Mid-Holocene emergence of southern Tunisian coasts

Marine Geology 220 (2005) 205 – 213 www.elsevier.com/locate/margeo

Mid-Holocene emergence of southern Tunisian coasts Christophe Morhange a, Paolo Antonio Pirazzoli b,* b

a CNRS-CEREGE, UMR 6635, Europoˆle de l’Arbois, BP 80, 13545-Aix-en Provence, France CNRS-Laboratoire de Ge´ographie Physique, UMR 8591, 1 Place Aristide Briand, 92195 Meudon cedex, France

Received 8 December 2004; received in revised form 12 May 2005; accepted 20 June 2005

Abstract We present new sea-level data from the coasts of southern Tunisia, between the Gulf of Gabe`s and the Libyan border. The work tests, previously, published evidence on Holocene shorelines, and confirmed that a distinct emergence has occurred in this area during this time. The emergence peak lies at least 186 F 11 cm above present and is inferred from: (1) AMS radiocarbon dates of subtidal vermetids and boring shells collected in growth position, and (2) careful assessment of tidal heights. Maximum emergence took place between about 6000 and 5000 14C years BP; it cannot be ascribed to tectonics and is probably related to post-glacial hydro-isostatic effects. It challenges the inference of a 3-m global sea-level rise since 6000 years BP due to residual Antarctic melting. D 2005 Elsevier B.V. All rights reserved. Keywords: sea-level change; bioconstructions; isostasy; eustasy; Holocene; Tunisia

1. Introduction In the framework of an ECLIPSE CNRS-funded research program, investigations are currently underway to test the possible occurrence of Holocene isostatic movements along a north–south transect between Marseilles (France) and Jerba Island in southern Tunisia. The aim of the project is to compare high resolution field data with estimated sealevel changes obtained from computer models. In this paper we present and discuss results obtained from * Corresponding author. Fax: +33 1 45075830. E-mail addresses: [email protected] (C. Morhange), [email protected] (P.A. Pirazzoli). 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.06.031

the southern coast of Tunisia, and challenge the inference of a hypothetical 3 m global sea-level rise since 6000 years BP due to possible residual Antarctic ice-melting.

2. Previous work In southern Tunisia, two distinct lithostratigraphic units, overlying Mio-Plio-Villafranchian deposits, have been distinguished by Jedoui et al. (2003). The lower unit, consisting of a fine-grained bioclastic quartz-rich sand, devoid of diagnostic last Interglacial fossils, and locally including well-developed aeolian sediment facies, is capped by a more extensive unit

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Fig. 1. Location map. Letters A to F correspond to localities for which tidal values have been computed (see Table 1). Numbers 1 to 12 relate to sites of radiocarbon-dated samples (see Table 2): 1: Bori Jilii; 2: Sidi Jmour; 4: Cheikh Yahyahia; 5: Humt Adjim; 6A: Oued Mellah; 8: Lella Meriem; 9–11: Sebkhet el Melah; 12: Bahiret el Bibane.

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comprising carbonate ooids, peloids and bioclasts dated from the last Interglacial period. Marine sediments with the marker fossil Strombus bubonius do not exceed an elevation of a few metres (Paskoff and Sanlaville, 1983; Jedoui et al., 2003). On the surface, slightly emerged Holocene marine deposits can be observed on the outer margin of the last Interglacial unit and many erosional features (e.g. coastal platforms) have probably a polycyclic origin. Recent research demonstrates that the southern Tunisian coastal area has remained relatively stable, with no significant tectonic activity for at least the last 130,000 years (Paskoff and Sanlaville, 1977; Oueslati et al., 1982; Oueslati, 1986; Jedoui et al., 1998, 2002; Bouaziz et al., 2003). Using dated evidence, various authors have reported Holocene crustal uplift from the coasts of southern Tunisia. Fontes and Perthuisot (1971) were the first to provide a radiocarbon date of about 6300 years BP for fossil high tide deposits, located at + 2 m in the Sebkha el Melah. The altimetric precision of their datum (bze´ro marin moyenQ) is unclear, because the tide gauge to which it may refer is not specified (no tide-gauge records are available in the Sebkha area) and because the zero of local marine charts is not mean sea level, but the lowest tide level. Furthermore, Fontes and Perthuisot’s (1971) assumption that storms cannot displace sediments above high tide level in an almost closed basin, is hardly acceptable; it is well known that persistent strong wind can cause decimetric differences in the elevation of lagoon water level. Such uncertainty was not clarified by Perthuisot (1975). In order to refine relative sea-level movements in this area, Dalongeville (1979) emphasized tidal variations, mentioning a spring tidal range of 2.0–2.3 m at

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Gabe`s (Loc. 6A in Fig. 1), 1.8 m south of Ajim (Loc. 5), 1.3 m 6 km offshore Oumet Essouˆq (Loc. D), 1.0 m near Ras Taguermess (Loc. C), 0.9 m at Aghir (SW of Loc. C) and 0.6 m in the gulf of Bougrara (Loc. A). The source of the latter values, some of which differ from tide values used in this study (Table 1), was not given. On the western coast of Jerba Island, at Cheikh Yahyia (Loc. 4), Paskoff and Sanlaville (1979) dated two shell samples to ca. 3100 conventional 14C years BP. These data were obtained from a fossil beach situated at an estimated altitude of ca. +1 m. They do not specify which datum they refer to and the range of uncertainty in their altitude estimation. In a subsequent publication, Dalongeville et al. (1980) ascribed the same data to a shoreline 0.8 m higher than the present one. They also reported beach deposits close to the present sea level near Gabe`s, dated 6600 F 100 and 6820 F 100 conventional 14C years BP. Beach deposits are however not reliable sea level indicators since they can be left at any elevation by the sea, both within the intertidal range and above it on the shore, or below it in the foreshore. Seven additional emerged beach deposits, collected between the Gulf of Gabe`s and the Libyan border, were dated by Jedoui et al. (1998). We checked most of their findings in the field and generally we agreed with their stratigraphical interpretation, though these authors did not attempt to differentiate the uncertainty range between the samples. All samples are reported as having been collected between 0.4 and 1 m above the high tide level, irrespective of local variations in tidal range and radiocarbon ages (between 4000 and 6000 14C years BP). They also dated two beach deposits collected near the present high tide level at ca. 2000–2200 14C years BP (Table 2).

Table 1 Astronomical tidal values predicted by the SHOM model for November 2002 at six locations near Jerba Island (Tunisia) Location (Fig. 1)

Geographical coordinates East

Mean spring range

North

A B C D E F

33835V 33842V 33848V 33855V 33840V 33838V

Highest

Mean

Lowest

10848V 10843V 11803V 10853V 10834V 11803V

98 110 92 103 120 84

+63 +67 +56 +66 +73 +49

+34 +36 +30 +34 +40 +28

+3 +6 +6 +9 +9 +6

Values (in cm) are related to the local mean tide level.

High tide

Low tide Highest

Mean

Lowest

7 10 4 14 13 7

34 36 30 33 40 28

52 61 52 55 68 49

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Table 2 Radio carbon ages of Holocene elevated shorelines from Southern Tunisia Location (numbers refer to Fig. 1)

Latitude N

Longitude Sample E no.

Material

4. Cheikh Yahyia

Shelly sand

7. El Grine

8. Lella Meriem

33844.44V 10844.09V Dj02-02

Beachrock shells Cardium Arca Coral Lamellibranchia Lamellibranchia 33839.32V 10834.13V DJ-02-12 Petricola shell in growth position 33839.32V 10834.13V DJ-02-13 V. triqueter shell in growth position 33839.32V 10834.13V DJ-02-14 Cerethium vulgatum shell inside a beachrock (cliona remarks) Conus

9. Sebkhet el Melah

24-6-69 5 Murex shells

10. 10. 11. 12.

B12 9A Pirenella B12 10A Pirenella Cardium

Sebkhet el Melah Sebkhet el Melah Sebkhet el Melah Bahiret El Bibane

Beach deposits Beach deposits Beach deposits

0.4–1.0 a.h.t.l. 0.4–1.0 a.h.t.l. Near present high tide

14 C age y13C (conventional years BP)a

4055 F 60 5530 F 60 2210 F 55 +1.0F?

Laboratory Source no.

2.31 Gif-10330 b 2.15 Gif-09923 b 3.65 Gif-10333 b

Intertidal Beach deposits Beach deposits Beach deposits Beach deposits Beach deposits Vlow tide

3180 F 70 3060 F 60 0.6 F 0.1 +0.40 F 0.68 4245 F 30 F0 6820F100 F0 6600F100 0.4–1.0 a.h.t.l. 5210F50 0.4–1.0 a.h.t.l. 4750F45 0.4–1.0 a.h.t.l. 5735F60 +1.08F0.10 z+1.72 F 0.11 5210 F 35

MC-2151 MC-2152 3.4 Poz-2590 MC-2155 MC-2154 1.39 Gif-09157 1.23 Gif-10335 2.46 Gif-09922 1.1 Poz-2596

a f f b b b a

Vlow tide

+1.08 F0.10

z+1.72F0.11 5065 F 30

3.4

Poz-2597

a

Intertidal (Vlow tide)

+1.08 F 0.10 +1.12 F 0.74 6810F40 (+1.22 F 0.11) (z1.86 F 0.10)

0.3

Poz-2598

a

Beach deposits

Near present high tide

3.13 Gif-10091 b

Associated to neolithic artefects, near fossil high tide deposits

Beach deposits Beach deposits

0.4–1.0 a.h.t.l. 0.4–1.0 a.h.t.l.

2040 F 35 +2.0 F 0.2

6320 F 120

ca. +1.0 ca. +0.8

5560 F 110 5680 F 190 5910 F 60 4220 F 60

Bold lines correspond to new data. a: This study; b: Jedoui et al., 1998; c: Paskoff and Sanlaville, 1979; d: Fontes and Perthuisot, 1971; e: Perthuisot, 1975; f: Dalongeville et al., 1980. a 14 C ages published by c, d, e, f have been increased of 400 years to be converted into conventional ages.

?

? ? 2.76 Gif-09921 3.27 Gif-10331

c

d,e

d,e d,e b b

C. Morhange, P.A. Pirazzoli / Marine Geology 220 (2005) 205–213

Lamellibranchia Arca Cerites

7. El Grine

Elevation (m) Sample (above Palaeo water level) sea level

1. Bori Jilij 2. Sidi Jmour 3. S of Sidi Jmour

5. Humt Adjim 6A. Qued Mellah 6A. Qued Mellah 6. Zarat 6. Zarat 7. El Grine 7. El Grine

Former position

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Our survey was undertaken in November 2002. We systematically visited the coasts around Jerba Island, and along the continent, between Zarat (Loc. 6) and Bahiret el Bibane. Special attention was given to the accuracy of altimetric measurements, with the aim of improving, if possible, the accuracy of previous emergence estimates. For this reason, we avoided surveying Sebkhet outcrops distant from the present-day shoreline, where elevation estimates were uncertain in the absence of accurate levelling devices, such as a differential GPS (Strasser et al., 1989).

3. Methods Geographical coordinates were determined in the field using a GPS. Elevations were measured, with reference to the sea surface, by means of a spirit level and a folding ruler. Sea level broadly corresponds to the upper biological limits of living infralittoral populations (e.g. the upper limit of studied bioindicators such as Lithophaga or Vermetids). This measure was then corrected according to tidal predictions from the nearest of six locations around Jerba Island (Table 1), or at interpolations between these locations. In particular, the tide at Locality 5 (sample Dj02-02) has been assumed to be the same as in locality B and that at Locality 7 (samples DJ02-12, -13 and -14) the same as in locality E. The astronomical tidal values at the six locations, deduced from a high-resolution tidal model of the Mediterranean, were kindly provided by the French Service Hydrographique et Oce´anographique de la Marine. Highest and lowest tidal levels reported in Table 1 correspond to a significant range on the 5th–6th November 2002 (tidal coefficient 111/120). The mean spring tidal ranges in Table 1 correspond to a tidal coefficient 95/120. At an annual scale, spring mean low tide corresponds to the lowest mareographic levels. Additional corrections in elevation take into account changes in air pressure measured in the field with a pocket altimeter. These were subsequently referred to published synoptical air pressure maps. During the fieldwork, local surface air pressure varied between 1019 and 1006 h Pa, thus implying corrections in the sea-level height between 6 F 2 and + 4 F 2 cm for the samples collected. On the other hand, the almost absence of wind, swell and waves

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during the survey enabled us to exclude their possible effects on the coasts. Beach sediments, beach conglomerates and beachrocks have been assumed to correspond to sediments deposited at any elevation in the local tidal range or even in the foreshore zone (Bernier and Dalongeville, 1996; Coudray and Montaggioni, 1986). Subtidal biological features (such as erosion marks left by Cliona, subfossil Vermetids and Lithophaga in growth position) have developed below a maximum height approximately corresponding to the former spring mean low tide level (Laborel and LaborelDeguen, 1994). We have taken these as evidence of minimal emergence above former mean spring low tide level. All the shell samples collected were cleaned using an ultra-sound bath. Radiocarbon dates were performed at the Poznan Radiocarbon Laboratory, Poland.

4. Results Dates from emerged Holocene shorelines are reported in Table 2. The new radiocarbon datings obtained from the El Grine area are considered especially meaningful in defining the former sea-level peak. Sample DJ-02-12, dated 5210 F 35 14C years BP, corresponds to a Petricola lithophaga shell, still in growth position in the limestone outcrop (Fig. 2C). The latter could only have developed below the former low tide level in the subtidal zone (Laborel and Laborel-Deguen, 1994). In addition, the nearby sample DJ-02-13, a well preserved, in situ Vermetus triqueter, was dated to 5065 F 30 14C years BP (Fig. 2D). Both types of indicators mean that sea level at the time they were living was at least 1.72 F 0.11 m higher than present. A Cerithium vulgatum shell (sample DJ-02-13, dated to 6810 F 40 14C years BP) collected nearby from a beachrock at a similar elevation, may be older than the beachrock’s lithification. More interesting are erosion marks left by Cliona (boring sponges) found on the surface of the same beachrock. These demonstrate that a period of submergence followed the beachrock lithification. The latter observation indicates that the former sea level was at least 1.86 F 0.11 m higher than present. A tentative mean sea level (MSL) curve comprising all the available evidence has been drawn in

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Fig. 2. A and B: Location of samples DJ-02-12/13 collected at El Grine. B: General view of the El Grine elevated platform; C.M. gives scale; water level is near mean sea level. C: Petricola shells are visible in their holes. Articulated shells demonstrate that their growth position corresponds to a former sea level at least 1.72 F 0.11 m above the present one (sample DJ-02-12, dated 5210 F 35 14C years BP; photograph by P.A.P. No. G902). D: A Vermetus triqueter shell, still preserved in growth position, also indicates a former sea level at least 1.72 F 0.11 m above the present one (sample DJ-02-13, dated 5,065 F 30 14C years BP; photograph by P.A.P. No. G903).

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Fig. 3. Tentative curve (dotted line) of MSL Holocene changes in southern Tunisia. Numbers correspond to sites (see Fig. 1 and Table 2). Vertical discontinuous lines indicate dated beach deposits that may have been left by the sea in the intertidal range or in the foreshore. The peak of the curve is situated at an elevation z1.86 F 0.11 m by the samples DJ-02-12, 13, and 14. The star indicates the position proposed by Fontes and Perthuisot (1971) for their sample 24.6.69/5, for which a more realistic uncertainty range in the paleo sea-level reconstruction should however be greater than the F0.2 m range as proposed by these authors.

Fig. 3. The highest Holocene sea-level peak has been reached between 6000 and 5000 14C years BP and was almost 2 m higher than at present, thus confirming previous conclusions drawn by Fontes and Perthuisot (1971) and Paskoff and Sanlaville (1983). This peak was apparently short lived, and was probably followed by a subsequent sea-level fall. This fall, however, cannot be documented in detail due to the absence of accurate related sea-level indicators. Most previously dated samples seem to correspond to beach material deposited slightly below MSL.

5. Discussion Evidence of Holocene emergence is a rare phenomenon in Mediterranean areas non affected by tectonic activity. According to Lambeck and Johnston (1995, p. 8): bWithin a distance of typically 408 to 608 of the ice centres, the glacio-isostatic term is important in shaping the overall pattern of apparent land subsidenceQ. Accordingly, no Holocene emergence should be expected here because the Jerba area lies within

these latitudes relative to the Gulf of Bothnia, centre of the former Fennoscandian ice-sheet. On the other hand, according to Peltier et al. (in preparation) bthe Mediterranean region is sufficiently remote from the Northwest European Centre of Deglaciation as to be little influenced by the collapse of the forebulge that lay outboard of the Fennoscandian ice-sheet at LGMQ. Mediterranean relative sea-level history would therefore be primarily influenced by hydro-isostatic effects due to local water depth. As this depth is shallow over a wide area around the Gulf of Gabe`s and Jerba Island (water depths of at least 20 m are only found 20 km north of Jerba Island, and of 200 m only at 300–400 km offshore), the observed Holocene emergence should be linked to hydro-isostasy. Local tidal changes since the Holocene sea-level peak may also have led to low-magnitude regional differences in the emergence pattern. In particular, according to isostatic models, emergence is expected to slightly decrease seawards and to have been slightly lower in Jerba Island (especially in its outer part) than along the continental shoreline. Lambeck (2002) remarks that ocean volumes continued to increase by 3 m between 6000 and 3000 14C

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years BP, possibly due to residual melting of the Antarctic ice sheet. According to the map published by Lambeck et al. (2004) for b6 ka BPQ (Lambeck’s maps are based, like Fig. 3, on uncalibrated radiocarbon ages) sea level at that time would have been 2– 4 m below present in the Jerba area. These results are however clearly challenged by the sea-level data from southern Tunisia.

6. Conclusions New sea-level data on the shorelines between the Gulf of Gabe`s (southern Tunisia) and the Libyan border have improved the Holocene relative sealevel tendencies outlined in previous publications. Clear emergence has occurred during the Holocene, reaching ca. + 2 m since 6000–5000 14C years BP. Holocene uplift is an unusual feature in the Western Mediterranean. Evidence of Holocene shorelines higher than at present has been reported from just a few sites, including Almeria, Spain (e.g.: Goy et al., 2003) or of the Messina Strait area, Italy (Pirazzoli et al., 1997; Antonioli et al., 2003, 2004). The other coasts of Spain, France and Italy show no evidence for Holocene uplift. Even in Tunisia, relative sea-level rise has prevailed in the Holocene along most of the coasts (Paskoff and Sanlaville, 1983). Still in the northernmost part of the Gabe`s Gulf, the Kerkenna Islands, affected by tectonic movements, indicate a submergence of at least 2 m since Antiquity (Slim et al., 2004). However, south of Fax, and especially of Gabe`s, evidence of Holocene emergence is widespread along the coast and continues at least until the Libyan border. Such emergence cannot be ascribed to tectonics and is probably related to post-glacial hydro-isostatic effects. Less loaded by the postglacial sea-level rise than most of the Mediterranean sea floor, the exceptionally wide, shallow and tectonically stable continental shelf of the Gabe`s Gulf and around Jerba Island was uplifted during the late Holocene, compared to most other Mediterranean coasts where the continental shelf was much narrower. Nevertheless, such a hydro-isostatic uplift cannot exceed certain limits. If Lambeck’s assumption (Lambeck, 2002) that global sea level has risen 3 m since 6000 BP was correct, then total uplift in southern

Tunisia should have been around 5 m, leading to 2 m of emergence today. Indeed, this seems much greater than what could be expected in coastal areas from hydro-isostatic effects. Other Mediterranean data do not support the assumption of a global sea-level rise after 6000 years BP. This is the case of the Provence coast, where the gradual deceleration in the sea-level rise during the last 5000 years (Lambeck and Bard, 2000; Morhange et al., 2001) does not support a change of trend since 4000 or 3000 years BP due to a delayed eustatic sea-level rise. Finally, on the Levant coasts, there is clear morphological and radiometric evidence from Turkey, Syria and the Lebanon showing that sea level remained stable, with an accuracy of F0.3 m, between ca. 6000 and 3000 14C years BP (Pirazzoli, in press; Morhange et al., in preparation).

Acknowledgments The authors kindly thank the Centre National de la Recherche Scientifique (Eclipse Program) for financial assistance. We thank Bernard Simon (Service Hydrographique et Oce´anographique de la Marine) for having provided detailed tide predictions in the Jerba Island area, J. Laborel and an anonymous reviewer for very useful suggestions and N. Marriner for revision of the English text.

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