Environmental radioactivity in a sediment core from Algiers Bay: Radioecological assessment, radiometric dating and pollution records

Environmental radioactivity in a sediment core from Algiers Bay: Radioecological assessment, radiometric dating and pollution records Djamel Taieb Errahmani a , b , Abdelkader Noureddine b , Jos�e María Abril-Hern�andez c , * , Mostefa Boulahdid b a Centre de Recherche Nucl�eaire d’Alger (CRNA), Commissariat �a l’Energie Atomique. 02 Bd. Frantz Fanon, Box.399, Algiers, Algeria b Laboratoire des Ecosyst�emes Marins et littoraux, Ecole Nationale Sup�erieure des Sciences de la Mer et de l’Am�enagement du Littoral (ENSSMAL), BP 19, Campus Universitaire de Dely Brahim, Bois des Cars, Alger, Algeria c Departamento de Física Aplicada I, ETSIA, University of Seville, Spain Keywords: Algiers bay Sediment core Radiometric dating 210 Pb 137 Cs Pollution records ABSTRACT The sedimentary system of the Algiers Bay has been impacted in the last decades by the exponential increases in population, urbanism and industrial activities of the city of Algiers, and by episodic floods, mudflows, earth-quakes and tsunamis. Activity concentrations of environmental gamma-emitters, namely 210 Pb, 226 Ra, 40 K, 228 Ra, 228 Th and 137 Cs, were measured in a sediment core sampled in 2015 at the central zone of this bay, at 73 m depth from the surface water. The 210 Pb-based radiometric dating was challenging due to the incomplete recovery of the inventory, the absence of 137 Cs peaks, and the failure of classical models. The analysis of clusters in the logarithmic plot of the excess 210 Pb versus mass depth profile allowed for establishing a reliable chronology. Fluxes of excess 210 Pb and sediment accumulation rates (SAR) have both varied over the last five decades. Recent SAR values are of 0.73 � 0.21 g cm  2 y  1 . Major and trace elements were determined by X-Ray Fluorescence in some selected sediment slices. Moderate enrichments have been observed for Mn, Zn, Br, Rb, Sr, Y and Pb in the 0–1 cm sediment layer (when using Al as normalization element), being governed by Fe-rich carriers. The major impact is observed in the carbon content, which increased by two orders of magnitude (from their basal values) for three decades since early 70’s. The dataset also allowed for a radioecological assessment in the area, performed using the ERICA software. 1. Introduction The study in sediment cores of the 210 Pb found in excess with respect to its parent radionuclide 226 Ra (labelled 210 Pb exc hereafter) has shown to provide useful insights on the functioning of the sedimentary systems at a centennial time scale (Robbins, 1978; Carroll and Lerche, 2003; Mabit et al., 2014). Independent chronostratigraphic information can be provided by 137 Cs, a man-made radionuclide first introduced in the environment after the atmospheric nuclear weapon tests (Robbins and Edgington, 1975; S� anchez et al., 1992; Abril, 2004). A summary of the most known 210 Pb-based dating models can be found in Mabit et al. (2014). The most widely used models are CRS (assuming a constant 10 Pb exc flux), CIC (assuming a constant 10 Pb exc initial activity concentration) and the CF-CS (assuming constant flux and constant sedimentation rate). A detailed presentation of these models can be seen, among others, in S� anchez-Cabeza and Ruíz-Fern� andez (2012). In anthropogenic-impacted and/or high energy systems the 210 Pb flux onto the sediments may show non-random temporal variability, leading to the failure of classical dating models (Abril, 2019). Studies on environmental radioactivity in the Algerian marine coastal zone are scarce. They include the distribution of anthropogenic radionuclides in the water column (Lee et al., 2006, Noureddine et al., 2008), 137 Cs in seawater, surficial sediments and sediment cores (Noureddine et al., 2006). The 210 Pb-based radiometric dating of recent sediments was applied to some sediment cores (Noureddine et al., 2008). The nearest one to the Algiers Bay was collected at a depth of 1220 m from the surface water. During the last decades, the city of Algiers has experienced an intense increase in population, urbanization and industrialization, noticeably affecting the flows of matter and pollutants to its bay. An important harbour is integrated in the Algiers Bay. It has been continuously expanded, with major works carried out from the beginning of the 50’s * Corresponding author. ETSIA, Universidad de Sevilla, Carretera de Utrera km 1, D.P, 41013, Sevilla, Spain. E-mail address: [email protected] (J.M. Abril-Hern� andez). 2 2. Materials and methods 2.1. Sampling and sample treatment The studied site in the Algiers Bay has been briefly described above. Concerning climate, Algiers has a Mediterranean climate with hot and dry summers, while winters are cool and wet. The average temperature is 17.7 �C and the average annual rainfall is 707 mm, although it is very irregular and unevenly distributed in space. A cylindrical sediment core of 45 cm length and 8.5 cm diameter was collected in September 2015 in the Algiers Bay at geographical coordinates 36�47.9970N, 3�7.7930E. The water depth was 73 m from the surface water. The sampling site is shown in Fig. 1. An Uwitec corer was used for sampling. It was operated from the Grine Belkacem research vessel, belonging to the Centre National de Recherche et de D� eveloppement de la P^ eche et de L’Aquaculture (CNRDPA). The sediment core was frozen on board at a temperature of 18 �C, and sectioned at the laboratory into sections of 1–2 cm thick using a core slicer. Samples were put into plastic containers till oven drying at 70 �C to constant weight. They were grounded to fine powder using centrifugal grinder Retsch S100, and homogenized before analysis. The water content in each sediment slice was calculated from the wet and dry weights, while bulk densities were determined by the ratio between the dry weight and the known volume for each slice. Prior to gamma analysis, the sediment samples were sealed in Petri dishes to avoid 222 Rn escape, and stored for a period of at least three weeks to ensure secular equilibrium among 226 Ra and its progeny radionuclides 214 Bi and 214 Pb. Loss on ignition (LOI) analysis were used as a proxy for estimating the organic matter content of selected sediment core samples. Two grams of dry sample were ignited at 480 �C for 4 h (Luczak et al., 1997). 2.2. Gamma spectrometry analysis Gamma spectrometry measurements were performed to determine 210 Pb, 212 Pb, 214 Pb, 228 Ac, 40 K and 137 Cs activity concentrations using a coaxial High purity Germanium (HPGe) detector with a 40% of relative efficiency (a GX4018 from Canberra), and Genie-2000 software. Activity concentrations of 226 Ra were estimated through its progeny radionuclide 214 Pb. For radionuclides from the 232 Th series, 228 Ra was estimated through 228 Ac, and 228 Th through 212 Pb. Unsupported 210 Pb was determined for each slice by subtracting 226 Ra from the total 210 Pb activity concentration. The energies considered for the activity calculations were: 46.5 KeV for 210 Pb, 238.6 KeV for 212 Pb, 295.2 and 351.9 KeV for 214 Pb, 661.6 KeV for 137 Cs, 911.2 KeV for 228 Ac and 1460.8 KeV for 40 K. Counting times ranged between 20.3 and 48 h. Energy and resolution calibrations were determined using pointsources of 241 Am, 137 Cs and 60 Co.The detector efficiency was calibrated using in–house calibration sources prepared by spiking separately a local marine sediment from the bay with liquid radioactive sources of 152 Eu and 133 Ba. Due to the mass variations of core samples, four standards for each tracer at different masses (25, 50, 75 and 100 g) were prepared with the same geometry as the samples. EFFTRAN software (Vidmar, 2005) was used for true coincidence summing corrections. This allowed for fitting the efficiency as a continuous function of the energy, and this for each mass. Thus, for any given photo-peak (e.g., the 46.5 keV emission for 210 Pb) there were four values for the efficiency as a function of the mass, which in all the cases followed a well-defined linear trend (e.g., r ¼- 0.998, p ¼0.0021 for 210 Pb). For samples with intermediate masses, interpolations in the efficiency vs mass curve for each gamma emission were performed. The analytical procedure was checked using the proficiency test soil sample IAEA TEL 2018 (natural soil spiked with anthropogenic gamma emitter radionuclides). Two samples of the IAEA soil, of 50 g and 100 g, were prepared and measured as the samples from the studied sediment core. Table S-1, in electronic supplementary material, reports the measured activity concentrations until late 80’s. Historically, Algiers was mainly supplied with water from Mitidja and Sahel Dunes aquifers, and with surface waters from the C^otier Alg�erois basin. In the mid-nineties water shortages were a prevailing phenomenon, so since 2002, new dams, surface water transfer systems, and desalination plants have been built (Naimi-Ait-Aoudia and Berezowska-Azzag, 2014). Two major oueds flow to the Algiers Bay, namely Oued El-Harrach (regulated by a dam) and that of El-Hamiz, fed by waste waters of the city and the waters of the plains of the Metidja. Benamar et al. (1999) have reported high concentrations of copper and lead in surficial sediments from the central and western sectors of the bay, near the main emission of waste waters and of Algiers harbour. For their part, Boulahdid et al. (2013) found anomalous concentrations of zinc, copper and chromium at 6 cm depth in the sediments of a core taken at a depth of 30 m in the vicinity of the mouth of El Harrach River. In 1999, as part of the activities of the Mediterranean Action Plan (MAP) and the Mediterranean Pollution program (MEDPOL), the bay of Algiers was identified by UNEP/WHO as one of the hotspots priority pollution among 8 in Algeria and 131 in the Mediterranean Sea (UNEP/WHO, 1999). In addition to the anthropogenic pressure, the Algiers Bay has been impacted by a diversity of natural hazards. The most important historical floods recorded in the area are those of 1954, 1960, 1974 and 2007 (Aroua and Berezowska-Azzag, 2009). On 10 November 2001, northern Algeria was hit by a heavy rainstorm and the Bab El Oued district of Algiers was devastated by cataclysmic floods and mudflows (Machane et al., 2008). The Bay of Algiers is located in a seismic zone with active faults triggering important earthquakes and sometimes small tsunamis. The latest important earthquakes took place in 1980 (at El Asnamin, Ms 7.3) and 2003. This last, with Ms 6.8, and with epicentre located a few km offshore, triggered a tsunami which was recorded in a large number of stations in the central and western Mediterranean Sea (Vecchio et al., 2014). On August 2014, a moderate-sized earthquake struck the city of Algiers, likely triggered by and active fault in the same bay of Algiers, according to Benfedda et al. (2017). Additionally, along the dunar cordon of the Algiers Bay, the combination of natural erosion and the intensive sand extraction produce the backward displacement of the coastal string (Chemrouk and Chabbi, 2016). The aim of this paper is getting some insights on the sedimentary conditions and the recent pollution history in the Algiers Bay through the measurement of natural and artificial radionuclides, namely 210 Pb, 226 Ra, 228 Ra, 228 Th, 40 K and 137 Cs, along with major and trace elements, in a sediment core sampled at 73 m depth from the surface water. It has the value of reporting temporal trends of pollution in this coastal area, affected by anthropogenic and natural stresses, and scarcely studied. The 210 Pb-based radiometric dating is challenging since the high sediment accumulation rate (SAR) prevented the total recovery of the 210 Pb exc profile (and inventory) with our coring device, and it lacked of a clear trend-line in the deepest layers to support the reference-SAR method. The 137 Cs profile did not show distinct peaks for establishing independent reference dates. Under the dynamic sedimentary conditions above described the classical dating models failed, since their assumptions of constant values over time for fluxes (CRS model), initial activity concentrations (CIC model) or both (CF-CS model) did not hold. The holistic analysis of the whole dataset along with updated modelling approaches, including the analysis of clusters in the 210 Pb exc vs. mass depth profile (Abril, 2019) have shown to be useful for deriving a reliable chronology. Thus, the methodological procedures may be also of interest for the international scientific community dealing with environmental studies requiring the radiometric dating of recent sediments. The measured activity concentrations for the studied radionuclides also allowed for the first radioecological assessment in the area, performed with the ERICA tool (Larsson, 2008). 3 for 210 Pb, 214 Pb and 137 Cs, compared against the target values. 2.3. Chemical and mineralogical compositions Major and trace elements in sediment samples from selected depths in the core were analyzed by Wavelength –Dispersive X-Ray Fluorescence Spectrometer (WDXRF), using a Panalytical Philips Magix Pro and PW2440 model. Two grams of each sediment sample were prepared as pressed pellets using a licowax binder at a pressure of 18 tons, and then measured for 24.5 min time. The mineralogical composition of sediment samples was determined by powder X-Ray diffractometry, using a PANalytical X’pert pro diffractometer equipped with a Cu-K α radiation (1.54 Å) operated at 45 KV and 40 mA. Scans were performed in 2θ range from 5.01 to 84.99�, with a step of 0.02�and 1.8 s for each step. Phases were identified using Philips Highscore software. 2.4. Radioecological risk assessment Because of the lack of previous studies, present results will be used for a radio-ecological assessment conducted with the ERICA tool. The mean activity concentrations found in the 0–10 cm of the core for 137 Cs, 210 Pb, 226 Ra, 228 Ra and 228 Th will support the application of the Tier 2 of version 1.2.1 of ERICA-tool to estimate activity concentrations and doses in eight selected biota (benthic fish, crustaceans, macroalgae, molluscbivalve, pelagic fish, phytoplankton, polychaete worm and zooplankton). We used the ERICA library for K d values, (i.e., the ratio of activity concentration in sediment to that in water) and concentration ratios, CR (i.e., the ratio of activity concentration in the whole body of an organism to that in media) for the marine environment. The derived activity concentrations in the selected biota were then further processed by the ERICA-tool by selecting defined occupancy factor(s) of each biota and the dose conversion coefficients, DCCs (i.e., dose rate per unit activity concentration in the organism or media) – see Brown et al. (2016). Default settings were used, as in Klubi et al. (2017). The above estimated concentrations also served for calculating the dose to human due to the consumption of seafood products. The ingestion rate of each biota class, k, denoted as IR k , was documented from local uses. The 50 year committed effective dose, E ing (given in Sv), is then (UNSCEAR, 2008): Eing ¼X k;j Ck;jej;ingIRk;(1) where j refers to radionuclides and k to the type of food; C k,j is the activity concentration (Bq/kg) and e j;ing is the coefficient from conversion from intake to committed effective dose integrated for 50 years for adults (UNSCEAR, 2008). 3. Results and discussion 3.1. The experimental dataset Table 1 reports results for the water content, LOI percentage, dry mass thickness and the activity concentrations for the target radionuclides in the sediment core from Algiers Bay. The elemental composition determined by WD-XRF is reported in Table 2 for samples from five selected depth intervals within the core. The mineralogical composition (determined by X-R diffractometry) is reported in Table 3 for the same depth intervals. Activity concentrations in the top sediment slices are of the same order than those reported by Benamar et al., (1999) for surficial sediments sampled within the bay. The 228 Ra/ 226 Ra isotopic ratio was found to be 1.39 �0.14 (mean and standard deviation, all the sediment slices). Benamar et al., 1999 reported values ranging from 0.6 to 2.2 for this ratio, being of 1.6 around the sampling site of the core studied in this work. It is also similar to the mean value of 1.6 reported by Klubi et al. (2017) for sediment cores sampled in the Pra and Volta estuaries, in Ghana. The 228 Th/ 228 Ra isotopic ratio was 0.98 �0.18 (mean and standard deviation), being close to the radioactive secular equilibrium. The mineralogical composition of crystalline phases (Table 3) remains quite uniform downcore, and dominated by Illite, Quartz, Muscovite and calcium-carbonates. The elemental composition (Table 2) reveals noticeable changes with depth in the concentration of C and some metals, such as Cu, Zn and Pb, which will be discussed after the radiometric dating of the sediment core. 3.2. Radioecological risk assessment Table 4 reports the activity concentrations in reference organisms and the total dose rate per organism after applying ERICA with its default values for model parameters. The total dose rates to all the reference organisms fell below the screening value of 400 mGyˑh 1 , and therefore being unlikely to cause harmful effects (UNSCEAR, 2008). The representative seafood product from the Algiers Bay are benthic fish (Mullus barbatus), pelagic fish (Sardina pilchardus; Sardinella aurita) and crustacean (Aristeus antennatus), with relatively low mean annual consumption rates of 0.5, 2.0 and 0.5 kg per person (data from the Ministry of Agriculture, Rural Development and Fisheries of Algeria; Dr. Fig. 1. The Algiers Bay with indication of the sampling site for the studied core. Satellite image from USGS/NASA Landsat, October 2nd , 2015. It shows clouds of suspended particulate matter. 4 Kacher, personal communication). The ERICA-derived activity concentrations in these biota classes (Table 4) for the set of studied radionuclides served to estimate (by Eq. (1)) the committed effective dose integrated for 50 years for adults by using the conversion coefficients recommended by UNSCEAR (2008) for natural radionuclides, and ICRP (2012) for 137 Cs. It resulted of 3.2⋅10 5 Sv, which is far from producing any harmful effects (UNSCEAR, 2008). 3.3. Sediment dating by 210 Pb-based classical models Fig. 2 plots the activity concentrations for 210 Pb exc and 137 Cs versus the mass depth, m, in the sediment core. The CF-CS model assumes constant 210 Pb exc flux onto the sedimentwater interface (SWI), and a constant sediment accumulation rate (SAR), w [ML 2 T 1 ], which results in an exponential decrease of the activity concentration, A(m), with m: AðmÞ ¼ A0eλm=w;(2) where λ is the radioactive decay constant for 210 Pb. The best fit is shown in Fig. 2, first panel. The scaling factor isλ=w ¼0.084 �0.006 cm 2 g -1 , which leads to a sedimentation rate of w ¼0.371 �0.025 g cm 2 y 1 . The Pearson’s correlation coefficient is r ¼  0.948, but it is worth noting that high r values are a necessary, but not a sufficient condition for the reliability of the CF-CS model assumptions. The corresponding chronology (ages, t ¼m/w) is plotted in Fig. 2, second panel. The CF-CS age of the central point of the last measured slice is 102 �7 y (a date of 1914 Depth (cm) Water (%) LOI (%) Δm 210 Pb 226 Ra 137 Cs 228 Ra 212 Pb 4 K (g⋅cm 2 ) Bq/kg 0–1 45.4 15.0 0.576 133 �9 21.8 �0.8 3.8 �0.4 26 �10 36.4 �1.1 479 �15 1–2 47.4 7.5 0.701 136 �6 20.0 �0.7 3.5 �0.3 25 �4 26.0 �0.8 441 �27 2–3 45.9 7.2 0.628 149 �7 21.5 �0.7 3.4 �0.3 27 �7 28.0 �0.9 481 �30 3–4 47.2 0.817 127 �7 21.3 �0.6 3.3 �0.3 26 �6 27.8 �0.8 461 �29 4–5 44.1 7.5 0.714 136 �6 20.3 �0.7 3.5 �0.2 26 �4 27.8 �0.8 447 �28 5–6 45.1 7.2 0.906 131 �5 19.6 �0.6 3.8 �0.2 26 �4 26.7 �0.7 466 �30 6–7 42.8 7.5 0.644 122 �5 18.7 �0.7 4.0 �0.3 26 �4 26.7 �0.8 462 �28 7–8 41.6 7.1 0.747 116 �5 20.6 �0.7 3.8 �0.2 27 �5 26.7 �0.8 466 �29 8–9 41.8 7.1 1.076 110 �5 20.0 �0.7 3.8 �0.2 24 �4 25.7 �0.8 462 �30 9–10 41.3 4.7 0.923 89 �4 21.0 �0.6 4.0 �0.3 29 �8 26.0 �0.7 474 �30 10–12 40.6 6.2 1.806 80 �8 20.1 �1.6 4.6 �0.4 30 �6 26.8 �2.0 471 �35 12–14 40.2 1.941 70 �8 21.1 �1.9 4.7 �0.5 30 �10 26.8 �2.4 490 �38 14–16 39.1 1.614 69 �6 20.9 �1.3 5.4 �0.4 31 �5 28.8 �1.7 500 �36 16–18 39.0 1.895 60 �7 22.7 �2.0 5.1 �0.5 32 �6 27.6 �2.3 504 �38 18–20 39.7 1.753 56 �6 21.8 �1.6 5.4 �0.5 31 �10 30.9 �2.2 468 �35 20–22 41.5 1.842 51 �6 21.2 �1.7 4.9 �0.4 32 �6 27.1 �2.2 480 �36 22–24 41.5 1.959 39 �5 20.0 �1.9 4.3 �0.4 28 �5 18.3 �1.7 430 �33 24–26 41.4 1.750 33 �3 21.0 �1.6 5.0 �0.4 22 �4 34.6 �2.5 493 �37 26–28 40.8 1.806 38 �4 22.3 �1.8 4.9 �0.4 30 �5 22.8 �1.7 510 �38 28–30 42.2 1.758 37 �4 21.6 �1.6 4.1 �0.4 33 �6 28.1 �2.0 506 �38 30–32 43.1 8.4 1.609 28 �3 21.5 �1.4 4.3 �0.3 32 �6 28.0 �1.7 531 �38 32–34 44.4 1.967 28 �4 18.1 �1.7 4.4 �0.5 24 �5 22.0 �2.0 432 �34 34–36 43.0 1.899 28 �4 18.7 �1.6 5.6 �0.6 28 �10 29.3 �2.5 443 �34 36–38 42.6 1.803 33 �4 20.1 �1.6 6.0 �0.5 33 �6 29.2 �2.2 526 �39 38–40 41.6 1.773 26 �4 22.2 �1.7 6.6 �0.6 33 �11 33.9 �2.5 500 �37 40–42 40.3 1.844 33 �4 21.9 �1.8 7.5 �0.7 35 �6 31.5 �2.5 531 �40 42–44 38.0 8.3 1.783 36 �4 22.1 �1.7 7.4 �0.7 34 �11 35.6 �2.6 506 �38 1 Mean and 1 σ counting uncertainties. Δm is the dry mass thickness, estimated from the total dry mass of sediment in each slice and the core cross sectional area (56.72 cm 2 ). Table 2 Elemental composition determined by WDXRF in the sediment core from Algiers Bay. Analyte Depth interval in the sediment core (0–1 cm) (9–10 cm) (18–20 cm) (28–30 cm) (42–44 cm) C (%) 2.27 14.07 8.54 0.03 0.18 Na (%) 0.45 0.48 0.63 0.44 0.45 Mg (%) 3.65 3.89 5.86 4.31 4.18 Al (%) 5.26 5.62 6.38 7.30 7.13 Si (%) 64.80 70.31 73.28 82.17 82.32 P (%) 0.11 0.13 0.14 0.14 0.13 S (%) 0.19 0.22 0.30 0.36 0.32 Cl (%) 0.56 0.65 0.53 0.45 0.53 K (%) 0.33 0.36 0.48 0.53 0.47 Ca (%) 2.36 2.70 2.59 2.45 2.40 Ti (%) 0.12 0.12 0.14 0.16 0.16 V (%) 17.35 0.01 N.M. 0.01 0.00 Cr (ppm) N.M. 63.4 N.M. 41.6 55 Mn (ppm) 241.2 79.4 48.4 139.8 73.6 Fe (%) 2.495 1.435 1.119 1.642 1.691 Cu (ppm) 80.7 15.8 22.7 N.M. N.M. Zn (ppm) 57.5 28.9 7.5 24.1 21.5 Ga (ppm) 8.6 9.1 3.8 N.M. N.M. Br (ppm) 23.9 13.7 5.6 10.3 12.8 Rb (ppm) 27.7 13.9 5.9 12.4 14.3 Sr (ppm) 82.1 57.8 27.3 57.1 53.4 Y (ppm) 6.9 2.2 1.9 6.1 3.1 Pb (ppm) 19.5 12 N.M. N.M. 10.5 N.M.: Non measured. Table 3 Mineralogical composition determined by X-Ray diffractometry in the sediment core from Algiers Bay. Phases are reported in %. Phase Depth interval in the sediment core (0–1 cm) (9–10 cm) (18–20 cm) (28–30 cm) (42–44 cm) Illite 34 35.6 34.3 37 36 Quartz 23 22.8 24.2 21 24 Muscovite 19 17.8 17.2 20 17 Calcium carbonate 14 13.9 15.2 12 14 Al Mg Hydroxide Silicate 7 6.9 7.1 8 7 Potassium Iron (III) oxide 2 2 1 1 1 Anatase 1 1 1 1 1 Table 1 Sediment characterization and activity concentrations 1 of the target radionuclides in the sediment core from Algiers Bay. 5 Table 4 Radionuclide activity concentrations in sediment 1 and ERICA derived radionuclide activity concentrations and total dose rates in reference organisms. Radionuclide (Bq⋅kg 1 ) Reference organisms (Bq⋅kg 1 fresh weight) benthic fish crustaceans Macro-algae mollusc-bivalve pelagic fish phytoplankton polychaete worm zooplankton 137 Cs 3.7 0.029 0.018 0.033 0.017 0.029 0.003 0.062 0.045 210 Pb 124.9 15.475 9.848 0.469 2.954 15.475 225.095 18.758 7.972 226 Ra 20.5 0.537 0.329 0.344 0.249 0.537 4.102 0.537 0.310 228 Ra 26.3 0.690 0.424 0.443 0.320 0.690 5.633 0.690 0.399 228 Th 27.8 0.005 0.133 0.016 0.006 0.005 2.552 0.006 0.025 Total Dose Rate ( μ Gy.h 1 ) 0.089 0.087 0.067 0.051 0.072 1.061 0.111 0.046 1 Radionuclide activities in sediments correspond to the 0–10 cm depth interval, and they are reported in a dry weight basis. Fig. 2. 210 Pb exc and 137 Cs activity concentrations versus mass depth in the studied sediment core (first panel). The continuous line is the exponential fit from the CFCS model (Eq. (2)). The second panel shows the chronologies obtained from the CF-CS, CIC and CRS models (see text for details). For the sake of simplicity error-bars are only depicted for CIC ages. 6 � absolute maximum at the deepest layers, and so, they cannot be older than the first significant appearance of 137 Cs in the environment (middle 50’s). The CIC model assumes constant initial activity concentration, A0, so the age of the sediment slice centered at mass depth m can be derived from the measured A(m) value: t¼1 λLN�A0 AðmÞ�:(3) The choice of A0 is problematic, and here we selected the value of 128 Bq/kg with the criteria that the linear trend line of the CIC ages intercepts the origin at m ¼0. The corresponding CIC chronology is shown in Fig. 2 (second panel). The age of the deepest layer is 72 �11 y (date 1944 �11), which is also inconsistent with the 137 Cs profile. It is worth noting that when A0 shows random variability around a mean value, the CIC ages show random dispersion around a trend-line, which is the one with physical meaning. The CRS model assumes a constant flux 210 Pb exc onto the SWI, which is maintained for long enough as to achieve a steady-state for the total inventory of 210 Pb exc . The age of a sediment slice which ends at mass depth m is t¼1 λLN�Σ0 Σm�;(4) where Σm¼R∞ mAðm’Þdm’ is the 210 Pb exc inventory laying below mass depth m; and Σ0 is the total inventory below the SWI (m ¼0). As seen in Fig. 2 the core length, m L , is not long enough as to reach the equilibrium depth ( 210 Pb exc concentrations remains null below such a depth). Consequently the recovered inventory (RmL 0Aðm’Þdm’ ¼15.0 �0.4 kBq⋅m 2 ) is lower than Σ0, and it is necessary estimating under some assumptions the missing part of the inventory. The usual correction method by the reference SAR (Appleby, 1998) is problematic in this case since 210 Pb exc activity concentrations show a flattening at the deepest layers, limiting the use of extrapolations. But using the exponential trend holding for the whole core (Fig. 2, first panel), a summing correction of 0.64 kBq⋅m 2 can be estimated. The resulting chronology is plotted in Fig. 2, second panel. The age of the last measured slice results of 103 �8 y, also in contradiction with 137 Cs data. It is worth noting that the agreement between CF-CS and CRS chronologies is not a proof of their reliability for the studied core. Implicitly the above three classical models assume ideal deposition (in the terms described by Abril and Gharbi, 2012) and no post-depositional redistribution. Diffusion of particle bound radiotracers within the sediment requires the physical reworking of solids, moving up and down with a null mass flow; and this needs a forcing agent. Thus, invoking diffusion for 137 Cs but not for 210 Pb does not seem to be physically consistent (Abril, 2003a). Non-ideal deposition depends on the physic-chemical form of the incoming fluxes and on the sediment structure; and it can work with different intensities for these two radionuclides. It produces long penetrating tails in the 137 Cs profile (which cannot be observed in this case because the short core length), and a flattening and/or a sub-surface maximum in the 210 Pb exc profile (Abril and Gharbi, 2012). Alternatively, a flattening in the 210 Pb exc profile could reflect a homogenite – a relatively thick sediment layer deposited in an episodic event. But this last uses to correlate with discontinuities in the depth profiles of bulk density and/or other radiotracers (Abril et al., 2018). These questions will be discussed further below, within the holistic analysis of all the available sources of evidences from the present dataset. 3.4. Bulk density and normalized depth-distributions of multi-tracers Fig. 3 plots the bulk density versus true depth in the sediment core. Method A uses the total dry mass and the known volume of the sediment slice, as described in Section 2.1. The salinity of the pore fluid was not directly measured, but it is expected to be close to the mean salinity of seawater in the Algiers Bay, which is about 36.5 PSU (Noureddine et al., 2008). This, along with the measured water content (Table 1), allows correcting by the precipitated salts from the pore fluid (Dadey et al., 1992). Corrections are small, of 2.7% in average (range 2.2%–3.3%). It can be helpful comparing these estimates with the bulk density ( ρ b) arising from a two-phases model of the sediment, with uniform densities for solids ( ρ sÞand pore water ( ρ w), and additive volumes (method B): ρ b¼1fw 1fw ρ sþfw ρ w (5) In Eq. (5) fw is the fraction in weight of water. Results are plotted in Fig. 3, for ρ s ¼2.2 g cm 3 – a value lower than the commonly assumed of 2.5–2.6 g cm 3 , what highlights the contribution of organic matter and the remaining water films after drying at 70 �C. Method A is not limited by the assumptions involved in method B, but it is sensitive to slight uncertainties in the thickness of the layer when slicing. Thus, the discontinuity observed in the 7–9 cm interval does not appear with method B. The bulk density profile roughly follows the expected trend for a steady state compaction (Abril, 2003b, 2011), without clear evidences of discontinuities which could suggest homogenites or other drastic changes in the sedimentary conditions. It is worth noting that corrections by precipitated salts are not necessary for the estimation of the CRS ages, since this model works with inventories (Eq. (4)), so the corrections in the mass thickness and in the activity concentrations mutually compensate. These corrections properly apply to the CRS-SARs and to the CF-CS and CIC models. In the studied core the corrections are small (about 2.7%) and thus, the chronologies (Section 3.3) remain essentially unchanged (e.g., an age of 101 �6 y is estimated for the deepest sediment slice, with the salt-corrected CF-CS model). Fig. 4 plots the normalized (to their arithmetic means) activity concentrations of 226 Ra, 212 Pb ( 228 Th), 40 K and LOI vs. mass-depth in the studied sediment core. Normalization facilitates the comparison of the depth-distribution patterns for different analytes by using the same vertical and dimensionless scale. It is worth mention that 228 Ra has not been included since it is close to the secular equilibrium with 212 Pb, and thus, it does not add new information. 226 Ra and 40 K show quite uniform distributions with mass-depth. The same is true for 212 Pb but few excursions out of the main trend line (e.g., an anomalous high value at the top sediment slice). The LOI content of the uppermost sediment slice is Fig. 3. Bulk density versus true depth in the sediment core. Method A uses the total dry mass and the known volume of the sediment slice; method B uses a two-phases sediment model with an uniform density for solids of 2.2 g cm 3 . Both estimates are corrected by the precipitated salts from the pore fluid (estimated from the water content and the salinity of seawater in the bay, 36.5 PSU). The continuous line plots the steady-state trend. 7). This is in contradiction with the 137 Cs profile, which shows an 7 also much higher than in the rest of the core. Overall, Fig. 4 does not reveal any noticeable disruptions in the sedimentary sequence. On the light of present results, a homogenite can be discarded as the likely explanation of the plateau observed in 210 Pb exc in the deepest portion of the core, since it is not compatible with the structure of varying activity concentrations observed for 137 Cs in the same region (Fig. 2). Similarly, homogenite and physical mixing seem to be unlikely for explaining the flattening in the 210 Pb exc activity concentrations in the top sediment, because the fingerprint of active early compaction is preserved (Fig. 3), and because the singular high value of LOI in the first sediment slice. 3.5. 210 Pb exc and 137 Cs inventories and fluxes onto the SWI The incompletely recovered 210 Pb exc inventory was 15.0 �0.4 kBq⋅m 2 , what implies an equivalent constant 210 Pb exc flux onto the SWI (Feq ¼λΣ0) greater than 465 �13 Bq⋅m 2 y 1 . This is much higher than the expected atmospheric deposition in the area, knowing that the worldwide atmospheric deposition is in the range 40–250 Bq⋅m 2 y 1 (Appleby, 1998). This discards pure water-column scavenging process, and suggests an important contribution of the 210 Pb exc already bound to the mass flows reaching the SWI. In these cases, 210 Pb exc fluxes are expected to be correlated with SAR (Abril and Brunskill, 2014). Satellite images of the Algiers Bay often allow visualizing clouds of suspended particulate matter supplied by the oueds and by runoff and erosion along the shoreline (e.g., see Fig. 1). The transport can progress through deeper layers as the combined effect of horizontal advection and vertical settling. It is not unlikely that under some particular conditions these mass flows could reach the sampling site. The episodic floods and associated mudflows in the area, along with the anthropogenic impacts described in the Introduction section, may result in SAR and 210 Pb exc fluxes largely varying over time. Nevertheless, the application of the CRS model would be still possible in the case that such variability in 210 Pb exc fluxes was randomly distributed over time (Abril, 2019). This will be discussed further in Section 3.6. The 137 Cs inventory measured in the 0–45 cm interval was 1920 �40 Bq⋅m 2 , but there is an unknown missing part of the total inventory due to the non-null 137 Cs concentrations below such 45 cm depth. It is known that 137 Cs fallout largely varied with geographical latitude and local rainfall, and that in deep sea a large fraction of the integrated deposition remains for long in the water column (Noureddine et al., 2008). The integrated atmospheric deposition in European areas not affected by the Chernobyl accident were about 1400 Bq⋅m 2 (corrected by radioactive decay to the date of sampling in 2015; data for UK, after Wright, 2016). Consequently, it is also expected that the 137 Cs fluxes onto the SWI were contributed by both water column scavenging, and by the 137 Cs bound to the mass flows reaching the SWI. Although there exist good records for historic 137 Cs atmospheric deposition, several pre-depositional and/or within-system processes may lead to different temporal patterns for the 137 Cs fluxes onto the SWI (Robbins et al., 2000). Here we explore some modelling approaches in an attempt to understand the main features of the measured 137 Cs vs mass depth profile. Let us consider a simple model consisting in a well-mixed water column of cross-sectional area S [L 2 ] and depth h [L], receiving the atmospheric deposition, F at [BqˑL 2 T 1 ], and transferring a flux F S [BqˑL 2 T 1 ] to the underlying sediment (physical dimensions in brackets). F S can be written in terms of a mean settling-contributed SAR, wS[MˑL 2 T 1 ], the k d value for the studied radionuclide, and the concentration in the water column, Cw[BqˑL 3 ]: FS¼wSSkdCw. Thus, the following differential equation holds for CwðtÞ: dCw dt ¼Fat hwSkd hCwλCw(6) where λ [T 1 ] is the radioactive decay constant for 137 Cs. The activity concentration of 137 Cs in seawater at the site and time of sampling was measured, being of 1.00 �0.04 Bq⋅m 3 (data from authors’ personal records). Using the activity concentration in the first sediment slice (Table 1), a local k d value of 3.8⋅10 3 L/kg arises. Fig. 5 plots the solution of Eq. (6) using for Fat the historical records for UK (Wright, 2016), corrected by a normalization factor, Z, globally accounting the different geographical latitude and local rainfall, h ¼73 m, and wS ¼0.06 g cm 2 y 1 (Model 1 in Fig. 5, with Z~1). The value of wS has been selected to better fit the available data (Fig. 5). Similar results can be obtained by a modified water-column model considering horizontal inputs (by exchanges with the surrounding water masses), as in Laissaoui et al. (2008), at a rate of 0.05 y 1 , and increasing the value of wS to 0.15 g cm 2 y 1 (Model 2 in Fig. 5). The above numerical exercises only have the value of a concept- Fig. 4. Normalized (to their arithmetic means) activity concentrations of 226 Ra, 212 Pb ( 228 Th) and 40 K, and normalized LOI content versus mass depth in the sediment core from Algiers Bay. 8 demonstration model, where SAR values account for the settling fluxes. These mass flows are much lower than the ones estimated from the classical models (Section 3.3), so the contribution of other mass flows (from continental runoff and shallower depositional areas) must be considered. Nevertheless, the integration of 137 Cs atmospheric fluxes in the catchment area has been widely described (McCall et al., 1984; Abril and García-Le� on, 1994; Robbins et al., 2000). The simplest formulation can be found in Robbins et al. (2000), where it is designated as the system time averaging (STA) model. The fluxes onto the SWI, F S , are expressed as a function of the atmospheric deposition, F at , involving the radioactive decay constant of the radionuclide, λ, and a system’s constant, k r , with physical dimension of T 1 : dFS dt ¼krFat  ðλþkrÞFS:(7) Fig. 5 plots the solution of Eq. (7) with Fat from Wright (2016) and kr ¼0.026 y 1 (this value has been selected to get a temporal pattern similar to that of Model 1 in Fig. 5). As conclusion, it is expected an integration effect of the atmospheric 137 Cs fluxes in the water column and in the erosional areas contributing to the mass flows reaching the SWI at the core site. This integration effect would result in a 137 Cs profile in the sediment being a mirrorimage of the curves depicted in Fig. 5, but to some extent distorted by varying sedimentary conditions. This is, the core is not long enough as capturing the peak, but only the declining trend line. Consequently the age of the deepest measured sediment-slice must be younger than the age of the 137 Cs peak in Fig. 5, which, due to the pre-depositional effects (Eqs. (6) and (7)), is slightly displaced to ~1965. Direct contribution of 137 Cs fallout from Chernobyl is expected to be negligible in this area, but it affected the northern and eastern Mediterranean basins (Delfanti et al., 2003). Thus, the Levantine Intermediate Water (LIW) circulating through the Algerian Basin could carry some signal from Chernobyl, but their effects in the Algiers Bay are unlikely, since the Modified Atlantic Water (MAW) occupies the top 200 m layer of the Alboran Sea (Noureddine et al., 2008). The above analyses suggest that 210 Pb exc and 137 Cs inputs on the SWI have been largely contributed by their activities bound to the mass flows from continental runoff and shallower depositional areas. Non-ideal deposition arises mainly from diffusion and the kinetic uptake of dissolved radionuclides through the connected water pores and, to less extend, by depth distribution of colloidal and small grain size fractions (Abril and Gharbi, 2012), but it is quite unlikely for those radionuclides already bound to solid particles reaching the SWI. Consequently, the flattening observed in the 210 Pb exc profile in the top region of the core must obey to varying SAR and initial activity concentrations. 3.6. Analysis of clusters and piecewise models Based upon a wide and systematic survey on varved sediments from marine, riverine and lacustrine environments, Abril and Brunskill, (2014) found that 210 Pb exc fluxes onto the SWI were highly variable in time and statistically correlated with SAR. As above discussed, this seems to be also the case for the studied core. This supports the application of the methodology of analysis of clusters developed by Abril (2019). Briefly, when persistent changes in environmental conditions shift the initial concentrations and/or the SARs towards different mean values, the plot Ln[A(m)] can show jump discontinuities and/or changes in the slope –A(m) being the activity concentration of 210 Pb exc . Fig. 6, first panel, shows the Ln[A(m)] plot for the studied core, with two jump discontinuities and three transects. A linear fit has been applied to each transect, what allows estimating the mean sedimentary conditions (i.e., the mean values for SAR, initial activity concentrations and fluxes) – see Abril (2019). Results are summarized in Table 5. For transects 1 and 2 the linear fit is statistically significant at a confidence level higher than 95%, but it is poor for transect 3. Although with large propagated uncertainties, the mean values of initial activity concentrations have increased with time (from transect 3, in the deeper and older portion of the core, to transect 1, in its upper and younger portion). Mean SAR was high and with also high variability in the third transect (0.94 �0.69 gˑcm 2 y 1 ), smaller and quite uniform in the second (0.385 �0.027 gˑcm 2 y 1 ), and rising up to 0.74 �0.21 gˑcm 2 y 1 in the first transect. As result, the mean values of fluxes have changed over time. Consequently, under these sedimentary conditions, the CRS model hardly can provide reliable results, even under its piecewise version – in the case that a reference date was known (Abril, 2019). The piecewise versions of the CF-CS and CIC models have been applied to the studied core using, respectively, the SAR values and the initial activity concentrations reported in Table 5. The resulting Fig. 5. 137 Cs concentration in seawater (in Bqˑm 3 ) around the Algiers Bay (data for 1999 and 2001 are from Noureddine et al., 2008; while data for 2015 is from authors’ personal records). Model 1 is the solution of Eq. (6) with local k d value of 3.8⋅10 3 L/kg and atmospheric deposition from Wright (2016), with h ¼73 m and wS ¼0.06 g cm 2 y 1 . Model 2 includes horizontal exchanges at a rate of 0.05 y 1 , with wS ¼0.15 g cm 2 y 1 . STA fluxes (in Bqˑm 2 y 1 ) are the solution of Eq. (7) with the same atmospheric deposition and kr ¼0.026 y 1 . The vertical dashed line delimits the maximum time-interval recorded in the sediment core (Fig. 2). 9 chronologies, along with the propagated uncertainties, are depicted in Fig. 6, panel 2. This must be understood as providing a reasonable proxy to the true chronology, being particularly limited for the third transect. Again, for the CIC model the physically meaningful chronology is the trend line defined by the cloud of model ages. The piecewise CF-CS age for the central point of the last sediment slice is 59 �15 y, which is compatible with the age of <51 y expected from the 137 Cs profile and the analysis of Fig. 5. The third transect is poorly described by both models. The CIC chronology shows two ages being too old and out of the trend-line. This suggests that true initial activity concentrations in these two layers should have been lower by a factor 2 than the mean value assumed for this transect (Table 5). They are likely linked to large depositional events. Thus, sedimentary conditions in this transect seem to have been characterized by a sequence of depositional stages involving varying SARs and varying initial activity concentrations, spanning for a decade since middle 60’s (the limit of the chronology). This corresponds with the early stage of urban development of the city after the independence of Algeria, and it may comprise the large flood of 1974. The second transect comprises from middle 70’s till ~2006. Sedimentary conditions seem to have been relatively regular in this period, without distinguishable fingerprints neither from the earthquake of 1980 nor for the tsunami of 2003. The same is true for the 2001 flood in the Bab El Oued district of Algiers. Peri� a~ nez and Abril (2013) reported results from numerical simulations of the 2003 tsunami in Algiers. The maximum amplitude of the tsunami wave was close to 0.5 m along the bay shoreline, and below 0.2 m in the sampling site. The maximum water currents were of 0.25 m/s along the shoreline and below 0.1 m/s at the sampling site. Their associated shear stresses were of 0.16 Pa and 0.025 Pa, respectively; the last figure being below the threshold value Fig. 6. Logarithmic plot of 210 Pb exc activity concentrations in the sediment core from Algiers Bay, with the identification of three clusters defined by jump discontinuities and changes in the slope (first panel). The fitting parameters are reported in Table 5. The chronologies from piecewise CF-CS and CIC models are shown in the second panel.