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U dating of marine and continental mollusk shell, and travertine samples in quaternary deposits in Morocco
Radiation Physics and Chemistry 61 (2001) 697–699
Th/U dating of marine and continental mollusk shell, and travertine samples in quaternary deposits in Morocco S. Semghoulia,*, A. Choukrib, R. Cherkaoui El Mourslia, E. Jahjouhb, A. Chouaka, A. Ben Mohammadic, M. Latirisd, J.-L. Reysse, J.L. Plaziatf ! Laboratoire de Physique Nucleaire, Faculte! des Sciences, BP.1014, Rabat, Morocco ! et d’Environement, Faculte! des Sciences, BP.133, Kenitra, ! Laboratoire des Faibles Radioactivites Morocco c ! ! ! Departement de Geologie, Faculte! des Sciences,BP.133, Kenitra, Morocco d ! ! Laboratoire de Geomorphologie, Faculte! des Lettres, Kenitra, Morocco e LSCE / Labo Mixte CNRS/CEA, Gif-sur-Yvette, France f Laboratoire de Petrologie Sedimentaire, Universite! Paris-sud, URA 723, Orsay Cedex, France a
b
Abstract Th/U measurements by alpha spectrometry have been carried out on samples of quaternary deposits in Morocco Ages yielded by marine samples allow identification of two high sea levels: (1) Last Interglacial B133 ka (ii) Harounien sea level B266 ka. The scatter of ages around 133 and 266 ka has been discussed and corrected. For continental samples, the ages allowed identification of humid episodes during which these samples have been deposited. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Th/U dating; Mollusk shell; Travertine; Quaternary deposits
1. Introduction U-series dating of reef corals and fossil mollusk shells by alpha and thermal ionisation mass spectrometric methods has been used to assess the time scale of sea level and continental changes occurring over the last 350,000 years. Additional data from coastal and continental areas from Morocco are needed in order to better document the regional specifities of Moroccan quaternary materials. In this study, we have undertaken U-series measurements by alpha spectrometry on fossil mollusks and travertines from the region in order to: (i) better document the behaviour of the Th/U system in such fossils, and; (ii) to determine the chronology of these areas.
2. Results of the study Radiochemical results of nine mollusk shells sampled from the Mediterranean coast of Morocco are presented *Corresponding author. Fax: +212-7-778973.
in Table 1. The age range is between 54,000 and 266,000 years. These ages can be subdivided into two populations: 54,000–132,000 years and; 183,000–266,000 years. The first group, applying to samples taken above shorelines, at elevations of 672 m, can be compared to other sites in the world, specifically to the 5e stage highsea level dated at about 122,000 years (Reyss et al., 1993). The second population, provided by mollusk shells sampled at B15 m, corresponds to the harounien stage sea-level previously dated to be between 220,000 and 280,000 years (Choukri et al., 1995). The ages To122,000 years applying to the first group and the ages To260,000 years applying to the second group were rejuvenated by uranium post-incorporation after fossilisation. The correction of these rejuvenated ages by two models (Table 1) based, respectively, on continual and episodic uranium post-incorporation assumptions, shows that mollusk shell samples taken from the same shoreline cannot be corrected in the same way: such results have been discussed elsewhere (Choukri et al., 2000). Results of radiochemical dating for four mollusk shells of rumina, one aspitaria and one ostrich egg sampled on the Moroccan south-east region (Errachidia)
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 3 7 8 - 4
698
S. Semghouli et al. / Radiation Physics and Chemistry 61 (2001) 697–699
Table 1 Isotope data, U-series ages in mollusks from the Mediterranean coast of Morocco. Age CCM: age corrected by continual model. Age CEM: age corrected by episodic model. Age CS: Age calculated by equation of closed system 230Th/234U. ND: species non-identified Species
Altitude (m)
U (ppm)
230
Th/232Th
234
U/238U
230
Th/234U
Age CS (ky)
Age CCM (ky)
Age CEM (ky)
ND ND
+6 +6
1.5070.06 1.4370.06
416 310
1.2570.04 1.2170.05
0.470.02 0.4770.02
54þ3:07 2:99 68þ4:56 4:39
120þ10 8 154þ11 2
74þ4:21 4:1 101þ6:8 6:5
ND
+6
1.7870.07
45
1.4070.05
0.4670.06
66þ12:2 11
147þ33 27
159þ29:4 26:5
133þ8:27 7:76 120þ9:73 9:01 80þ4:14 4 81þ5:44 5:2 266þ38 29 183þ30:6 24:5
F
F
Lamilli Branche
+6
0.7670.02
71
1.5370.37
0.7570.02
Lamilli Branche
+6
0.8670.04
125
1.3970.07
0.7070.03
Thais
+6
1.7870.04
121
1.3470.02
0.5370.02
Thais
+6
2.0270.08
360
1.2570.05
0.5470.02
ND
+15
1.1670.09
225
1.2870.11
0.9770.03
ND
+15
1.2670.07
239
1.3870.08
0.8670.05
F
F
191þ14 13
114þ7:6 7:29
193þ38 12:8
115þ7:7 7:38
F
F
664þ336 164
267þ44:6 35:75
Table 2 Radiochemical results of fossils from the Moroccan south-east region (Errachidia) Species
U (ppm)
234
U/238U
230
Th/234U
Age(ky)
Rumina Rumina
0.3470.06 0.5970.05
3.6270.68 1.8170.15
0.0670.02 0.0670.02
6:59þ2:35 2:31 7:31þ2:38 2:34
Rumina
0.1770.01
1.5970.16
0.0870.02
9:26þ1:86 1:83
Rumina
0.2270.03
1.9170.33
0.0970.02
10:27þ2:57 2:52
Ostrich egg
0.2570.01
1.4370.06
0.0570.005
5:65þ0:55 0:55
Aspitaria
0.5370.02
1.7170.06
0.0770.006
7:35þ0:65 0:65
Fig. 1. U–Th isochrons constructed from the data obtained for four aliquots of analysed travertine: (a) 230Th/232Th vs. 234U/232Th and (b) 234U/232Th vs. 238U/232Th.
are listed in Table 2. The results, ranging from between 6000 and 10,000 years, are in good agreement with 14C ages obtained by Ben Mohammadi et al. (2001), indicating that this region experienced a humid episode, being in good correlation with the Holocene stage. The arid conditions subsequent to the humid episode have likely ensured a fair closure of the radioactive system of such fossils since that time.
The Total Sample Dissolution (TSD) method was used for U–Th isochron dating of four aliquots of the travertine material sampled from the right bank of the Korifla River, north-west of the Moroccan Central Plateau. The isochrons of Fig. 1 show good linear correlation between the date ratio for 230Th/232Th and the date ratios 234U/232Th, 234U/232Th and 238U/232Th. We regard this to be an evidence for the travertine
S. Semghouli et al. / Radiation Physics and Chemistry 61 (2001) 697–699
materials not being disturbed by migration of radionuclides following their deposition. The authigenic 230 Th/234U and 234U/238U ratio values of this sample are (0.05870.042) and (2.16570.07), respectively. The U–Th isochronal age calculated from these ratios is 6.45+4.88 4.70 ky. This shows that formation of the analysed travertine material occurred during the last humid episode, as dated by the continental mollusk shells. These radiochemical analyses of various fossils allow us to improve the existing analytical techniques and to understand transformation mechanisms which may affect the U–Th isotopic systems of these fossils.
References Ben Mohammadi, A., Riser, J., Ballais, J.L., Fontugne, M., Semghouli, S, and Choukri, A., 2001. L’Holoc"ene du bassin
699
de Taghbalt, des Feijas et de la vall!ee de l’oued Mird (retomb!ee saharienne du Jebel Sarhro). Z Geomorph. N.F. (in revision). Choukri, A., Jahjouh, E., Semghouli, S., Hakam, O., Reyss, J.L., 2000. Modelling of the relationship between apparent and real ages of fossil mollusk shells related to the posterior incorporation of the uranium: application to study of marine level in the past. International Conference on Environmental Modelling and Simulation San Diego, CA, 23–27 January. Choukri, A., Reyss, J.L., Plaziat, J.C., Sperber, F.O., Berrada, M., 1995. Reliability of sea level dating using Th/U method for mollusks from the West Coast of the Red Sea and from the Atlantic Coast of the Moroccan High Atlas; Appl. Radiat. Isot. 7, 653–654. Reyss, J.L., Choukri, A., Plaziat, J.C., Purser, B.H., 1993. Datation radiochimiques des r!ecifs coralliens de la rive occidentale du Nord de la Mer Rouge, premi"eres implications stratigraphiques et tectoniques. C. R. Acad. Sci. Paris s!er. II 317, 487–492.
Th/U dating of marine and continental mollusk shell, and travertine samples in quaternary deposits in Morocco S. Semghoulia,*, A. Choukrib, R. Cherkaoui El Mourslia, E. Jahjouhb, A. Chouaka, A. Ben Mohammadic, M. Latirisd, J.-L. Reysse, J.L. Plaziatf ! Laboratoire de Physique Nucleaire, Faculte! des Sciences, BP.1014, Rabat, Morocco ! et d’Environement, Faculte! des Sciences, BP.133, Kenitra, ! Laboratoire des Faibles Radioactivites Morocco c ! ! ! Departement de Geologie, Faculte! des Sciences,BP.133, Kenitra, Morocco d ! ! Laboratoire de Geomorphologie, Faculte! des Lettres, Kenitra, Morocco e LSCE / Labo Mixte CNRS/CEA, Gif-sur-Yvette, France f Laboratoire de Petrologie Sedimentaire, Universite! Paris-sud, URA 723, Orsay Cedex, France a
b
Abstract Th/U measurements by alpha spectrometry have been carried out on samples of quaternary deposits in Morocco Ages yielded by marine samples allow identification of two high sea levels: (1) Last Interglacial B133 ka (ii) Harounien sea level B266 ka. The scatter of ages around 133 and 266 ka has been discussed and corrected. For continental samples, the ages allowed identification of humid episodes during which these samples have been deposited. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Th/U dating; Mollusk shell; Travertine; Quaternary deposits
1. Introduction U-series dating of reef corals and fossil mollusk shells by alpha and thermal ionisation mass spectrometric methods has been used to assess the time scale of sea level and continental changes occurring over the last 350,000 years. Additional data from coastal and continental areas from Morocco are needed in order to better document the regional specifities of Moroccan quaternary materials. In this study, we have undertaken U-series measurements by alpha spectrometry on fossil mollusks and travertines from the region in order to: (i) better document the behaviour of the Th/U system in such fossils, and; (ii) to determine the chronology of these areas.
2. Results of the study Radiochemical results of nine mollusk shells sampled from the Mediterranean coast of Morocco are presented *Corresponding author. Fax: +212-7-778973.
in Table 1. The age range is between 54,000 and 266,000 years. These ages can be subdivided into two populations: 54,000–132,000 years and; 183,000–266,000 years. The first group, applying to samples taken above shorelines, at elevations of 672 m, can be compared to other sites in the world, specifically to the 5e stage highsea level dated at about 122,000 years (Reyss et al., 1993). The second population, provided by mollusk shells sampled at B15 m, corresponds to the harounien stage sea-level previously dated to be between 220,000 and 280,000 years (Choukri et al., 1995). The ages To122,000 years applying to the first group and the ages To260,000 years applying to the second group were rejuvenated by uranium post-incorporation after fossilisation. The correction of these rejuvenated ages by two models (Table 1) based, respectively, on continual and episodic uranium post-incorporation assumptions, shows that mollusk shell samples taken from the same shoreline cannot be corrected in the same way: such results have been discussed elsewhere (Choukri et al., 2000). Results of radiochemical dating for four mollusk shells of rumina, one aspitaria and one ostrich egg sampled on the Moroccan south-east region (Errachidia)
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 3 7 8 - 4
698
S. Semghouli et al. / Radiation Physics and Chemistry 61 (2001) 697–699
Table 1 Isotope data, U-series ages in mollusks from the Mediterranean coast of Morocco. Age CCM: age corrected by continual model. Age CEM: age corrected by episodic model. Age CS: Age calculated by equation of closed system 230Th/234U. ND: species non-identified Species
Altitude (m)
U (ppm)
230
Th/232Th
234
U/238U
230
Th/234U
Age CS (ky)
Age CCM (ky)
Age CEM (ky)
ND ND
+6 +6
1.5070.06 1.4370.06
416 310
1.2570.04 1.2170.05
0.470.02 0.4770.02
54þ3:07 2:99 68þ4:56 4:39
120þ10 8 154þ11 2
74þ4:21 4:1 101þ6:8 6:5
ND
+6
1.7870.07
45
1.4070.05
0.4670.06
66þ12:2 11
147þ33 27
159þ29:4 26:5
133þ8:27 7:76 120þ9:73 9:01 80þ4:14 4 81þ5:44 5:2 266þ38 29 183þ30:6 24:5
F
F
Lamilli Branche
+6
0.7670.02
71
1.5370.37
0.7570.02
Lamilli Branche
+6
0.8670.04
125
1.3970.07
0.7070.03
Thais
+6
1.7870.04
121
1.3470.02
0.5370.02
Thais
+6
2.0270.08
360
1.2570.05
0.5470.02
ND
+15
1.1670.09
225
1.2870.11
0.9770.03
ND
+15
1.2670.07
239
1.3870.08
0.8670.05
F
F
191þ14 13
114þ7:6 7:29
193þ38 12:8
115þ7:7 7:38
F
F
664þ336 164
267þ44:6 35:75
Table 2 Radiochemical results of fossils from the Moroccan south-east region (Errachidia) Species
U (ppm)
234
U/238U
230
Th/234U
Age(ky)
Rumina Rumina
0.3470.06 0.5970.05
3.6270.68 1.8170.15
0.0670.02 0.0670.02
6:59þ2:35 2:31 7:31þ2:38 2:34
Rumina
0.1770.01
1.5970.16
0.0870.02
9:26þ1:86 1:83
Rumina
0.2270.03
1.9170.33
0.0970.02
10:27þ2:57 2:52
Ostrich egg
0.2570.01
1.4370.06
0.0570.005
5:65þ0:55 0:55
Aspitaria
0.5370.02
1.7170.06
0.0770.006
7:35þ0:65 0:65
Fig. 1. U–Th isochrons constructed from the data obtained for four aliquots of analysed travertine: (a) 230Th/232Th vs. 234U/232Th and (b) 234U/232Th vs. 238U/232Th.
are listed in Table 2. The results, ranging from between 6000 and 10,000 years, are in good agreement with 14C ages obtained by Ben Mohammadi et al. (2001), indicating that this region experienced a humid episode, being in good correlation with the Holocene stage. The arid conditions subsequent to the humid episode have likely ensured a fair closure of the radioactive system of such fossils since that time.
The Total Sample Dissolution (TSD) method was used for U–Th isochron dating of four aliquots of the travertine material sampled from the right bank of the Korifla River, north-west of the Moroccan Central Plateau. The isochrons of Fig. 1 show good linear correlation between the date ratio for 230Th/232Th and the date ratios 234U/232Th, 234U/232Th and 238U/232Th. We regard this to be an evidence for the travertine
S. Semghouli et al. / Radiation Physics and Chemistry 61 (2001) 697–699
materials not being disturbed by migration of radionuclides following their deposition. The authigenic 230 Th/234U and 234U/238U ratio values of this sample are (0.05870.042) and (2.16570.07), respectively. The U–Th isochronal age calculated from these ratios is 6.45+4.88 4.70 ky. This shows that formation of the analysed travertine material occurred during the last humid episode, as dated by the continental mollusk shells. These radiochemical analyses of various fossils allow us to improve the existing analytical techniques and to understand transformation mechanisms which may affect the U–Th isotopic systems of these fossils.
References Ben Mohammadi, A., Riser, J., Ballais, J.L., Fontugne, M., Semghouli, S, and Choukri, A., 2001. L’Holoc"ene du bassin
699
de Taghbalt, des Feijas et de la vall!ee de l’oued Mird (retomb!ee saharienne du Jebel Sarhro). Z Geomorph. N.F. (in revision). Choukri, A., Jahjouh, E., Semghouli, S., Hakam, O., Reyss, J.L., 2000. Modelling of the relationship between apparent and real ages of fossil mollusk shells related to the posterior incorporation of the uranium: application to study of marine level in the past. International Conference on Environmental Modelling and Simulation San Diego, CA, 23–27 January. Choukri, A., Reyss, J.L., Plaziat, J.C., Sperber, F.O., Berrada, M., 1995. Reliability of sea level dating using Th/U method for mollusks from the West Coast of the Red Sea and from the Atlantic Coast of the Moroccan High Atlas; Appl. Radiat. Isot. 7, 653–654. Reyss, J.L., Choukri, A., Plaziat, J.C., Purser, B.H., 1993. Datation radiochimiques des r!ecifs coralliens de la rive occidentale du Nord de la Mer Rouge, premi"eres implications stratigraphiques et tectoniques. C. R. Acad. Sci. Paris s!er. II 317, 487–492.