Ορυκτολογική και γεωχημική μελέτη της αιωρούμενης σωματιδιακής ύλης και των ιζημάτων του πυθμένα στο βόρειο τμήμα του Θερμαϊκού Κόλπου

Abstract

The shallow marine environment of northern Thermaikos Gulf is characterized by significant inputs of energy and particulate matter. Biogeochemical processes affect the quality of SPM while physical conditions control its distribution. The study of SPM composition and chemical transformations promotes the understanding of transport and settling processes in the marine environment. The distribution of SPM in the Gulf is controlled by water circulation patterns, the proximity to the river outfalls and seabed topography. Higher SPM concentrations recorded at the surface (mean = 1.45 mg/l, sd = 0.75 mg/l, maximum value = 11.60 mg/l) and close to the bottom (mean = 1.49 mg/l, sd = 0.67 mg/l, maximum value = 11.72 mg/l), creating surface and bottom nepheloid layer (BNL), respectively. At the intermediate water column of 10 m depth, SPM concentrations were always lower than surface and bottom (mean = 0.74 mg/l, sd = 0.10 mg/l, maximum value = 2.56 mg/l), while small fluctuations were recorded between sampling stations. During the sampling period, the highest SPM concentrations of Surface Nepheloid Layer (SNL) were recorded in the Axios and Aliakmon prodelta. Axios River is considered as the main contributor of particulate matter in the Gulf (measured SPM in Axios Channel 35 – 461 mg/l). Axios and Aliakmon rivers supply the Gulf with terrestrial particulate matter during the whole sampling period; the supply continues even during the dry summer season. Due to cyclonic circulation, fluvial inputs concentrate along the west coast of the gulf without entering in the interior of the Gulf. On rare occasions, the concentration of particulate matter in the NW part of the Gulf of Thessaloniki was observed. The areas of the Gulf where particulate matter is concentrated are possible spots of eutrophic events under favorable conditions. At the intermediate water depth of 10 m, SPM concentrations are significantly lower than those recorded at the surface and at the bottom; intermediate nepheloid layers were not distinguished. Fluvial inputs as well as biogenic particles created at the surface layer are retained in suspension through lateral transport; lateral transport overcomes settling. Benthic nepheloid layer results from bottom currents that bring into lateral movement the unconsolidated sediment of the seabed. The fluffy surface layer at the sediment–water interface will be resuspended at lower critical shear velocities, compared to the underlying sediment. Wind-driven resuspension does not affect significantly the sampling locations with the exception of shallow station TP22A. Wind-driven resuspension takes place at shallow areas of the Gulf; then resuspended material is transported through benthic currents to deeper 142 areas of seafloor. Bottom topography shapes the distribution of BNL. Grain size analysis of SPM was undertaken throughout SEM image processing. ImageJ a public domain, Java-based image processing program developed at the National Institutes of Health (USA) was used. Unconformable and various relief of the filter create false shadows and lights preventing reliable recognition of particle shapes by the program. From the visual description of several particles it is concluded that the mode is 3 - 6 κm, the big dimension lies at a width of 2 - 30 κm and the size of aggregates fluctuates between 10 κm and 150 κm. Grain size analysis of seabed sediments classified them as muds. Sand content is small varying between 0,3% and 8,7%. The high percentage of fine-grained sediments implies predominance of low hydrodynamic conditions. Their sorting varies from very poor to moderate, being indicative of the poly-modal populations, reflecting different input sources. The mineralogical examination of SPM reveals the existence of the minerals illite, chlorite, smectite, I/S mixed layer, kaolinite, quartz, plagioclase, K-feldspar, calcite and tremolite. Semi-quantitative analysis of SPM samples shows that the most abundant clay mineral is illite (12-47%) followed by chlorite + kaolinite (11-28%) and smectite (2-9%). Ordered illite/smectite (I/S) with 30 - 40% smectite layers is present in most SPM samples. In addition, randomly interstratified I/S with 50% smectite layers have been recognized in some samples. The significant presence of quartz (on average 18%) reveals the effective transport processes in this part of the Gulf. Among the feldspars plagioclases exceed significantly K-feldspar. The presence of calcite is attributed to fragments of coccolithophores and carbonate shells. In the fine fraction of surface sediments (<0.2 κm), the minerals that were identified are illite, chlorite, smectite, I/S mixed layer, kaolinite, quartz, plagioclase and calcite. Among clay minerals, illite (49 - 76%) predominates over smectite (12 – 36%) and smectite over chlorite + kaolinite (12 – 18%). Clay mineral distribution is quite similar in the sampling locations of the Gulf and Bay of Thessaloniki as well as in the prodelta areas of Axios and Aliakmon Rivers. The abundance of smectite in the sampling locations may be enhanced by the physicochemical conditions of the seawater, which result in rapid flocculation and settling of smectite flocks. The remaining part of smectite is transported in suspension to the inner plateau of the Gulf. The presence of illite in SPM and seabed sediments is attributed to the weathering of parent rocks in the drainage basins. Smectite is derived from the volcanic terrain of the Axios River catchment, and other, more sporadic volcanic outcrops. Finally, the significant presence of the interstratified illite/smectite confirms the limited reworking and weathering of the primary minerals during their transport and deposition from the drainage basins to the Gulf. 143 The coarse fraction of surface sediments (>63 κm), is composed of angular quartz grains, plagioclase, chlorite and mica. Sericitized crystals of plagioclase are found. No K-feldspar was identified. Mica is abundant and is found as discrete fine sheets. Biogenic particles comprise subrounded shell fragments of benthic microfauna; their distribution is relatively uniform. Pyrite and goethite are the most abundant metallic minerals in seabed sediments of the Gulf. Pyrite occurs in the form of framboids as well as cement between sediment grains. Framboidal pyrite formed during transport or after deposition of sediment rich in organic matter under reduced conditions. The formation of framboids was observed in the interior of shells; they were created during the decomposition of marine organisms and the consequent reduced conditions. Framboids were also observed on goethite structure. XRF analysis of SPM revealed the significant correlation between the elements Al, Si, Fe, Ti, K, V, Mg and Ba; they were grouped as lithogenic elements and their presence is attributed to the structures of aluminosilicate minerals. Aluminium has been used as index of terrigenous discharges in the Gulf. Si is attributed to quartz and feldspars as well as to the silicon shells of phytoplankton species such as diatoms. Iron is predominantly held in aluminosilicate lattices, mainly chlorites. Some small amounts of Fe are present as hydroxide FeO(OH), coming from the weathering of Fe bearing minerals or created authigenically in the surface sediments. In suboxic and anoxic interfaces of surface sediments, Mn(IV) dissolved and released into sediment pore water in the form of Mn(II) and/or Mn(IIΗ). When dissolved Mn2+ reaches oxidizing interfaces, it returns in particulate form as Mn oxide and precipitates as grain coating. During resuspension events, release of dissolved Mn2+ by diffusion into the water column is taken place. Barium is associated with the weathering of feldspars and mica drained by the rivers. There is no marine barite BaSO4 formation in the water column. Barium seems to be correlated with detrital aluminosilicates rather than organic matter. At the surface sediments of the Bay of Thessaloniki, where organic matter respiration occurs, sulphate enrichment could result in the formation of barite. Organic matter concentrations are elevated, exhibiting strong spatial and temporal variations. The highest POC concentrations were recorded at the sea surface of the Gulf and Bay of Thessaloniki from March to June 2005, reflecting spring phytoplanktonic bloom. Elevated pigments at the surface of the water column prevent sunlight irradiance and decrease primary production at deeper layers. The wastewater treatment plant effluents discharged close to TP5 sampling station, are a major source of organic particles that results in increased POC concentrations at the intermediate layer of 10 m depth. Particulate P is associated with 144 organic matter as showed by a good correlation with POC as well as by poor correlation with aluminosilicates. Measured concentrations of P and total N are elevated characterizing the Northern part of Thermaikos Golf as mesotrophic and the Bay of Thessaloniki as eutrophic during summer. The ratio N:P shows that the limiting parameter of phytoplankton growth in the Gulf is nitrogen. Calcium has mainly a terrigenous source, but also a biogenic: biogenic carbonate minerals constitute the tests of coccolithophores. Sulfur is associated with organic matter and particulate phosphorus. In the water column, sulfur occurs mainly as a constituent of proteins (primarily amino acids). Organic sulfur compounds are converted by some phytoplankton species into dimethylsulfoniopropionate (DMSP). Then DMSP can be microbially converted into DMS by bacteria. At the surface sediments, sulfur is related to the formation of sulfides FeS and FeS2 as well as sulfate SO42- and thiosulfate S2O32-. These sulfate ions react with metal cations to form sulfates. Trace metals of the water column are attributed to terrigenous sources. Cobalt is associated strongly with iron; Fe-Co minerals are abundant in the Aliakmon drainage basin. Nickel and chromium show similar distributions in the water column; the highest concentrations recorded at the prodelta of Axios and Aliakmon Rivers. Their origin is the weathering products of ultramafic rocks that are abundant the in Axios and Aliakmon drainage basins. Nickel comes from the weathering of peridotites while Cr is attributed to chromite minerals. The increased concentrations of Co, Ni and Cr in the water column are attributed to terrestrial supply through river basins and they are not considered as subject if anthropogenic impact. Zinc and copper are correlated each other, while their distributions follow those of organic matter and sulfur. This evidence implies that a significant portion of Cu and Zn is related to anthropogenic impact to the marine environment. At the surface sediments, the spatial distribution of elements Cu, Zn, Pb, As and S reveals higher concentrations at the Bay of Thessaloniki than those measured in front of the mouths of Axios and Aliakmon Rivers. Arsenic is in the form of As(III) and As(V) and is bound to iron hydroxides or is held in the structure of arsenopyrite. Copper, Zn and Br are associated with degraded organic matter that settles on the seabed. The index of geoaccumulation was used to evaluate the quality of surface sediments. All of them were not contaminated with Mn, Co and Cu. The bay of Thessaloniki was contaminated with respect to Pb and Zn contrary to prodelta area that was characterized as uncontaminated. The prodelta area of Axios and Aliakmon Rivers found moderately contaminated with Cr and Ni; this is due to terrestrial supply through river basins and is not subject to pollution. The NW part of the Bay and Axios outfall were moderately contaminated 145 with arsenic. Sediment core was sampled at Axios prodelta and the concentrations of V, Fe, Co, Cr, Ni, Pb, Cd, Zn, Cu and As was measured in the fine fraction throughout the core. The vertical distribution of metals is relative homogeneous. Cadmium and Pb concentrations show an increasing trend with depth while Co concentration decreases with depth. The concentrations of all metals studied were found higher than the ‘average shale’ values implying anthropogenic interference as well as terrestrial inputs through the Axios and Aliakmon river drainage basins.