Investigation of sedimentation rates and sediment dynamics in Danube Delta lake system (Romania) by 210Pb dating method

Journal of Environmental Radioactivity 192 (2018) 95–104

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Investigation of sedimentation rates and sediment dynamics in Danube Delta lake system (Romania) by 210Pb dating method

T

R.-Cs Begya,b, H. Simona,∗, Sz Kelemena, L. Preoteasac a

Babeș-Bolyai University, Faculty of Environmental Science and Engineering, Fântânele 30, 400294, Cluj-Napoca, Romania Babeș-Bolyai University, Interdisciplinary Research Institute on Bio-Nano-Science, Treboniu Laurean 42, 400271, Cluj-Napoca, Romania c University of Bucharest, Faculty of Geography, M. Kogălniceanu Blvd 36-46, Sector 5, 050107, Bucharest, Romania b

A R T I C LE I N FO

A B S T R A C T

Keywords: Danube delta 210 Pb dating method Sedimentation rates

Being a dynamic environment associated with complex costal, fluvial and marine processes, only a few studies regarding the evolution of the Danube Delta and the human impacts on its ecosystem have been carried out. Being a sensible to all processes occurring in its catchment area, information is stored in the deposited sediments, which can be used as tracers for natural and anthropogenic processes. The aim of this study is to determine a detailed reconstruction of the sedimentation rates in the last century by applying the 210Pb dating method validated by 137Cs profiles. Additionally, the impacts of the construction of river-regulating structures (mainly the Iron Gates Hydro-Energetic Power Plants) are investigated, along with the assessment of natural phenomena (floods, storms etc.). To achieve this, 26 sediment cores from seven lakes were collected. 210Pbsup and 137Cs were determined using gamma spectrometry, while 210Pbtot was measured via alpha spectrometry (210Po), using the CRS model for age determination. From the assessed lakes, the most affected was the Matița Lake with a maximum sedimentation rate of 10.93 g cm−2 yr−1 and the least affected was the Isac Lake. Average sedimentation rates are: 0.95 g cm−2 yr−1 for Cruhlig Lake, 0.70 g cm−2 yr−1 for Uzlina Lake, 0.44 g cm−2 yr−1 for Isac Lake, 0.47 g cm−2 yr−1 for Cuibida Lake, 0.51 g cm−2 yr−1 for Iacob Lake, 1.00 g cm−2 yr−1 for Matița Lake and 0.76 g cm−2 yr−1 for Merhei Lake. Physical parameters (water content, porosity and bulk density) and LOI (organic matter and inorganic carbon content) were determined for each core to differentiate organic and non-organic sedimentation. Beside the natural influences, it is difficult to track the effects of the Iron Gates and not all analysed lakes were suitable for this task. The 1940–1970 period and the following ten years were compared in means of sedimentation: a decrease in sedimentation can be observed in four of the lakes: 59% in Cruhlig Lake, 16% in Uzlina Lake, 10% in Iacob Lake and 42% in Isac Lake, leading to an average 32% for the four lakes. The other three lakes show increasing tendencies of 39% in this period: 87% in Matița Lake, 6% in Merhei Lake and 24% in Cuibida Lake. Sedimentation rates show growths of 3 times after 1989, the most affected being the two northern lakes (3 times increase in both Matița Lake and Merhei Lake) and the four central lakes (2 times in case of Cuibida Lake, 3 times in Iacob Lake, 3 times in Isac Lake and 4 times in Uzlina Lake) with an average increase of 3 times, while the southern one (Cruhlig Lake) 2 times.

1. Introduction

changes within the Danube Delta and along the deltaic coast are complex and little understood. The rates of the sedimentation processes are continuously changing on different time-scales (e.g. macro-scale changes related to climate and sea level; meso-scale changes of the solid discharge, hydrodynamic characteristic or physical conditions of the river bed) due to both natural and anthropogenic factors. Major human interventions (e.g. dams, hydro-energetic power plants, meanders cut offs, artificial channel cuttings, protection walls) have been performed within the Danube Basin in the last century (Coman, 2002). Insights into the spatial and temporal sedimentation rate dynamics during the

Lake ecosystems react sensibly to the processes occurring in their catchment areas. Therefore the information stored in the sediments is often useful tools for exploring the natural and anthropogenic changes in their environment. Such an ecosystem is represented by the Danube Delta. Having a catchment area of 4152 km2, which includes regions of Central and Eastern Europe, it is the second largest delta of the continent, being part of the UNESCO world heritage (Sommerwerk et al., 2009). The sedimentation processes and the associated morphological

∗

Corresponding author. E-mail address: [email protected] (H. Simon).

https://doi.org/10.1016/j.jenvrad.2018.06.010 Received 15 September 2017; Received in revised form 9 June 2018; Accepted 9 June 2018 0265-931X/ © 2018 Elsevier Ltd. All rights reserved.

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While between 1971 and 1980 the Danube River's solid discharge was of 1308 kg s−1, between 1981 and 1990 this value decreased to 926 kg s−1. The maximal value was recorded in 1941 (192 × 106 tons yr−1) and the minimal in 1921 (19.8 × 106 tons yr−1). The average transported sediment quantity per year shows a decreasing tendency (Coman, 2002). The present study aims to present the changes in the sediment dynamics of the Danube Delta by analysing 7 lakes and 26 sediment cores and pointing out the period before and after the construction of the Iron Gates, indicating its effects. Some of the results are already presented in published articles, which partially (the dynamic of Iacob and Merhei lakes (Begy et al., 2014, 2015; Simon et al., 2016), or integrally include the description and justification of the changes in sedimentation rates (Begy et al., 2016).

last two centuries represent a major challenge for the better understanding the development phenomenology of the Danube Delta. Comprehending the feed-back between the sedimentation processes and the morphological changes is a crucial step in predicting how a sedimentary system will evolve in the near future and in assessing its vulnerability to the extreme events (e.g. sea level rise, storms, floods, droughts) related to the widely acknowledged climate changes. A series of published papers on shelf mud focus on the large dispersal systems associated with subaqueous-deltas, e.g. Amazon, Ganges–Brahmaputra, Changjiang–Huanghe, Fly River and describe continental shelves in west of the Mississippi Delta (Kuehl et al., 1982, 1986; DeMaster et al., 1985; Alexander et al., 1991; Harris et al., 1993; Lesueur et al., 2001; Tichenor et al., 2016). Starting with the end of the 1970's researches were made on the sedimentation of the rivers forming delta (Nittrouer et al., 1979; Michels et al., 1998), some of these being still in progress (Jweda and Baskaran, 2011; Humphries et al., 2010). The ongoing studies have extended from the recently deposited sediment to the maritime shelves in the screening of the anthropic factors (Xu et al., 2008; Canuel et al., 2009). The human impact on the sedimentation processes from the Danube Delta has been previously approached by means of some geographic and geomorphologic methods such as the comparison of historical bathymetric maps (Constantinescu et al., 2010) or the behaviour of hydrodynamic modelling of extreme events (e.g. floods). The application of the radiometric method on Danube Delta sediments can be found in the literature investigating the pollutant agents (heavy metals) in recent sediment layers (Woitke et al., 2003; Vukovic et al., 2013). The accumulation rate of recent sediments using radiocesium profiles was not yet done using the 210Pb dating methods (Dinescu and Duliu, 2001; Florea et al., 2011). For aquatic sediments, the use of 210Pb (T1/2 = 22.2 yr) (Duenas et al., 2003), originating from the decay of atmospheric 222Rn, is a wellestablished method to estimate sediment ages and sedimentation rates on a time scale of up to 100 years. The 210Pb method was first developed by Goldberg (1963), then applied on lake sediments by Krishnaswamy et al. (1971) and subsequently introduced to marine sediments by Koide et al. (1972). More recent studies applied the 210Pb radiometric method in studies of changes due to human-induced geomorphic processes and climate changes in riverine-lacustrine systems, estuaries or bays in different parts of the world (Appleby and Oldfield, 1978; Sert et al., 2012; Jweda and Baskaran, 2011; Sabaris and Bonotto, 2011; Bruschi et al., 2012; Mabit et al., 2014; Delbono et al., 2016). The detailed reconstruction of the sedimentation rates by means of high resolution radiometric methods during the last two centuries will help isolate and quantify the impact of the human interventions. Of all the anthropogenic interventions, the Iron Gate Hydro-Energetic Power Plants have the highest effect on the sediment quantity which arrives in Danube Delta. The dams were constructed in 1972 (Iron Gate I located at km 942 of the Danube) and in 1986 (Iron Gate II localized 68 km downstream) and represent the largest hydropower dam and reservoir system along the entire Danube. The reservoir of Iron Gate I of 3.2 billion m3 volume and 270 km total length (up to Novi Sad, Serbia) (ICPDR, 2015), trapping some 20 million tons of sediment per year (Laszlo, 2007). The total drainage area upstream of the Iron Gate I is 577,000 km2, representing 250–300 km upstream the Danube River. The annual water flow of the Danube River is 110–220 109 m3, while daily discharges range between 1500 and 15000 m3 s−1. The suspended sediment concentrations in the Danube River are in the 10−3 to 10−1 kg m−3 range, while the sediment volumes entering the reservoir are considerably larger: 7–30 million tons per year (Laszlo, 2007). The dams interrupted the natural sediment transport in the Upper Danube, retaining approximately two-thirds of the suspended solids. Therefore, sediment delivery to the Delta decreased from 53 to 18 million tone per year (Duțu et al., 2014), resulting in severe coastal erosion (Sommerwerk et al., 2009), and therefore assuming that there should be an observable variation in the sedimentation rates of the lakes after their implementation, downstream of the two dams.

2. Materials and methods 2.1. Study site More than 300 lakes are situated between the three branches of the Danube Delta. Halfway through the delta, the branches of the Danube River cut through a marine levee, where they form the natural boundaries of four hydrological sub-units or lake complexes, namely from west to east: Sontea-Furtuna, Gorgova-Uzlina, Matița-Merhei and Roșu-Puiu (Oosterberg et al., 2000). Through this study seven lakes with an average of four sediment columns were investigated. A gravity corer was used for sampling with a sampling tube of 60 cm and, respectively, 120 cm. The location of the lakes and the sampling points can be seen in Fig. 1, and a detailed description of these and sediment cores are presented in Table 1. Situated north to the Sfântu Gheorghe Branch in the marine delta, the genesis of the Cruhlig Lake is in correlation with the gradual closing of a lagoon. The lake can be accessed through a channel south to the Sfântu Gheorghe Branch. Placed in Grogova-Uzlina hydrological sub-unit close to the Mahmudia Meander, the Uzlina Lake is connected to the Sfântu Gheorghe Branch and joined by two lateral channels to the Isac Lake. The Iacob and Isac lakes are both situated in the first part of the marine Danube Delta, south to the Sulina Branch, but without having a direct connections to it. In the present, the basin of the Iacob Lake is connected to the rest of the deltaic system with a secondary channel in the south of the lake, being indirectly connected to the Sulina Branch. The basin of the Isac Lake is a part of the Uzlina and Isac lake system and has three entry channels: one in the north-eastern, one in north-western and the last one in the southern part of lake. This has indirect connection to Sfântu Gheorghe Branch through the Uzlina Lake, while the north-eastern and north-western channels are connected to each other and start at the Caraorman Channel. The Cuibida Lake is situated between the Isac Lake and Caraorman Channel. This lake is connected to the Sulina Branch with two entry channels in the southern and the western part of the lake. The sediment cores taken from this lake are situated in the proximity of the entry channels, which connect the lake to the Sulina Branch. The Matița and Merhei lakes are situated in the fluvial part of the Danube Delta, between the Chilia and Sulina branches. Although their positions are further away from the main branches, the sources of suspended sediments are provided by numerous secondary channels, diminishing the sediment quantity reaching the two lakes. 2.2. Physical parameters Every sediment core was sub-sectioned into 1–2 cm layers. Wet and dry weight were measured (sediments were dried at 75 °C in a drying oven for 24 h) and physical parameters such as porosity, water content and wet and dry bulk density were calculated. LOI (Loss On Ignition) measurements were carried out on each sediment core. Sub-samples were dried in a drying oven at 105 °C to 96

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Fig. 1. Location of the lakes and sampling points.

content (210Pbsup) of each sediment core. Samples were put in sealed 7 cm diameter, 1 cm height aluminium boxes and stored for at least 28 days prior to measurement. The activity concentration of 226Ra was measured after a month of storage using the gamma lines of the shortlived radionuclide daughters of 222Rn (214 Pb at 295 keV and 351 keV and 214Bi at 609 keV). Samples were analysed with a high resolution gamma spectrometer equipped with a HPGe type ORTEC GMX detector, having a FWHM of 1.92 keV at 1.33 MeV and a Be window of 0.5 mm permitting the detection of low gamma energies. The activity concentration was calculated using the relative method with IAEA 385 standard. The 210Po content of each sediment sample was determined using an aliquot of 0.5 g dry sample. 0.3 ml of 100 mBq ml−1 209Po tracer (having an alpha energy of 4.9 MeV) for determination of chemical

eliminate the water content from the crystalline structure of the dry samples, incinerations at 350 °C and 750 °C were made to eliminate the organic material (OM) and, respectively, inorganic carbon compounds (IOC). After each step samples were weighted and precentral data was calculated for each of the parameters. The total carbon content is given by the sum of the organic matter and inorganic carbon content. 2.3.

210

Pb dating

The 210Pbtot content from the sediment was measured by its daughter radionuclide 210Po, the two elements reaching the equilibrium after 2 years. In case of higher sedimentation rates than 0.5 cm y−1, the first layer from the sediment column is measured twice. Gamma spectrometric measurements were made in order to determine the 226Ra Table 1 Sampled cores and their characteristics. Name

Code

Location N

Cruhlig Lake

Uzlina Lake

Isac Lake

Cuibida Lake Iacob Lake

Matița Lake

Merhei Lake

CR1 CR2 CRII1 CRII2 CRII3 UZII7 UZ8 UZ10 ISII9 ISII10 IS11 IS12 CU7 CUII11 IA3 IA4 IAII15 IAII16 MA18 MA20 MAII17 ME15 ME16 MEII19 MEII20 MEII21

44° 44° 44° 44° 44° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45°

53′ 0.75″ 52′ 59.05″ 53′ 3.3″ 52′ 54.5″ 52′ 48.8″ 04′ 43.2″ 5′ 45.7″ 5′ 13.6″ 06′ 55.9″ 07′ 01.5″ 5′ 51.7″ 5′ 57.4″ 8′ 8.28″ 17′ 47.7″ 7′ 49.3″ 7′ 49.7″ 8′ 9.5″ 9′ 8.7″ 17′ 54.93″ 17′ 22.61″ 18′ 26.3″ 19′ 8.71″ 19′ 38.64″ 19′ 3.6″ 19′ 37.2″ 19′ 27.7″

Depth

Reference

E

Length (cm)

Water (m)

29° 32′ 1.6″ 29° 31′ 59.75″ 29° 32′ 5.1″ 29° 32′ 0.2″ 29° 31′ 53.2″ 29° 15′ 46″ 23° 15′ 39.6″ 23° 15′ 28.7″ 45° 06′ 55.9″ 29° 17′ 36.7″ 23° 16′ 35.3″ 23° 16′ 28.7″ 29°20′27.6″ 29° 20′ 33.7″ 29° 23′ 38.8″ 29° 23′ 43.2″ 29° 24′ 3.1″ 29° 24′ 2.7″ 29° 23′ 0.94″ 29° 21′ 28.73″ 29° 22′ 22.4″ 29° 24′ 26.44″ 29° 25′ 0.93″ 29° 25′ 35.4″ 29° 27′ 02.2″ 29° 27′ 33.8″

73 74 38 36 44 60 62 78 52 45 40 55 80 80 60 69 70 35 70 60 40 58 66 75 81 75

4 4 3.8 3.8 3.5 3.5 4 4.2 3.1 3.2 3 3 2.7 2.5 1.9 2 2.2 2 1.8 1.8 1.7 1.9 1.8 1.7 1.7 1.7

97

Begy et al., 2016

Described in this work

Begy et al., 2018

Described in this work Begy et al., 2014 Begy et al., 2018 Simon et al., 2016

Begy et al., 2015 Described in this work

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yield was added to each sample. The dry sediment was digested by a series of acids (3 × 10 ml 65% HNO3, 3 × 10 ml 35% HCl, 10 ml 6N HCl and 10 × 3 ml 8:1 35% H2O2: 35% HCl), after which the 210Po was deposited on high nickel content stainless steel discs (3 h at 85 °C in a drying oven), interferrents (iron ions) being eliminated by ascorbic acid. The obtained alpha sources were measured by an ORTEC SOLOIST 900 mm2 PIPS detector, having a resolution of 19 keV and an ASPEC-92 data acquisition system. The age depth model and the sedimentation rate obtained from each sediment column were calculated using the CRS model (Appleby, 2001). 3. Results and discussion 3.1. Cruhlig Lake Being connected through only one channel to the south of Sfântu Gheorghe Branch of the Danube, this lake is prone to recent impacts because of its genesis and secluded position. The recent flooding events influence the sediment formation in such manner, that the effects of the Iron Gates remained visible until the present. A detailed study of this lake is presented in Begy et al. (2016). LOI measurements show that the cores have a total carbon (TC) concentrations above 26% for CRII1 and CRII3 and, respectively, 35% for CR1, CR2 and CRII2 in the upper 12 cm. Some cores show a high carbon content throughout the entire profile (CR2 and CRII2), however, this is negligible in terms of sediment quantity, since very little deposed material is organic (less than 2%). Average sedimentation rates are 0.63 g cm−2 y−1 for CR2, 0.92 g cm−2 y−1 for CR1 and CRII3 and 1.13 g cm−2 y−1 for CRII1 and CRII2. Some local maximums are visible in the cores and are accounted to major flooding periods: the CR1 and CR2 situated near the western bank were susceptible to the 1940–1947 floods, while the 1960–1970 floods are visible in the sedimentation rates of CRII1, CRII2 and CRII3 cores. However, flooding events from the last 10 years are visible in all cores. Because of the construction of the Iron Gates, sedimentation rates decreased with an average of 59%. The most impacted sampling point was CRII1 situated nearest to the inflow channel, where sedimentation rates decreased with 75%, followed by CRII3, CRII2 and CR2 (55%, 51% and 52%), all three being situated in near-shore areas in the southern part of the lake. The least affected was CR1 located near the western shore, with only 25% decreasing. An increase in the sedimentation rates of 2.25 times after 1989 (from an average of 0.14 g cm−2 yr−1 to 0.31 g cm−2 yr−1) is visible compared to the period before. The most sediment was deposited in the north of the lake in the past 14 years: the sedimentation rate of CR1 grew from 0.07 g cm−2 yr−1 to 0.26 g cm−2 yr−1 (2.48 times) and that of CRII1 from 0.12 g cm−2 yr−1 to 0.50 g cm−2 yr−1 (3.32 times). The least affected is the western bank, represented by CR2, where sedimentation decreased with 59%: from 0.24 g cm−2 yr−1 to 0.10 g cm−2 yr−1.

Fig. 2. The carbon content of the Cuibida Lake cores.

Fig. 3. The

210

Pb and

226

Ra activity concentration of the Cuibida Lake cores.

3.2. Cuibida Lake Two sediment cores were taken from the Cuibida Lake. An 28% organic matter (OM) peak is visible, leading to the conclusion that 210 Pb may be diluted (Fig. 2). Based on the above it can be stated that the lacustrine vegetation in case of the Cuibida Lake leads to serious changes in the deposition of the sediments. The exponential profile of 210Pb according to depth is visible in both cores, having some minimums (Fig. 3). The distribution of ages according to depth shows that the CU7 sampling point is subjected to a more serious sedimentation than CUII11 (Fig. 4). A total of 27 cm sediment was deposited in the past 100 years in the CUII11 point, which can be found in the middle of the lake.

Fig. 4. The age depth model and cores.

137

Cs activity concentration for Cuibida Lake

Compared to the ages according to depth of the CU7 core it is visible that the same amount of sediment was deposited at the entrance of the north-eastern channel (CUII11) over a period of 10 years: the last 100 98

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Fig. 5. Sedimentation rates of the Cuibida Lake cores.

years are accountable for a sediment deposition of 60 cm. A plateau was formed in case of CU7 in the age-depth distribution after 2000 and, until 2013, 40 cm sediment was deposited, meaning a 3 cm y−1 linear sedimentation. The high sedimentation of this sampling point can be explained with its localization and the frequency of the floods on the Sulina Branch (Rossi et al., ). The 137Cs activity concentration of the CU7 core can be seen in Fig. 4. The two peaks of the 137Cs concentration are visible at 1986 and 1963, validating the 210Pb dating method. It is important to mention, that caesium measurements were carried out for all sediment cores and the values for the 210Pb and 137Cs were in good agreement in all cases. Average mass sedimentation rates are 0.51 g cm−2 yr−1 for CU7 and 0.43 g cm−2 yr−1 for CUII11 (Fig. 5). Both sampling points show an increasing in the mass sedimentation. The minimums are visible in the 1930–1940 period, values being 0.15 ± 0.01 g cm−2 yr−1 in case of CUII11 and 0.10 ± 0.01 g cm−2 yr−1 in case of CU7. The maximums are visible in both cases after 2000: in the period of 2005–2007 for CU7 with 1.78 ± 0.18 g cm−2 yr−1 and for CUII11 with 1.05 ± 0.09 g cm−2 yr−1, corresponding to the 2006 flood, when the debit of the Danube reached 15,800 m3 s−1 (Gan et al., 2012). The other maximum is visible in the 2010–2011 period, when the sedimentation was 1.04 ± 0.09 g cm−2 yr−1 and the maximal debit of the Danube reached 16,480 m3 s−1 according to ANAR (2013). Drastic increasing can be observed after 1989, where the 14 year average increased on average from 0.38 ± 0.06 g cm−2 yr−1 to 0.68 ± 0.10 g cm−2 yr−1. This increasing is caused by the uncontrolled lumbering and floodings. Due to the sediment dynamics it is visible that the sedimentation rate increased after the construction of the Iron Gates: comparing the 1940–1972 data with an average sedimentation of 0.19 ± 0.02 g cm−2 yr−1 to those of the 1972–1980 period (0.24 ± 0.03 g cm−2 yr−1), a 24% increasing is visible. The peaks of 2006 and 2011 are visible in both organic and inorganic sedimentation and show similar tendencies, proving that the carbon content of the sediment arrives simultaneously with the sediment and is not present due to the decay of the vegetation. It is visible that in the 1988–2004 period, the sediment containing carbon is second to the non-carbon one, the first having a sedimentation of 0.05 g cm−2, while the second is in the 0.4–0.8 g cm−2 range, proving that a great amount of sediment arrived from the catchment area in this period.

Fig. 6. The carbon content of the Uzlina Lake cores.

Fig. 7. The

210

Pb and

226

Ra activity concentration of the Uzlina Lake cores.

(between 19% and 38%), being situated in an area characterized by high vegetation. The other cores present lower carbon content because of the more active sediment exchange and less deposition (Fig. 6). The 210Pb distributions can be seen in Fig. 7. 210Pbsup shows values in the 20–30 Bq kg−1 range, while 210Pbtot has maximum values 68–138 Bq kg−1 range. The highest values are registered in case of the UZII7 sampling point due to the more secluded position of this are in the southern part of the lake. Local minimums in the 210Pbtot concentration are visible at 8 cm and 16 cm. UZ8 and UZ10 are both situated in areas near inflow channels, therefore the activity concentration of the 210Pbtot show more variation. All three sampling points show increasing in the sedimentation rate (Fig. 8). The minimums are visible in the 1940–1960 period, values being between 0.11 and 0.21 g cm−2 yr−1. In all three cases the maximums are visible after 1994. The most affected areas are those near the inflow-channels: most flooding events are recorded by UZ8 and UZ10 in the periods of 1995–1996 (UZ8: 1.60 ± 0.20 g cm−2 yr−1, UZ10: 1.47 ± 0.18 g cm−2 yr−1), 1999–2000 (UZ8: 1.51 ± 0.18 g cm−2 yr−1, UZ10: 2.10 ± 0.29 g cm−2 yr−1), 2005–2006 (UZ8: 2.41 ± 0.26 g cm−2 yr−1, UZ10: 1.79 ± 0.21 g cm−2 yr−1) and 2008–2010 (UZ8: 1.61 ± 0.18 g cm−2 yr−1, UZ10: −2 −1 1.81 ± 0.21 g cm yr ). Being situated at nearly 1 km distance from

3.3. Uzlina Lake From the three sediment cores analysed from the Uzlina Lake, the UZII7 sediment core presents the highest organic material content 99

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1972–1989, the sedimentation increases to 0.27 ± 0.04 g cm−2 yr−1. Until 2013, an increasing in the sedimentation pattern is visible (to 0.72 ± 0.06 g cm−2 yr−1). Before 1989, the sedimentation rate is relatively constant 0.11 ± 0.01 g cm−2 yr−1, while after 1989 it increases to 0.51 ± 0.06 g cm−2 yr−1. 14 years later, the sedimentation rate increases 4.3 times, the maximum of 3.95 ± 0.06 g cm−2 yr−1 being found in 2002. 3.5. Iacob Lake Four sediment cores were collected from this lake: one (IAII16) from the northern part, one from the centre (IAII15) and two from the southern (IA3, IA4) part of the lake. Analysing the sedimentation rates of the Iacob Lake, four flooding events are visible, namely 1980, 1991–1993, 2004 and 2011. The deposited sediment quantity and its velocity show that the 1991–1993 flooding was the most significant. The most affected area is the southern part of lake, close to the inflow channel. Maximal sedimentation values exceed 2.20 ± 0.09 g⋅cm−2⋅yr−1. The 2004, 2011 peaks are also the consequences of major floodings, having maximum debits of 10,800 m3 s−1 and, respectively, 10,200 m3 s−1. Similar four sedimentation peaks have been measured (Babic Mladenovic et al., 2013) in the suspended sediment behind Iron Gate 2. The mean sedimentation rate of the Iacob Lake is 0.51 ± 0.08 g⋅cm−2⋅yr−1, with a maximum of 3.84 ± 0.05 g⋅cm−2⋅yr−1 and a minimum of 0.03 ± 0.01 g⋅cm−2⋅yr−1. The sedimentation rates were divided into four major periods: namely 1940–1972, 1972–1980, 1972–1989 and 1989–2013. The first period had an average sedimentation of 0.42 ± 0.14 g⋅cm−2⋅yr−1, the second showing a decreasing to 0.38 ± 0.11 g⋅cm−2⋅yr−1 and the third to 0.21 ± 0.02 g cm−2 yr−1, which means a 10.04% decreasing of the sedimentation. Therefore, this lake was not highly influenced by the construction of the Iron Gates, since it is not directly connected to the main branches of the Danube. Comparing the 1972–1989 (0.21 ± 0.05 g cm−2 yr−1) and 1989–2013 (0.68 ± 0.07 g cm−2 yr−1) periods, an abrupt increasing can be observed. After 1989, the number of floodings has increased, which was highly influenced by the lumbering processes. More information about Iacob (cores IAII15 and IAII16) and Isac lake's sedimentation processes can be found in the Begy et al., 2018 and a detailed description of the IA3 and IA4 cores can be found in Begy et al. (2014).

Fig. 8. Sedimentation rates of the Uzlina Lake cores.

the lake entry, the area of UZII7 is less affected by the Sfântu Gheorghe Branch, however the two local minimums in the 210Pbtot activity concentration are visible in the two peaks representing the 2002–2004 (1.23 ± 0.21 g cm−2 yr−1) and 2008–2010 (1.62 ± 0.27 g cm−2 yr−1) periods. The decreasing effect on sedimentation of the Iron Gate's construction can be observed in the UZ8 (from 0.20 g cm−2 yr−1 to 0.15 g cm−2 yr−1) and UZII7 (from 0.28 g cm−2 yr−1 to 0.18 g cm−2 yr−1) cores situated near the inflow channel, while its effect is not visible on the UZ10 (from 0.27 g cm−2 yr−1 to 0.34 g cm−2 yr−1) core. This can be caused by a series of factors such as the location of the sampling point and the local natural and anthropogenic influences. The mean sedimentation of the lake (0.26 ± 0.02 g cm−2 yr−1) in the period of 1940–1972 decreases 16% in the period of 1972–1980. The sedimentation rate increases drastically after the 1980's, which could have been majorly influenced by the straightening of the Mahmudia Meander (1984–1988) and the increased lumbering. The mean sedimentation rate (0.27 ± 0.02 g cm−2 yr−1) calculated for the 1972–1989 period increases 4 times in the 1989–2013 period (1.00 ± 0.10 g cm−2 yr−1).

3.4. Isac Lake According to the sampling areas, the four sampled cores can be grouped into two groups: samples ISII9 and ISII10 from the northeastern part of the lake, close to the entry channel, and IS11 and IS12 from the south-eastern part, close to the inflow channel. In a case of IS11, the mean sedimentation is 0.45 g cm−2 yr−1. From the five visible peaks, the one caused by the 1991–1993 flooding is the most significant, having an average sedimentation of 0.82 ± 0.07 g cm−2 yr−1. The IS12 sediment core is prone to an increasing sediment deposition tendency starting from the 1980's, having a series of local maximums. The Mahmudia Meander straightening (1984–1988) increased the sediment intake to almost double (0.63 ± 0.05 g cm−2 yr−1) in 1986. The first ten years for the ISII10 core show a decreasing from 0.86 ± 0.12 g cm−2 yr−1 to 0.13 ± 0.01 g cm−2 yr−1 by the 1950's. In 1991 the sedimentation decreases to a minimum of 0.14 g cm−2 yr−1 until 2014, while simultaneously the sedimentation of the ISII9 sampling point increases. Until the 1990's the western part of the entrance of the north-eastern channel was more exposed to sediment deposition than the eastern part, building a natural barrage throughout time. Therefore, more sediment deposited on the eastern part of the lake. In the period before the construction of the Iron Gates (1940–1972) the sedimentation rate had an average of 0.45 ± 0.06 g cm−2 yr−1; this value decreases to 0.20 ± 0.04 g cm−2 yr−1 in the 1972–1980 period, leading to a 42% retention of sediment. Afterwards, in

3.6. Merhei Lake The Merhei Lake has an average water content of 70% higher and a porosity of 66% higher. The bulk density of the analysed samples shows an average of 0.49 g cm−3, having an indirect correlation with water content, porosity and organic matter concentration. Activity concentrations for the 210Pbunsup and 210Pbtot are visible in Fig. 9. Due to the positioning of the sampling points, MEII19 has an exponential decreasing of the supported 210Pb values in the first 10 cm, being situated in the centre of the lake, furthest from the entry channels. The other two sampling points are in the proximity of the NE channel entry, MEII21 being in right of it, explaining therefore the scattering in the activity concentrations in both cases. Because of the similarities in porosity and water content (both being in the 33–35% range), a constant density (0.65 g cm−3) and TC content (11%) in the middle section of each core and the non-exponential activity concentrations, grain size analysis was performed. The upper 10 cm and lower sections (below 60 cm) are composed of very fine sand of 25–39 μm (53%) and coarse silt of 21 μm (11%), while the middle section is made up by very fine silt of 8–11 μm (34%) and clay of 2 μm (15%). The obtained dates show a two stepped distribution, the first period being visible from the present to 1968 ± 8 y for MEII19, 1980 ± 5 y in case of MEII20 and 1975 ± 7 y in case of MEII21, 100

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Fig. 11. Sedimentation rates of the Merhei Lake cores.

Fig. 9. The activity concentrations of the Merhei Lake cores.

disrupted the entire equilibrium of the delta. Results for the ME15 and ME16 cores are presented in Begy et al. (2015), stating that the sedimentation rates from the more secluded ME6 sampling point are lower, than those of ME15, situated near an inflow channel. Both cores were affected by floods which took place more than 30 years ago (such as the major flood in 1940 and the multiple floods in the 1980–1990 period). Additionally, physical parameters (decreasing of water content and porosity and increasing of bulk density with depth) confirm the exponential decreasing of the unsupported 210Pb concentration of the ME16 sampling point, while the three peaks of unsupported 210Pb are also visible in the distribution of the physical parameters. In case of all sediment cores from the Merhei Lake, the non-carbon mass sedimentation is responsible for the majority of the sediment (Fig. 12). The organic matter content of the young sediment has an average of 10%, the great changes being caused by the sediments of non-carbonate origin. The organic mass containing sediment tracks the

confirming a high sediment income for these periods. Average sedimentation rates for the three cores are 1.17 g cm−2 −1 yr , having average sedimentation rates of 1.11 g cm−2 yr−1 for MEII19, 1.82 g cm−2 yr−1 for MEII20 and 0.58 g cm−2 yr−1 for MEII21. The period with the highest sediment income is 1973–1988 in case of MEII19 with 1.95 g cm−2 yr−1 (values before having an average of 0.21 g cm−2 yr−1, and 0.34 g cm−2 yr−1 after), 1992–2012 in case of MEII20 with 2.53 g cm−2 yr−1 (values before being 0.40 g cm−2 yr−1) and 1973–2013 in case of MEII21 with 1.08 g cm−2 yr−1 (values before being 0.212 g cm−2 yr−1) (Fig. 10). Positioned in the eastern to the lake, the Letea Sandbank has a significant contribution to the sediment accumulation rates in the eastern part of Merhei Lake, storms being able to carry amounts of sand into the lake, as visible from the grain size analysis (Fig. 10) and the high sedimentation rates (Fig. 11). The years, when massive storms were registered, are 1981, 1987, 1991, 1995 and 1998 (periods being accounted for sedimentation rates well above 1 g cm−2 y−1), when the storm form 21–24 January has

Fig. 10. Grain size distribution of the Merhei Lake cores. 101

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Fig. 12. Carbon and Non-Carbon sedimentation for MEII19 and MEII21 cores.

trend of the non-organic sediment, meaning that the incoming sediment contains the carbon and it is not originating from the decomposing organic vegetation. There were several major flooding periods with maximal debits over 104 km3 s−1 in the last century. The yearly appearance of these starting with 2005 also contributed to the increase of the sediment income, visible in all the cores of the Matița-Merhei lake system. Other noticeable floods were registered yearly in the 1974–1982 and 1965–1970 periods, however, there were no major floods in the 1970–1974 period, when the Iron Gates Hydro-Energetical Power Plant was built. The effect of the Iron Gates is visible only in cores MEII19 and MEII20, where sedimentation decreased 22% and, respectively, 4% in the period after 1972 (from 0.69 g cm−2 yr−1 to 0.54 g cm−2 yr−1 and, respectively, from 0.23 g cm−2 yr−1 to 0.22 g cm−2 yr−1). Sediment cores ME15 and ME16 do not show significant variations in the sedimentation rates for these periods (Begy et al., 2015), while MEII21 shows an increasing of 1.68 times (from 0.21 g cm−2 yr−1 to 0.35 g cm−2 yr−1). The 1989 regime change lead to an increasing of 3 times compared to the period before: the average 0.43 g cm−2 yr−1 increased to 1.14 g cm−2 yr−1 until 2013. The most affected area was the eastern part of the lake, where sedimentation rates increased 3 times for MEII19 (from 0.37 g cm−2 yr−1 to 1.26 g cm−2 yr−1), 2 times for MEII20 (from 1.18 g cm−2 yr−1 to 2.13 g cm−2 yr−1) and 6 times for MEII21 (0.33 g cm−2 yr−1 to 2.01 g cm−2 yr−1).

Fig. 13. The carbon content of the Matița Lake cores.

Lake have already been published (Simon et al., 2016), where an intercomparison was made between lakes of different origin from Romania in order to establish the changes in the sedimentation pattern over the last three decades. The supported 210Pb quantity in the Matița Lake has an average of 17 ± 3 Bq kg −1, with scattered values similar to those of the Merhei Lake. The age/depth profiles of the cores can also be divided in two periods: the one before and the one after 1995 ± 3 yr, the more recent having elevated sedimentation levels. Sedimentation rates increased on average 2 times after 1972: from 0.19 g cm−2 yr−1 to 0.29 g cm−2 yr−1 for MA18, from 0.24 g cm−2 yr−1 to 0.42 g cm−2 yr−1 for MA20 and from 0.22 g cm−2 yr−1 to 0.48 g cm−2 yr−1 for MAII17, leading to the conclusion that, other local influences were much more accentuated in this area in the period of the construction of the Iron Gates. The regime change of 1989 lead to an increase of 3 times (from 0.57 g cm−2 yr−1 to 1.38 g cm−2 yr−1). The MAII17 sampling point situated in the proximity of the northern inflow channel was influenced most, namely 3.81 times: from 0.48 g cm−2 yr−1 to 1.82 g cm−2 yr−1, the other two sampling points show increasing of 2 times. In case of the Carbon and Non-Carbon sedimentation, the two trends are similar, showing no significant contribution of organic matter from the decaying of the vegetation (Fig. 14). As a summary it can be stated that in the period before the construction of the Iron Gates (1940–1972) the mean sedimentation rate was 0.29 ± 0.02 g cm−2 yr−1. The lowest sedimentation is accounted for the Cuibida Lake, where the sedimentation was 0.19 ± 0.01 g cm−2 yr−1, while the highest was visible in the Isac

3.7. Matița Lake In case of the three sediment cores taken from the Matița Lake, it is visible, that the MA20 sampling point the lowest porosity and water content (less than 38% in both cases), being situated in a more secluded area with no man-made or natural channels, while the other two sampling points are in the direct proximity of water inflows. Also, both MA20 and MAII17 show a decreasing tendency from 69% to 35%, while MA18 seems to have massive sediment mixing among its layers having a relatively constant bulk density of 0.49 g cm−3 and a high organic matter content of 8% throughout of its entire length, proving that this part of the lake is going through massive eutrophication. MAII17 has the highest inorganic carbon content its maximum value being higher than 23% in the first 15 cm, where the values for porosity range from 64 to 92% and water content also shows values of 70 ± 4%. MA20 is represented by one visible peak in the LOI measurements regarding the TC: 21% (4% OM, 17% IOC) at 12 cm (Fig. 13): A detailed description of the activity concentrations, age/depth correlations and sedimentation rates for the sediment cores from Matița 102

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Fig. 14. Carbon and Non-Carbon sedimentation of the Matița Lake cores.

sedimentation rate (Matița: 79%; Merhei 6%; Cuibida 24%) because of their further location and the impact of more local influences (floods, storms, decaying organic material, grains transported by the wind etc.). The highest decreasing after the construction of the Iron Gates is visible in the Cruhlig Lake, namely at the sampling area closest to the inflow channel where sedimentation decreased from 0.60 g cm−2 yr−1 to 0.13 g cm−2 yr−1. In the same period, the highest increase in sedimentation is visible in the Matița Lake (10.93 g cm−2 yr−1), since this lake is fed by several channels from the Sulina and Chilia Branches. Beside the effects of the Iron Gates, the straightening of the Mahumdia Meander 1984–1988 can also be seen in the Uzlina and Isac lakes. Also, all lakes show that in the period of the last 14 years, the sedimentation rates show increasing tendencies of 3 times the values before 1989. The highest increase is visible in case of the Uzlina Lake, where sedimentation grew 4 times after 1989. The Cuibida Lake was influenced the least, sedimentation rates growing only 2 times, since it is located between the Sulina and Sfântu Gheorghe branches, which have no direct connection to the lake itself. The political regime change caused, among others, the redistribution of forests, which resulted in accentuated lumbering in the hydrographic basin of the Danube.

Table 2 Mean sedimentation rates in g cm−2 y−1 for the analysed lakes in the periods of interest. Period

1940–1972 1972–1980 1972–1989 1989–2013

Lake Matița

Merhei

Cruhlig

Iacob

Uzlina

Isac

Cuibida

Mean

0.21 0.38 79% 0.57 1.38 3x

0.25 0.27 6% 0.43 1.14 3x

0.24 0.10 59% 0.14 0.31 2x

0.42 0.38 10% 0.21 0.68 3x

0.26 0.22 16% 0.27 1.00 4x

0.45 0.20 42% 0.27 0.72 3x

0.19 0.24 24% 0.38 0.68 2x

0.29 0.25 12% 0.32 0.84 3x

Lake (0.45 ± 0.01 g cm−2 yr−1). This difference can be observed because the Isac Lake is closer to the main branches of the Danube, therefore receiving a greater sediment amount than the Cuibida Lake. The higher sedimentation rates in the central danubian lakes are due to the higher debit of the Sulina and Sfântu Gheorghe branches compared to Chilia. The mean decreasing in the sedimentation rates of all seven lakes is of 12.16% comparing the 1940–1972 period to 1972–1980 period (Table 2). All lakes show a dramatic increasing in sedimentation after 1989 and have an average of 3 times growth. The highest increase is visible in the Uzlina Lake, where the sedimentation increased 4 times after 1989 (1.00 g cm−2 yr−1) compared to the period before (0.21 g cm−2 yr−1). Peaks can be observed in the individual sedimentation rates of the cores, which are in good agreement with the major recorded flooding periods of the past 14 years.

Acknowledgements This work was supported by the Ministry of National Education, Romania under the grant 61/30.04.2013, PN-II-RU-TE-2011-3-0351 project. Authors Begy, R.Cs. and Kelemen, Sz. acknowledge the financial support of PN-III-P3-3.6-H2020-2016-0016, contract 7/2016. Appendix A. Supplementary data

4. Conclusions

Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jenvrad.2018.06.010.

The 210Pb dating method was implemented for the first time on a study of this extension to determine sedimentation rates. This study denoted, that anthropic activities such as excessive lumbering and the construction of major dams (the Iron Gates in case of the Danube Delta) can influence the sedimentation rates of individual lakes. As a conclusion it can be stated that in the studies such as this, the proper choosing of the lakes is of great importance, since only half of the analysed lakes showed decreasing tendencies after the construction of the Iron Gates. The impacts on the sedimentation can be observed in the areas of the inflow channels, mostly by the decreasing of the sediment quantity. In these points the decaying vegetation influences the sedimentation in a negligible manner, therefore the Non-Carbon or inorganic sedimentation is present in a 98% extent. From the analysed lakes, four show the decreasing of the sedimentation after the construction of the Iron Gates: namely Cruhlig (59%), Iacob (10%), Uzlina (16%) and Isac (42%). These lakes are located closely to one of the main branches of the Danube or have a direct links to these. The rest of the lakes show an increase in the

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