Speciation of trace metals in seawater: The Mediterranean Sea
Del Core, Marianna (2010) Speciation of trace metals in seawater: The Mediterranean Sea. [Tesi di dottorato] (Inedito)
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Since the early 1970s, marine chemists have gained a first order understanding of the concentrations, distributions and chemical behaviour of trace metals in seawater (TMs). The concentration and distribution of TMs are controlled by a combination of processes including external sources (natural or anthropogenic) of elements delivered by rivers, by wind-blown dust, by hydrothermal circulation while processes removing TMs from seawater include active biological uptake or passive scavenging onto particulate matter with the ultimate sink of TMs in the marine sediments. All these processes are superimposed on the general circulation and mixing of the different marine systems, resulting in the characteristic distribution of each TM. In the last two decades many studies have been performed relatively to ocean chemistry, but, paradoxically, few studies have been carried out in the Mediterranean Sea that may be considered a miniaturised ocean system, where seawater response to environmental perturbations and/or evolution is rapid and generally amplified with respect to the global ocean. The Mediterranean Sea is characterised by i) the presence of all trophic regimes, from oligotrophy to coastal eutrophy, ii) a 3D ocean dynamics and iii) the near desert of Sahara that represents the world’s largest source of Aeolian soil dust. The present work attempts to review the most crucial aspects of TMs ocean chemistry on the basis of accurate sampling and analysis of dissolved [Med] and particulate [Mep] (Cd, Co, Mo, V, Ag, Pb, Ni and Cu for dissolved form and Al, Cu, Ni, Cd, Co, Pb, Mo and Ag for particulate form) from 15 sampling stations where a number of points along vertical profiles were collected during two basin scale oceanographic cruises (TRANSMED, summer 2007 and SESAME-IT4, spring 2008). The results evidence that the Mediterranean distribution of Cd, Cu and Ni (that show a nutrient-like behaviour in the world ocean) is dominated by land and atmospheric inputs rather than biogeochemical cycling. Particularly, although [Cdd] in the Gulf of Cadiz seems directly controlled by biogeochemical processes, in the Mediterranean sea, the external inputs play a primary role on the distribution of this element. The [Pbd] in the Mediterranean Sea, generally shows values in the range of those recorded in the world ocean, without any evidence of scavenging behaviour along the water column. Noteworthy, very high [Pbd] values were measured in the sampling stations of the Gulf of Lyon and in the S-E of the Spain as possible response to inputs from the large Rhone and Segura rivers. These results suggest a key role played by anthropic impact on the marine Lead chemistry of the Mediterranean Sea. The unprecedented collected dataset of [Agd] and [Vd] suggests a primary control of the Western Europe industrialised areas on the distribution patterns of these elements. Particularly, the depletion of [Vd] in the Eastern basin respect to Western part, reflects the different anthropogenic sources of [Vd] in the Mediterranean area. The [Alp] distribution pattern, clearly shows the direct influence of the Sahara dust at basin scale. The most enriched areas well-agree with wind patterns at regional scale that regulate a direct and primary impact of dust on [Alp] distribution at the surface and also along the entire water column. Fine regulation of particulate sinking speed clearly differentiate the easternmost part of the Mediterranean sea, more continuously affected by desert storms with respect to westernmost areas under control of more irregular dust inputs. The collected dataset represents an original contribution to knowledge of the ocean chemistry of the Mediterranean sea and a potential reference for future investigation of biogeochemical cycles of this, often, unexplored marine system. Natural and anthropic forces continuously change the “chemical physiognomy” of the Mediterranean Sea and it is mandatory to deeply explore mechanisms and processes that control the dynamic of the elements at the interface of the ocean with lithosphere, atmosphere, biosphere and “anthroposhere”. This research job is a tentative to add new pieces of knowledge in this complex field of cutting edge science.
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