Trace elements release from volcanic ashes to seawater. Natural concentrations in Central Mediterranean sea.

Distributions and concentrations of many minor and trace elements in epicontinental basins, as Mediterranean Sea,are mainly driven to atmospheric fallout from surroundings. This mechanism supplies an estimated yearly flux ofabout 1000 kg km-2 of terrigenous matter of different nature on the whole Mediterranean basin. Dissolution ofthese materials and processes occurring at solid-liquid interface along the water column drive the distributions ofmany trace elements as V, Cr, Mn, Co, Cu, and Pb with contents ranging from pmol l-1 (Co, Cd, Pb) to nmol l-1scale in Mediterranean seawater, with some local differences in the basin.The unwinding of an oceanographic cruise in the coastal waters of Ionian Sea during the Etna’s eruptive activityin summer 2001 led to the almost unique chance to test the effects of large delivery of volcanic ash to a coastal seawater system through the analyses of distribution of selected trace elements along several seawater columns. Thecollection of these waters and their analyses about V, Cr, Mn, Co, Cu, and Pb contents evidenced trace elementconcentrations were always higher (about 1 order of magnitude at least) than those measured concentrations inthe recent past in Mediterranean seawater, apart from Pb. Progressive increase of concentrations of some elementswith depth, sometimes changing in a “conservative” behaviour without any clear reason and the observed higherconcentrations required an investigation about interaction processes occurring at solid-liquid interface betweenvolcanic ash and seawater along water columns.This investigation involving kinetic evaluation of trace element leaching to seawater, was carried out during a6 months time period under laboratory conditions. X-ray investigations, SEM-EDS observations and analyseson freshly-erupted volcanic ash evidenced formation of alteration clay minerals onto glass fraction surfaces.Chemical analyses carried out on coexisting liquid phase demonstrated that trace element leaching occurs througha first quick followed by a slow second step that attaints to an apparent equilibrium after 6 months. Amplitude ofkinetic rate constant measured for SiO2 release during the first step and behaviour of Ti/Si and Cr/Si rations inprimary volcanic minerals, glass fraction and leaching solutions during the first 1 month stage of the experimentalinteraction allowed to demonstrate that trace element release mainly occurs from glassy materials and Ti-richmagnetite.