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206 L. TRANCHINA et al.

7 min. The mixture was then kept at 106 8C for 5 min, at 120 8C for 4 min, at 140 8C for
2 min and at 180 8C for 2 min. Digested samples, filtered through 0.2 mm pores, were
cooled and diluted to 25 ml with distilled and deionized water. All the glassware was
cleaned by soaking in a 10% HNO3 solution for 24 h, then rinsed three times with deionized
water to prevent any contamination. All the chemical reagents were of Merck suprapure
grade.

Zn, Cu, Cd, and Pb concentrations were measured using a Varian AA20 Plus FAAS. The
spectrophotometer was equipped with a deuterium background corrector and an ACT 80
atom concentrator tube, which was positioned in the air/acetylene flame to make the sensi-
tivity two to three times greater than that of a standard FAAS. The detection limits of Zn, Cu,
Cd, and Pb are 0.6, 0.6, 0.25, 0.9 mg g21, respectively.

The National Research Council of Canada PACS-2 marine sediment was used as a certi-
fied reference material for sediment measurements. The Community Bureau of Reference
(European Union) RM 60 (Lagarosiphon major) was used as a certified reference material
for P. oceanica measurements.

2.4 Radioactivity Measurements

Gamma activity counts of 40K, 228Ac, 214Bi and 137Cs were carried out by high-resolution
gamma spectrometry using three different HPGe detectors. The HPGe detector used to
analyze rhizomes and sediments had an operability range of 150 –10,000 keV, a relative effi-
ciency of 32% and a resolution of 1.8 keV at 1.33 MeV. A second detector, with an opera-
bility range of 5– 400 keV and an active area of 20 cm2, was used to detect 46.5 keV gamma
rays emitted by 210Pb in rhizome samples.

The first detector was calibrated against a standard soil (SRM4353 from the National
Bureau of Standards) containing all the radionuclides of interest except 210Pb. The second
detector was calibrated for 210Pb against a standard source provided by the Italian Laboratory
for the Metrology of Ionizing Radiations. A third low-background germanium detector was
used for scale analysis. This was an Ortec GMX-50220, with an operability range of 3–
10,000 keV, a relative efficiency of 51%, 1.99 keV at 1.33 MeV and 658 eV at 5.9 keV
resolutions. The same sources as those above were used for calibration.

Data were corrected to take into account different sample densities (which, for sediments,
ranged between 0.4 and 1.6 g cm23; Cutshall et al., 1983; Rizzo and Puccio, 1992).

The small amount of Posidonia available for activity determinations, about 8 – 15 g dry
weight for each sample, required a counting time of 3.5 Â 105 s to yield (and only in a
few cases) values of specific activity above the detection limit (Currie, 1968).

About 150– 200 g for each sample of sediment was used for radioactivity determination;
the counting time was 2.5 Â 105 s.

3 RESULTS AND DISCUSSION

The geochemical features of the sediments for major mineralogical composition, obtained by
XRF, are listed in Table I. These refer to hydrated samples. The mineralogical data obtained
by XRD are listed in Table II. The values shown in the two tables are the mean values and
ranges (in parenthesis) of percent concentration for three samples at the same depth.

The XRF and XRD data of the sediments show a fairly homogeneous percentage of CaO,
indicating a common carbonate lithology; statistical differences have been tested using the
standard F-test, showing a P-value of 0.2765 for a 95% confidence level (see, for instance,