Rapp. Comm. int. Mer Médit., 36,2001
120
Introduction
137
Cs is an anthropogenic radionuclide, conservative in the open
sea. Its main sources to the Mediterranean area are fallout from past
nuclear weapon testing and the Chernobyl accident in 1986. From
1963 to 1986, the radionuclide concentration in surface water has reg-
ularly decreased, re?ecting the decrease of the atmospheric input and
vertical transport processes. The
137
Cs vertical profiles were charac-
terised by decreasing concentrations from surface to bottom. In the
Eastern Mediterranean, the only vertical profiles before the Deep
WaterTransient were obtained by Fukai et al. (1) and Livingston,
(unpublished) in the period 1975-77.
137
Cs surface concentrations
ranged between 4.2 and 5.5 Bq m
-3
over the entire basin. The profiles
demonstrated subsurface maxima in the upper 400 m and an exponen-
tial decrease towards the bottom. The concentrations in deep Ionian
Sea (0.3-0.5 Bq m
-3
) were about double those of the Levantine Sea. It
is reasonable to assume that the spatial distribution of
137
Cs was sim-
ilar to that of tritium in 1978 (2), which showed concentrations below
1500 m decreasing from the Western Ionian Sea towards the Levantine
basin. These distributions re?ected the circulation in the Eastern
Mediterranean, with tracer-rich dense waters of Adriatic origin
(Eastern Mediterranean Deep Water, EMDW) ?owing first into the
bottom of the Western Ionian Sea and then spreading eastward. In
1986 the fallout from the Chernobyl accident produced a sharp
increase in
137
Cs concentration at the surface of the Eastern and
Northern basins. Pre-Chernobyl levels were reached again in 1990. In
1995 and 1999 two sampling campaigns have been carried out in the
Eastern Mediterranean to determine the recent distribution of
137
Cs
and its relation to the Chernobyl input and to the water circulation in
the area.
Materials and methods
The location of the sampling stations is shown in Fig. 1. After CTD
casts, water samples (60-100 l) were collected in different water mass-
es, to determine
137
Cs concentration.
137
Cs was pre-concentrated
onboard by co-precipitation on ammonium molibdo-phosphate (AMP)
at pH 1.5. 134Cs was used as yield determinant.
137
Cs was determined
by gamma spectrometry with high purity germanium detectors (rela-
tive efficiency 60%, resolution 2.1 keV at 1332 keV). The accuracy of
the results is regularly checked by participation in IAEA intercompar-
ison exercises.
Results and discussion
The 1995 campaign (UNESCO-POEM) found a dramatic change in
the thermohaline circulation of the E-Mediterranean: a new type of
water originating in the Aegean Sea, the CSOW, was ?owing through
the Cretan Arc straits and spreading throughout the bottom layer of the
entire Levantine basin, ?owing to the West and forcing upward the old
deep water of Adriatic origin (3). The vertical profiles of
137
Cs at the
two sides of Crete (St. 3 and 4) showed that the CSOW was marked by
relatively high concentrations of
137
Cs (2-2.5 Bq m
-3
). The most
recent vertical profiles in the Ionian and Levantine Seas (St. 1 and 2)
showed surface concentrations around 3 Bq m
-3
and, as in 1977, a sub-
surface maximum in the LIW. However, in the bottom layer, they sig-
nificantly differed from the old profiles. In the Ionian Sea, minimum
concentrations (1-1.5 Bq m
-3
) were found in the depth interval 750-
1500 m, followed by an increase up to 2.5 Bq m
-3
from 2000 m to the
bottom. The deep layer was still characterised by the presence of
EMDW, and the increase in
137
Cs concentration was due to its contin-
uous transport from surface to bottom through convection processes in
the Adriatic Sea. In the Levantine Sea (St. 2) minimum concentrations
(1-1.5 Bqm
-3
) characterised a larger depth interval, from 750 to 2000
m, but below this depth, where salinity and temperature data indicated
the intrusion of CSOW, the levels increased reaching 2.5 Bq m
-3
near
the bottom. The inventory of
137
Cs at these two locations has more
than doubled with respect to 1977, indicating an efficient transport
from surface water and from the shelf areas to the deep sea.
In 1999, a new sampling campaign (SINAPSI) covered 4 stations
along a transect from the Malta sill to the western side of Crete. The
vertical profiles of
137
Cs were very similar at all stations (Fig. 2):
almost constant concentrations (3-3.5 Bq m
-3
) from surface to 300 m,
decrease to a minimum value of 1-1.5 Bq m
-3
around 1000 m and a
new increase to an average value of 2.5 Bq m
-3
from 1500 m to the
bottom. Salinity and temperature profiles showed that the deep layers
at the twoWestern Ionian stations (St.6 and 1) were characterised by
EMDW, while at the other two stations CSOW dominated. As
137
Cs
concentration was very similar in the two types of deep water, a
marked homogenisation of the tracer concentration in the whole
Ionian Sea is presently observed. Most of the
137
Cs inventory is now
confined in the deeper part of the Eastern Mediterranean.
References
1- Fukai R., Ballestra S., Vas D., 1980. Distribution of caesium-137 in
the Mediterranean Sea. pp. 353-360 In:Management of Environment,
Wiley Eastern Ltd., New Delhi.
2- Roether W. and Schlitzer R., 1991. Eastern Mediterranean deep water
renewal on the basis of chloro?uoromethane and tritium data. Dynamics
of the atmosphere and the oceans, 15: 333-354.
3- Klein B., Roether W., Manca B.B., Bregant D., Beitzel W., Kovacevich
W. and Luchetta A., 1999. The large deep water transient in the Eastern
Mediterranean.Deep Sea Res.I 46: 371-414.
THE NEW DISTRIBUTION OF THE TRACER
137
CS IN THE EASTERN MEDITERRANEAN:
RELATIONSHIPTOTHE DEEP WATER TRANSIENT.
R. Delfanti*, C. Papucci, S. Salvi, L. Torricelli
ENEA-CRAM, La Spezia, Italy - delfanti@santateresa.enea.it
Abstract
In the late seventies, the vertical profiles of the conservative tracer
137
Cs in the Eastern Mediterranean showed a relative maximum in the
Levantine Intermediate Water (LIW) and an exponential decrease towards the bottom. In the late nineties, the distribution of
137
Cs was
characterised by lower surface concentrations and higher levels at depth, related to efficient transfer of the tracer by convection processes.
Moreover, the input of the new Cretan Sea Over?owWater (CSOW) from the Aegean Sea produced a marked homogenisation of
137
Cs
concentrations all over the Ionian Sea below 1500 m.
Keywords: Eastern Mediterranean, radionuclides, deep water.
Fig.1 – Sampling points
Fig. 2 –
137
Cs distribution along a West-East section in the
Ionian Sea (1999).
