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U-series evidence for two high Last Interglacial sea levels in southeastern Tunisia
Quaternary Science Reviews 22 (2003) 343–351
U-series evidence for two high Last Interglacial sea levels in southeastern Tunisia Younes Jedouia,*, Jean-Louis Reyssb, Nejib Kallelc, Mabrouk Montacerc, Hedi Ben Isma.ıld, Eric Davaude b
a D!epartement de G!eologie, Ecole Nationale d’Ing!enieurs de Sfax, B.P. ‘‘W’’, 3038 Sfax, Tunisia Laboratoire des Sciences du Climat et de l’Environnement, UMR 1572, CNRS-CEA, Parc du CNRS, 91198, Gif-sur-Yvette cedex, France c Facult!e des Sciences de Sfax, Route de Soukra, B.P. 802, 3018 Sfax, Tunisia d Facult!e des Sciences de Tunis, Campus Universitaire, 1060 Tunis Bev!ed"ere, Tunisia e Department of Geology and Palaeontology, University of Geneva, CH-1211 Geneva 4, Switzerland
Received 18 September 2001; accepted 24 July 2002
Abstract Pleistocene raised marine deposits in southeastern Tunisia consist of a siliciclastic unit that culminates at +3 m asl, overlain by a carbonate-rich unit with Strombus bubonius that culminates at +5 m asl. 234U/238U ratios on fossil Ostraea shells from both units are compatible with a marine origin from the uranium incorporated into the shells and show narrowly clustered 230Th-ages, respectively, between 147 and 110 ka and 141 and 100 ka. The two units were therefore developed during Marine Isotopic Substage 5e (MISs 5e, Last Interglacial). Their heights are comparable to those of contemporaneous marine deposits found in many tectonically stable areas of the world such as in the Bahamas and in Bermuda and can therefore be used as indicators of eustatic changes during the Last Interglacial. It is argued that on the basis of this evidence, the Last Interglacial was characterised by two eustatic maxima. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction On the basis of geomorphological criteria, the raised Pleistocene marine deposits along Tunisian coastlines have been subdivided into three superposed formations: from oldest to youngest, the Douira, Rejiche and Chebba formations (Paskoff and Sanlaville, 1977, 1980, 1983; Oueslati et al., 1982; Ben Ouezdou, 1986; Oueslati, 1986). Their preliminary dating in the central Tunisia, using a-counting uranium series on mollusc shells and amino acid ratios, assigned the Rejiche and the Chebba formations, respectively, to the high sea level stands of the MISs 5e and 5c. The Douira formation has been attributed to a high eustatic sea level older than the Last Interglacial (Miller and Paskoff, 1986; Paskoff and Oueslati, 1988). Neither sedimentological nor geochronological studies have been conducted in southeastern Tunisia *Corresponding author. Tel.: +216-74-274-088; fax: +216-74-275595. E-mail address: [email protected] (Y. Jedoui).
(Fig. 1). Paskoff and Sanlaville (1983), and Jedoui et al. (1996, 1998) demonstrated that this area has been tectonically stable during the last 130 ka. If this is the case, it is difficult to explain the emergence of the deposits assigned to the MISs 5c, as sea level at that time was lower than today by more than 10 m (Chappell and Shackleton, 1986). This problem may be explained by imprecise age attribution caused by the fact that mollusc shells provide usually unreliable U-series ages (Broecker, 1963; Szabo and Rosholt, 1969; Kaufman et al., 1971, 1996; Bernat et al., 1985; Hillaire-Marcel et al., 1986; McLaren and Rowe, 1996, etc.). In this paper we applied a sedimentological approach to defining the different lithostratigraphic sequences within the coastal marine deposits of southeastern Tunisia. We also tested the reliability of dating different mollusc shell species using the U-series a-counting chronology. It is necessary to use shell species since corals are largely absent from these coastal deposits. Our results suggest that Ostraea shells are more reliable material than other shell material for U-series analysis.
0277-3791/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 2 ) 0 0 1 3 3 - 6
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Fig. 1. Location and map of marine Pleistocene deposits in southeastern Tunisia. Arrows indicate the analysed shell sites: (1) Oued Tmoula (Tm), (2) El Grine (Gt), (3) Henchir Damous (J), (4) El Gala (Gl), (5) Taguermess (Tg), (6) Tarbella (T), (7) Sidi Yati (Sy), (8) Ras Karboub (Kb), (9) El Bibane (BB), (10) S. el Briga (Bg).
2. Sedimentology Sedimentological analysis of marine Pleistocene deposits along the coastal area of southeastern Tunisia indicates two distinct lithostratigraphic units (Jedoui, 2000; Jedoui et al., 2002; Fig. 2). The lower unit is a finegrained bioclastic quartz-rich sand devoid of Strombus fossils (a warm water Senegalese fauna) and locally includes well-developed aeolian sediment facies. The lower unit onlaps an erosion surface which truncates Mio-Plio-Villafranchian deposits. The upper unit is more extensive and consists of carbonate ooids, peloids and bioclastes along with a warm water Senegalese fauna, specifically Strombus bubonius. This unit clearly displays a shallowing-upward sequence beginning with subtidal deposits and ending with an aeolian facies. In some localities on Jerba island,
a Strombus-rich boulder bed terminates the Pleistocene marine deposits. This unit includes well-rounded cobbles originating from calcareous Villafranchian deposits and also calcarenite blocks from the carbonate unit. The matrix is made of bioclastic calcarenite containing ooids and mollusc shells. The origin of the gravel has been extensively debated, and two hypothesis have received the greatest attention: (i) an independent transgression separated from that associated with the carbonate unit by a regression (Paskoff and Sanlaville, 1983); and (ii) the gravel deposits can result from a storm erosion following deposition of the carbonate unit (Mahmoudi, 1988; Jedoui et al., 2002). In this work, the second hypothesis is retained since our field investigations indicate that boulder beds, which are very similar to those found within the carbonate unit, fill erosional notches. This choice has no influence on the conclusions of this paper.
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Fig. 2. Synthetic cross-section of marine Pleistocene deposits in southeastern Tunisia. Quartz-rich unit: 1a, marine siliciclastic sands; 1b, eolian siliciclastic sands. Carbonate unit: 2a, marine oolitic sands with Strombus; 2b, eolian oolitic sands.
Therefore, two main lithostratigraphic units can be distinguished in southeastern Tunisia. Each one overlies an irregular erosion surface truncating underlying deposits. Assuming tectonic stability in the region over the past 130 ka, the units thus correspond to at least two sedimentation phases associated with two positive eustatic pulsations. In this study, the lower unit will be designated as the ‘‘siliciclastic unit’’ while the upper one will be designated as the ‘‘carbonate unit’’.
3. Elevation of the Pleistocene deposits and tectonic activity The coastal deposits of southeastern Tunisia have been mapped and their paleogeography has been reconstructed (Jedoui and Bouaziz, 1997; Jedoui and Perthuisot, 1997a, b; Jedoui, 2000). The elevation of the marine units at many coastal localities has also been determined. Deposits of the siliciclastic unit culminate at B2–3 m above the mean present sea level whereas those of the carbonate unit reach an elevation of B3 to 6 m. The heights of these paleobeaches are comparable to those of contemporaneous marine deposits found in many tectonically stable areas of the world such as Bermuda (Harmon et al., 1983; Vacher and Hearty, 1989), Bahamas (Chen et al., 1991), Western Australia
(Lambeck and Nakada, 1992) and the North Egyptian coast of Red Sea (Reyss et al., 1993). Moreover, detailed statistical analysis of the geometry of the fracture systems visible in these deposits has demonstrated that vertical tectonic movements were of weak magnitude (Bouaziz, 1995; Jedoui et al., 1996). Tectonics cannot therefore be responsible for their emergence. Thus, we conclude, in agreement with some previous studies (Paskoff and Sanlaville, 1983), that there has been no significant tectonic activity for at least the last 130,000 yr in southeastern Tunisia. A high sea level stand has been dated at 12677 ka in central Tunisia using the U-series a-counting chronology on a coral fragment (Cladocora caespitosa) collected from the Rejiche formation (Miller and Paskoff, 1986). On the basis of petrographic composition and stratigraphic position, this dated sea level can be correlated with the carbonate unit in southeastern Tunisia (Jedoui, 2000). The sedimentation period of the carbonate unit should thus be associated with the sea level highstand of MISs 5e. The underlying siliciclastic unit however, devoid of Strombus and having a slightly lower elevation, should correspond either to a high eustatic sea level at the beginning of MISs 5e, or to a more ancient high sea level such as that characterised by the end of MIS 7. To clarify the depositional age of the marine Pleistocene coastal sediments of Tunisia, we have conducted U-series analyses of numerous mollusc shells sampled from the deposits.
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4. Dating of the Pleistocene deposits The two Pleistocene units described above have been dated by the U-series isotope method using a-spectrometry at the Laboratoire des Sciences du Climat et de L’Environnement de Gif-sur-Yvette (France) following the procedure of Ku (1965) with some minor modifications. As corals are scarce or missing from these deposits, determinations have been performed on mollusc shells. The mollusc species analysed are listed in Table 1. To avoid any artefact due to recrystallisation of calcitic shell samples, only well-preserved entire shells are used in this study. Each shell was systematically hand-cleaned by mechanical abrasion to eliminate all impurities. Depending on uranium content, the weight of the samples used for radiochemical analysis ranged from B10 to 20 g. The results are given in Table 1. All uncertainties given are based on propagated errors from counting statistics and are quoted at the 71s (standard deviation) level. 4.1. Reliability of radiochemical analysis It is well demonstrated that U-series disequilibrium dating of mollusc shells is less reliable than dating of corals mainly because of the mobility of uranium and its incorporation into carbonate shells after their death (Broecker, 1963; Szabo and Rosholt, 1969; Kaufman et al., 1971, 1996; Bernat et al., 1985; Hoang and Hearty, 1989; McLaren and Rowe, 1996, etc.). However, the situation appears different for some marine mollusc species such as in southern Spain (Bernat et al., 1978; Hillaire-Marcel et al., 1986), in the Atlantic coast of Marocco (Choukri, 1994) and in the Balearic Islands (Hillaire-Marcel et al., 1996), which yield U-series ages comparable to those obtained from contemporaneous coral samples. The reliability of the uranium-series dating method (230Th/234U) for carbonate samples depends on many parameters, including mineralogy, 232 Th content, uranium concentration, 234U/238U activity ratios, morphostratigraphic data and the coherence of results for the same unit. The mineralogical composition of samples was determined by X-ray diffraction. Estimates of aragonite and calcite content are reported in Table 1. Except for Ostraea shells, which have a calcitic primary composition, the calcite content of most shells is relatively low, often lower than 1% and rarely exceeding 4%. Continental contamination is evaluated using the 230 Th/232Th ratio. When 230Th/232Th values are higher than 20, this indicates that the influence of thorium derived from detrital origin is negligible (Chen, 1985). Studied samples mostly show 230Th/232Th values higher than this threshold. This excludes any continental contamination in the studied shells.
By contrast, U concentrations of the analysed shells display a wide range (between 0.2170.01 and 4.4270.15 dpm/g; Table 1). They were found to be much higher in the aragonitic mollusc shells than in the oyster shells (mainly the species Ostraea lamellosa), the only mollusc species with initially calcitic shells. Comparison of the uranium contents measured in Cardium, Conus, Murex, Glycymeris and Cerithium shows that values are 8–40 times higher in fossil than in modern shells (Table 2). This phenomenon can only be explained by U-uptake during diagenesis. Our observations are in agreement with the recent studies (Labonne and Hillaire-Marcel, 2000) which display that the difference in the U concentrations between aragonitic and calcitic layers in mollusc shells is directly linked to the diagenetic decay of the organic matrix of the shells. A faster degradation of this matrix is suggested in the aragonitic layers. The decay of the organic matrix should induce low-redox conditions in the micropores created and thus drive U precipitation. This process could explain the differential U-uptake rates observed between various mollusc species in given diagenetic situations (Hillaire-Marcel, 1995; Hillaire-Marcel et al., 1996). As the incorporation of uranium into fossil shells requires the presence of some pore water in the embedding sediment, most of the uranium incorporated into the shell is more likely taken up during a very short and early diagenetic episode, when marine pore water was still present (Labonne and Hillaire-Marcel, 2000). Moreover, measured 234U/238U activity ratios (ARs) of initially aragonitic fossil mollusc shells (1.1470.02 and 1.3670.01; Table 1) are found to be higher than that of the modern sea water value (1.135–1.155; Chen et al., 1986). These results, which are in agreement with the observations made from other Mediterranean coastal deposits (Causse et al., 1993; Hillaire-Marcel et al., 1996; McLaren and Rowe, 1996), indicate also that uranium has been incorporated into shells after death. By contrast, this AR is lower in Ostraea lamellosa shells and ranges from 1.0970.03 and 1.1570.02, with a mean value of 1.11470.02. When corrected for the decay of 234U, taking into account the estimated ages, the calculated mean initial value of about 1.1670.03 becomes close, within experimental error, to the presentday seawater value. This suggests that post-depositional exchanges of uranium are reduced in Ostraea shells, which can be considered as relatively closed chemical and isotopic systems during diagenesis. Only radiometric analyses obtained from Ostraea shells will therefore be considered in this study to estimate the ages of the Pleistocene coastal deposits of southeastern Tunisia. 4.2. Age of the carbonate unit Eight Ostraea shells samples collected from the carbonate unit have been analysed. The ages obtained
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Table 1 Radiochemical data and ages of Pleistocene shells. Underlined values correspond to the ages used in this study; nd—not detected Sample
Species
% calcite
238
234
232
230
230
(234U/238U)o 1.2370.02 1.2770.04 1.2570.02 1.4870.03 1.2870.05
50.8(+3.4; 3.2) 76.2(+12.6; 11.3) 39.6(+2.4; 2.4) 55.5(+3.5; 3.4) 47.4(+3.3; 3.2) 50.3(+2.7; 2.6) 86.8(+8.8; 8.2) 54.0(+4.6; 4.5)
U (dpm/g)
U/238U
Th(dpm/g)
Th(dpm/g)
Th/232Th
Age (ka)
Carbonate unit Tg1 Strombus Tg2 Thais Tg3 Glycymeris Tg4 Conus Tg5 Spondylus
1 15 1 2 1
2.0570.05 0.9270.05 1.7570.07 2.0970.07 0.3870.02
1.2070.02 1.2270.04 1.2270.02 1.4170.02 1.2470.04
0.00270.001 0.02170.005 0.00270.001 nd 0.00170.001
0.9370.03 0.5770.05 0.6670.02 1.2170.05 0.1770.01
4677234 27.277 3327166
Gl1a Gl1b Gl1c
Glycymeris Cardium Strombus
1 1 1
1.3170.05 1.3170.07 2.5370.14
1.1670.02 1.1270.03 1.1470.02
0.00270.001 0.00170.001 nd
0.5770.01 0.8870.03 1.1470.04
2847142 8837884
1.1970.02 1.2570.04 1.1770.03
Tm1 Tm2 Tm3 Tm4 Tm5
Arca Cardium Murex Conus Ostrea
1 1 1 1 100
0.7770.03 0.5370.02 1.8870.05 2.0670.1 0.3470.02
1.3370.03 1.3370.04 1.3370.01 1.3470.03 1.1170.04
0.04970.003 0.00870.001 0.08670.004 0.01270.002 0.07170.005
0.6170.03 0.470.01 0.9970.03 1.3670.06 0.2770.02
12.470.9 49.976.4 11.670.6 114720 3.7370.3
1.4370.04 1.4370.05 1.3970.02 1.4170.03 1.1570.06
92.6(+8.8; 86.8(+7.1; 53.8(+2.8; 71.5(+6.5; 127.6(+19;
8.3) 6.7) 2.8) 6.1) 16.3)
T11a T11b T11c T11d T11e T11f T11g
Ostrea Ostrea Thais Trochus Cardium Glycymeris Arca
100 100 13 4 1 1 3
0.3170.01 0.3570.01 4.4270.15 0.7670.03 1.370.05 1.4270.07 2.5970.05
1.1070.02 1.1270.04 1.2070.01 1.1970.03 1.2070.02 1.1670.03 1.1870.01
0.00570.001 0.00270.001 0.00170.002 nd 0.00670.002 0.00870.002 0.00370.002
0.2270.01 0.2470.01 2.5170.09 0.5470.02 0.8570.04 0.6270.03 1.0670.04
44.279 120760 >2500 — 141748 78720 3537236
1.1370.03 1.1670.05 1.2470.02 1.2570.04 1.2570.03 1.1970.03 1.2170.01
109.1(+8.8; 101.8(+6.9; 68.3(+4.7; 96.9(+8.9; 83.4(+8.1; 50.8(+4.1; 45.6(+2.3;
8.2) 6.5) 4.5) 8.3) 7.5) 3.9) 2.3)
G12a G12b G12c G12d G12e G12f
Arca Spondylus Spondylus Cardium Glycymeris Cerithium
1 50
1.1770.02 1.2270.03 1.2870.03 1.2570.03 1.2270.02 1.2370.03
0.00470.001 0.00270.001 0.00170.001 nd 0.00170.001 0.00670.001
0.7870.02 0.6470.02 0.7970.03 0.8470.02 0.7270.03 1.0870.02
196749 3217161 7867787
1 1 4
1.4670.07 1.6170.07 0.9570.05 1.3370.08 1.2170.05 2.1170.08
1.2170.03 1.2570.03 1.3870.04 1.3170.04 1.2670.03 1.2770.03
65.2(+5.0; 4.8) 42.6(+2.7; 2.6) 106(+11.5; 10.5) 74.6(+6.5; 6.1) 71(+5.2; 4.9) 56.6(+3.2; 3.1)
J1 J2 J3 J4
Ostrea Ostrea Chlamys Cardium
100 100 1 1
0.2170.01 0.2670.01 1.4170.08 1.5370.08
1.1470.03 1.1170.03 1.2970.03 1.2570.03
0.00470.001 0.00170.001 0.03770.006 0.00470.001
0.1870.01 0.1770.01 0.8770.06 1.1270.04
1.2070.05 1.1470.04 1.3570.04 1.3270.03
141.5(+15.8; 13.9) 90.9(+5; 4.8) 68.6(+9.1; 8.4) 92.4(+8.7; 8.1)
Kb1 Kb2 Kb3
Ostrea Ostrea Ostrea
100 100 100
3.5270.17 1.8370.03 1.3670.03
1.0970.02 1.1270.02 1.0970.02
0.00270.001 0.00870.001 nd
2.7770.07 1.5270.03 0.9970.02
44.377.9 168711 23.47168 28074.2 70 13867694 190724
1.1370.02 1.1870.02 1.1270.02
135.8(+15.4; 13.6) 141.5(+7.1; 6.7) 118.2(+6.6; 6.2)
Gta1 Gta2
Cardium Cardium
1.4970.02 1.41270.04
1.3670.01 1.2770.02
0.00870.001 0.00270.001
1.6570.02 1.3770.02
206726 6857343
1.5670.02 1.4070.04
163.4(+7.5;–7.01) 144.9(+9.2; 8.5)
100 100 100 1
0.4870.02 1.0970.04 0.8070.02 2.4270.12
1.0970.03 1.1270.03 1.1570.02 1.3070.02
0.03570.003 0.00470.001 0.00470.001 nd
0.3370.02 0.8770.02 0.6270.01 1.7370.08
9.570.9 218755 155.7739
1.1270.04 1.1770.04 1.2070.03 1.3770.03
109.3(+11.5; 10.5) 131.2(+10.6; 9.7) 119(+5.4; 5.2) 83.9(+8.7; 8.1)
Quartz rich unit Sy1 Ostrea Sy2 Ostrea Sy3 Ostrea Sy4 Cardium
2
1707170
7177717 179730
Bg1 Bg2
Ostrea Ostrea
100 100
1.0370.02 0.4970.03
1.1270.02 1.0970.05
0.00270.001 nd
0.7870.01 0.3870.02
387.57194
1.1770.03 1.1370.08
117.5(+5.3; 5.0) 132.2(+24; 20)
BB1 BB2 BB3 BB4 BB5 BB6
Ostrea Ostrea Ostrea Ostrea Cardium Cerithium
100 100 100 100 1 2
0.4470.02 0.3170.01 0.3470.01 0.8970.02 0.7770.04 1.1970.06
1.2170.04 1.1270.03 1.1370.04 1.1270.02 1.2670.04 1.2570.04
0.00370.002 nd 0.00470.001 0.00370.001 0.00570.001 0.0170.002
0.3570.02 0.2570.01 0.2970.01 0.7370.01 0.8270.04 1.1170.04
116778
1.2870.06 1.1870.05 1.1970.06 1.1870.03 1.4370.07 1.3670.06
112.2(+13.7; 12.3) 132.9(+9.6; 8.8) 147.5(+10.7; 9.7) 139(+6.7; 6.3) 180.2(+38; 28.8) 140.1(+19.3; 16.6)
73.3718 243.7781 164734 111723
Y. Jedoui et al. / Quaternary Science Reviews 22 (2003) 343–351
348 Table 2 Radiochemical data on modern shells Species
238
Calcite (%)
Conus Murex Glycymeris Cardium Cerithium
0.1970.01 0.0770.01 0.0370.003 0.0770.01 0.0870.01
0 0 0 0 0
U (dpm/g)
U-series ages of 27 other mollusc species display higher variability, showing ages ranging from 39.6 to 163.4 ka and most of the results are younger than the Last Interglacial (Table 1; Fig. 3). Eighteen samples yield ages ranging from 40 to 75 ka, seven display ages ranging from 83 to 106 ka whereas two samples give 144.9 and 163.4 ka. This variability is similar to that reported for mollusc shells (particularly Strombus) collected from coastal deposits in central Tunisia (Bernat et al., 1985; McLaren and Rowe, 1996), confirming the unreliability of U-series determination on this type of material. When constrained by stratigraphy, we can suppose that all analysed mollusc shells of the carbonate unit should be developed during the MISs 5e. After decay of excess 234U, the initial 234U/238U activity ratio in 125 ka old seawater (1.144) becomes about 1.103. Fig. 4 displays the distribution of 234U/238U activity ratio for all studied mollusc species. As expected, only the Ostraea shells yield values compatible with the marine origin of uranium. These consistent radiometric results are probably linked to the stability, during diagenetic processes, of initially calcitic Ostraea shells. 4.3. Age of the siliciclastic unit For this unit, ages were obtained using nine Ostraea shells samples. Estimated ages range between about 147 and 110 ka (Fig. 3). Taking into account this age range and the tectonic stability of southeastern Tunisia, we can conclude that this unit could not have been deposited during a high sea-level stand earlier than that associated with the Last Interglacial, as the penultimate Interglacial is older than 190 ka and deposition during MISs 5e is likely. The stratigraphic position of the unit suggests that it was the oldest Last Interglacial coastal sediment in southeastern Tunisia. Thus the Last Interglacial was characterised by two eustatic high sea levels.
5. Discussion
Fig. 3. Frequency histograms of ages for analysed mollusc shells: (A) obtained ages for all mollusc species (except Ostraea shells); (B) ages of Ostraea shells from the carbonate unit; (C) ages of Ostraea shells from the quartz-rich unit.
range between about 141 and 100 ka (Fig. 3; Table 1) and suggest that this unit formed during the Last Interglacial high sea-level stand.
Evidence for two high sea levels during the Last Interglacial exists elsewhere in the Mediterranean area (Balearic Islands: Hillaire-Marcel et al., 1996; Sardinia: Kindler et al., 1997), in Hawaii (Sherman et al., 1993), in the Bahamas (Hearty and Kindler, 1995; Kindler and Hearty, 1996) and along the Red Sea shorelines (Plaziat et al., 1998). Independently, two positive oxygen isotope oscillations are observed in high-resolution planktonic foraminifera records obtained from deep sea cores (Labeyrie et al., 1987; Martinson et al., 1987). The concept of a single high sea level stand during the Last Interglacial is still retained by a number of authors (Mesolella et al., 1969; Edwards et al., 1987; Ku et al.,
Y. Jedoui et al. / Quaternary Science Reviews 22 (2003) 343–351
Fig. 4. Distribution of 234U/238U ratios vs. age for different mollusc species. Only the Ostraea shells display theoretical ratio for 125 ka old sea water of 1.103.
1990; Gallup et al., 1994; Muhs and Szabo, 1994, etc.), possibly as a consequence of early lower resolution marine oxygen isotope records (Imbrie et al., 1984). Variations in petrography and faunal content of the carbonate and siliciclastic units of southeastern Tunisia brought evidence for significant changes in climatic and hydrological conditions at the time of their deposition (Jedoui et al., 2001). In the lower siliciclastic unit, the predominance of detrital material suggests an important contribution to sediment supply from continental runoff carrying quartz-rich material towards the coast. This would reflect the establishment of wetter climatic conditions at the beginning of MISs 5e than today. The presence of similar deposits, attributed to the same period, along some Mediterranean coasts (Central coast of Tunisia: Mahmoudi, 1988; Sardinia: Kindler et al., 1997) suggests the occurrence of such pluvial conditions in all the Mediterranean borderlands. Our data are in agreement with the strong sea surface salinity lowering observed in both eastern and western Mediterranean basins at the beginning of the Last Interglacial (Kallel et al., 2000). At that time, estimated surface salinities were almost homogeneous over the whole Mediterranean basin and not significantly different from those of the North Atlantic Ocean off Gibraltar (Kallel et al., 2000). An increase in precipitation was thus sufficient to balance the water loss by evaporation and transformed the Mediterranean Sea into a non-concentration basin. Such conditions were probably responsible for the recharge of the Sahara–Sahel deep aquifers (Sultan et al., 1997) and the development of an extensive paleolake in Libya (Gaven et al., 1981). The change in these climatic conditions during the second half of the Last Interglacial was associated with a strong decrease in the input of terrigeneous material and the development, along the coast, of carbonate deposits.
349
234
U/238U ratios compatible with the
The strong production and accumulation of ooids and peloids indicate that climatic conditions warmer than today prevailed in this coastal area (Jedoui et al., 2001).
6. Conclusion A sedimentological study of the Pleistocene marine coastal deposits of southeastern Tunisia has permitted the identification of two distinct lithostratigraphic units separated by an erosion surface. U-series determinations of Ostraea lamellosa indicate that the two units were probably developed during two different phases of sedimentation associated with two separate sea level highstands. During the first phase, the relative sea level was about 3 m higher than today whereas it was at about +5 m during the second sedimentation phase. U-series dating on Ostraea shells displays initial 234 U/238U ratios close to that of present-day seawater, in contrast to other mollusc species (Cardium, Glycymeris, Chlamys, Arca, Strombus, Spondylus, Conus, Cerithium, Thais, Murex, Trochus), which display a wide range of initial 234U/238U ratios, significantly different from that of modern marine water. Ostraea appears to take up uranium immediately after death whereas the other aragonitic species do so over a large period of time and behave as open geochemical systems. Ostraea shell ages suggest that the two Pleistocene units have been deposited during the MISs 5e. The coastal deposits of southeastern Tunisia, in which they are found, can therefore be used as indicators of eustatic changes during the Last Interglacial. We suggest that the Last Interglacial was characterised by at least two eustatic maxima. Owing to the uncertainties on estimated ages, the exact duration of each of the two sea level highstands cannot be determined.
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Variations in the petrography and fossil content of these two units show that important climatic and hydrologic changes occurred during the Last Interglacial and humid conditions were prevalent at the beginning of this period and were responsible for an enhanced supply of terrigeneous siliciclastic material from the continent, whereas a change in climate towards more arid conditions during the second half of the Last Interglacial favoured carbonate sedimentation. At this time the younger beach was formed. The voluminous production and accumulation of ooids and peloids within carbonate sediments suggest that the coastal waters of southeastern Tunisia during this period were warmer than at present.
Acknowledgements We are grateful to Professor Jim Rose, Professor Roland Paskoff and an anonymous referee for their critical and constructive reviews and to Professor Dalila Turki for palaeontological determination of oyster samples. The present study was financially supported in Tunisia by ENIS, FSS, DGRST (CMCU), FST and in France by CNRS-CEA and Ministe" re des Affaires Etrange" res (Institut Franc-ais de coope! ration, Tunis, Projet CMCU no. 00/F 1003).
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McLaren, S.J., Rowe, P.J., 1996. The reliability of uranium-series mollusc dates from the Western Mediterranean basin. Quaternary Science Reviews 15, 709–717. Mesolella, K.J., Matthews, R.K., Broecker, W.S., Thurber, D.L., 1969. The astronomical theory of climatic change: Barbados data. Journal of Geology 77, 250–274. Miller, G.H., Paskoff, R., 1986. Amino-acid geochronology of Pleistocene littoral deposits in Tunisia. Zeitschrift fur . Geomorphologie, Supplementum 62, 197–207. Muhs, D.R., Szabo, B.J., 1994. New uranium-series ages of the Waimanalo Limestone, Oahu, Hawaii: implications for sea level during the last interglacial period. Marine Geology 118, 315–326. Oueslati, A., 1986. Jerba et kerkena: leur e! volution g!eomorphologique au cours du Quaternaire. Publication de la Facult!e des Lettres, Tunis, 2, 210p. Oueslati, A., Paskoff, R., Sanlaville, P., 1982. Le Tyrrh!enien de la Tunisie: essai de synth"ese. Bulletin de la Soci!et!e G!eologique de France 7, 173–178. Paskoff, R., Oueslati, A., 1988. Acquisitions r!ecentes a" propos # du quaternaire sup!erieur des cotes de la Tunisie. M!editerran!ee 2, 79–84. Paskoff, R., Sanlaville, P., 1977. Les formations quaternaires de l’#ıle de Jerba (Tunisie); essai sur les lignes de rivage. Comptes Rendus Sommaire de la Soci!et!e G!eologique de France 4, 217–219. Paskoff, R., Sanlaville, P., 1980. Le Tyrrh!enien de la Tunisie: essai de stratigraphie. Comptes Rendus de l’Acad!emie des Sciences, Paris 290, 393–396. # Paskoff, R., Sanlaville, P., 1983. Les cotes de la Tunisie: variation du niveau marin depuis le Tyrrh!enien. Ed. Maison de l’orient, Lyon, France, 192p. Plaziat, J.C., Reyss, J.L., Choukri, A., Orszag-Sperber, F., Baltzer, F., # r!ecifale d’Egypte, Purser, B.H., 1998. Mise en e! vidence, sur la cote d’une r!egression interrompant bri"evement le plus haut niveau du Dernier Interglaciaire (5e): un nouvel indice de variations glacioeustatiques a" haute fr!equence au Pl!eistoc"ene. Bulletin de la Soci!et!e G!eologique de France 169, 115–125. Reyss, J.L., Choukri, A., Plaziat, J.C., Purser, B.H., 1993. Datations radiochimiques des r!ecifs coralliens de la rive occidentale du nord de la mer Rouge, premi"eres implications stratigraphiques et tectoniques. Comptes Rendus de l’Acad!emie des Sciences, Paris 317, 487–491. Sherman, C.E., Glenn, C.R., Jones, A.T., Burnett, W.C., Schwarcz, H.P., 1993. New evidence for two highstands of the sea during the last interglacial oxygen isotope substage 5e. Geology 21, 1079–1093. Sultan, M., Sturchio, N., Hassan, F., Hamdan, M., Mahmood, A.M., El Alfy, Z., Stein, T., 1997. Precipitation source inferred from stable isotopic composition of Pleistocene groundwater and carbonate deposits in the western desert of Egypt. Quaternary Research 48, 29–37. Szabo, B.J., Rosholt, J.N., 1969. Uranium-series dating of Pleistocene molluscan shells from southern California—an open system model. Journal of Geophysical Research 74, 3253–3260. Vacher, H.L., Hearty, P., 1989. History of stage 5 sea level in Bermuda: review with new evidence of a brief rise to present sea level during substage 5a. Quaternary Science Reviews 8, 159–168.
U-series evidence for two high Last Interglacial sea levels in southeastern Tunisia Younes Jedouia,*, Jean-Louis Reyssb, Nejib Kallelc, Mabrouk Montacerc, Hedi Ben Isma.ıld, Eric Davaude b
a D!epartement de G!eologie, Ecole Nationale d’Ing!enieurs de Sfax, B.P. ‘‘W’’, 3038 Sfax, Tunisia Laboratoire des Sciences du Climat et de l’Environnement, UMR 1572, CNRS-CEA, Parc du CNRS, 91198, Gif-sur-Yvette cedex, France c Facult!e des Sciences de Sfax, Route de Soukra, B.P. 802, 3018 Sfax, Tunisia d Facult!e des Sciences de Tunis, Campus Universitaire, 1060 Tunis Bev!ed"ere, Tunisia e Department of Geology and Palaeontology, University of Geneva, CH-1211 Geneva 4, Switzerland
Received 18 September 2001; accepted 24 July 2002
Abstract Pleistocene raised marine deposits in southeastern Tunisia consist of a siliciclastic unit that culminates at +3 m asl, overlain by a carbonate-rich unit with Strombus bubonius that culminates at +5 m asl. 234U/238U ratios on fossil Ostraea shells from both units are compatible with a marine origin from the uranium incorporated into the shells and show narrowly clustered 230Th-ages, respectively, between 147 and 110 ka and 141 and 100 ka. The two units were therefore developed during Marine Isotopic Substage 5e (MISs 5e, Last Interglacial). Their heights are comparable to those of contemporaneous marine deposits found in many tectonically stable areas of the world such as in the Bahamas and in Bermuda and can therefore be used as indicators of eustatic changes during the Last Interglacial. It is argued that on the basis of this evidence, the Last Interglacial was characterised by two eustatic maxima. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction On the basis of geomorphological criteria, the raised Pleistocene marine deposits along Tunisian coastlines have been subdivided into three superposed formations: from oldest to youngest, the Douira, Rejiche and Chebba formations (Paskoff and Sanlaville, 1977, 1980, 1983; Oueslati et al., 1982; Ben Ouezdou, 1986; Oueslati, 1986). Their preliminary dating in the central Tunisia, using a-counting uranium series on mollusc shells and amino acid ratios, assigned the Rejiche and the Chebba formations, respectively, to the high sea level stands of the MISs 5e and 5c. The Douira formation has been attributed to a high eustatic sea level older than the Last Interglacial (Miller and Paskoff, 1986; Paskoff and Oueslati, 1988). Neither sedimentological nor geochronological studies have been conducted in southeastern Tunisia *Corresponding author. Tel.: +216-74-274-088; fax: +216-74-275595. E-mail address: [email protected] (Y. Jedoui).
(Fig. 1). Paskoff and Sanlaville (1983), and Jedoui et al. (1996, 1998) demonstrated that this area has been tectonically stable during the last 130 ka. If this is the case, it is difficult to explain the emergence of the deposits assigned to the MISs 5c, as sea level at that time was lower than today by more than 10 m (Chappell and Shackleton, 1986). This problem may be explained by imprecise age attribution caused by the fact that mollusc shells provide usually unreliable U-series ages (Broecker, 1963; Szabo and Rosholt, 1969; Kaufman et al., 1971, 1996; Bernat et al., 1985; Hillaire-Marcel et al., 1986; McLaren and Rowe, 1996, etc.). In this paper we applied a sedimentological approach to defining the different lithostratigraphic sequences within the coastal marine deposits of southeastern Tunisia. We also tested the reliability of dating different mollusc shell species using the U-series a-counting chronology. It is necessary to use shell species since corals are largely absent from these coastal deposits. Our results suggest that Ostraea shells are more reliable material than other shell material for U-series analysis.
0277-3791/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 2 ) 0 0 1 3 3 - 6
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Fig. 1. Location and map of marine Pleistocene deposits in southeastern Tunisia. Arrows indicate the analysed shell sites: (1) Oued Tmoula (Tm), (2) El Grine (Gt), (3) Henchir Damous (J), (4) El Gala (Gl), (5) Taguermess (Tg), (6) Tarbella (T), (7) Sidi Yati (Sy), (8) Ras Karboub (Kb), (9) El Bibane (BB), (10) S. el Briga (Bg).
2. Sedimentology Sedimentological analysis of marine Pleistocene deposits along the coastal area of southeastern Tunisia indicates two distinct lithostratigraphic units (Jedoui, 2000; Jedoui et al., 2002; Fig. 2). The lower unit is a finegrained bioclastic quartz-rich sand devoid of Strombus fossils (a warm water Senegalese fauna) and locally includes well-developed aeolian sediment facies. The lower unit onlaps an erosion surface which truncates Mio-Plio-Villafranchian deposits. The upper unit is more extensive and consists of carbonate ooids, peloids and bioclastes along with a warm water Senegalese fauna, specifically Strombus bubonius. This unit clearly displays a shallowing-upward sequence beginning with subtidal deposits and ending with an aeolian facies. In some localities on Jerba island,
a Strombus-rich boulder bed terminates the Pleistocene marine deposits. This unit includes well-rounded cobbles originating from calcareous Villafranchian deposits and also calcarenite blocks from the carbonate unit. The matrix is made of bioclastic calcarenite containing ooids and mollusc shells. The origin of the gravel has been extensively debated, and two hypothesis have received the greatest attention: (i) an independent transgression separated from that associated with the carbonate unit by a regression (Paskoff and Sanlaville, 1983); and (ii) the gravel deposits can result from a storm erosion following deposition of the carbonate unit (Mahmoudi, 1988; Jedoui et al., 2002). In this work, the second hypothesis is retained since our field investigations indicate that boulder beds, which are very similar to those found within the carbonate unit, fill erosional notches. This choice has no influence on the conclusions of this paper.
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Fig. 2. Synthetic cross-section of marine Pleistocene deposits in southeastern Tunisia. Quartz-rich unit: 1a, marine siliciclastic sands; 1b, eolian siliciclastic sands. Carbonate unit: 2a, marine oolitic sands with Strombus; 2b, eolian oolitic sands.
Therefore, two main lithostratigraphic units can be distinguished in southeastern Tunisia. Each one overlies an irregular erosion surface truncating underlying deposits. Assuming tectonic stability in the region over the past 130 ka, the units thus correspond to at least two sedimentation phases associated with two positive eustatic pulsations. In this study, the lower unit will be designated as the ‘‘siliciclastic unit’’ while the upper one will be designated as the ‘‘carbonate unit’’.
3. Elevation of the Pleistocene deposits and tectonic activity The coastal deposits of southeastern Tunisia have been mapped and their paleogeography has been reconstructed (Jedoui and Bouaziz, 1997; Jedoui and Perthuisot, 1997a, b; Jedoui, 2000). The elevation of the marine units at many coastal localities has also been determined. Deposits of the siliciclastic unit culminate at B2–3 m above the mean present sea level whereas those of the carbonate unit reach an elevation of B3 to 6 m. The heights of these paleobeaches are comparable to those of contemporaneous marine deposits found in many tectonically stable areas of the world such as Bermuda (Harmon et al., 1983; Vacher and Hearty, 1989), Bahamas (Chen et al., 1991), Western Australia
(Lambeck and Nakada, 1992) and the North Egyptian coast of Red Sea (Reyss et al., 1993). Moreover, detailed statistical analysis of the geometry of the fracture systems visible in these deposits has demonstrated that vertical tectonic movements were of weak magnitude (Bouaziz, 1995; Jedoui et al., 1996). Tectonics cannot therefore be responsible for their emergence. Thus, we conclude, in agreement with some previous studies (Paskoff and Sanlaville, 1983), that there has been no significant tectonic activity for at least the last 130,000 yr in southeastern Tunisia. A high sea level stand has been dated at 12677 ka in central Tunisia using the U-series a-counting chronology on a coral fragment (Cladocora caespitosa) collected from the Rejiche formation (Miller and Paskoff, 1986). On the basis of petrographic composition and stratigraphic position, this dated sea level can be correlated with the carbonate unit in southeastern Tunisia (Jedoui, 2000). The sedimentation period of the carbonate unit should thus be associated with the sea level highstand of MISs 5e. The underlying siliciclastic unit however, devoid of Strombus and having a slightly lower elevation, should correspond either to a high eustatic sea level at the beginning of MISs 5e, or to a more ancient high sea level such as that characterised by the end of MIS 7. To clarify the depositional age of the marine Pleistocene coastal sediments of Tunisia, we have conducted U-series analyses of numerous mollusc shells sampled from the deposits.
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4. Dating of the Pleistocene deposits The two Pleistocene units described above have been dated by the U-series isotope method using a-spectrometry at the Laboratoire des Sciences du Climat et de L’Environnement de Gif-sur-Yvette (France) following the procedure of Ku (1965) with some minor modifications. As corals are scarce or missing from these deposits, determinations have been performed on mollusc shells. The mollusc species analysed are listed in Table 1. To avoid any artefact due to recrystallisation of calcitic shell samples, only well-preserved entire shells are used in this study. Each shell was systematically hand-cleaned by mechanical abrasion to eliminate all impurities. Depending on uranium content, the weight of the samples used for radiochemical analysis ranged from B10 to 20 g. The results are given in Table 1. All uncertainties given are based on propagated errors from counting statistics and are quoted at the 71s (standard deviation) level. 4.1. Reliability of radiochemical analysis It is well demonstrated that U-series disequilibrium dating of mollusc shells is less reliable than dating of corals mainly because of the mobility of uranium and its incorporation into carbonate shells after their death (Broecker, 1963; Szabo and Rosholt, 1969; Kaufman et al., 1971, 1996; Bernat et al., 1985; Hoang and Hearty, 1989; McLaren and Rowe, 1996, etc.). However, the situation appears different for some marine mollusc species such as in southern Spain (Bernat et al., 1978; Hillaire-Marcel et al., 1986), in the Atlantic coast of Marocco (Choukri, 1994) and in the Balearic Islands (Hillaire-Marcel et al., 1996), which yield U-series ages comparable to those obtained from contemporaneous coral samples. The reliability of the uranium-series dating method (230Th/234U) for carbonate samples depends on many parameters, including mineralogy, 232 Th content, uranium concentration, 234U/238U activity ratios, morphostratigraphic data and the coherence of results for the same unit. The mineralogical composition of samples was determined by X-ray diffraction. Estimates of aragonite and calcite content are reported in Table 1. Except for Ostraea shells, which have a calcitic primary composition, the calcite content of most shells is relatively low, often lower than 1% and rarely exceeding 4%. Continental contamination is evaluated using the 230 Th/232Th ratio. When 230Th/232Th values are higher than 20, this indicates that the influence of thorium derived from detrital origin is negligible (Chen, 1985). Studied samples mostly show 230Th/232Th values higher than this threshold. This excludes any continental contamination in the studied shells.
By contrast, U concentrations of the analysed shells display a wide range (between 0.2170.01 and 4.4270.15 dpm/g; Table 1). They were found to be much higher in the aragonitic mollusc shells than in the oyster shells (mainly the species Ostraea lamellosa), the only mollusc species with initially calcitic shells. Comparison of the uranium contents measured in Cardium, Conus, Murex, Glycymeris and Cerithium shows that values are 8–40 times higher in fossil than in modern shells (Table 2). This phenomenon can only be explained by U-uptake during diagenesis. Our observations are in agreement with the recent studies (Labonne and Hillaire-Marcel, 2000) which display that the difference in the U concentrations between aragonitic and calcitic layers in mollusc shells is directly linked to the diagenetic decay of the organic matrix of the shells. A faster degradation of this matrix is suggested in the aragonitic layers. The decay of the organic matrix should induce low-redox conditions in the micropores created and thus drive U precipitation. This process could explain the differential U-uptake rates observed between various mollusc species in given diagenetic situations (Hillaire-Marcel, 1995; Hillaire-Marcel et al., 1996). As the incorporation of uranium into fossil shells requires the presence of some pore water in the embedding sediment, most of the uranium incorporated into the shell is more likely taken up during a very short and early diagenetic episode, when marine pore water was still present (Labonne and Hillaire-Marcel, 2000). Moreover, measured 234U/238U activity ratios (ARs) of initially aragonitic fossil mollusc shells (1.1470.02 and 1.3670.01; Table 1) are found to be higher than that of the modern sea water value (1.135–1.155; Chen et al., 1986). These results, which are in agreement with the observations made from other Mediterranean coastal deposits (Causse et al., 1993; Hillaire-Marcel et al., 1996; McLaren and Rowe, 1996), indicate also that uranium has been incorporated into shells after death. By contrast, this AR is lower in Ostraea lamellosa shells and ranges from 1.0970.03 and 1.1570.02, with a mean value of 1.11470.02. When corrected for the decay of 234U, taking into account the estimated ages, the calculated mean initial value of about 1.1670.03 becomes close, within experimental error, to the presentday seawater value. This suggests that post-depositional exchanges of uranium are reduced in Ostraea shells, which can be considered as relatively closed chemical and isotopic systems during diagenesis. Only radiometric analyses obtained from Ostraea shells will therefore be considered in this study to estimate the ages of the Pleistocene coastal deposits of southeastern Tunisia. 4.2. Age of the carbonate unit Eight Ostraea shells samples collected from the carbonate unit have been analysed. The ages obtained
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Table 1 Radiochemical data and ages of Pleistocene shells. Underlined values correspond to the ages used in this study; nd—not detected Sample
Species
% calcite
238
234
232
230
230
(234U/238U)o 1.2370.02 1.2770.04 1.2570.02 1.4870.03 1.2870.05
50.8(+3.4; 3.2) 76.2(+12.6; 11.3) 39.6(+2.4; 2.4) 55.5(+3.5; 3.4) 47.4(+3.3; 3.2) 50.3(+2.7; 2.6) 86.8(+8.8; 8.2) 54.0(+4.6; 4.5)
U (dpm/g)
U/238U
Th(dpm/g)
Th(dpm/g)
Th/232Th
Age (ka)
Carbonate unit Tg1 Strombus Tg2 Thais Tg3 Glycymeris Tg4 Conus Tg5 Spondylus
1 15 1 2 1
2.0570.05 0.9270.05 1.7570.07 2.0970.07 0.3870.02
1.2070.02 1.2270.04 1.2270.02 1.4170.02 1.2470.04
0.00270.001 0.02170.005 0.00270.001 nd 0.00170.001
0.9370.03 0.5770.05 0.6670.02 1.2170.05 0.1770.01
4677234 27.277 3327166
Gl1a Gl1b Gl1c
Glycymeris Cardium Strombus
1 1 1
1.3170.05 1.3170.07 2.5370.14
1.1670.02 1.1270.03 1.1470.02
0.00270.001 0.00170.001 nd
0.5770.01 0.8870.03 1.1470.04
2847142 8837884
1.1970.02 1.2570.04 1.1770.03
Tm1 Tm2 Tm3 Tm4 Tm5
Arca Cardium Murex Conus Ostrea
1 1 1 1 100
0.7770.03 0.5370.02 1.8870.05 2.0670.1 0.3470.02
1.3370.03 1.3370.04 1.3370.01 1.3470.03 1.1170.04
0.04970.003 0.00870.001 0.08670.004 0.01270.002 0.07170.005
0.6170.03 0.470.01 0.9970.03 1.3670.06 0.2770.02
12.470.9 49.976.4 11.670.6 114720 3.7370.3
1.4370.04 1.4370.05 1.3970.02 1.4170.03 1.1570.06
92.6(+8.8; 86.8(+7.1; 53.8(+2.8; 71.5(+6.5; 127.6(+19;
8.3) 6.7) 2.8) 6.1) 16.3)
T11a T11b T11c T11d T11e T11f T11g
Ostrea Ostrea Thais Trochus Cardium Glycymeris Arca
100 100 13 4 1 1 3
0.3170.01 0.3570.01 4.4270.15 0.7670.03 1.370.05 1.4270.07 2.5970.05
1.1070.02 1.1270.04 1.2070.01 1.1970.03 1.2070.02 1.1670.03 1.1870.01
0.00570.001 0.00270.001 0.00170.002 nd 0.00670.002 0.00870.002 0.00370.002
0.2270.01 0.2470.01 2.5170.09 0.5470.02 0.8570.04 0.6270.03 1.0670.04
44.279 120760 >2500 — 141748 78720 3537236
1.1370.03 1.1670.05 1.2470.02 1.2570.04 1.2570.03 1.1970.03 1.2170.01
109.1(+8.8; 101.8(+6.9; 68.3(+4.7; 96.9(+8.9; 83.4(+8.1; 50.8(+4.1; 45.6(+2.3;
8.2) 6.5) 4.5) 8.3) 7.5) 3.9) 2.3)
G12a G12b G12c G12d G12e G12f
Arca Spondylus Spondylus Cardium Glycymeris Cerithium
1 50
1.1770.02 1.2270.03 1.2870.03 1.2570.03 1.2270.02 1.2370.03
0.00470.001 0.00270.001 0.00170.001 nd 0.00170.001 0.00670.001
0.7870.02 0.6470.02 0.7970.03 0.8470.02 0.7270.03 1.0870.02
196749 3217161 7867787
1 1 4
1.4670.07 1.6170.07 0.9570.05 1.3370.08 1.2170.05 2.1170.08
1.2170.03 1.2570.03 1.3870.04 1.3170.04 1.2670.03 1.2770.03
65.2(+5.0; 4.8) 42.6(+2.7; 2.6) 106(+11.5; 10.5) 74.6(+6.5; 6.1) 71(+5.2; 4.9) 56.6(+3.2; 3.1)
J1 J2 J3 J4
Ostrea Ostrea Chlamys Cardium
100 100 1 1
0.2170.01 0.2670.01 1.4170.08 1.5370.08
1.1470.03 1.1170.03 1.2970.03 1.2570.03
0.00470.001 0.00170.001 0.03770.006 0.00470.001
0.1870.01 0.1770.01 0.8770.06 1.1270.04
1.2070.05 1.1470.04 1.3570.04 1.3270.03
141.5(+15.8; 13.9) 90.9(+5; 4.8) 68.6(+9.1; 8.4) 92.4(+8.7; 8.1)
Kb1 Kb2 Kb3
Ostrea Ostrea Ostrea
100 100 100
3.5270.17 1.8370.03 1.3670.03
1.0970.02 1.1270.02 1.0970.02
0.00270.001 0.00870.001 nd
2.7770.07 1.5270.03 0.9970.02
44.377.9 168711 23.47168 28074.2 70 13867694 190724
1.1370.02 1.1870.02 1.1270.02
135.8(+15.4; 13.6) 141.5(+7.1; 6.7) 118.2(+6.6; 6.2)
Gta1 Gta2
Cardium Cardium
1.4970.02 1.41270.04
1.3670.01 1.2770.02
0.00870.001 0.00270.001
1.6570.02 1.3770.02
206726 6857343
1.5670.02 1.4070.04
163.4(+7.5;–7.01) 144.9(+9.2; 8.5)
100 100 100 1
0.4870.02 1.0970.04 0.8070.02 2.4270.12
1.0970.03 1.1270.03 1.1570.02 1.3070.02
0.03570.003 0.00470.001 0.00470.001 nd
0.3370.02 0.8770.02 0.6270.01 1.7370.08
9.570.9 218755 155.7739
1.1270.04 1.1770.04 1.2070.03 1.3770.03
109.3(+11.5; 10.5) 131.2(+10.6; 9.7) 119(+5.4; 5.2) 83.9(+8.7; 8.1)
Quartz rich unit Sy1 Ostrea Sy2 Ostrea Sy3 Ostrea Sy4 Cardium
2
1707170
7177717 179730
Bg1 Bg2
Ostrea Ostrea
100 100
1.0370.02 0.4970.03
1.1270.02 1.0970.05
0.00270.001 nd
0.7870.01 0.3870.02
387.57194
1.1770.03 1.1370.08
117.5(+5.3; 5.0) 132.2(+24; 20)
BB1 BB2 BB3 BB4 BB5 BB6
Ostrea Ostrea Ostrea Ostrea Cardium Cerithium
100 100 100 100 1 2
0.4470.02 0.3170.01 0.3470.01 0.8970.02 0.7770.04 1.1970.06
1.2170.04 1.1270.03 1.1370.04 1.1270.02 1.2670.04 1.2570.04
0.00370.002 nd 0.00470.001 0.00370.001 0.00570.001 0.0170.002
0.3570.02 0.2570.01 0.2970.01 0.7370.01 0.8270.04 1.1170.04
116778
1.2870.06 1.1870.05 1.1970.06 1.1870.03 1.4370.07 1.3670.06
112.2(+13.7; 12.3) 132.9(+9.6; 8.8) 147.5(+10.7; 9.7) 139(+6.7; 6.3) 180.2(+38; 28.8) 140.1(+19.3; 16.6)
73.3718 243.7781 164734 111723
Y. Jedoui et al. / Quaternary Science Reviews 22 (2003) 343–351
348 Table 2 Radiochemical data on modern shells Species
238
Calcite (%)
Conus Murex Glycymeris Cardium Cerithium
0.1970.01 0.0770.01 0.0370.003 0.0770.01 0.0870.01
0 0 0 0 0
U (dpm/g)
U-series ages of 27 other mollusc species display higher variability, showing ages ranging from 39.6 to 163.4 ka and most of the results are younger than the Last Interglacial (Table 1; Fig. 3). Eighteen samples yield ages ranging from 40 to 75 ka, seven display ages ranging from 83 to 106 ka whereas two samples give 144.9 and 163.4 ka. This variability is similar to that reported for mollusc shells (particularly Strombus) collected from coastal deposits in central Tunisia (Bernat et al., 1985; McLaren and Rowe, 1996), confirming the unreliability of U-series determination on this type of material. When constrained by stratigraphy, we can suppose that all analysed mollusc shells of the carbonate unit should be developed during the MISs 5e. After decay of excess 234U, the initial 234U/238U activity ratio in 125 ka old seawater (1.144) becomes about 1.103. Fig. 4 displays the distribution of 234U/238U activity ratio for all studied mollusc species. As expected, only the Ostraea shells yield values compatible with the marine origin of uranium. These consistent radiometric results are probably linked to the stability, during diagenetic processes, of initially calcitic Ostraea shells. 4.3. Age of the siliciclastic unit For this unit, ages were obtained using nine Ostraea shells samples. Estimated ages range between about 147 and 110 ka (Fig. 3). Taking into account this age range and the tectonic stability of southeastern Tunisia, we can conclude that this unit could not have been deposited during a high sea-level stand earlier than that associated with the Last Interglacial, as the penultimate Interglacial is older than 190 ka and deposition during MISs 5e is likely. The stratigraphic position of the unit suggests that it was the oldest Last Interglacial coastal sediment in southeastern Tunisia. Thus the Last Interglacial was characterised by two eustatic high sea levels.
5. Discussion
Fig. 3. Frequency histograms of ages for analysed mollusc shells: (A) obtained ages for all mollusc species (except Ostraea shells); (B) ages of Ostraea shells from the carbonate unit; (C) ages of Ostraea shells from the quartz-rich unit.
range between about 141 and 100 ka (Fig. 3; Table 1) and suggest that this unit formed during the Last Interglacial high sea-level stand.
Evidence for two high sea levels during the Last Interglacial exists elsewhere in the Mediterranean area (Balearic Islands: Hillaire-Marcel et al., 1996; Sardinia: Kindler et al., 1997), in Hawaii (Sherman et al., 1993), in the Bahamas (Hearty and Kindler, 1995; Kindler and Hearty, 1996) and along the Red Sea shorelines (Plaziat et al., 1998). Independently, two positive oxygen isotope oscillations are observed in high-resolution planktonic foraminifera records obtained from deep sea cores (Labeyrie et al., 1987; Martinson et al., 1987). The concept of a single high sea level stand during the Last Interglacial is still retained by a number of authors (Mesolella et al., 1969; Edwards et al., 1987; Ku et al.,
Y. Jedoui et al. / Quaternary Science Reviews 22 (2003) 343–351
Fig. 4. Distribution of 234U/238U ratios vs. age for different mollusc species. Only the Ostraea shells display theoretical ratio for 125 ka old sea water of 1.103.
1990; Gallup et al., 1994; Muhs and Szabo, 1994, etc.), possibly as a consequence of early lower resolution marine oxygen isotope records (Imbrie et al., 1984). Variations in petrography and faunal content of the carbonate and siliciclastic units of southeastern Tunisia brought evidence for significant changes in climatic and hydrological conditions at the time of their deposition (Jedoui et al., 2001). In the lower siliciclastic unit, the predominance of detrital material suggests an important contribution to sediment supply from continental runoff carrying quartz-rich material towards the coast. This would reflect the establishment of wetter climatic conditions at the beginning of MISs 5e than today. The presence of similar deposits, attributed to the same period, along some Mediterranean coasts (Central coast of Tunisia: Mahmoudi, 1988; Sardinia: Kindler et al., 1997) suggests the occurrence of such pluvial conditions in all the Mediterranean borderlands. Our data are in agreement with the strong sea surface salinity lowering observed in both eastern and western Mediterranean basins at the beginning of the Last Interglacial (Kallel et al., 2000). At that time, estimated surface salinities were almost homogeneous over the whole Mediterranean basin and not significantly different from those of the North Atlantic Ocean off Gibraltar (Kallel et al., 2000). An increase in precipitation was thus sufficient to balance the water loss by evaporation and transformed the Mediterranean Sea into a non-concentration basin. Such conditions were probably responsible for the recharge of the Sahara–Sahel deep aquifers (Sultan et al., 1997) and the development of an extensive paleolake in Libya (Gaven et al., 1981). The change in these climatic conditions during the second half of the Last Interglacial was associated with a strong decrease in the input of terrigeneous material and the development, along the coast, of carbonate deposits.
349
234
U/238U ratios compatible with the
The strong production and accumulation of ooids and peloids indicate that climatic conditions warmer than today prevailed in this coastal area (Jedoui et al., 2001).
6. Conclusion A sedimentological study of the Pleistocene marine coastal deposits of southeastern Tunisia has permitted the identification of two distinct lithostratigraphic units separated by an erosion surface. U-series determinations of Ostraea lamellosa indicate that the two units were probably developed during two different phases of sedimentation associated with two separate sea level highstands. During the first phase, the relative sea level was about 3 m higher than today whereas it was at about +5 m during the second sedimentation phase. U-series dating on Ostraea shells displays initial 234 U/238U ratios close to that of present-day seawater, in contrast to other mollusc species (Cardium, Glycymeris, Chlamys, Arca, Strombus, Spondylus, Conus, Cerithium, Thais, Murex, Trochus), which display a wide range of initial 234U/238U ratios, significantly different from that of modern marine water. Ostraea appears to take up uranium immediately after death whereas the other aragonitic species do so over a large period of time and behave as open geochemical systems. Ostraea shell ages suggest that the two Pleistocene units have been deposited during the MISs 5e. The coastal deposits of southeastern Tunisia, in which they are found, can therefore be used as indicators of eustatic changes during the Last Interglacial. We suggest that the Last Interglacial was characterised by at least two eustatic maxima. Owing to the uncertainties on estimated ages, the exact duration of each of the two sea level highstands cannot be determined.
350
Y. Jedoui et al. / Quaternary Science Reviews 22 (2003) 343–351
Variations in the petrography and fossil content of these two units show that important climatic and hydrologic changes occurred during the Last Interglacial and humid conditions were prevalent at the beginning of this period and were responsible for an enhanced supply of terrigeneous siliciclastic material from the continent, whereas a change in climate towards more arid conditions during the second half of the Last Interglacial favoured carbonate sedimentation. At this time the younger beach was formed. The voluminous production and accumulation of ooids and peloids within carbonate sediments suggest that the coastal waters of southeastern Tunisia during this period were warmer than at present.
Acknowledgements We are grateful to Professor Jim Rose, Professor Roland Paskoff and an anonymous referee for their critical and constructive reviews and to Professor Dalila Turki for palaeontological determination of oyster samples. The present study was financially supported in Tunisia by ENIS, FSS, DGRST (CMCU), FST and in France by CNRS-CEA and Ministe" re des Affaires Etrange" res (Institut Franc-ais de coope! ration, Tunis, Projet CMCU no. 00/F 1003).
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