*4.4. Core Sediments*

Organic carbon content varied widely with depth and ranged from 2.4% to 11.9% (Figure 7). The lower values found in the sediments below 25 cm depth, indicate the decomposition of organic matter with time, while the higher values found within the 4–20 cm sediment interval imply buried organic matter not ye<sup>t</sup> degraded [56]. High OC contents result in high oxygen consumption and, subsequently, in the establishment of sub-oxic, anoxic, and/or sulfidic conditions in the sub-surface sediments. Black-gray bands and dots were observed within the 6–24 cm sediment interval, which indicate the presence of sulfides. Deeper in the sediment column, in the 24–31 cm interval, sediments had a reddish-brown color that indicates the presence of iron oxyhydroxides [45,57].

**Figure 7.** Vertical distributions of total, 0.5N HCl extractable, and normalized to Al metal contents in the core sediments.

The levels of Al, Fe, Cd, Cu and Zn in the core sediments were similar to those found in the surficial ones, whereas the levels of Mn were slightly lower (Table 4). The extractability of metals by 0.5 N HCl in the core sediments followed the order (median): Mn (17%) < Fe (25%) < Zn (28%) < Cu (49%) < Pb (71%) < Cd (83%). The lower percentages of extracted metals, particularly for Mn and Fe, compared to the surface sediments, signify their presence in di fferent forms. This could be attributed to: (a) the processes occurring in the water column that involve the complexation of metals with organic and inorganic ligands and the formation of metal precipitates; and (b) the processes occurring in the sediment column that involve the redistribution of metals on the geochemical substrates, or even a partial release from the sediments to the water column, triggered by diagenetic redox processes.

The extractability of Mn by the dilute HCl decreased from 44% of the total content at the surface layer to less than 20% within the 9–30 cm sediment interval. The profile of 0.5N HCl extracted Mn (Figure 7) shows an increasing trend from the depth of 30 cm towards the surface, which is typical of the progressive dissolution of Mn oxyhydroxides, upward di ffusion of at least part of Mn(II) dissolved ions, and re-oxidation/precipitation at the surface layer where higher redox potentials are met [58]. With this process, the least labile fraction of Mn is left behind, thus the extractability of Mn is lower in the subsurface sediments than in the surficial ones.

The amount of Fe extracted by the dilute HCl varied widely with depth from 9% to 71% of the total Fe content. The highest percentages coincide with the observed orange- bands, which are attributed to accumulations of Fe oxyhydroxides that are fully recovered by the extractant [59]; the lower percentages coincide with the black-grey bands, which could signify the presence of pyrite, the end-product of metastable Fe-monosulfides with hydrogen sulfide [60], which are not extracted by HCl [59]. Previous research at the site [23] confirmed the presence of pyrite in the subsurface sediments of the Antinioti Lagoon by means of X-ray Di ffraction and Scanning Electron Microscope analysis. However, within the pyritized sediments, micro-environments of authigenic Fe/Mn oxides (Mn containing goethite) were identified through synchrotron radiation micro X-ray fluorescence (SR μ-XRF).

Total and 0.5 N HCl extracted Cd contents were significantly correlated with OC content (r = 0.695 and 0.783, respectively; < 0.0005). The profiles were similar to each other and showed lower contents near the water-sediment interface, a substantial increase in the 4–20 cm sediments layers, followed by a further decrease after this depth. This distribution pattern has been described in previous studies [19,58], and is consistent with Cd dissolution after the oxic degradation near the surface of fresh organic carbon with which Cd was originally bound, and downward di ffusion and fixation, in the thick zone of sub-oxic to anoxic grey-black colored, underlying sediments, which are depleted in Mn. This pattern is typical in sub-oxic sediments where authigenic accumulations, as CdS precipitates, take place [19,58,61,62]. Cadmium sulfide minerals are fully recovered by HCl [63], and this explains the high amounts of Cd extracted by the procedure (median value: 85%) in relation to the total metal.

The vertical distribution of total and 0.5 N HCl-extracted Pb is dominated by two extreme values at 23 cm (203 and 161 mg·kg−1, respectively) and 30 cm (566 and 300 mg·kg−1, respectively). These peaks are most probably attributed to distinct and occasional events of pollution, which are unknown to us.
