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Implications for the 210Pb Geochronology: CF – constant flux model, CFCS- constant flux/constant sedimentation
model
The constant flux (CF, also known as CRS model) 210Pb chronology agrees well with the proposed 239,240Pu, 241Am and 137Cs onset fallout, when taking into account a slump event in 1982.
The vertical distribution of the Pu and Am radionuclides in the sediment core appear disturbed with not sufficient
resolution to show a well developed peak corresponding to the expected radionuclide fallout maximum in 1963.
Despite the potential for 137Cs mobility in the sediments, there is no evidence of 137Cs penetration below the depth of
onset fallout of Pu.
The total inventory of 239,240Pu in the core (60 Bq m-2) is about two to four times higher than the expected Pu inventory
(15 - 35 Bq m-2) from the NWT fallout in Havana and in other nearby regions. The high 239,240Pu inventory in the
sediments reflects then that the Havana Bay is capturing particle-reactive materials, and therefore has a high
sedimentation rate.
Pu radioisotopes and 241Am as alternative chronostratigraphic markers in tropical
regions: An application in Havana Bay(Cuba)
J.A. Corcho Alvaradoa, *, M. Diaz-Asenciob, C.M. Alonso Hernàndezb, P. Froidevauxa, F. Bochuda, J.A. Sanchez-Cabezac, d
a Institute of Radiation Physics (IRA), University of Lausanne – CHUV, Lausanne, Switzerland b Centro de Estudios Ambientales de Cienfuegos (CEAC), Cienfuegos, Cuba
c Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain.d Institute of Environmental Science and Technology, and Physics Department, Universitat Autónoma de Barcelona, Barcelona, Spain.
ABSTRACT: The low 137Cs activity observed in marine sediments of tropical regions often precludes its use as chronostratigraphic marker. Here we
present a study on the use of Pu and Am radioisotopes as alternative markers to constrain the 210Pb ages in a sediment core of the Havana Bay (Cuba).
Mean activity ratios of 238Pu/239,240Pu, 241Am/239,240Pu and 241Pu/239,240Pu indicated that the nuclear weapon tests (NWT) fallout is the main source of the
anthropogenic radionuclides. While the inventory of 137Cs in the sediments is lower than the expected fallout inventory, 239,240Pu accumulates in the
sediments with inventories higher than the expected fallout inventory. The high fluxes of 239,240Pu are nevertheless corroborated here through use of210Pb, and confirm that focusing of solid particles is of great importance in the investigated site. 239,240Pu showed to be a useful time tracer in marine
sites where the 137Cs signal is very low.
CONCLUSIONS
1. The Pu, Am and Cs distribution profiles in a sediment core from
the Havana Bay are characteristic for: (a) not showing the peaks
corresponding to the maximum fallout deposition (1963) and (b)
presenting homogenous concentrations in the mid- layers of the
core (3-15 cm), and decreasing concentrations in the sections
above and below.
2. Pu and Am measurements by alpha spectrometry showed a
greater sensitivity than 137Cs measurements by gamma
spectrometry, allowing a better identification of the first
appearance of the NWT fallout signal in the sediments.
3. The 238Pu/239,240Pu and 241Am/239,240Pu activity ratios indistinctly
showed that the origin of the regional Pu contamination is the
global fallout of the NWT.
4. The high fluxes of 239,240Pu and 210Pb estimated for the site
confirm that focusing of solid particles is of great importance in
the investigated site of the bay. This process has possibly an
important impact on the application of the simple 210Pb models.
Cuba
Havana Bay
1974
1961
1946
19261912
0
1
2
3
4
5
6
1890 1910 1930 1950 1970 1990 2010
MA
R (
kg
m-2
y-1
)
Age (A.D.)
Hurricane1924-1926
Cyclone1910
Hurricane1944
Heavy rains1957 1965 1972 1982
Slump
1890
1910
1930
1950
1970
1990
2010
0 40 80 120 160 200
Ag
e (
A.D
.)
Massic depth (kg m-2)
CFCS model
Onset fallout (1952)
2 CFCS models
CF model
a
Onset fallout
0
5
10
15
20
25
30
35
1 10 100 1000
Dep
th (cm
)
log Activity (Bq kg-1)
Po-210
Ra-226
0
5
10
15
20
25
30
35
0.0 1.0 2.0
Activity (Bq kg-1)
137Cs
0
5
10
15
20
25
30
35
0 200 400 600Activity (Bq kg-1)
241Am
a cb
onset fallout
Figure 2. Profile of 239,240Pu and 241Am
in a sediment core from Havana Bay
239,240Pu 241Am
High sensitivity measurements by α-spectrometry.
239,240Pu and 241Am activities ranged
between 0.06 - 0.50 Bq/kg, and 0.04 - 0.16
Bq/kg.
Low mobility in the sediments
(Particle-reactive)
Relatively low measurement uncertainties
Complexpreparation of the
source before measurement The first appearance
of Pu and Am in the sediments (onset
fallout) is found at ca. 23 cm.
137Cs
γ-ray at 662 keV, easily detected by γ-spectrometry. It can be measured concurrently with
210Pb.
In the Caribbean region, NWT fallout is about two-three times lower than in
the 40o-50o N
The onset of 137Cs is found at 22cm,,
although it is difficult to
positively identify it
Potential mobility in the sediment
profile, especially in saline sediments
The 137Cs signal is very low (< 1.8 Bq/kg). Relatively high measurement uncertainties
associated with low 137Cs activities
Due to its relative short half-life (30 yr), about
70% of the fallout deposited in the
sediments has already decayed.
0
5
10
15
20
25
30
35
0.0 1.0 2.0Activity (Bq kg-1)
137Cs
Figure 1. Profile of 137Cs in a sediment
core from Havana Bay
1. Mean 238Pu/239,240Pu and 241Am/239,240Pu activity ratios of 0.04
and 0.3, respectively, indicate that the origin of the regional Pu
and Am contamination is the global fallout of the NWT.
2. 239,240Pu/137Cs activity ratios in the sediments range from 0.2 to
0.4, which are much higher than those expected (0.03-0.04) from
global fallout after decay correction. The high 239,240Pu/137Cs
activity ratios in the sediments may be explained by the different
particle affinities and biological uptake of these radionuclides.
Due to their high particle affinity, there is a preferential removal
of Pu isotopes from the water column to the sediments.
0.00
0.02
0.04
0.06
0.08
0.0 0.2 0.4 0.6 0.8
23
8P
u/2
39
,24
0P
u
241Am/239,240Pu
Typical global NWT fallout238Pu/239,240Pu: 0.02 - 0.04241Am/239,240Pu: 0.2 - 0.4
Pu = 0.31 (Cs)
R² = 0.15
0
0.1
0.2
0.3
0.4
0.5
0.6
0.0 0.5 1.0 1.5 2.0
239,2
40P
u (B
q/k
g)
137Cs (Bq/kg)
ACKNOWLEDGMENT
We would like to acknowledge the financial supports of the IAEA,
through the international project RLA7012 “Use of Nuclear
Techniques to Address the Management Problems of Coastal Zones
in the Caribbean Region“, and of the Swiss Federal Office of Public
Health.
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