Isotope analyses of the lake sediments in the Plitvice Lakes, Croatia

Cent. Eur. J. Phys.DOI: 10.2478/s11534-014-0490-7

Central European Journal of Physics

Isotope analyses of the lake sediments in the PlitviceLakes, Croatia

Research Article

Nada Horvatincić1∗, Andreja Sironić1† , Jadranka Barešić1‡ , Ines Krajcar Bronić1§, Jovana Nikolov2¶ ,Nataša Todorović2∗∗, Jan Hansman2†† , Miodrag Krmar2‡‡

1 Laboratory for Low-level Radioactivity,Ruđer Bošković Institute, 10000 Zagreb, Croatia

2 University of Novi Sad,Faculty of Sciences, Department of Physics, 21000 Novi Sad, Serbia

Received 18 March 2014; accepted 19 May 2014

Abstract: The analyses of radioactive isotopes 14C, 137Cs and 210Pb, and stable isotope 13C were performed in thesediment cores, top 40 cm, taken in 2011 from karst lakes Prošće and Kozjak in the Plitvice Lakes NationalPark, central Croatia. Frozen sediment cores were cut into 1 cm thick layers and dried. 14C activity inboth carbonate and organic fractions was measured using accelerator mass spectrometry technique withgraphite synthesis. 137Cs, 210Pb, 214Pb and 214Bi were measured by low level gamma spectrometry methodon ORTECHPGe detector with the efficiency of 32%.Distribution of 14C activity from both lakes showed increase of the 14C activity in the top 10− 12 cm in bothcarbonate and organic fractions as a response to thermonuclear bomb-produced 14C in the atmosphere inthe sixties of the 20th century. Anthropogenically produced 137Cs was also observed in sediment profiles.Sedimentation rates for both lake sediments were estimated based on the unsupported 210Pb activity.Different 14C activity of the carbonate fraction (63−80 pMC, percent of modern carbon) and organic fraction(82 − 93 pMC) is the result of geochemical and biological processes of the sediment precipitation in thelake waters. This is also confirmed by the δ13C values of both fractions. Carbon isotope composition, a14Cand δ13C , was compared with the lake sediments from the same lakes collected in 1989 and 2003.

PACS (2008): 29.30 Kv, 91.65.Dt, 91.67.Qr, 91.67.Rx, 92.20.Td, 92.20.Xy

Keywords: Plitvice Lakes • lake sediments • 14C • 137Cs • 210Pb • 13C • carbon cycle© Versita sp. z o.o.

∗E-mail: [email protected]†E-mail: [email protected]‡E-mail: [email protected]§E-mail: [email protected]¶E-mail: [email protected] (Corresponding author)∗∗E-mail: [email protected]††E-mail: [email protected]‡‡E-mail: [email protected]

1. IntroductionRadioactive isotopes 137Cs (half-life 30.2 yr) and 14C (half-life 5730 yr) are used as suitable tool in lake sedimentstudies [1–3]. 137Cs is an anthropogenic isotope intro-duced into the atmosphere by nuclear weapon tests infifties and sixties of the last century and additionally in1986 as Chernobyl fallout. The 137Cs fallout activities inCroatia after the Chernobyl accident were much higher(6200 Bqm−2) than in sixties during the period of inten-

Isotope analyses of the lake sediments in the Plitvice Lakes, Croatia

sive atmospheric nuclear weapon test (baseline level in1985 was 1.5 Bqm−2) [4]. 14C is basically of natural ori-gin, but significant 14C contamination of the atmosphericCO2 by nuclear weapons testing also occurred with max-imum contamination in 1963 [5, 6]. 14C activity (a14C ) ofatmospheric CO2 and in tree rings measured in Croatiashowed the same trend, with maximum in 1963 and thensteady decrease towards present-day a14C values closeto the natural activity [7].Global changes in the atmosphere have been reflected inthe whole environment and they are particularly noticablein lake sediments that preserve records of anthropogenicalterations. Therefore, the record of the 137Cs and 14C inrecent carbonate sediments can be used to determine sedi-mentation rate, response of the lake environment/sedimentto anthropogenic contamination of the atmosphere, as wellas to follow environmental processes of carbon geochem-istry.The largest fraction of lake sediments in karst area is sec-ondary carbonate precipitated from water. Stable isotoperatios of 13C/12C in carbonate (δ13C values) depend on thecarbon isotope composition of dissolved inorganic carbon(DIC) in water and the conditions under which the pre-cipitation occurs [8]. Organic fraction in the karst lakesediments is much less abundant, only by a few percentin the Plitvice Lakes [9] but its δ13C values give an addi-tional information about the processes of bioactivity andproductivity in the lake waters [10–12].Radioactive fallout 137Cs deposited across the landscapefrom atmospheric nuclear tests is strongly absorbed on soilparticles, limiting its movement by chemical and biologicalprocesses. Most 137Cs movement in the environment is byphysical processes; therefore, 137Cs is a tracer for studyingerosion and sedimentation.210Pb occurs naturally as one of the radioisotopes in the238U decay series. In sediment 210Pb has two sources: partof 238U progenitor, 222Rn, produced in local rocks and sed-iments, escapes to the atmosphere and decays there viashort-lived isotopes to 210Pb. 210Pb is removed from theatmosphere by adsorption to aerosols and in precipitationor dry deposition and is incorporated into the upper sedi-ment layer (unsupported or excess 210Pb)[13]. The secondsource is 210Pb produced in the sediments in situ (sup-ported). 210Pb method is based on the radioactive decayof unsupported 210Pb which has been calculated as thedifference between total 210Pb and supported 210Pb. Thehalf-life of 210Pb of 22.3 yr makes it an ideal indicatorof modern sedimentation rates. Several models based on210Pbunsup measurement in sediment columns have beenextensively used [14]. As 137Cs is derived from a differentsource than 210Pb it provides a valuable cross-check ofthe 210Pb results.

In this paper we investigated changes in the uppermost40 cm thick layers of lake sediments retrieved in 2011from two karst lakes Prošće and Kozjak in the PlitviceLakes system using radioactive isotopes 14C, 137Cs and210Pb and stable isotope 13C. The results were comparedwith previous studies of surface sediment cores (top 30−40cm) retrieved from the Lakes Prošće and Kozjak in 1989and 2003 [15, 16]. All previous studies included 14C and13C isotopes of carbonate fraction, while 137Cs and 210Pbwere determined in short sediment cores retrieved in 2003.However, the top 10 cm thick layer of the sediment corefrom Lake Kozjak was perturbed during sampling and theresults were inconclusive [16]. In this study we used ac-celerator mass spectrometry technique (AMS) for a14Cmeasurement (in previous studies GPC [15] and LSC [16]techniques were used). This technique provided: 1) newa14C results of organic fraction in sediment which is moresuitable and relevant material for a14C measurement andin correlation with a14C of carbonate fraction gave addi-tional information of lake sediment formation/changes, 2)better resolution of 14C results of carbonate and organicfractions along the sediment profile.

2. Site description

The Plitvice Lakes are spectacular series of sixteen lakes,tufa barriers and waterfalls situated in the karst region ofthe northwestern Dinarides in central Croatia (Figure 1a).The lake water is characterized by intense precipitationof calcium carbonate forming tufa barriers between thelakes and fine-grained lake sediments at the bottom of thelakes. For this study we selected two biggest lakes fromthe Plitvice Lakes system (Figure 1b,c): the uppermostLake Prošće (0.68 km2, maximal depth 39 m) which is fedby water from Matica stream, and Lake Kozjak (0.82 km2,maximal depth 46 m) fed by water from the upper lakes.The seismic records of both lakes showed 10−12 m thick-ness of lacustrine deposits below which the pre-lacustrinebase can be easily identified [17].

Qualitative mineralogical analyses have shown that dom-inant component of inorganic fraction of the sediment iscalcite (70 − 85%) with a few percent of dolomite andquartz, and the minor components, detected in some sam-ples, are aragonite, quartz, feldspars and minerals of thefilosilicates group [9]. Calcite is mainly authigenic, i.e., itprecipitates from the DIC in water. Organic matter in lakesediments is present in low amount (2 − 8%) and is alsomainly authigenic [18].

Nada Horvatincić et al.

Figure 1. a) Geographical setting of Croatia, Dinarides and Plitvice Lakes. b) Map of the Plitvice Lakes. c) Bathymetric maps of Lake Prošće andLake Kozjak [19, 20] with the sampling sites.

3. Material and methodsSediment cores, top 40 cm, were taken by the gravity corerat the depth of 39 m (July 2011) in Lake Prošće and at22 m in Lake Kozjak (May 2011). The sediment coreswere frozen immediately. The frozen sediment cores werecut into 1 cm thick layers, dried and then analyzed. Allisotope analyses presented in this study were performedin the same sediment core. a14C and δ13C were measuredin carbonate and organic fractions of sediment. Organicfraction was separated by dissolution of carbonate fractionin 4% HCl, the rest was washed and dried. a14C was mea-sured by accelerator mass spectrometry technique (AMS).Graphite targets for the AMS measurement were preparedby the following chemical procedures: 1) CO2 productionby dissolution of carbonate fraction and oxidation of or-ganic fraction of sediment, 2) reduction of CO2 by Zn at450◦C to graphite [21]. a14C and δ13C analyses were per-formed on 250 kV SSAMS [22] and Isotope Ratio MassSpectrometers, respectively, at the Scottish UniversitiesEnvironmental Research Center, SUERC, East Kilbride,Scotland, UK. a14C was corrected for δ13C and expressedin percent of modern carbon (pMC) [23]. Isotope ratio13C/12C was expressed in standard delta notation (δ13C )

as per mil (h) deviation from the V-PDB standard. Mea-surement uncertainties in a14C and δ13C were between0.2 and 0.4 pMC and 0.1h, respectively.For 137Cs and 210Pb analyses dry sediment samples withthe mass ranging from 2.5 g to 32 g were packed instandard geometry, cylindrical plastic containers (0.7 mmwall thickness), with diameter 70 mm and height up to15 mm. A low-background extended range HPGe detec-tor (shielded by 18 cm of lead, 1 mm of tin and 1.5 mm ofcopper) equipped with a Be window was used in mea-surements. The relative efficiency of the detector was32%. Detection efficiency was established using stan-dard IAEA radioactive materials positioned in the samedetection geometry as that used for sample measurements.Correction for self-attenuation effects was not necessarybecause of the very similar composition and density ofthe sample and reference materials. The characteristicmeasurement time was up to 80 · 103 s, depending onthe mass of the sample. A well-known 137Cs 661.66 keVgamma line was followed in spectra, where intensity wasdetermined using Genie Canberra software. The specificactivity of 210Pb (in Bqkg−1) was determined in all spec-tra where 46.5 keV was observed. Several of the mostintense gamma lines from 214Bi and 214Pb were used to

Isotope analyses of the lake sediments in the Plitvice Lakes, Croatia

calculate the activity of both radionuclides in all sam-ples. Obtained mean values were subtracted from cor-responding values of measured 210Pb activities to ob-tain values of unsupported 210Pb in sediment samples,A(210Pbunsup) = A(210Pb)− 1

2 [A(214Pb)+A(214Bi)].

4. Results and discussionResults of measured 137Cs and 210Pb activities, as wellas a14C and δ13C of organic and carbonate fractions insediment profiles of Lake Prošće and Lake Kozjak are pre-sented in Figure 2.In both sediment profiles 137Cs has been detected only inthe upper layers. Its activity has maximum of 125 Bqkg−1

at depth 1−2 cm in Lake Prošće, and 131 Bqkg−1 at depth4− 5 cm in Lake Kozjak. These maxima can be attributedto the Chernobyl accident in 1986. At depths below 17 cmfor Lake Prošće and below 11 cm for Lake Kozjak presenceof 137Cs in sediment samples was below the minimum de-tectable limit of the spectroscopy system. The measureddepth distribution of unsupported 210Pb in Lake Prošćesediments shows a decreasing trend with some small butstill measurable values (order of magnitude of about 80Bqkg−1) even at the highest depths of 40 cm. Almost threetimes lower values of unsupported 210Pb were detected inLake Kozjak than in Lake Prošće.Measured depth distributions of unsupported 210Pb wereused for estimation of sedimentation rate as well as thechronology of sedimentation (Figure 2). When plotted onlogarithmic scale, unsupported 210Pb activities versus cu-mulative dry mass of sediment (in kgm−2) for Lake Prošćeappear pretty linear. The simplest Constant Sedimen-tation Rate (CSR) model was used for calculation. Itwas estimated that sedimentation rate in Lake Prošće is0.9 kgm−2yr−1. This value can be compared with the massaccumulation rate of 1.3±0.4 kgm−2yr−1 obtained for thecores retrieved in 2003 [16]. The approximate age of thesediment calculated by the CSR model is shown on theright scale in Figure 2.Logarithmic plot of unsupported 210Pb activity measured inLake Kozjak, as a function of depth (in kgm−2) is not linearand Constant Rate of Supply (CRS) model was applied incalculations. It was estimated that sedimentation rate inLake Kozjak fluctuated from 0.59 kgm−2yr−1 in period 10years ago (at depth 3− 4 cm) to 1.3 kgm−2yr−1 about 35years ago at depth 7−8 cm (right scale in Figure 2). Theaverage mass accumulation rate for sediment retrieved in2003 was 0.8± 0.1 kgm−2yr−1 [16]. The approximate ageof the sediment calculated by the CRS model is shown onthe right scale in Figure 2.The most direct method using 137Cs dating markers was

not used considering that 1963 peak was not prominent inLake Prošće and not visible at all in sediment column ofLake Kozjak. It is well known that 210Pbunsup and 137Csbehave differently (chemically) in the sediment grains,however in recent publication [24] it was shown that thebulk contents in the samples of mentioned radionuclidesshowed good correlation, despite different chemical states.In some cases [25] when 137Cs and 210Pbunsup chronolo-gies are not consistent (probably due to sediment mixing),210Pbunsup dating results were adopted without 137Cs val-idation.

a14C and δ13C values of carbonate and organic fractionof sediments for both lakes showed significant variationsin top ∼12 cm and ∼10 cm, for Lakes Prošće and Koz-jak, respectively (shaded areas in Figure 2) and gener-ally stable values in lower layers. The increase of a14Cwas observed in both carbonate and organic fractions withmaximum values at the depth of 4− 5 cm in Lake Kozjakand at 5− 6 cm in Lake Prošće. This maximum a14C wasthe response to 14C produced in atmospheric thermonu-clear bomb tests in the sixties of the last century. Thea14C peak in sediments was damped in comparison withthe atmospheric peak, which almost doubled natural a14Cin 1963. According to the estimated sediment chronol-ogy based on unsupported 210Pb, the peak in a14C in thesediment was delayed by approximately 30 - 35 years inboth lakes Kozjak and Prošće. It is in good correlationwith the result for Lake Prošće sediment core retrievedin 2003 [16]. Delayed and damped response of a14C inthe lake sediment is the consequence of the geochemi-cal processes involved in the lake sediment formation, i.e.the process of soil CO2 and limestone dissolution in therecharge area of springs, exchange of carbon isotopes ofDIC and atmospheric CO2, photosynthetic activity in thewater and water-sediment interaction.

The mean a14C value for carbonate fraction for LakeProšće sediment was (68.2± 3.6) pMC and for Lake Koz-jak it was (73.0 ± 3.2) pMC (Table 1). The difference of∼5 pMC can be attributed mainly to the processes involv-ing DIC in the lake waters since its isotope compositionwas reflected in the isotope composition of carbonate sed-iment. Lake Prošće is the uppermost lake in the PlitviceLakes system, while the Lake Kozjak is ∼5 km distantin downstream direction, with several lakes and numer-ous waterfalls in between (Figure 1). In this turbulentwater flow the CO2 degassing from the water and carbonisotope exchange processes between DIC in water and at-mospheric CO2 affected the increase of a14C of DIC andas a consequence an increase of a14C of lake carbonate[8]. a14C of atmospheric CO2 in the Plitvice Lakes areain period 2011-2013 was 100-110 pMC, and a14C of DICfor Prošće and Kozjak Lake were (73.9 ± 0.4) pMC and

Nada Horvatincić et al.

Figure 2. 137Cs and 210Pbunsup activities, a14C and δ13C in carbonate and organic fractions in sediment profiles from Lake Prošće and Lake Kozjak.Vertical error bars indicate the layer thickness, and horizontal error bars indicate measurement uncertainties that are often smaller thansymbol width. Additional right axis shows the approximate age of the sediment determined by applying CSR and CRS model for LakeProšće and Lake Kozjak, respectively (210Pb sediment chronology).

(79.8±0.4) pMC, respectively [26]. The exchange with theatmospheric CO2 was also confirmed by the δ13C valuesof carbonate fractions: higher δ13C values of Lake Kozjaksediment, mean δ13C (−9.0 ± 0.4)h, compared with theLake Prošće δ13C (-9.7 ± 0.5)h (Table 1). δ13C values

of DIC in Prošće and Kozjak Lake in the site of samplingwere (−11.2±1.2)h and (−10.88±0.64)h, respectively[8]. The photosynthetic activity of aquatic plants couldalso influence the a14C increase of DIC and carbonatefraction in downstream flow.

Isotope analyses of the lake sediments in the Plitvice Lakes, Croatia

Table 1. Comparison of carbon isotopic composition (a14C and δ13C ) of carbonate and organic fractions of the sediments from Lake Prošće andLake Kozjak retrieved in 2011 with the corresponding results of the sediment cores retrieved in 1989 [15] and 2003 [16].

year ofsampling water depth

length ofsediment

cores

δ13C (h)carbonate δ13C (h) organic a14C (pMC)

carbonatea14C (pMC)

organic ref.

Lake Prošće

1989 20 m 30 cm −9.2± 0.2 −33 to −30 66− 79 – [15]

2003 19 m 40 cm −8.8± 0.2−9.2 to −8.5

– 65.8± 2.761.7− 69.8

– [16]

2011 39 m 40 cm −9.7± 0.5−10.4 to −8.7

−32.0± 1.5−34.0 to −29.9

68.2± 3.663.4− 72.2

87.7± 3.482.2− 91.8

Lake Kozjak

1989 21.5 m 30 cm −8.9± 0.1 −33.5 to −30 72 to 91 – [15]

2003 21 m 40 cm −8.6± 0.2−9.0 to −8.2

– 72.9± 3.368.5− 79.2

– [16]

2011 22 m 37 cm −9.0± 0.4−9.7 to −8.2

−31.8± 1.0−33.9 to −30.4

73.0± 3.270.6− 79.8

87.4± 3.081.9− 93.1

For organic fraction of sediment a14C was in average15 – 20 pMC higher than that for carbonate fraction inboth lakes (Table 1). This was the result of the carbonorigin and/or processes involved in sediment formations.The a14C and δ13C values of carbonate fraction were theresults of the geochemical processes occurred mainly inthe karst spring/aquifier area [8, 16]. Significantly highera14C of organic fraction for both lake sediments and δ13Cvalues of −34 to −30h showed that the organic fractionwas mainly composed of aquatic plants and influenced bybioproductivity of the lake waters. A slight decrease ofδ13C in top 10 – 12 cm of the sediments, from −30 to−34h for both lakes can be explained by the increase inprimary productivity in the lake waters in recent period,i.e. in last ∼50 years (Figure 2). It also correlated withslight increase of organic carbon concentration in top 5cm of sediment, 3.7 – 4.5% and 3% for lakes Prošće andKozjak, respectively, and below 10 cm it was 2.9 – 3.2%for Lake Prošće and 1.6 – 2.2% for Lake Kozjak [9].

The carbon isotopic composition of both carbonate and or-ganic fraction of sediment from both lakes presented in thiswork was compared with the carbon isotope compositionsof sediments from the same lakes retrieved in 1989 and2003 (Table 1). All sampling campaigns resulted in similarrange of δ13C values of carbonates and organic fractions,and slight differences could relate to uncertainty of ana-lytical procedure (measured in different laboratories) and

spatial variability of δ13C of lake sediment collected indifferent time. The trend of slight decrease of δ13C of bothfractions in upper layers was confirmed, which can be at-tributed to the increase of primary productivity in the lakewaters. In all cores a peak in a14C of carbonate fractionwas observed, and the cores taken in 2011 showed in ad-dition that the organic fraction of the sediments reflectedthe atmospheric a14C in a similar way. Slight differencesin measured a14C values could be explained by differentsampling locations and different sampling and measure-ment techniques.

5. ConclusionThe presented isotope study of recent lake sedimentsformed during the last ∼200 years from two karst lakesin the Plitvice Lakes National Park, Croatia, showed thatsediments reflected global atmospheric changes in 14C and137Cs activities caused by anthropogenic influence. Dis-tribution of a14C in sediment profiles of lakes Prošće andKozjak showed increase of the a14C in the top ∼10− 12cm in both carbonate and organic fractions. Based on theactivities of unsupported 210Pb, a sedimentation rate of0.9 kgm−2yr−1 was determined for Lake Prošće, while forLake Kozjak it ranged between 0.6 and 1.3 kgm−2yr−1.Different 14C activities of the carbonate fraction (63− 80

Nada Horvatincić et al.

pMC) and organic fraction (82 − 93 pMC) in both sedi-ment profiles were the result of geochemical and biologicalprocesses of the sediment precipitation in the lake waters.The δ13C values of both fractions (-10 to -8h for carbon-ate and −34 to −30h for organic) were good indicators oforigin of carbon and processes involved in lake sedimentprecipitation.Comparison of a14C and δ13C values for Prošće and Koz-jak lake sediments collected in previous investigations(1989 and 2003) with sediments from 2011 (this study)showed similar range of δ13C for carbonate and organicfraction. The slight decrease of δ13C of both fractions inupper layers indicated a possible increase of primary pro-ductivity in the lake waters in last ∼50 years. Both car-bonate and organic fractions of the sediments reflected theanthropogenic variations in atmospheric a14C in a similarway showing damped and delayed response to the atmo-spheric a14C peak.

AcknowledgmentThe work was performed within the project with NationalPark Plitvice Lakes and Bilateral scientific project be-tween Croatia and Serbia, The Project 098-0982709-2741, Ministry of Science, Education and Sport of theRepublic of Croatia and Projects No.43002 and 43007,Ministry of Education, Science and Technological Devel-opment of the Republic of Serbia.We thank Prof. Ivan Sondi, Momir Milunović and SrećkoKarašić for retrieving the sediment cores from lakes Prošćeand Kozjak and the staff of the Plitvice Lakes NationalPark for logistic help.

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