Sea extreme events during the last millennium in north-east of Morocco

Introduction Conclusions References


Introduction
The Mediterranean region has experienced numerous extreme coastal events, weather events (Lionello et al., 2006) as well as tsunamis events (Papadopoulos and Fokaefs, 2005) wich caused casualties and economic damages.Preventing the adverse impacts of such events requires to study their past frequency and intensity in order to predict future trends and determine their recurrence interval.However, the meteorological and marine instrumental records are limited to only a few decades especially in South Mediterranean countries.Thus, geological data offer the best way for reconstructing the historical records of intense hurricanes and tsunamis activity (Liu and Fearn, 1993).The most appropriate sites for such studies seem to be the coastal lagoons with a sandy barrier, which provide a source of material for transport and depo-Introduction

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Full sition as an overwash layer (Woodruff et al., 2008;Yu et al., 2009;and Dezileau et al., 2011).The objective of this research is to reconstruct paleo-sea extreme events (sea storms or tsunamis) using a multi-proxy analysis in the Nador lagoon, located along the Moroccan Mediterranean coast.This area is part of the Western Mediterranean sea which presents a high cyclogenetic character (Lionello et al., 2006;Campins et al., 2006).It is also prone to risks related to tsunami events; (Gonzalez et al., 2010) have studied potential tsunamigenic sources in the Alboran sea and identified Nador area as one of the most exposed to a large tsunami wave elevation.The Nador lagoon thus offers a real potential for reconstitution of sea paleo-events, and for that an undisturbed core (MC4.5)was successfully sampled using the UWITEC coring platform in the NW of the Nador lagoon; superficial sediments were also collected during coring campaign (Fig. 1).

Study area
The Nador lagoon, also called Sabkha Bou Areg or Mar Chica, is the largest lagoon of Morocco with a surface area of approximately 115 km 2 and the only one located along the Moroccan Mediterranean coast between Melilla and the district of Kariat Arekman (Fig. 1).The lagoon is surrounded by a dense urbanization which houses a large population of approx.450 000 inhabitants (RGPH, 2004).The actual development is significant and mainly related to the ongoing project of MarchicaMed, which covers an area of over 20 km 2 and aims to develop seven distinct cities, around the lagoon and on its barrier island.The watershed, draining an area of 220 km 2 , is formed by four morphological structures: (i) in the northwest the volcanic massif of Gourougou composed of Messinien to Plio-Quaternary calc-alkaline rocks (Morel, 1985) (Mahjoubi, 2001); and (iv) the Bou-Areg Plain in the southwest which is of Neogene to Plio-Quaternary age, characterized by salt marshes, several intermittent streams with irregular flow some of which presently serve as sewage outflows for urban areas (Irzi, 2002).Dominant climatic conditions prevailing in the study area are that of typical Mediterranean climate, with contrasted hot and dry summers, and mild and rainy winters.The strongest winds and waves are from the NE and E. Mean significant waves height is less than 1 m, although during storm condition it can reach 4.5 m (WANA Puertos del Estado wave data network).The lagoon is isolated from the Mediterranean Sea by a 25 km long sandy barrier crossed by an artificial inlet recently opened and stabilized (2011).The NW area of this lido is marked by many overwashes, which indicate how intensely it has been exposed to the adverse sea events through history.

Methods
The MC4.5 core was first split, photographed and logged in detail.Then, a multiproxy approach was adopted by the combination of sedimentological, geochemical and geochronological data.
The sediment core was analyzed by X-ray fluorescence (XRF) using an XRF Core Scaner.With a step size of 0.5 mm, a semi-quantitative analysis was performed directly on the surface of sediment; this surface had to be covered with a 4 mm thin Ultralene to avoid contamination of the XRF measurement unit and desiccation of the sediment.
The analysis of particle size was performed on samples taken directly from the sediment core every cm.Using a Beckman Coulter © LS 13 320, each sample was sieved at 1 mm, suspended in deionized water and gently shaken to achieve disaggregation.layers of the core.The use of this natural radionuclide 210 Pb ex to determine sedimentation rate is now a well established technique (Goldberg, 1963;Krishnaswamy et al., 1971;Robbins and Edgington, 1975).The dating by 137 Cs was conducted according to Robbins and Edgington (1975).For the long scale time: the Radiocarbon ( 14 C) chronology was adopted using shell samples. 14C analyses were conducted at the Laboratoire de Mesure 14 C (LM 14 C) on the ARTEMIS Accelerator Mass Spectrometry in CEA Institute at Saclay (Atomic Energy Commission).These 14 C analyses were done with the standard procedure described by Tisnérat-Laborde (Tisnérat-Laborde et al., 2006).

Results and discussion
4.1 MC45 Core description

Lithostratigraphy
The MC45 sediment core of 115 cm long shows visual variations in sediment composition.Based on photography, digital X-ray radiography, stratigraphic log and grain size results (Fig. 2), the core was roughly divided into four units: -Unit 1 (115-105 cm) composed by silty fine laminae, three colors are observed; dark grey layers and lighter grey layers correspond to fine grained sediments, while white layers associated with gypsum microcrystals correspond to relatively coarse sediments.
-Unit 2 (105-60 cm) is characterized by fine grained sediment and brownish gray color; it consists mainly of homogeneous silts with large number of shell fragments mainly at 75-100 cm interval.
-Unit 3 (60-54 cm) composed by fine parallel-laminated silts, red layers relatively fine-grained and grey layers.These laminae are well observed in digital X-ray radiography (as intercalation of dark and light laminae).2083 Introduction

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Full -Unit 4 (54-0 cm) display several lithological changes.Three main facies can be identified: (i) silty sand characterized by coarse sediment with grey color (very dark in radiographic images) (ii) sandy silts show brownish gray color, relatively coarse sediment and is dominated by shell fragments and (iii) red silts consists in fine grained sediment with relatively absence of shell fragments.

Geochemistry
25 elements (Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, As, Br, Rb, Sr, Y, Zr, La, Ta, Bi) were detected in the Nador MC45 core sediment; however most of them exhibit noisy profiles.Therefore, only elements which present the most important variations through depth were selected and down-core plots of signal intensity (cps, counts per second) vs. depth are presented in Fig. 3. Based on their profiles, the group composed by Al, Fe, Ti, K, Rb and Si, exhibit similar variations, intensity values are high in fine-grained intervals while low values in coarse-grained intervals.The Zr is also related to intervals rich in fine particles.However, the maximum values are only detected at the bottom of the core.The highest values of Sr and Ca are also found in the bottom but unlike to the first group, both elements show peaks in sandy layers.Eight shells sampled from MC45 lagoon core were dated using conventional 14 C-AMS measurements and obtained dates were calibrated using the Marine13 curve (Reimer et al., 2013) 3, Fig. 3).The ∆R of about −218 years is thus estimated and adopted (Table 2).The negative ∆R value is common in shallow oceanic regions or in enclosed lagoons where (i) the 14 C in seawater DIC (dissolved inorganic carbon) are higher than the average global ocean, and (ii) 14 C mixing with depleted water from deeper ocean is limited (Dutta, 2008).However, this value should be taken with caution, due to: (i) intra-shell variation in radiocarbon age, which can be significant (∼ 200 y) (Jones et al., 2007) and (ii) to possible variations in relation to palaeo-environmental changes (Sabatier et al., 2010).

Age-depth model and sedimentation rate
The age model of MC45 core was realized by fitting a linear regression using the "clam" R-code package version 2.2 (Blaauw, 2010) on 14 C ages (without inversed ages) and 210 Pb ex / 137 Cs (Fig. 5).The 14 C mean sedimentation rate calculated is 0.10 cm yr −1 .

Sediments source
The sedimentation in Nador lagoon is mostly controlled by terrestrial and marine inputs.
Rivers are the main source of fine sediments dispersed within the lagoon while marine sediments consist mainly of coarse carbonate sand (Inani, 1995;Mahjoubi, 2001 andBloundi, 2005).In order to characterize the lagoon barrier beach sand samples, the Introduction

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Full spectrum of the grain size percentages vs. grain size was plotted; the results show unimodal distribution with one mean grain population ranging between 133 and 282 µm.The percentages of this grain size class in surface samples decrease considerably from the sea to the lagoon (Fig. 6), therefore, it can be used successfully to trace marine sand in lagoon sediments.
The geochemical data obtained by XRF portable analyzer allowed detection of 11 elements (Ti, Fe, Sr, Cu, Zn, Zr, Mn, Co, As, Rb, and Pb).Variation of the concentration of the most important detected elements from sea to continental samples (Fig. 7) shows that the elemental composition of barrier beach sediments are enriched in Sr and depleted in Fe, Ti, Mn and Rb.Although, these results are based on a limited number of samples and detected elements, they are in agreement with previous studies in the area.Indeed, in Nador lagoon two groups can be identified: the first group is composed of Al, Fe, K, Mn, S, Ti and Mg, these elements are associated with clay and Feldspar and the minimum concentrations are found mainly on the continental part of the lagoon.The second group is composed of Ca and Sr the calcium being mainly of marine origin, associated to biogenic calcareous (Bloundi, 2005).Regarding Si, its terrestrial origin is mainly from feldspar and clay minerals while the marine Si comes from mainly from quartz.
In order to trace marine sediment geochemically, inter-elements ratios of terrestrial elements and marine elements can be used.Additionally, the use of inter-elements ratios presents the advantage to be insensitive to dilution and matrix effects (Richter et al., 2006;Croudace et al., 2006).

Identification of paleoevents
The evolution with depth of the 133-282 µm grain size population in MC45 core shows five peaks; all are over the last 600 years.However, only three events are confirmed by geochemical data (Fig. 8).Indeed, the selected inter-elements ratios (Si/Al, Ca/Fe and Sr/Fe) show important peaks for the events 1, 2 and 3.An increase of inter-elements ratios value is also observed in the bottom core, this is not related to the presence of 2086 Introduction

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Full Based on all of the above results, we can confidently assume that the events 1, 2 and 3 correspond to paleo-overwash events.The age model indicates that overwash layer "1" was probably deposited between Cal AD 1438 to 1524, the overwash "2" between Cal AD 1610 to 1799 and overwash "3" between Cal AD 1856 to 1899.

Storm or tsunami events?
Generally, both tsunamis and storms cause brief coastal flooding with high overland flow velocities.Distinguishing tsunami and storm deposits is still controversial and several studies have pointed out many hypotheses regarding the diagnostic characteristics of storm/tsunami deposits (Kortekaas and Dawson, 2007;Morton et al., 2007;Tappin, 2007;Engel and Brückner, 2011).
Local and regional historic documents with tsunami catalogues are one of the best ways to correlate identified events with historical recorded tsunamis or storm events.During the last 600 years four tsunamis were recorded in the western Mediterranean coast; (i) the 22 September 1522 tsunami near Almeria coast (Spain); recently evidences of this tsunami deposits were found in Cabo de Gata lagoon (Reicherter and Heidmann, 2009).In Moroccan Mediterranean coast, the affected areas are mainly Badis Ghomera (Al Hoceima); however, this information is not supported by Maghrebian documents (Kaabouben et al., 2009).Anyway, the geographic position of Nador lagoon directly exposed lets us not exclude its impact on Nador area.(ii) The 9 October 1680 tsunami (Malaga) is not supported by the available information in contemporary documents (Goded et al., 2008).Besides, Nador lagoon is not geographically exposed.(iii) The 9 October 1790 tsunami which caused flooding of the north shore of Africa and southern shore of Spain (Soloviev, 2000).No documents support tsunami waves on the Moroccan coast but the extent of the tsunami event on Algeria coast and its manifestation on the southern coast of Spain, leads to assume a possible impact on the northern Moroccan coast (Kaabouben et al., 2009).(iv) Finally, the 21 2087 Introduction

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Full August 1856 of this tsunami on the north-eastern shore of Algeria (Jijel, 300 km east of Algiers) (Salaviev, 2000); the effects of this event on the Moroccan coasts is improbable (Kaabouben et al., 2009).
In the Nador area, the well recorded sea storms are those who opened inlets along the lagoon lido.The oldest one is the 1889 strong storm caused by N to NE winds and which occasioned a large inlet of 150 m width.The paleo-inlets 3 and 5 (Fig. 1) were respectively the results of recent storms of 1941 and 1981.The inlet 1 was opened artificially in 1909 but enlarged by storms effects.In addition, it should be noted that in the Western Mediterranean, storms had a higher frequency in the Little Ice Age (LIA), with two lesser periods of high frequency in the 1400s and at the end of the 1700s (Camuffo et al., 2000).Other studies confirmed increased intensity and frequency of storms during the LIA in the Mediterranean, as for example in the French coast (Dezileau et al., 2011).Based on a set of six simulations of the LIA using an Ocean-Atmosphere General Circulation Model (OAGCM), Raible et al. (2006) consistently found an increase in cyclone occurrence in the Mediterranean during the LIA compared to present-day.
Considering all these historical data, we can confidently assume that the event 3 corresponds to 1889 storms because of its date located in "the event 3" age interval and also of the paleo-overwash generated by this event located in front of the coring site.For the event 1 and 2, the tsunami-storm hypothesis remains open.Indeed, even if they are probably related to sea storms because of the increase of these phenomena during this period in the Western Mediterranean, both 1522 and 1790 tsunamis seem to have also affected the Nador area during event interval age 1 and 2 respectively.Based on examples of modern tsunami and storm deposits, some sedimentological criteria can be used to identify tsunami from storm deposit (Morton et al., 2007).The tsunami deposit has a relatively thin bed consisting of a single structureless bed or a bed with only a few thin layers.The presence of internal mud laminae or intraclasts is more in favor of tsunami deposits.The storm origin deposit present a moderately thick sand bed composed of numerous subhorizontal planar laminations organized into multiple laminasets.The stratification associated with bed-load transport and abundant shell frag-Introduction

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Full ments organized in laminations also favor a storm origin.In our case, all of the events deposits identified are observed as a single homogeneous bed (∼ 2-5 cm thick).Within it, no sedimentary structures (ripple crossbeds, planar laminae or mud intraclasts) were observed; only the events 3 contain internal mud laminae within the deposit.However, even if these criteria favor tsunami origin, this observation should be taken with caution because the use of a single sediment core results make it inadequate for physical distinction process (Morton et al., 2007).About the use of geochemical signatures to distinguish between tsunami and storm deposits, no study was found.The increase in geochemical elements indicating marine origin as evidence of marine inundation is generally used in both deposits (Engel et al., 2010;Ramírez-Herrera et al., 2012).

Conclusions
The present study is a first attempt for the reconstitution of paleo-events in the Nador lagoon; he use of a multiproxy analysis by lithological evidences and X-ray fluorescence has revealed distinct anomalies and allowed differentiation of stratigraphic intervals which are interpreted as event horizons.Eight radiocarbon dates of shells samples combined with 210 Pb and 137 Cs chronologies allowed to establish a chronological framework, which was used to calculate a time-averaged sedimentation rate (0.10 to 0.28 cm year −1 ).
Three events were identified; all of them are concentrated over the last 500 years.Attribute these deposits to tsunami or storm events was not an easy task; historically only the modern event can be attributed with certainty to a 1889 major storm.The sedimentological and geochemical data were not very helpful in this process because of the single core studied but they could probably be more indicative by correlation with other cores in the area.Figures  Full Discussion Paper | Discussion Paper | Discussion Paper | ; (ii) in the west, the massif of Beni-Bou-Iffrour consists of limestones, Jurassic calcareous sandstones and marls, and of Cretaceous sandy shales and volcano-sedimentary series covered by the Miocene; (iii) in the south, the Jurassic to Mio-Pliocene Kebdana massif Discussion Paper | Discussion Paper | Discussion Paper | After introduction of sediment into the fluid module of the granulometer, ultrasounds were used to avoid particles flocculation.The 137 Cs and 210 Pb ex activities analysis were measured in the fine-grained fraction (< 150 µm) by gammaspectrometry using BEGe detector (CANBERRA BEGe 3825).The 210 Pb ex dating is based on the determination of the 210 Pb excess activities in the Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Measured 210 Pb values in the uppermost 30 cm of the MC45 core range from 132.6 to 1.7 mBq g −1 .In general, the downcore distribution of excess 210 Pb values follows a relatively exponential decrease with depth.Therefore a constant flux, constant supply CF : CS sedimentation model was applied (Appleby, 2000).The indicated sedimentation rate is about of 0.28 cm yr −1 .The profile distribution of 137 Cs activity (Fig. 4) shows the maximum at 16 cm depth, this may represent the period of maximum radionuclide fallout in the Northern Hemisphere associated with the peak of atomic weapons testing in 1963.The 137 Cs-derived sedimentation rate (0.34 cm yr −1 ) is more important than 210 Pb rate.Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | coarse sediments but to the gypsum crystals (enriched en Ca and Sr and depleted en Al and Fe).
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | study.The authors wish to thank also the staff of MarChica Med Company (Rabat and Nador) for their assistance during the entire period of the expedition.This study is funded by the MIS-TRAL project (Coord.L. Dezileau) and PHC-Volubilis (MA/11/253; coord.: L. Dezileau and MDiscussion Paper | Discussion Paper | Discussion Paper |

Fig. 8 .
Fig.8.Identification of paleo-events based on photography, radiography X, log, grain size and geochemical inter-elements ratios.The identified events show coarse grained texture in photography, dark layer in radiography X, high percentage of 133-282 µm fraction and peaks of Si/Al, Ca/Fe and Sr/Fe ratios.
(Table1).The modern 14 C reservoir age in Nador lagoon was estimated using the radiocarbon date within 210 Pb ex -time domain.
14 C age of the shell (235 ± 30 14 C yr, Table

Table 1 .
14C data for mollusk shells from MC45. 14 C ages are calibrated using the Marine13 calibration curve with reservoir age ∆R of −218, model.Ages were obtained by Clam codepackage.