Luminescence dating of ephemeral stream deposits around the Palaeolithic site of Ifri n'Ammar (Morocco)

doi:10.1016/j.quageo.2015.02.012

Quaternary Geochronology

Volume 30, Part B, October 2015, Pages 460-465

https://doi.org/10.1016/j.quageo.2015.02.012 Get rights and content

Abstract

The prehistoric site of Ifri n'Ammar is situated in northeastern Morocco, in the northern prolongation of the Middle Atlas Mountains. It is a key location in unravelling the history of anatomically modern humans (AMH) in northern Africa as it reveals Middle and Late Palaeolithic occupation phases since ∼170 ka. Whilst the archaeological sequence within the rock shelter has been well studied, the timing of landscape dynamics around Ifri n'Ammar is still poorly understood. This study therefore aims to establish a detailed chronology of the Wadi Selloum profile at the apron of the shelter, based on optically stimulated luminescence (OSL) dating of ephemeral stream deposits. Coarse-grain quartz was used for single-grain and multiple-grain dating procedures to investigate the luminescence properties of these deposits and to get more accurate age information concerning the phases of human occupation. Continuous wave OSL (CW-OSL) revealed a dominant fast component for all quartz samples. The dose distribution of the uppermost samples showed overdispersion values >25% and significant positive skewness. We identified partial bleaching as the main source of scatter in the equivalent dose (D_e) distribution. The lowermost sample appeared to be close to signal saturation. The shapes of the dose response curve varied widely between aliquots and coarse quartz grains exhibited therefore very different dose saturation behaviours among aliquots. With fully saturated dose response curves (DRCs), meaningful D₀ values were assumed for D_e estimation.

The eight OSL samples yielded stratigraphically consistent ages ranging from 1.3 ± 0.2 ka to 76 ± 5 ka, thus reaching the Middle Palaeolithic period. Moreover, a pottery shard dated to 7.4 ± 0.6 ka (Early Neolithic period) by thermoluminescence (TL), perfectly matched the Holocene OSL samples extracted at the same depth of the profile. In summary, our results point to fluvial aggradation during OIS 5.1, the late glacial period, and the Holocene.

Introduction

The impact of environmental changes on the emergence and dispersal of anatomically modern humans (AMH) is particularly important in North Africa, since the Straits of Gibraltar might have served as western migration route from Africa to Europe (Linstädter et al., 2012). The prehistoric rock shelter of Ifri n'Ammar in NE Morocco (Fig. 1a) reveals discontinuous Middle and Late Palaeolithic occupation phases since ∼170 ka (Moser, 2003, Nami and Moser, 2010, Richter et al., 2010), thereby constituting one of the oldest AMH settlement sites in this part of the continent. Contrasting with the archaeological background, the Pleistocene and Holocene palaeoenvironmental conditions in this area are as yet poorly reconstructed.

Given that the ephemeral stream deposits of the Wadi Selloum represent the only available archive in the direct vicinity of the rock shelter, this study aimed at evaluating the suitability of such sediments to establish a robust chronology on off-site archives. We applied optically stimulated luminescence (OSL) dating, a technique that has been widely used for dating fluvial sediments (e.g. Wallinga, 2002, Rittenour, 2008). However, especially when working on fluvial systems characterised by episodic flow regimes such as ephemeral streams (Abdalla et al., 2014), complex luminescence characteristics must often be taken into account. Whilst Bubenzer et al. (2007) yielded consistent OSL dating results for fluvio-lacustrine sediments of Wadi Asfura (Egypt) by using small aliquots to account for partial bleaching, Klasen et al. (2013) could not determine the particular contribution of partial bleaching, heterogeneous beta microdosimetry and weak OSL signals to the D_e scatter within sediments from Wadi Sabra (Jordan). A second challenge for dating Pleistocene fluvial sediments is the comparably low saturation level of the quartz fast component (Wintle and Murray, 2006). McLaren et al. (2004) applied optical dating to Wadi Faynan (Jordan) sediments and showed minimum ages of some samples due to quartz saturation. The saturation level of quartz was investigated by Timar-Gabor et al. (2012) and Timar-Gabor and Wintle (2013), who demonstrated the significance of fully saturated dose response curves (DRCs) to determine the quartz saturation level D₀.

Our study is based on eight fluvial samples from a profile at the apron of Ifri n'Ammar, focussing on (i) the investigation of potential sources of scatter within the D_e distributions; (ii) approaches to assess the saturation level of the oldest quartz sample; and (iii) the suitability of the Wadi Selloum deposits as potential archive for reconstructing local palaeoenvironmental changes.

Section snippets

Study area, archaeological background and sampling strategy

The prehistoric site of Ifri n'Ammar (34°47′03.68″ N 3°05′32.42″ W, elevation: 470 m above sea level) is located in the northern prolongation of the Middle Atlas Mountains in NE Morocco (Fig. 1a, b). The archaeological layers of the rock shelter reveal discontinuous phases of modern human settlements during the last ∼170 ka. The lower part of a Middle Palaeolithic sequence is dated from 171 ± 12 ka to 83 ± 6 ka by TL dating of heated flint (Richter et al., 2010). The uppermost part of the Middle

Sample preparation and measurement equipment

Quartz grains (100–150 μm) for OSL were separated using solutions of sodium polytungstate (ρ₁ = 2.62 g/cm³, ρ₂ = 2.68 g/cm³) after leaching with H₂O₂, HCl and Na₄P₂O₇. Afterwards, the quartz grains were etched with hydrofluoric acid (40% for 40 min) plus a final HCl (10% for 1 h) wash. TL sample preparation was carried out on the polymineralic 4–11 μm grain-size fraction following the procedure described in Stoneham (1991). High-resolution γ-spectrometry using a high-purity germanium detector

Luminescence properties

Based on preheat-plateau tests, appropriate preheat temperatures between 200 and 240 °C were selected for further SAR procedures (Tab. S1). The laboratory doses were recovered with measured/given dose ratios of 0.91–1.05, with overdispersion values <5% (Tab. S1). Transfer of charge to optically sensitive traps due to preheating prior to optical stimulation was negligible, and IR depletion tests (Duller, 2003) showed no influence of feldspar contamination. Oriented on the trap parameters and

Source of scatter in D_e distributions

The two uppermost samples C-L3396 and C-L3390 revealed significant scatter within their D_e distributions (Fig. 5a). Since the distribution of the dose recovery results yielded no influence of significant internal sources of scatter (Tab. S1), external sources of scatter must be taken into account. For fluvial samples, in particular partial bleaching may result in an apparent overestimation of the true burial dose (e.g. Olley et al., 1998). The interpretation of scatter as the result of partial

Conclusion

Partial bleaching and signal saturation represent the main challenges for OSL-dating the Late Pleistocene quartz samples from the ephemeral stream deposits of Wadi Selloum. Single-grain measurements in combination with the minimum age model (MAM) were successfully applied to cope with partial bleaching affecting the youngest sediments. Careful investigation of the oldest sample, which almost reached saturation, showed that a meaningful D_e can be estimated if the D₀ values are properly assessed.

Acknowledgements

This project is affiliated to the CRC 806 “Our way to Europe”, which is generously funded by the German Research Foundation (DFG) (CRC 806). The support by the “Institut National des Sciences de l'Archéologie et du Patrimoine du Maroc” (INSAP) and by the “Commission for Archaeology of Non-European Cultures” (KAAK) of the German Archaeological Institute (DAI) is gratefully acknowledged. We thank the anonymous reviewer for the constructive comments that helped to improve the paper.

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Based on a combination of Electron Spin Resonance (ESR) dating of quartz, luminescence dating of K-feldspar and palaeomagnetism, this study presents the first chronostratigraphic framework for the Pleistocene fluvial deposits of the lower Moulouya river in the Triffa basin (NE Morocco). K-feldspar pIRIR₂₂₅ and pIRIR₂₉₀ signals of all samples are saturated, suggesting fluvial deposition at least as early as the Middle Pleistocene (∼0.39–0.80 Ma). Consequently, further chronological information was obtained with ESR dating of quartz grains from the ancient Pleistocene fluvial deposits. As for ESR, the multiple centres approach provides equivalent dose values derived from the Al and Ti centres that mostly agree within 1σ-error, suggesting complete signal resetting from the former during fluvial transport. ESR dating results yield Calabrian deposition ages for all river profiles from ∼1.1 to ∼1.5 Ma. These ages are remarkably consistent with the palaeomagnetic results: the occurrence of mostly reversed polarity in the deposits indicates a Matuyama age (>0.78 Ma). While low incision rates in the Triffa basin (0.025 ± 0.003 mm/a) related to thrusting activity during the Calabrian could be inferred, the fluvial record points to an acyclic and discontinuous sedimentation pattern over the last ∼1.3 Ma. It thereby probably rules out climate as the main driver for fluvial aggradation in the lowermost sedimentary basin. At a regional scale, several indicators point to transient fluvial response resulting from major Quaternary tectonic activity along the Beni Snassen gorge, located directly upstream of the investigated basin. We suggest that a capture event at the margin of the uplifting Beni Snassen massif occurred between 1.04 and 1.36 Ma at the latest and subsequently led to the creation of the gorge.
Controls on dryland mountain landscape development along the NW Saharan desert margin: Insights from Quaternary river terrace sequences (Dadès River, south-central High Atlas, Morocco)
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In northern Africa, Richter et al. (2010) have reported new dates for the Ifri n’Ammar rock shelter, in the Western Maghreb region of Morocco, a site that has now extended the chronological range of Aterian technological attributes. Thermoluminescence dates on associated heated artefacts indicate the stratigraphical presence of tanged artefacts at ∼145 and ∼83 ka and their absence at ∼171 ka and ∼130 ka, respectively (also see Bartz et al., 2015, for a ∼75 ka OSL estimate from the nearby Wadi Selloum). The youngest dated context also includes personal ornaments and the site represents the oldest known occurrence of tanged Aterian artefacts.
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Eight micromorphological samples (thin section, TS) were taken at depths of 1.97, 2.32, 2.52, 2.92, 3.22, 3.57, 3.92 and 4.22 m b.s. (TS 2.1–2.8). The profile IAM 2 was sampled in 10 cm intervals down to 4.80 m b.s. Two luminescence samples from profile IAM 6 (Fig. 4), previously dated by OSL (C-L3393 and C-L3395; Bartz et al., 2015), were measured with the pIRIR290 dating technique for an inter-method comparison. In addition, two samples for micromorphological studies were taken at 1.05 and 4.50 m b.s. (TS 6.1–6.2).
Our study focuses on the ephemeral stream deposits of Wadi Selloum to identify phases of morphodynamic stability (pedogenesis) and activity (flooding) in the direct vicinity of the rock shelter of Ifri n'Ammar (NE Morocco). As one of the oldest settlement sites of anatomically modern humans (AMH) in North Africa, Ifri n'Ammar documents periodical occupations since ~ 170 ka. Since the discontinuous settlement record may reflect climate forcing with subsequent landscape changes, we aim at reconstructing the palaeoenvironmental variability recorded in the ephemeral stream deposits. In addition to the use of micromorphological, sedimentological, geochemical and mineralogical methods, the geochronological framework of ephemeral stream deposits was established by the application of different luminescence dating techniques.
The deposition ages between 102 ± 8 ka and 1.3 ± 0.2 ka span different morphodynamically stable and active phases. Periods of enhanced aggradation occurred around ~ 100 ka, ~ 75 ka, ~ 55 ka, after the LGM, and during the Holocene, whilst sedimentation ended after ~ 1.3 ka. The Wadi Selloum might be characterised by enhanced flooding during humid phases. Pedogenesis may be used as environmental indicator for more humid climate conditions during MIS 3 (palaeo-Calcisol), the early Holocene (Calcisol) and the late Holocene (Fluvisol). This study thus provides first insights into the palaeoenvironmental changes around the rock shelter of Ifri n'Ammar during the last glacial-interglacial cycle.

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Luminescence dating of ephemeral stream deposits around the Palaeolithic site of Ifri n'Ammar (Morocco)

Abstract

Introduction

Section snippets

Study area, archaeological background and sampling strategy

Sample preparation and measurement equipment

Luminescence properties

Source of scatter in De distributions

Conclusion

Acknowledgements

Quat. Geochronol.

Quat. Geochronol.

Radiat. Meas.

Quat. Geochronol.

Radiat. Meas.

Nucl. Tracks Radiat. Meas.

J. Archaeol. Sci.

Radiat. Meas.

Quat. Geochronol.

Quat. Int.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Radiat. Meas.

Radiat. Meas.

Quat. Sci. Rev.

Nucl. Tracks Radiat. Meas.

Radiat. Meas.

J. Hum. Evol.

Radiat. Meas.

Radiat. Meas.

Source of scatter in D_e distributions