominance, corresponds to the beginning of the ever-increasing confinement. As a result (second group) all individuals are small in size except for the Lenticulina obonensis mg. Planularia which greatly developed in the coarse fraction. This species, which appears at this critical stage is deriving from Lenticulina inaequistriata mg. Planularia by simplification of the ribs (fig. 2). When the tolerable limit of the confinement is exceeded, the foraminifera disappear in the coarse fraction and in the small fraction. They reappear in conditions similar to those that governed their distribution in the second group. We note, however, the extreme scarcity of large forms. This can be put in parallel with what Phleger and Soutar (1973) observed in some oxygen-deficient environments: the dominant foraminifera are smaller than 250 μm. In the Oued Bou-Zit section (5 km SE Ain Reggada), in the lower part of the Serpentinus zona,
first group, characterized by the Lenticulina dominance, corresponds to the beginning of the ever-increasing confinement. As a result (second group) all individuals are small in size except for the Lenticulina obonensis mg. Planularia which greatly developed in the coarse fraction. This species, which appears at this critical stage is deriving from Lenticulina inaequistriata mg. Planularia by simplification of the ribs (fig. 2). When the tolerable limit of the confinement is exceeded, the foraminifera disappear in the coarse fraction and in the small fraction. They reappear in conditions similar to those that governed their distribution in the second group. We note, however, the extreme scarcity of large forms. This can be put in parallel with what Phleger and Soutar (1973) observed in some oxygen-deficient environments: the dominant foraminifera are smaller than 250 μm. In the Oued Bou-Zit section (5 km SE Ain Reggada), in the lower part of the Serpentinus zona, foraminifera are also very small, and mostly in uncoiled form such as Lingulina, Dentalina and Nodosaria. Coiled forms tend to disappear and to be replaced by uncoiled forms. The quantitative and qualitative evolution first tends towards the development of flattened-test forms, then in the course of the Serpentinus zona, to that of elongated-test foraminifera. Leutenegger and Hansen (1979) demonstrated that the gas exchange is effected in the pores. Consequently the non-perforated tests like Miliolids cannot tolerate oxygen-deficient environments. This partly tells why the Ophtalmidium are known to be indicative of shallow and well-oxygenated waters (Boudchiche, 1986; Brooke and Braun in Haynes, 1981; El Kanchoufi, 1988; Gordon, 1970; Gregory, 1990). We point out here that this element is true only when the Ophtalmidiums are big; those whose size is inferior to 250 μm can prosper in a more-or-less confined environment (Boudchiche's thesis). The development of flattened and elongated forms in the course of Serpentinus Zona is similar: increased exchange surface facilitated respiration. The succession of three species of Planularia (fig. 2) within a short time could indicate that the loss of ribs favours life in this kind of environment. However, the reduction of the size, which we consider as belonging to
that of elongated-test foraminifera. Leutenegger and Hansen (1979) demonstrated that the gas exchange is effected in the pores. Consequently the non-perforated tests like Miliolids cannot tolerate oxygen-deficient environments. This partly tells why the Ophtalmidium are known to be indicative of shallow and well-oxygenated waters (Boudchiche, 1986; Brooke and Braun in Haynes, 1981; El Kanchoufi, 1988; Gordon, 1970; Gregory, 1990). We point out here that this element is true only when the Ophtalmidiums are big; those whose size is inferior to 250 μm can prosper in a more-or-less confined environment (Boudchiche's thesis). The development of flattened and elongated forms in the course of Serpentinus Zona is similar: increased exchange surface facilitated respiration. The succession of three species of Planularia (fig. 2) within a short time could indicate that the loss of ribs favours life in this kind of environment. However, the reduction of the size, which we consider as belonging to an early sexual maturity acquisition, constitutes another kind of adaptation, which enables reduction of energy expenditure. Indeed, the study of present foraminifera shows that individuals whose size is inferior to 250 μm have a low oxygen consumption (Bradshaw, 1961). The tendency towards uncoiled forms of Lenticulina found in the highstand sea level (Cubaynes and Ruget, 1985; Cubaynes et al., 1989), as well as the development of the pores and the increase of the diameter of the Orbulina with the increase of temperature shown by Bee et al. (1973), could be explained similarly: as breathing conditions became more difficult, the individuals reacted to it by an increase of the oxygen-exchange surface. INTRODUCTION. — L'étude des variations quantitatives et qualitatives des peuplements de foraminifères des Béni-Snassen orientaux (Maroc nord-oriental) a permis, lors de travaux antérieurs (Boudchiche, 1986, 1990; Boudchiche et al., 1987), de mettre en évidence un confinement biologique au cours du Toarcien inférieur. Ce phénomène est connu à des degrés différents dans d'autres régions d'Afrique du Nord (Boutakiout, 1990; Sebane, 1984). Il coïncide avec la remontée eustatique constatée au cours du Toarcien basal (Hallam, 1981; Haq et al., 1987) et avec l'événement anoxique connu en plusieurs points de la Téthys occidentale