. Introduction
The use of 14C ages from samples that grow in marine environments (i.e. molluscs, fish bones, etc.) requires special consideration like calibration, which is important for interpreting or comparing historical or climatic records. Mixing of water masses (i.e. during upwelling) may dilute the amount of 14C in the water. Marine organisms that absorb their carbon from dissolved inorganic carbon (DIC) typically have relatively older ages due to the dilution effect in the ocean, as compared to atmospheric ages. Models of exchange between the atmosphere and the ocean have been proposed for surface waters (0–75 m), thermocline waters (75–1000 m) and deep waters (1000–3800 m) (Stuiver and Braziunas, 1993). From the model of surface waters, verified by 14C dates on shells of known ages, one can calculate a global mean value for pre-AD 1950 marine reservoir age correction R(t) of 400 years. Some laboratories publish 14C ages without reservoir effect corrections; however, this approximation is not sufficient for archaeological applications that require calibration using programs such as CALIB7.1 (Stuiver et al., 2013) or CALIB8.2 (Heaton et al., 2020).
It has been demonstrated that the variability of reservoir ages at a particular site depends on oceanic water mass circulation and mixing (Siani et al., 2000; Southon et al., 2002). In upwelling zones, for instance, mixing of deep waters with surface waters produces important local reservoir effects, with ΔR values of several hundred years or more. Previous studies (Ndeye, 2008) focussed on the estimation of the mean value for the reservoir effect for coastal Senegal and Mauritania, western Africa. It has been suggested that the reservoir effect in this part of Africa could be high because it should be affected by upwelling phenomena (Goodfriend and Flessa 1997). That study area extends from Port Etiénne (21°01′N, 17°02′W) (coastal Mauritania) in the north to Rufisque (14°42′N, 17°15′W) on the Cap-Vert (Cape Verde) peninsula (coastal Senegal). 14C dates were calculated from AD 1837 and 1945 using gastropods from which are calculated reservoir ages. The weighted mean results of R for Senegal is 511 ± 50 Before Present (BP) and ΔR is 176 ± 15 BP; for Mauritania, R is 421 ± 15 BP and ΔR is 71 ± 13 BP. The stations studied are part of the family-face distribution of archaeological sites in the West African Neolithic. From this classification and dating, the settlement of Khant, Chami and Tintan stretches from the ancient Neolithic to the historical period. However, the problem of these chronologies, with periods of hiatus, is the consideration of the influence of the marine effect on the samples collected. Hence, the question of the absolute accuracy of the dates obtained. From the BP age classification, the sites are ‘old’ and the chrono-cultural and taphonomic conceptuality of the stations are poorly defined. Therefore, the application of the local reservoir effect allows greater consistency in the ‘timing’ of the calibrated samples and occupancy. This article aims to provide revisited chronology of two human settlements in Mauritania and Senegal, applying the age characteristics of the reservoirs previously published from the Mauritanian and Senegalese coast.
. Material and Methods
Description of the Studied Sites and excavations
Khant site (Senegal)
Ravisé discovered the site of Khant in 1968. It is a depression located at 22 km (16°04′02.8″N; 16°20′25.7″W) of the city of Saint-Louis (Fig. 1). Human occupation of the coastline at the beginning of the Holocene is facilitated by the availability of coastal resources (molluscs, fish and marine mammals). This is the archaeological essence (shellfish) of the Khant site (excavated in 1970 by Ravisé) whose duration of occupation begins between 5650 ± 140 BP (Ly 990) and 5248 ± 177 PB (Ravisé et al., 1975; Hebrard, 1978). Similarly, radiocarbon dating on Anadara Senilis shells of 5340 ± 120 BP (Ly 988), 4352 ± 123 BP (Dk60) and 4225 ± 119 BP (Dk69) were obtained. In addition, a charcoal sample, taken at a depth of 150 cm (Ravisé, id.) is dated at 1751 ± 13 BP.
Other dates obtained in the vicinity of the Khant depression, despite the inaccuracy of their location, range from 5000 BP to 1500 BP (Table 1; Mbow 1987). On this site remains of burials of a Negroid type subject have been reported. Therefore, based on calibrated ages (Ravisé et al., 1975), the settlement of Khant stretches from the ancient Neolithic to the protohistorical era (1000 BC to 1500 AD).
The Khant site has delivered polished bone axes, harpoons, hooks and ceramics whose properties match those of the Atlantic coast’ (Ravisé, 1970). The ground shell and its derivatives were used as technical elements (tool shaping) and as a basis for dating the site.
Tintan and Chami sites (Mauritania)
Tintan and Chami integrate into the entire Mauritanian coast (Fig. 1). These two sites have affinities with Khant in Senegal. In the seventh millennium, human occupation affected part of the coastline. From the fifth and fourth millennia, the coast, entirely occupied, encompasses Tintan and Chami. Tintan and Chami integrate into the entire Mauritanian coast. Traces of prehistoric settlement (epipaleolithic and neolithic) are found in the area of Foum Arguin, in Chami but novelties, heralds of the Neolithic make their appearance—arrow frames, often bifacial, polished axes, ceramics, grinding equipment (Vernet, 2004). The Tintan site, on the Tintan peninsula, is 7 km from Foum Arguin, north of Banc d’Arguin. It is part of the Senegalese-Mauritanian coastal strip from Saint-Louis to Nouadhibou. Tintan is limited to the north, from Cap Blanc, by Nouadhibou-Cansado, the Banc d’Arguin to the south, to the East Foum Arguin and to the West the Atlantic coast. Tintan, ‘older’, like Khant are Neolithic stations. Thus, the dating obtained in this region, by J.P. Charbonnel are in the intervals from 6020 ± 150 BP (on shells) to 5670 ± 300 BP (on Arca Senilis), at 4270 ± 100 BP (on cymbium) and/or 3530 ± 130 BP (on Arca Senilis). Fonte obtained a date of 4860 ± 160 BP. Thus, in the evolution of the occupation, the culture of Tintan slips into the peninsula of the same name and the Tasiast.
Chami site
Chami is a «Hasi 1», (20° 03′03″N; 15°58′0.3″W) gateway to the Banc d’Arguin, located on the Nouadhibou-Nouakchott axis, east of Cape Tafarit. In addition, while
Chami does not have a real food discharge area, there are, however, many deteriorated shells on the surface of the occupied dunes (Farida, 2013). Consequently, Petit-Maire in 1979, who searched the site, collected the following dates on cymbium: 2360 ± 100 BP, 3220 ± 110 BP, 3950 ± 80 BP. While J.P. Charbonnel collected in Chami Tafarit, on Arca Senilis 3570 ± 120 BP. Most of the population lived on the inland dunes (Chami, Tijirit, etc.). Chami is presented in the form of wind mounds specific to the dune industries that outcrop on the whole great coast from Dakar to Banc d’Arguin. As a result, the lithic industry is carved on flint. The industry is made up of flash nuclei. The tooling is composed of scrapers, collectors, scales and denticula, geometric pieces (segments, trapezes and triangles), common to the dune industries of Senegal. Arrowhead reinforcements are preferentially pedunculated (Vernet, 1993). Traces of prehistoric settlement (epipaleolithic and neolithic) are found in the area of Foum Arguin, in Chami but novelties, heralds of the Neolithic make their appearance—arrow frames, often bifacial, polished axes, ceramics, grinding equipment (Vernet, 2004). This lithic production is associated with ceramics, with many decorated shards.
Tintan site
The Tintan site (16°24′0″N; 10°10′0″), on the Tintan peninsula is 7 km from Foum Arguin, north of Banc d’Arguin. It is part of the Senegalese-Mauritanian coastal strip from Saint-Louis to Nouadhibou. Tintan is limited to the north, from Cap Blanc, by Nouadhibou-Cansado, the Banc d’Arguin to the south, to the East Foum Arguin and to the West the Atlantic coast. Tintan, ‘older’, like Khant are Neolithic stations. Thus, the radiocarbon datings obtained in this region, by J. P. Charbonnel are in the intervals 6020 ± 150 BP (on shells), 5670 ± 300 BP (on Arca Senilis), at 4270 ± 100 BP (on cymbium) and/or 3530 ± 130 BP (on Arca Senilis). JC Fontes obtained a date of 4860 ± 160 BP. Thus, in the evolution of the occupation, the culture of Tintan slips into ‘peninsula of the same name and from Tasiast to the region of Chami. Further south, the Nouakchott region and the Aftout es Sahili region are attracting more and more men’ (Vernet and Tous, 2004). In addition, the geological configuration of the region allows the outcropping of raw materials, of rocks suitable for size. Thus, the Tintan deposit is of great lithic wealth and collections of Senelia, which greatly contribute to the activity of specialised groups (Vernet et al., 2004).
Methods
Literature data
We used archaeological reports (Vernet, 1998), articles (Delibrias and Evin, 1980) which allowed us to collect 18 dates for Chami, 27 dates for Tintan. Radiocarbon dating was mostly carried out on marine shells in the 1980s. Only 21 dates are out of continental macrorests (Human bones; Pottery degreaser; charcoal sand).
Table 1
Calibration of marine samples collected at the Khant, with the atmospheric curve and the Marine curves (AR = 0 BP or AR = 176 ± 15 BP or AR = 27 ± 56 BP).
[i] Note: The calibrated ages chosen are those with the highest probability density.
BP, Before Present.
aAtmospheric calibration curve: I ntcal13.14C (Reimer et al., 2013).
bMarine calibration curve with world marine reservoir age: Marinel3.14C (Reimer et al., 2013).
cMarine calibration curve with estimated marine reservoir age: Marinel3.14C (Reimer et al., 2013).
dMarine calibration curve with estimated marine reservoir age: Marine20.14C (Heaton et al., 2020).
A few rare dates were given with the corresponding isotopic fractionation δ13C. With regard to sea shells, the hypothesis put forward is that for δ13C close to 0‰ then the correction to be applied of 400 years is compensated by the apparent age of the oceanic surface waters, which is about 400 years except in very special cases of deep water ascents.
Radiocarbon laboratory new data (IFAN Ch.A.Diop, Dakar)
Since 2003, a new dating system has been installed at the Carbon 14 laboratory of the IFAN Cheikh Anta Diop of the University Cheikh Anta Diop of Dakar. It is equipped with a liquid scintillation meter (Tri-carb 3170TR/LS) with BGO tube (Bi4GeO12) to reduce background noise in super low-level mode.
The performance of this dating system (background, merit factor) and the calibration were evaluated by the use of the standards of the International Atomic Energy Agency (IAEA) but also by the age known samples from the University Paris VI (France). The counter is supported by software called Quanta Smart for the Windows operating system (Packard 1999). Counting is programmed for 100 min per cycle.
In the energy range 13–85 keV, the efficiency of the meter is 68% and the background noise is 0.2 cpm. The E2/B merit factor is 23.400. This clearly shows the high performance of the counting system. In super low-level counting mode using BGO tubes, the background noise is reduced considerably and goes down to 0.1 cpm for the same output. The newly renovated laboratory has had its first date with a new laboratory code DK.
Physical and chemical pretreatments
Before determining the age of a sample, it undergoes physicochemical pretreatments that depend not only on the nature of the sample but also on its degree of pollution. These pretreatments reduce the level of pollution. Once the samples are pretreated physically, that is, cleaned, stripped or crushed, a chemical pretreatment is usually performed in a conventional way by making acid-base-acid attacks to eliminate any pollution.
For this study, the collected marine samples are chemically pretreated. The sample is immersed in 8% HCl for about half an hour and then rinsed with swapped water. Since the sample is a shell, after a good rinse to a neutral pH, the pretreatment is completed and the sample is ready to be synthesised.
Benzene Sample Synthesis
After the pretreatments, the samples are placed in an oven at an appropriate temperature and for a time which allows to dry depending on the type of the sample. The synthesis of benzene is carried out by means of a device whose operating procedure is as follows: first of all, a primary vacuum within the synthesis bench is made, thanks to the pumps installed on the bench.
The sample is then attacked by ortho-phosphoric acid (H3PO4) to give carbon dioxide (CO2). The CO2 is sent to the lithium-containing furnace and the carburation starts at 800°C to give lithium carbide (Li2C2) and lithium oxide (Li2O). Acetylene (C2H2) is obtained by hydrolysing the obtained lithium carbide. Finally, the trimerisation of acetylene in the presence of catalyst (vanadium–chromium) at 50°C makes it possible to obtain benzene. The benzene obtained at the end of this synthesis is put into the counting chamber of the liquid scintillation counter (Tri-carb 3170 TR/LS with BGO tube) produced by Packard in 1999, which makes it possible to obtain, after a few days (15 cycles), the activity of the sample necessary for the determination of the radiocarbon age.
Calculation of sample activity by liquid scintillation
Two grams of synthesised benzene is mixed with mixed scintillator, Bis MSB + Buthyl PBD in proportions of 6 mg each. The scintillator converts the energy of the incident particle, the β-radiation released by benzene, into photons. The incident energy carries the atoms or a molecule of the scintillator in an excited state, and their return to the ground state is accompanied by the emission of photons. These photons are received by the photocathode of the photomultipliers that emit electrons that come to hit a dynode and re-emit several electrons to another dynode.
Each time a photon reaches a dynode, it triggers a pulse or strokes. All the electrons are collected by the anode, which is the output signal of the scintillation detector. This signal is proportional to the energy released by benzene and makes it possible to determine the total number of strokes recorded over a longer or shorter period of time. The calculation of the average activity of the sample makes it possible to determine the corresponding radiocarbon age.
The radiocarbon age is calculated not only assuming the constancy of the 14C content but also the consideration of isotopic fractionation (13C). During the photosynthesis process, plants proportionally absorb less 13C and 14C isotopes than what is available in the carbon reservoir, resulting in fractionation reflected in the consumption of living beings. 14C is half as absorbed as 13C. Thus, the 12C/13C ratio can be used to compensate for the initial exhaustion of the 14C. Radiocarbon dating must then be corrected for this isotopic fractionation: this is normalisation. Radiocarbon age is calculated using the following equation:
with ASN and A0N, respectively, the normalised sample activity at −25‰ and the normalised standard activity (Stuiver and Polach, 1977). The carbon14 half period used is 5568 years. The year 1950 is automatically used as the base age, and the ages are given in units BP, the present being the year 1950 AD (year Domini). These calculated ages take into account the isotopic fractionation corrections due to the difference in isotopic velocity in various environments.
Age calibration processes by appropriate programs such as Calib 7.04 and Calib 8.1 can then convert the radiocarbon age into real age, that is, BC units or AD units.
For the samples in this study, failing to be able to calculate the 12C/13C ratio by mass spectrometry, an isotopic fractionation correction was not made. We consider that the correction (400 years) is thus compensated by the apparent age of the surface waters. Most of the samples (DK-1 to DK-39) were analysed in the radiocarbon laboratory of IFAN Ch.A.Diop of Cheikh Anta Diop University of Dakar (Sénégal) and the rest by the Lyon dating laboratory (Ly-988, Ly-990) in France.
Calibration
The previously estimated reservoir ages (R) have been determined using the marine mollusc shell of historically known age from 1837 to 1945 AD (Ndeye, 2008). However, it has been demonstrated that R changes over time in response to changes in ocean circulation and climate conditions (Druffel and Griffin 1993; Dunbar and Cole 1996; Stuiver et al., 1986). Unfortunately, there are no published R values from archaeological (marine/terrestrial samples) in our sampling sites for the determination of the past reservoir age. The archaeological sites investigated in our study are under the NW African coastal upwelling system characterised by a complex and heterogeneous oceanographic pattern that extends south to Cape Verde in winter and north to the Iberian Peninsula in summer (Wooster et al., 1976; Láiz et al., 2000; Pelegrí et al., 2006). Some studies on the determination of the past reservoir age have been done in Cap Verde (Soares et al., 2011) using pair samples show that the reservoir age is preserved (70 ± 70 BP). Taking into account these comparisons, we applied the previously estimated modern reservoir age in the Senegalese and Mauritania archaeological sites for this study. We present the tables giving the calibrated ages for each archaeological site using three types of calibration curve (Reimer et al., 2013; Heaton et al., 2020), and the CALIB program has been used for these calibrations. To highlight the importance of the reservoir effect, the use of chronomodel 2.0.18 (Lanos and Philippe, 2015) with the option « phase » was made for the dates of the different sites. Then, four results from the calibration curves have been obtained in this chronomodel: the phase modelling results using IntCal13; the phase modelling results using Marine13; the phase modelling results with local reservoir age applied on Marine13 and the phase modelling results using Marine20 (Heaton et al., 2020) with local ΔR applied.
We applied the local reservoir age of 71 ± 3 BP for Mauritanian sites and 176 ± 15 BP for Senegalese site for the marine13 calibration curves (Ndeye, 2008). These reservoir values have been recalculated and became 27 ± 56 BP and −75 ± 42 BP for Senegal and Mauritania, respectively, based on the new calibration curve Marine20 (Heaton et al., 2020). The duration of the phase is symbolised by the bold line, and the probability distribution of the beginning and the end of the phase is also shown.
. Results and discussion
Khant Site (Senegal)
Comments 1
Looking at the four phases, we see a chronological shift towards the youngest phases. All beginnings and ends of occupation are rejuvenated. Thus, a comparison between the results of the calibration curves gives between Intcal13 and Marine13 with ΔR = 0 a difference of rejuvenation of the occupation at the beginning of 459 years. This difference is 62 years between Marine13 with ΔR = 0 and Marine13 with ΔR = 176 ± 15 BP. The comparison between Marine13 with ΔR = 176 ± 15 BP and Marine20 with ΔR = 27 ± 56 BP gives a gap of 38 years, 5119 years for Intcal13, 4660 years for Marine13 with ΔR = 0, 4598 years for Marine13 with ΔR = 176 ± 15 BP and 4560 years for Marine20.
With the Marine13 calibration curve (Fig. 2), the beginning of the human occupation of the Khant is around 4660 BC or the second half of the fifth millennium BC. Therefore, based on the established chronology, the dynamics of the establishment extends to the Khant beyond the protohistoric epoch, the beginning of history. In addition, with the application of the local reservoir age on Marine13, occupation begins in the fifth millennium BC (4598 BC), therefore a 521-year rejuvenation. The information provided by comparing the Marine13 and Marine20 calibration curves with the application of the local reservoir effect shows a 7-year rejuvenation 4044 BC to 4037 BC (Table 1). The end of the occupation, despite a relative extension, coincided with upper, and 175 AD ageing limits on lower limits of Khant samples (238–513). In filigree, the procedural reconstruction of the Khant settlement, based on the three approaches used, shows that the occupation extends from the fifth millennium BC to the birth of the Age of Metals. In addition, the duration of occupation of Khant ranges from 5119 BC to 118 AD or from 4560 AD to 513 AD with caesura phases. The occupation is continuous since the ancient Neolithic and extends beyond the Protohistory (dates AD, early antiquity). The taphonomy of the site raises, despite the ‘doubts’ about the dating issued, the problem of the construction (occupation), deconstruction (abandonment) and reconstruction (reoccupation) of the Khant station.
Chami Site (Mauritania)
For the Chami, the collections of the samples were made by Petit-Maire in 1979 and the dates by the laboratories of Gif sur Yvette (France) (Gif-1856 and Gif 2524) and only one (Ly-346) by the laboratory of Lyon (France).
Comments 2
The first 3 phases (Intcal13, Marine13 + ΔR = 0 BP, Marine13 + ΔR = 71 ± 13 BP) show a chronological shift to the youngest phases while with Marine20 taking into account reservoir effect the phase is aged. A comparison between the results of the calibration curves gives between Intcal13 and Marine13 with ΔR = 0 gives a rejuvenation gap of the beginning of occupation of 507 years. This difference is 103 years between Marine13 with ΔR = 0 and Marine13 with ΔR = 71 ± 13 BP. The comparison between Marine13 with ΔR = 71 ± 13 BP and Marine20 with ΔR = -75 ± 42 BP gives a gap of 88 years. For the duration of the occupations, they are 2988 years for Intcal13, 2481 years for Marine13 with ΔR = 0, 2378 years for Marine13 with ΔR = 71 ± 13 BP and 2466 years for Marine20. From the Intcal13 curve to the Marine13 curve with the consideration of the reservoir effect, we have a decrease in the duration of occupancy of the site. But Marine13 with ΔR = 71 ± 13 BP, the sample ages 88 years. Nevertheless, the prehistoric occupation dynamics of the Chami at the Marine13 base ranged from 2481 to 629 AD (Fig. 3). The occupation extends from 2481 BC to 2378 BC the third millennium BC, while the lower limits of the dates obtained confirm the long period of settlement of Chami, from the third millennium until the beginning of antiquity, with a period of interruption Gif-2487 and Gif-2164 [930 BC–153 AD] (Table 2). The marine calibration 13 provides a beam dated between the third millennium (Middle Neolithic), hence a hiatus extending from 1500 BC to 500 BC. Hence the complexity of interpretative considerations of the curves (GIF 1856), including the chronological wasteland, in Mauritania, is the first period of the ‘Protohistory’ coinciding with the Bronze Age, in the West. From Ly-346 to Gif-2487, the developed culture is linked to iron metallurgy. Sample GIF-2164 is of historical age, birth of ancient ‘kingdoms’. Thus, the population of the region ranges from 2481 BC to 1500 BC. From there, Chami was abandoned for more than a millennium, possibly because of the drying up of the region. From there, Chami was abandoned for more than a millennium perhaps because of the drying up of the region. The revival will occur with the rise of iron metallurgy in Africa, in 469 AD. From the lower limits, 462–629 AD, the settlement of Chami begins with the end of the Neolithic, first period of Protohistory and continues until the appearance of the first historical texts. The dates obtained by Marine13 rejuvenate Chami by just over a century: from 2481 BC to 2378 BC.
Tintan Site (Mauritania)
For the Tintane site, the excavations were made by Carbonnel in 1979 and the dating by the laboratory of Gif sur Yvette (France), Lyon (France). J.C. Fontes from the Dynamique de Paris VI geology laboratory analysed most of the samples by, but it did not adopt a laboratory code – reference ‘FC’ according to the inventory studies by Vernet (1998) was adopted.
Comments 3
For Tintan, there is also a chronological shift towards the younger phases for the first three calibration curves (Intcal13, Marine13 + ΔR = 0 BP and Marine13 + ΔR = 71 ± 13 BP) and an ageing phase with Marine20 + ΔR = −75 ± 42 BP (Fig. 4). A comparison of the results of the calibration curves gives between Intcal13 and Marine13 with ΔR = 0 a 428-year rejuvenation difference of the start of occupation. This difference is 116 years between Marine13 with ΔR = 0 and Marine13 with ΔR = 71 ± 13 BP. The comparison between Marine13 with ΔR = 71 ± 13 BP and Marine20 with ΔR = −75 ± 42 BP marks an increase of 170 years, or 5666 years for Intcal13, 5238 years for Marine13 with ΔR = 0, 5122 years for Marine13 with ΔR = 71 ± 13 BP and 5143 years for Marine20 with reservoir effect. From the Intcal13 curve to the Marine13 curve, taking into account the reservoir effect, we have a decrease in the duration of occupancy of the site. However, with Marine20, we have an increase in site occupancy time compared to the previous two Marine13 curves. On the Marine13 base, without application of the Tintan Reservoir Effect, the period of occupancy ranges from 5666 BC to 629 BC with an interruption period of 1620 BC to 900 BC. This gives, with the local reservoir effect on Marine13, the intervals [5238 BC–680 BC].
Table 2
Calibration of marine samples collected at the Chami site with the atmospheric curve, and the Marine curves (ΔR = 0 BP or ΔR = 71 ± 13 BP or ΔR = -75 ± 42 BP).
[i] Note: The calibrated ages chosen are those with the highest probability density.
BP, Before Present.
aAtmospheric calibration curve: Intcal13.14C (Reimer et al., 2013).
bMarine calibration curve with world marine reservoir age: Marinel3.14C(Reimer et al., 2013).
cMarine calibration curve with estimated marine reservoir age: Marinel3.14C(Reimer et al., 2013).
dMarine calibration curve with estimated marine reservoir age: Marine20.14C(Heaton et al., 2020).
Fig 4
Calibration of the marine sample collected from Tintan site using chronomodel 2.0.18 with phase option.
Consequently, taking into account the ‘local reservoir’ (Marine13 or Marine20) rejuvenates the duration of Tintan’s occupation by 116 years and 184 years at its lower limits (813 BC and 680 BC). The marine20 calibration curve makes Tintan a site of the sixth millennium BC. This long human occupation of Tintan ended at the beginning of the Iron Age in West Africa. The lower limits or end of occupancy are, according to Marine13, 813 BC and 629 BC if the local reservoir effect is applied, i.e. a rejuvenation of more than a century and an ageing of 21 years with Marine20.
According to the Marine13 curve with ΔR = 0 (Table 3), the calibrated ages are the intervals [4658 BC–4338 BC] at 1σ and [4845 BC–4173 BC] at 2σ. This chronocultural periodisation of Tintan is confirmed by the intervals of Ly553 of ΔR = 71 ± 13 BP: [4588 BC–4261 BC] at 1σ and [4756 BC–4068 BC] at 2σ. This long human occupation of Tintan ended at the beginning of the Iron Age in West Africa. This chronology (second–third millennium) is confirmed by ΔR = 0, [624 BC–1302 BC] at 1σ and [1799 BC–1125 BC] at 2σ and ΔR = 71 ± 13 BP with intervals [539 BC–1212 BC].
Table 3
Calibration of marine samples collected at Tintan with the atmospheric curve, and the Marine curves (ΔR = 0 BP or ΔR = 71 ± 13 BP or ΔR = -75 ± 42 BP).
[i] Note: The calibrated ages chosen are those with the highest probability density.
BP, Before Present.
aAtmospheric calibration curve: Intcal13.14C (Reimer et al., 2013).
bMarine calibration curve with world marine reservoir age: Marinel3.14C (Reimer et al., 2013).
cMarine calibration curve with estimated marine reservoir age: Marinel3.14C (Reimer et al., 2013).
dMarine calibration curve with estimated marine reservoir age: Marine20.14C (Heaton et al., 2020).
Comparison of Archaeological Sites
We use the Marine20 calibration curve for each site in case the reservoir effect is not applied (ΔR = 0) and in case it is applied (ΔR ≠ 0).
Comments 4
The use of the Marine20 calibration curve taking into account the reservoir effect shows a rejuvenation of the beginnings of human occupations (a gap of 10 BC) but also of the ends of occupations (a gap of 92 AD) for the Khant site On the other hand, for the two sites of Mauritania, taking into account the reservoir effect causes ageing of the occupation phases. (Fig. 5). In a phylogenetic approach, the phylogenetic data (cymbium, Arca senilis, oyster) dated confirm that the end of the occupation of Tintan coincides with the beginning of the occupation of the ‘populations’ of Chami. Tintan is the oldest of the three stations studied. If ΔR = 0, occupation begins in 5238 BC, followed by a period of hiatus [1268 BC–1870 BC], 578 years before the start of the Khant stand (4660 BC), based on samples collected. The long period of abandonment lasted less than a millennium (860 BC). The reoccupation of the site, starting from 500 BC, is contemporary, in its upper margins, of the Chami station whose human occupation dynamic covers the phase [2481 BC–1500 BC]. Thus, the beginning of the occupation in Chami, by the dating obtained, suggests a transition or migration of the populations from Tintan to Chami. Indeed, the abandonment of Tintan coincides with the displacement of isohyetes, or aridity, from north to south of Mauritania, corresponding to the marine transgression of 2200 BC. Long period of climatic drying involves migration of populations around the Tintan Peninsula.
Fig 5
Calibration by Marine20 curve of the marine sample collected from the 3 sites (Chami, Tintan, Khant) using chronomodel 2.0.18 with phase option.
This period of aridity will be followed by a probable recovery of the isohyetes, as the region repopulates [2000 BC–685 BC] until the beginning of the first phase of the age of metals. Moreover, the contemporaneity between Tintan and Khant goes from the fifth to the fourth millennium BC. The period of interruption in Khant is long [3200 BC–400 BC]. This is likely the result of the sample collection context (Tables 1 and 3). Thus, in its upper layers, Khant is contemporary with Chami, if ΔR = 0, during the phase of protohistoric occupation. This raises the question of the contextualisation of the samples and the probable reoccupation of Khant. Then, the phase of interruption of Tintan, with the aridity of the region linked to the transgression of Nouakchottien. The ‘revitalisation of the people’ of Tintan [2880 BC–629 BC] is clear evidence of its precedence on other sites whose dates obtained vary from 1100 BC to 118 BC for Khant and from 1500 BC to 462 BC for Chami. Thus, taking into account the reservoir effect with marine20 does not affect the calibration of the marine samples, even if there is a «longer life expectancy» of the samples, but confirms the rejuvenation of the dates of a few centuries. Therefore, the application of the ‘reservoir effect correction’ shows that Chami is more historical than Tintan.
. Conclusion
This study shows the need and importance of applying reservoir age to the chronology of Senegalese-Mauritanian coastal sites. In general, the application of the marine reservoir effect to the different Senegalese-Mauritanian sites shows a rejuvenation of the samples if Marine13 is used. However the calibration curves of Marine20 of the chronological phases cause the ageing of the samples of Chami and Tintan.
From the simplified synthesis, the interpretations made on the samples taken, taking into account the chronological phases of the local reservoir age (Marine13 and Marine20), confirm that the dynamics of the human settlement at Khant is a process that extends from prehistory to history from the fifth millennium BC (4598 BC for Marine13 with ΔR = 71 ± 13 BP and 4560 BC for Marine20 with ΔR = 27 ± 56 BP) at the beginning of history or even beyond (medieval) with prolonged interruption phases. As a result, the dates are relatively young.
By using Marine13 with ΔR = 71 ± 13 BP, the Tintan site is rejuvenated (5238 BC to 5122 BC) despite this there remains an older site than Khant. Tintan is the oldest of the Neolithic sites studied (5143 BC with Marine20), despite a phase of abandonment. The end of its occupation was at the beginning of the early age of iron metallurgy in West Africa (629 BC, Marine20). The displacement of the occupation dynamics of the Mauritanian coastline from Tintan to Chami, after the transgression of the Nouakchottians led the populations towards the southwest of Mauritania. The settlement of Chami is attested to in the third millennium BC (from 2378 BC with Marine13, to 2466 BC with Marine20). It ends in medieval times (from the 5th to the 15th century AD), therefore, the passage from Marine13 ΔR = 71 ± 13 BP to Marine20, ΔR = 75 ± 42 BP, shows an ageing of the samples collected. This raises the problem of reviewing the dates obtained.
In turn, in its lower limits, Chami is contemporary with Khant. In filigree, the ‘markers’ taken and dated provide information on the duration of human occupation of the three sites studied. Thus, if ΔR = 0, the human occupation in Khant goes from the fifth to the third millennium with periods of interruption. In Tintan, it extends from the fifth to the second millennium. During the terminal phase of this occupation, the Chami site was populated from the third millennium to the second century (Table 2). On the other hand, if the reservoir effect is taken into account, it does not alter the prehistoric periods too much despite the differences in methods applied. Khant and Tintan are contemporaries on long chronocultural sequences (from 4598 BC to 238 BC with ΔR = 71 ± 13 BP and from 3770 BC to 680 BC, with ΔR = 75 ± 42 BP) even if the occupation of the site of Khant continues until the beginning of the protohistory. The calibration by Marine20 gives Tintan a 21-year increase in Tintan samples. Moreover, in its terminal phase, Khant (third millennium–second century AD) is contemporary with Chami, whose occupation is a priori linked to the taphonomic mastery of the populations.
Indeed, the passage from Tintan to Chami occurred between 3600 BC and 2378 BC, therefore with the Marine20 calibration, the occupation of the Chami deposit is in 2466 BC or an ‘extended life expectancy’ of 88 years.
