Sediment reworking rates in deep sediments of the Mediterranean Sea.

Different pelagic areas of the Mediterranean Sea have been investigated in order to quantify physical and biological mixing processes in deep sea sediments. Herein, results of eleven sediment cores sampled at different deep areas (> 2000 m) of the Western and Eastern Mediterranean Sea are presented. ²¹°Pb(xs) and ¹³7Cs vertical profiles, together with ¹4C dating, are used to identify the main processes characterising the different areas and, finally, controlling mixing depths (SML) and bioturbation coefficients (D(b)). Radionuclide vertical profiles and inventories indicate that bioturbation processes are the dominant processes responsible for sediment reworking in deep sea environments. Results show significant differences in sediment mixing depths and bioturbation coefficients among areas of the Mediterranean Sea characterised by different trophic regimes. In particular, in the Oran Rise area, where the Almeria-Oran Front induces frequent phytoplankton blooms, we calculate the highest values of sediment mixing layers (13 cm) and bioturbation coefficients (0.187 cm² yr⁻¹), and the highest values of ²¹°Pb(xs) and ¹³7Cs inventories. Intermediate values of SML and D(b) (~6 cm and ~0.040 cm² yr⁻¹, respectively) characterise the mesothrophic Algero-Balearic basin, while in the Southern Tyrrhenian Sea mixing parameters (SML of 3 cm and D(b) of 0.011 cm² yr⁻¹ are similar to those calculated for the oligotrophic Eastern Mediterranean (SML of 2 cm and D(b) of ~0.005 cm² yr⁻¹).

Anthropogenically derived mercury in sediments of Pialassa Baiona, Ravenna, Italy.

Pialassa Baiona, a coastal wetland near the city of Ravenna, has been impacted for two decades (1958-1978) by industrial discharges containing mercury and floating agglomerates of residues of polymerization processes. Although the industrial use of mercury completely ceased in the early 90's, surface sediments are still highly contaminated, mercury concentrations decreasing from the southern sub-basin, close to the discharge point, to the farthest northern border. Concentrations of total mercury, synthetic polymers (determined by pyrolysis-GC/MS), total organic carbon, C/N ratio, total sulfur and 210Pb dating, were determined in sediment cores collected in the southern and northern sub-basins. Mercury and polymers exhibited parallel profiles with a peak corresponding to the historic emission record in the southern core, while in northern cores peaks of maximum concentration display a delay reflecting the time required for the pollutants to migrate. A recently developed mercury sequential extraction procedure was applied to the most polluted layers to study inorganic mercury speciation. Results indicate differences between the southern and northern areas, suggesting a more efficient binding of mercury to sediments in the southern sub-basin.

Sediment composition and normalisation procedures: an example from a QUASH project sediment exercise.

The QUASH UE-Project was designed to assess the reliability of normalisation approaches to compensate the influence of natural process affecting the distribution and concentration of contaminants in sediment. The focus of this paper was to test the influence on normalisation procedures of an inorganic matrix using a sample collected in the Venice Lagoon, Italy.

Critical Aspects of Sedimentary Phosphorus Chemical Fractionation.

Sequential leaching procedures have been widely used as an environmental tool to quantify sedimentary P reservoirs. Nevertheless, they still present important issues that need to be investigated such as readsorption of leached ions and organic P hydrolysis. Bottom sediments of a lagoonal environment were analyzed following some selected extraction procedures that are based on chelating agents at the same pH as the sediment (GB) or a mixture of reducing and acid extractants (AAC and SEDEX). Readsorption was estimated by washes with MgCl2 after each extraction, and turned out to be significant in the step of the SEDEX sequence that quantifies the authigenic Ca-bound P. Here, readsorbed P accounted for up to 46% of the fraction. In contrast, in the GB sequence readsorbed P accounted for only 1.7 to 4.3% of the Fe-bound P and Ca-bound P. The AAC and SEDEX sequences yielded relative organic P contents ranging from 24.5 to 30.5% of the total P, and the GB sequence yielded values of 41.3 to 50.7%. Despite the large difference between the two sequences, the relative proportions among the samples were consistent. The difference may result from the combination of hydrolysis sedimentary organic P by some extractants (mostly citrate dithionite bicarbonate [CDB] and HCl) in the AAC and SEDEX sequences, and of incomplete dissolution of Ca-bound phases by the relevant chelating agent (Na-EDTA [ethylene dinitrilo tetracetic acid]) in the GB sequence. In fact, when detrital apatite of magmatic origin was used as an analogue phase, only about 48% of it was leached during two Na-EDTA extractions. The undissolved detrital apatite was therefore wrongly attributed to the organic P reservoir and produced values of organic P overestimated by 2.9 to 8.8%. The organic P values obtained with the different sequences, although corrected for this effect, still showed differences of 5.4 to 13.9%, that were attributed to the hydrolysis of labile organic compounds. These results point out that the best sequence to use should be chosen taking into account the peculiarities of each sequence and the contents of detrital apatite and organic matter of the sediments under study.