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.