Authigenic iron sulphides in recent sediments of the Venice Lagoon (Northern Italy)
Frizzo, Pietro; Rampazzo, Giancarlo; Molinaroli, Emanuela
European Journal of Mineralogy Volume 3 Number 3 (1991), p. 603 - 612
published: Jun 14, 1991
manuscript accepted: Jan 19, 1991
manuscript received: Nov 21, 1990
ArtNo. ESP147050303012, Price: 29.00 €
Abstract Sediments of the upper 10-cm of the Venice Lagoon are made up of sandy-silty deposits, of siliceous-calcareous composition with a clayey matrix, containing abundant organic remains. The examined layer may be divided into a 0-3 cm thick, brownish-ochre coloured superficial horizon, with positive Eh for most of the year, and a blackish-grey lower horizon, 3-10 cm thick, with persistently negative Eh in Spring and Summer and slightly positive Eh in Autumn and Winter. Its pH is around 7.5 during Autumn and Winter, and fluctuates between 7.1 and 6.7 in Spring and Summer. Authigenic iron sulphides are ubiquitous, although they are present in variable quantities in the lower layer. From the mineralogical and textural viewpoints, the following varieties may be distinguished : - polyframboidal aggregates of microcrystalline pyrite, sometimes filling foraminifer tests ; - mixed Fe sulphide aggregates, with variable optical characteristics, in colloform, botryoidal and radial growths, frequently enveloping framboidal pyrite and typically growing on vegetal remains whose cell structures are sometimes preserved ; - rhythmic growths of pyrite, with pervasive replacement of clastic Fe oxide and hydroxide grains. The data indicate early formation of microcrystalline pyrite, in both the framboids and the grains which are disseminated in the organic matter ; it is frequently surrounded by massive pyrite growths and/or mixed Fe sulphides. Authigenic Fe sulphide formation in recent sediments (10-30 years old) in the Venice Lagoon is due to the anoxic conditions which seasonally affect the surface sediments and lead to interactions between reactive iron in solution and H2S. Iron may also be solubilized in a reducing environment from ferric hydroxides adsorbed on clays, from organo-metallic complexes, or directly by reduction of clastic grains of Fe oxides and hydroxides ; H2S is produced in the same environment by bacterial reduction of sulfates in the pore-water.