Recent changes in sedimentation regime in Cienfuegos Bay, Cuba, as inferred from 210Pb and 137Cs vertical profiles

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Continental Shelf Research 26 (2006) 153–167 www.elsevier.com/locate/csr

Recent changes in sedimentation regime in Cienfuegos Bay, Cuba, as inferred from 210Pb and 137Cs vertical profiles C.M. Alonso-Hernandeza,, M. Diaz-Asencioa, A. Munoz-Caravacaa, R. Delfantib, C. Papuccib, O. Ferrettib, C. Crovatoc a

Centro de Estudios Ambientales de Cienfuegos, AP 5, Ciudad Nuclear, Cienfuegos, Cuba b ENEA, Centro Ricerche Ambiente Marino, P.O. Box 224, I-19100 La Spezia, Italy c ENEA, Centro Ricerche Casaccia, Santa Maria Galleria, Anguillara, SP. 056 Roma, Italy Received 2 August 2004; received in revised form 17 May 2005; accepted 24 August 2005 Available online 4 January 2006

Abstract Dating techniques based on the natural and anthropogenic radionuclides 210Pb and 137Cs were applied to the study of the sedimentation regime in Cienfuegos Bay, Cuba. Core samples were collected from different locations in the bay and analysed to evalute the accumulation rate. Results evidenced significant changes in the sedimentation rate during the last 40 years: the recent sediment accumulation rates (0.47–0.50 g cm 2 yr 1) in the Northen basin are almost double those estimated before 1965 (0.30 g cm 2 yr 1). The 210Pb profiles show significant deviations from a simple exponential decline and abrupt variations between 1966 and 1970. These irregularities match closely periods of changes in land use (intense deforestation and regimentation of the Arimao and Caonao rivers in the late 1960s and early 1970s) and exceptional natural events (Hurricane ‘‘Camille’’ in 1969 and the intense rainfall of 1988) which occurred in Cienfuegos. The 137Cs and chlorite minerals profiles validate the results obtained from 210Pb dating and confirm the effect of exceptional events and changes in the natural hydrological regime of the bay during the past 40 years. r 2005 Elsevier Ltd. All rights reserved. Keywords:

210

Pb;

137

Cs; Sedimentation rates; Cienfuegos Bay; Cuba; Caribbean Sea

1. Introduction Cienfuegos Bay, situated in the southern central part of Cuba, is a semi-enclosed bay with a surface area of 90 km2 and an average depth of 14 m. It is connected to the Caribbean Sea by a narrow channel 3 km long (Fig. 1). The bay is divided in two well-defined hydrographic basins, due to the Corresponding author. Tel./fax: +53 43 96146.

E-mail address: [email protected] (C.M. Alonso-Hernandez). 0278-4343/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2005.08.026

presence of a submerged ridge 1 m below the surface, just north of the connection channel. The northern basin receives most of the anthropic impact from the outfall of Cienfuegos city, industrial pole in the country, and the freshwater input of the Damuji and Salado rivers. The southern basin is subject to a smaller degree of anthropic pollution originating from the Caonao and Arimao rivers. Part of the southern basin is a natural park, a niche for protected migratory birds and marine species. The bay represents the most important natural resource in the province, due to fishing activities,

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C.M. Alonso-Hernandez et al. / Continental Shelf Research 26 (2006) 153–167

Fig. 1. Location of Cienfuegos Bay and the sampling sites.

maritime transport, tourism industry, and natural parks. In the last three decades, deleterious ecological signals in the area have been observed: loss of biodiversity, shift of benthic communities, reduction in size and capture levels of commercial marine species and erosion of the coastline (Morales-Claro, 1991) Up to now, only few studies have been carried out in the area, mainly concerning the fishing resources (Borrell et al., 2004; Gonzalez, 1991; Gonzalez et al., 1991; Fattorini et al., 2004) and levels of environmental radionuclides (Alonso-Hernandez et al., 1998, 2002; Sibello-Hernandez et al., 2002), but a comprehensive description of the bay ecosystem is missing. In recent years a large environmental programme has been initiated, to define the main physical, chemical and biological characteristics of the bay, whose knowledge will constitute the basis for the correct management of the area. As part of this programme, in 1999/2000 a first characterisation of the sediments of Cienfuegos Bay

has been carried out and the vertical distributions of Pb and 137Cs have been determined to derive information on the sedimentation regime in the area. Radiometric techniques have been widely used during the last three decades for these purposes (Oldfield et al., 1995; Fung and Lo, 1997; Fuller et al., 1999; Somayajulu et al., 1999; Saito et al., 2001; Ligero et al., 2002; Colman and Bratton, 2003; Theng et al., 2003; Huh and Chen, 1999). The naturally occurring radionuclide 210Pb is a member of the 238U decay series and is produced in the atmosphere from the decay of 222Rn emanated from continental rocks and soils. 210Pb deposited onto the sea surface is rapidly associated to settling particles and accumulates in sediments, where its vertical profile is a function of sediment accumulation rate and of its physical decay. Because of its 22.3 year half-life, 210Pb provides indication on sedimentation occurring over the past 100 years. The profiles of the anthropogenic 137Cs have also been used to test and complement the information

210

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derived from 210Pb. This radionuclide was introduced into the environment mainly by atmospheric nuclear weapon testing and nuclear accidents. Its input function is well defined in the Caribbean region (Alonso-Hernandez et al., 2004), showing a maximum in 1963 and a sharp decrease in the following years, after the ban of testing explosions in the atmosphere. This paper describes and discusses sedimentological and radionuclide data to derive a first picture of the sedimentation regime in the Cienfuegos Bay and its relationship to anthropogenic activities.

155

its emissions at 662 keV. Efficiency calibration was performed using a Standard U-Ore (CANMET) and QCY44 Certified Solution (Amersham). NBS and IAEA Reference Materials were used to check the accuracy of the results. The reported uncertainty in the measured activity was calculated from the random counting error at one standard deviation level. 210 Pb and 137Cs inventories were calculated by integrating concentrations from the surface to the deepest detectable activity. Concentrations in missing intervals were linearly interpolated from adjacent measured values.

2. Materials and methods Twenty-seven surface sediment samples were collected from Cienfuegos Bay in 1999 and 2000 by a Petersen grab, collecting the top 5 cm. Three sediment cores were collected by a scuba diver, slowly pushing a tube (12 cm inner diameter, 1 m length) into the sediment. The locations of grab Samples (squares) and cores (stars) are shown in Fig. 1. Core S1999 was collected at a water depth of 19 m, while cores N1999 and N2000 at 9 and 6 m, respectively. The cores were extruded and sliced immediately after sampling in sections of 1.5 cm thick. Grain size analysis of surface samples and core sections was carried out on wet sediment. The analysis was performed on the fraction o2 mm by a Micrometrics Sedigraph 5000ET. Water content was determined drying an aliquot of the wet sediment at 80 1C. Organic matter content was calculated as ignition loss at 500 1C. All other analyses were performed on samples dried at 40 1C to constant weight. The mineralogical composition of the bulk samples and of the clay fractions was studied by X-ray diffraction analysis (Laviano and Mongelli, 1996). Sub-samples of the surface sediments and of core sections were analysed for 210Pb, 226Ra and 137Cs contents, by gamma spectrometry, using low background intrinsic germanium coaxial detectors (60% efficiency, 2.1 keV resolution at 1333 keV) coupled with a multichannel analyser. The samples were placed in sealed containers and left for three weeks before counting, to reach the 222Rn/226Ra equilibrium. 210Pb was determined via its gamma emissions at 46.5 keV, and 226Ra by the 295 keV, 352 keV and 609 keV gamma rays emitted by its daughters 214Pb and 214Bi. 137Cs was measured by

3. Results and discussion 3.1. Grain size distribution and mineralogical composition of surface sediments The grain size distribution in surface sediments (Fig. 2) is closely associated with bathymetry, distance from the main source of terrigenous materials and hydrodynamics of the bay. Sandy sediments are distributed along the coastline and in the channel connecting the bay to the Caribbean Sea. Clay and silt clay characterise the central part of the northern and southern basins. Three mineralogical provinces can be identified in the bay (Fig. 3): the sediments of the northern basin are constituted by phyllosilicates—calcite, those of the south-eastern basin by silicate components

Fig. 2. Grain size distribution in surface sediments of Cienfuegos Bay.

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Pb concentration in the water column in the northern and southern basins. The possible reasons for this different behaviour will be discussed in more detail in paragraph 3.4. (Radionuclides vertical profiles). The 226Ra distribution is homogeneous in surface sediments and does not show any correlation with median grain size, organic matter or mineralogical composition. 3.3. Mineralogical composition of the cores

Fig. 3. Mineralogical composition of the surface sediments in Cienfuegos Bay.

(quartz, feldspars and phyllosilicates) while the south-western basin is dominated by carbonate minerals. The high carbonate fraction (calcite–aragonite–calcite Mg) found in the south-western part of the bay and in the connection channel is of organic origin, as indicated by the abundance of shell fragments. 3.2. Radionuclide spatial distribution The 137Cs, 226Ra and 210Pb concentrations in the surface sediments of Cienfuegos Bay are reported in Table 1. 137Cs was the only anthropogenic radionuclide detected by gamma spectrometry. As expected, the distribution of 137Cs is mostly dependent on the grain size of the sediments (see Fig. 4). The highest values appear to be strongly correlated with the clay fraction, while the lowest or not detectable levels are associated to sand. The 210Pbex distribution in the surface sediments is shown in Fig. 5. As for 137Cs, the 210Pbex concentration increases with decreasing grain size. However, for this radionuclide, the concentrations in surface sediments from the southern basin are two times higher than in the northern one. Fig. 4 shows that, while there is a single linear correlation between the 137Cs activity and median grain size in all basin, 210Pbex shows different lineal correlations in the south and north basins. This result could indicate different adsorption processes for these radionuclides in surface sediments or different

The depth profiles of organic matter, porosity and mineralogical composition are shown in Table 2. The porosity profile (Fig. 6) shows an exponential decrease with depth, typical of uniform fine grained sediment. The content of organic matter is relatively constant along the cores and ranges between 40% and 60% in the northern cores and between 20% and 40% in the southern one. The northern basin receives most of the anthropic impact from the outfall of Cienfuegos city and of the industrial pole. The high organic matter flux to the sediments has generated basin-wide anoxic conditions that might explain the observed depauperated benthic life in the sampling areas. The textural composition of the sediments in the cores was classified as clay and silt clay. The total clay material was 70–92%, having a mean grain size of 6 mm, and was uniformly distributed along the core. The mineralogical composition of the cores reflected that of surface sediments in the same area and no significant variation in the vertical profiles was observed (Fig. 7). Only the phyllosilicate component showed an irregular distribution in the cores. In the deepest part of the cores (below 50 cm in core N1999 and below 30 cm in core S1999) the phyllosilicate content was constant and represented about 35% and 20% in N1999 and S1999, respectively. In the overlying layer, the phyllosilicate content gradually decreased in N1999, reaching 20% in the surface layer, while, in S1999 it increased to about 30%. The mineralogical analysis of the phyllosilicate fraction o2 mm showed that chlorite was the only component with different vertical distributions in the northern and southern basin. While in core N1999 its content is constant, in S1999 it was only present in the upper layer, from surface to 30 cm depth. Chlorite is only present in the drainage basins of the northern rivers (Damuji and Salado) and its presence in the southern basin of the bay only in recent times must be related to a

Depth (m)

25 3 3 3 4 14 19 12 18 6 4 3 5 3 13 2 15 6 3 18 5 3 6 6 3 3 3

Sample

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

1

dry-

2.1170.34 3.4270.45 1171.3 2573.6 7.271.1 98712 97712 110713 3.1070.37 98712 6477.9 3.0570.40 1571.8 7.370.9 3173.8 7.270.9 3874.7 1772.1 8.171.0 5176.2 1371.6 6.17 0.7 4875.9 4775.8 2973.5 1972.3 9.071.1

Pbex (Bq kg wt)

210 1

dry-

15.071.01 13.170.99 12.070.96 13.070.99 11.070.84 11.970.91 11.570.88 11.870.90 15.171.12 11.370.86 12.170.92 12.370.93 8.970 .70 8.670.66 8.970.68 7.970.60 9.770.74 9.470.72 10.370.79 10.170.77 10.470.80 9.170.71 9.870.75 9.770.74 10.270.78 9.370.71 11.270.86

226 Ra (Bq kg wt)

ND ND ND 4.0170.63 5.1270.78 6.0570.94 5.1270.78 6.3270.94 ND 5.1970.78 4.2370.63 ND 4.0870.63 ND 3.1270.47 ND 6.0770.94 2.1170.31 ND 9.0871.41 3.1070.47 ND 5.2970.78 3.1670.47 4.0970.63 4.4570.63 3.3970.47

Cs (Bq kg dry-wt)

137

1

5 10 10 25 22 25 28 30 9 23 29 6 27 8 45 8 41 35 12 49 25 10 32 51 34 17 10

OM (%)

161 140 125 6.90 88 5.55 5.07 4.59 118 5.58 6.56 122 21 119 6.12 113 6.03 24 98 4.34 30 107 6.08 7.74 5.81 19 46

Mz (mm)

3 12 19 0 12 27 22 2

22

2 13 15

18 19 18

24 0 22 18 19 0

0

3 34 28

35 20 23

QZ (%)

0 0

Phyllosilicate (%)

53

50

45

51

51

31

23

35 25 39 30 37 59

44 41

0

0

0

3

0

18

10

6 14 12 0 0 10

19 14

Calcite (%) Ca–Mg (%)

4

4

2

0

2

2

3

12 1 7 25 22 2

0 3

Feldspar (%)

Table 1 Radionuclide concentration in surface sediments at Cienfuegos Bay, as well as the organic matter (OM), median grain size (Mz) and mineralogical composition

0

6

0

3

0

44

42

1 58 8 0 0 27

32 30

Aragonite (%)

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158 8

137

Cs North 137Cs South

7

137Cs

(Bq.kg-1)

6 5 4 3 2 1 0 10

100 Mz (µm)

120

210

Pbex North Pbex South

210

210Pb ex

(Bq.kg-1)

100 80 60 40 20 0 10

southern basins. Irregularities in the 210Pbex profile were present in all cores, which contained two major non-monotonic features and lower concentrations in the upper layers. These features are conventionally interpreted as indicators of accelerated and irregular sedimentation in recent years. The 137Cs vertical profiles this trend: both in the northern and southern basin, they do not reflect the atmospheric input function for the region (Fig. 9) and show three subsurface peaks. The inventories of this radionuclide (S1999: 0.14 Bq cm 2, N1999: 0.19 Bq cm 2; N2000: 0.20 Bq cm 2) are similar at all stations and are higher than those reported in soil cores (0.12 Bq cm 2) collected from undisturbed pasture fields near the bay (Sibello-Hernandez et al., 002). This suggest that lateral sediment transport processes must have been operating in the sampling areas during the last 50 years, and that this lateral transport might also be responsible for the observed irregularities in the radionuclides vertical profiles. In contrast to 137Cs, the inventory of 210Pbex in the southern core (1.4 Bq cm 2) is about double those measured in the northern ones (0.6 and 0.7 Bq cm 2) reflecting the trend of surface concentrations. Two main hypotheses can be suggested to explain this behavior:

100 Mz (µm)

Fig. 4. 210Pbex and 137Cs activity in surface sediments plotted as a function of the median grain size.

change in the hydrological conditions. This change could be associated to regimentation of the Arimao And Caonao rivers (southern rivers) in the late 60’s and early 70’s which caused a decrease in the freshwater input to the southern basin. Consequently, the Damuji River (northern basin) assumed a main role in the hydrodynamics and particles supply to the bay. 3.4. Radionuclides vertical profiles The depth profiles of 137Cs, 226Ra and 210Pb are shown in Fig. 8, and Table 2. The vertical profiles of 210Pb (Fig. 8) suggest significant variation in the sedimentation regime of Cienfuegos Bay, both spatially and temporally. They show significant deviation from a simple exponential decline and substantial differences both in the 210Pbex surface activity and inventories in the northern and

(a) enhanced scavenging of 210Pb in the central southern basin, related to longer residence time of particles in the water column. Actually the northern stations, chracterised by a smaller inventory, are closer than st. S1999 to the main point source of particles (river mouths); (b) additional input of natural radionuclides (in particular 226Ra and daughters) to the southern basin from the Arimao river or via groundwater. In fact, high levels of natural radionuclides have been measured in soils from the Arimao catchment, related to the presence of granitic material in the zone. Moreover, levels up to 1000 Bq 1 1 of 222Rn have been reported by Valdez et al (Valde´s and Olivera Acosta, 1995, 1997) in wells of the Arimao catchment. Further studies will be necessary for testing these hyptotheses.

3.5. The chronology of sedimentation 3.5.1. Core S1999 For dating purposes, the radionuclide concentrations were also plotted versus mass depth (g cm 2),

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Fig. 5.

159

210

Pbex (Bq kg 1) distribution in the surface sediments of Cienfuegos Bay.

to avoid corrections for sediment compaction. As described above, the vertical profiles of both 137Cs and 210Pbex in all cores show irregularities suggesting that, at least in recent years, sediment accumulation has not been constant and regular. 210Pbex vertical profiles show an exponential decrease only below 11.5 g cm 2 (Fig. 10). If we apply to this layer the CFCS model, assuming constant 210Pb flux of and constant sedimentation (Appleby and Oldfield, 1978), we get a sediment accumulation rate of 0.30 g cm 2 yr 1. More complex is the situation in the upper layer: two packages of sediments with the same 210Pbex activity are present and an abrupt increase in 210Pbex concentration is observed around 10.6 g cm 2 depth (28 cm). If we assume no mixing (being all sediment anoxic) and we ignore the sediment packages with constant 210Pbex activity, we get also for this layer a sediment accumulation rate around 0.3 g cm 2 yr 1, the same as in the deeper layer. The formation of ‘‘sediment packages’’ with the same 210Pbex activity can be the result of exceptional events, when thick layers of sediment are all deposited at the same time. When the vertical profile 210Pbex is compared to that of 137Cs, it is clear that the sediment packages with the same

activity of the former perfectly correspond to two of the subsurface maxima of the latter. The major discontinuity in 210Pbex profile is located between the two deepest 137Cs maxima. Plotting together the time trend of rainfall, fallout deposition and the vertical profile of 137Cs gives an insight on the processes that caused the irregularities in the observed radionuclides vertical distribution (Fig. 9). We can assume that the deepest maximum in 137Cs profile corresponds to the maximum deposition of 137Cs from global fallout in 1963. Then, the other two maxima and the sediment layers with constant 210Pbex activity likely correspond to exceptional meteorological events that took place in the last 40 years and possibly influenced heavily the sedimentary record. In fact, the meteo record for the region indicates two main episodes: the intense rainfall in June 1988 (1000 mm of rain in 7 days) and intense rain associated to the formation of Hurricane ‘‘Camille’’ in August 1969. These exceptional events have caused a significant increase in the rivers flow rate, which might have led to re-suspension and transport to the bay of old sediments (with higher 137Cs concentration and lower 210Pbex levels), initially deposited in the river bed.

Depth (g cm 2)

0.2 0.5 0.9 1.3 1.8 2.2 2.7 3.3 3.9 4.5 5.2 5.9 6.6 7.3 8 8.8 9.3 9.7 10.1 10.6 11.1 11.5 14.5 16.3 17.8 20.6 22.4 24.2 25.9 29.4 32 35.4 44.4

0.25 0.36 0.87 1.59 3.02

Depth (cm)

S1999 1.5 3 4.5 6 7.5 9 10.5 12 13.5 15 16.5 18 19.5 21 22.5 24.5 25.5 26.5 27.5 28.5 29.5 30.5 36.5 39.5 42.5 47 50 53 56 62 66.5 72.5 87.5

N1999 1 2.5 5.5 7 11.5

1

dry-wt)

48.0373.60 54.4573.90 42.3274.00 38.5674.60 39.0077.00

11076.05 9975.45 9575.23 9275.06 9175.01 8974.90 7073.85 7073.85 7073.85 7073.85 6573.58 6073.30 5773.14 5573.03 4772.59 3972.15 3972.15 5372.92 6673.63 6173.36 5773.14 5673.08 4272.31 3670.09 2773.80 2374.72 1773.49 1573.08 1472.87 871.68 771.44 4.370.88 1.770.35

Pbex (Bq kg

210 1

dry-wt)

7.9970.72 8.6770.71 9.0370.74 10.0271.23 9.3470 .76

11.0270.49 10.7270.47 10.9270.49 12.970.49 12.370.47 10.2670.50 9.2570.48 11.6670.49 12.2170.65 10.9270.46 11.3170.49 8.7470.49 10.8370.50 8.7770.44 8.7170.54 10.1070.44 10.2170.41 11.5170.43 10.3370.54 11.1970.51 12.2070.51 10.7870.58 11.7170.52 11.3270.60 10.2470.61 10.5670.56 10.7870.59 12.1670.64 11.2170.58 11.9170.63 10.9270.56 10.7870.56 12.1670.67

226 Ra (Bq kg 1

dry-wt)

3.6070.47 3.8070.42 4.0070.51 3.4070.52 3.0070.47

5.0370.56 4.5170.51 5.2370.59 5.3270.60 5.0870.57 6.5670.73 7.0670.79 13.0271.46 13.2371.48 9.0071.01 10.2371.15 10.4371.17 11.5671.29 12.2371.37 11.2971.26 15.0071.68 15.0071.68 11.5671.29 12.0171.35 16.0371.80 16.0371.80 11.2171.26 7.4570.83 5.3470.60 3.4570.39 1.0970.12 ND ND ND ND ND ND ND

137 Cs (Bq kg

0.94 0.93 0.91 0.90 0.90

0.77 0.78 0.79 0.77

0.77 0.79 0.79 0.79 0.77

0.77

0.90 0.89 0.88 0.87 0.85 0.85 0.84 0.83 0.81 0.80 0.78 0.79 0.78 0.78 0.78 0.79 0.78 0.77 0.77 0.78

Porosity

6.00 5.66 5.25 5.04 4.74 5.85 6.03 5.87 5.02 5.33

6.02

24 24 24 22 29 19 23 33 23 30

28

6.07 5.73 6.12

36 44

6.08 5.46 6.00

45

24 25 22

5.58 5.07 5.81 5.07 5.22 5.43 5.34 5.73

6.15

24

23 23 22 18 24 26 19 18

5.22

Mz (mm)

24

OM (%)

67

53

44 43 40

42

41

6 8 7

6

6

2

3 2

35 36 37

4

6

6

6

Ca–Mg (%)

34

35

36

35

Calcite (%)

15

18

15 16 17

17

18

18

15 17

15

18

19

17

QZ (%)

15

23

17 16 17

17

18

29

35 32

35

30

25

32

Phyllosilicate (%)

Cs, porosity, organic matter (OM), median grain size (Mz) and mineralogical composition data in cores at Cienfuegos Bay

137

Ra,

226

Table 2 210 Pbex,

2

4

14 12 14

15

14

13

11 12

11

11

13

11

Feldspar (%)

3 5 5

2

2

1

1 1

1

1

1

1

Aragonite (%)

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4.18 4.92 5.55 6.5 7.22 7.87 8.61 9.23 11.14 11.71 12.89 13.78 15.24 15.94 17.03 17.58 19.1 21.47 23.08 23.79 25.3 28.86 31.58 37.78

0.25 0.54 0.87 1.59 2.45 3.57 4.18 4.92 5.55 6.5 7.22 7.87 9.23 9.83 11.14 11.71 12.26 12.89 13.78 14.41 15.24

14.5 16 17.5 19 20.5 22 23.5 25 29.5 31 34 35.5 38.5 40 42.5 44 47 51.5 54.5 56 59 66.5 73.5 87.5

N2000 1 4 5.5 7 10 13 14.5 16 17.5 19 20.5 22 25 26.5 29.5 31 32.5 34 35.5 37 38.5

55.7676.00 51.2375.98 48.4477.00 48.3777.00 43.1476.00 38.0175.00 38.7575.19 38.7875.65 40.3074.67 38.5974.59 35.3474.97 32.0075.00 28.3374.00 35.0076.00 48.0076.00 45.0175.34 40.1676.00 38.0076.00 36.0075.00 28.1674.00 32.0075.00

37.4774.50 36.5674.30 36.3274.30 25.6773.00 28.0073.00 29.4173.50 24.1373.00 22.9072.70 22.3472.70 22.7572.60 22.4772.60 22.1373.30 40.3174.00 38.9173.90 30.2674.00 26.3573.10 25.5572.60 21.2272.80 17.3872.90 15.0071.80 11.3470.94 7.4071.00 3.9071.00 1.7070.50 8.5670.76 8.8270.78 9.0270.82 8.4570.75 7.5670.67 8.1570.73 7.9970.71 8.6770.77 9.0370.81 11.2171.00 9.3470.83 8.4570.75 9.0170.80 7.9270.71 9.1470.81 8.9970.81 8.6170.77 8.2370.73 8.7070.77 7.9170.71 9.3470.83

8.4570.74 9.0170.73 7.9270.80 9.1470.76 8.9970.79 8.0270.74 7.4570.80 7.9970.77 8.9370.80 8.4770.77 9.0170.73 8.5670.77 8.3470.77 8.2570.83 8.5670.75 8.7070.80 7.9170.76 9.3470.73 8.5670.83 8.1770.65 8.2370.83 8.9170.75 8.3470.81 9.0370.76 5.2370.55 4.9970.51 6.0970.66 6.3870.66 6.2170.66 5.0170.55 10.5571.16 11.5671.27 9.8571.08 10.1271.11 5.7070.63 4.1170.45 6.6670.73 12.7171.40 14.5371.60 16.0071.76 16.5771.82 15.0071.65 10.1171.11 8.0070.88 10.0071.10

4.0070.59 5.8070.70 6.6070.80 4.4070.65 4.0070.59 3.0070.47 1.6070.28 1.1070.10 1.9070.10 2.1070.20 7.3070.90 9.4071.10 14.071.00 15.071.00 7.4071.10 9.0071.30 16.071.90 15.071.80 10.070.90 4.7070.60 3.1070.40 ND ND ND 0.94 0.91 0.87 0.86 0.85 0.85 0.85 0.84 0.83 0.83 0.83 0.84 0.84 0.84 0.84 0.84 0.84 0.84 0.84 0.84 0.84

0.89 0.90 0.87 0.87 0.87 0.87 0.87 0.88 0.89 0.88 0.87 0.89 0.88 0.88 0.88 0.89 0.87 0.89 0.88 0.88 0.88 0.88 0.89 0.89

5.22 5.74

5.66

5.28

5.78

55 47

54

55

56

5.34 4.9

36 38 6.5

4.93

35

53

50

5.42 6.08

41

27

47

5.74

35

50

4.89

5.37

50

29

16

17

18

36

28

30

22

6

4

3

9

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Mz (mm)

6.00

5.98

43

48

16.9671.86 16.1771.78 15.9271.75 11.0071.21 8.0070.88 4.0070.44 2.0070.22 ND ND ND ND ND 8.4770.75 9.0170.80 8.5670.76 8.9170.79 8.3470.74 9.0370.80 8.1070.72 7.9970.71 8.3470.74 8.2570.73 8.5670.76 8.3070.74 15.94 16.77 17.03 18.39 19.1 20.5 22.26 25.99 27.46 30.29 33.6 36.91 40 41.5 42.5 45.5 47 50 53 60.5 63.5 70.5 78 85.5

23.0074.00 25.1374.50 23.0074.32 21.9074.08 21.0074.00 17.1473.00 15.0074.00 8.3572.50 6.5071.60 4.2070.60 3.1070.50 1.3070.35

Depth (g cm 2) Depth (cm)

Table 2 (continued )

210

Pbex (Bq kg

1

dry-wt)

226

Ra (Bq kg

1

dry-wt)

137

Cs (Bq kg

1

dry-wt)

0.83 0.83 0.82 0.82 0.82 0.8 0.82 0.83 0.83 0.82 0.82 0.82

Porosity

OM (%)

Calcite (%)

Ca–Mg (%)

QZ (%)

Phyllosilicate (%)

Feldspar (%)

Aragonite (%)

162

The major discontinuity in 210Pbex profile can then be dated between 1963 and 1969 (Fig. 9). The discontinuity also corresponds to a significant change in the mineralogical composition of the sediment: at this depth, the fraction of phyllosilicate increases from 18% to 35% and chlorite minerals are first detected, indicating the beginning of a contribution of the northern rivers to sedimentation in the southern basin. This is likely related to a change in the hydrological regime of the bay in the late 1960s and early 1970s, following the regimentation of the Arimao and Caonao rivers, which caused decrease of freshwater input (approximately 50%), particle load and associated radionuclides to the southern basin. In the same period, the intensive deforestation for agricultural uses of the areas drained by the northern rivers, might have greatly increased the amount of particulate material transported to the bay. As a result, 210Pbex concentration at station S1999 decreased, while sediment accumulation remained constant, because the diminished particle supply from the southern rivers was balanced by the new input from the northern part of the bay. 3.5.2. Northern cores The vertical radionuclide profiles in the northern cores (Fig. 11) are very similar to that in the southern one and, to derive sediment accumulation rates, the same assumptions can be made. In the northern cores, the main discontinuity in the 210Pbex vertical profile is at a depth of 11–15 g cm 2 (30–40 cm). Also in these cases, two layers with different sediment accumulation rates were identified. The CFCS model indicated, for the deepest layer, sedimentation rates of 0.26 g cm 2 yr 1 and 0.27 g cm 2 yr 1 for the N1999 and N2000, respectively. Again, the relative maximum in 210Pbex profiles is located between the two deepest peaks in the 137Cs profile, associated to the maximum fallout deposition in 1963 and to the intense rain in 1969. Between these dates the sedimentation regime in the area has changed. Core N2000 in the upper layer shows a regular decline of 210Pbex yielding a sediment accumulation rate (CFCS model) of 0.50 g cm 2 yr 1, almost double that calculated for the deepest layer (Fig. 11). In the same time period, sediment deposition is less regular at station N1999: thick sediment layers with constant 210Pbex activity correspond to the peaks in 137Cs profile and can be attributed to the effects of the meteorological

ARTICLE IN PRESS C.M. Alonso-Hernandez et al. / Continental Shelf Research 26 (2006) 153–167 Organic Matter (%) 20 40 60

80

0

Organic Matter (%) 20 40 60

0 0

10

10

10

20

20

20

30

30

30

40

Depth (cm)

0

40 50

50

N1999

N2000

70

70

80

80

80

Porosity

Porosity 0.9

1.0

0.7

0.8

Porosity 0.9

1.0

0.7 0

10

10

10

20

20

20

30

30

30

50

Depth (cm)

0

Depth (cm)

0

40

40 50

60

80

80

0.9

1.0

40 50

N1999 70

0.8

60

60 S1999

70

80

40

70

0.8

Organic Matter (%) 40 60

60

S1999

0.7

20

50

60

60

Depth (cm)

80

0

Depth (cm)

Depth (cm)

0

163

N2000 70 80

Fig. 6. Depth distribution of organic matter and porosity in the core samples.

events in 1988 and 1969. Moreover, the slow decrease of 210Pbex activities in the upper layer indicates an increase in the sediment accumulation rate also at this station, reaching a sedimentation rate of 0.43 g cm 2 yr 1. In this area, the discontinuity in the 210Pbex vertical profile also corresponds to a change in the phyllosilicate fraction, that at this depth starts decreasing towards the surface. 4. Summary and conclusions The vertical radionuclide profiles and the mineralogical analyses show that in the last 40 years significant changes have affected sedimentation processes in the Bay of Cienfuegos. In the first half

of 1900, sediments were regularly accumulated both in the northern and southern basins, at a rate around 0.3 g cm 2 yr 1. In the following period, the radionuclides vertical profiles significantly changed. Although the two cells of the bay are separated by a shallow ridge and receive particles input from different sources, the changes are strongly correlated in the two basins and closely timed (1963–1969). In recent years, the sediment accumulation rate almost doubled in the northern basin and the incidence of extreme meteorological events is marked by the deposition of uniform thick sediment layers. The northern rivers have assumed a dominant role in the bay and, after 1963, they have started to

ARTICLE IN PRESS 164

C.M. Alonso-Hernandez et al. / Continental Shelf Research 26 (2006) 153–167

Fig. 7. Depth distribution of mineralogical composition in the S1999 and N1999 cores.

supply fine particles also to the southern basin, as marked by the appearing of chlorite in the southern core.

All environmental changes are strictly correlated to substantial changes in land use occurred in the area in the early sixties: large de-forestation

ARTICLE IN PRESS C.M. Alonso-Hernandez et al. / Continental Shelf Research 26 (2006) 153–167

Fig. 8. Total activity of

210

Pb,

226

Ra and

for a agricultural uses in the drainage basins of the northern rivers Damuji and Salado and regimentation of the southern rivers Caonao and Arimao.

165

137

Cs in sediment cores from Cienfuegos Bay.

The high background levels of natural radioactivity in the Arimao catchment, could justify the existence of lateral and/or underwater sources of 226 Ra and daughters in the southern basin. This

ARTICLE IN PRESS C.M. Alonso-Hernandez et al. / Continental Shelf Research 26 (2006) 153–167

166

25 0.43 g.cm-2.y-1

N1999

10

Cs (Bq.kg-1)

15 10

137

0.26 g.cm-2.y-1

120

Pbex (Bq.kg-1)

20

5

210

Pbex

137

Cs

1

0 0

5

10

15

20

25

30

35

40

45

Depth (g.cm-2) 30

0.50 g.cm-2.y-1

N2000

15 10 137 210

1 0

5

10

15

20

25

30

35

Cs Pbex

40

Cs (Bq.kg-1)

10

137

20 0.27 g.cm-2.y-1

210

Pbex (Bq.kg-1)

25

5 0 45

Depth (g.cm-2)

Fig. 9. Cienfuegos Bay. (a) Precipitation (mm yr 1); (b) 137Cs fallout (Bq m 2 yr 1); (c) 137Cs vertical profiles in the corer S1999 (Bq kg 1 mass depth 1, g cm 2).

Acknowledgements

20

The authors wish to thank Dr. Massimo Setti from the Earth Science Dept. of the University of Pavia for the mineralogical analysis. One of the authors, C.M. Alonso Hernandez, undertook this work with the support of the ‘‘ICTP Programme for Training and Research in Italian Laboratories’’ (Trieste, Italy).

10

210

10

Cs (Bq.kg-1)

15 0.30 g.cm-2.y-1

137

0.30 g.cm-2.y-1 Pbex (Bq.kg-1)

25 S1999

0.32 g.cm-2.y-1

100

Fig. 11. Core N1999 and N2000: 210Pbex and 137Cs vertical profiles. Results are expressed in Bq kg 1 versus both mass depth (g cm 2).

5

210

Pbex

137

Cs

1

0 0

10

20

30

40

Depth (g.cm-2)

Fig. 10. Core S1999: 210Pbex and 137Cs vertical profiles. Results are expressed in Bq kg 1 versus both mass depth (g cm 2).

could explain the significant differences in 210Pbex inventories between the northern and southern part of the bay. Future studies will be carried out to better explain these differences.

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