Sulphide speciation in surface sediments of the Lagoon of Venice: A geochemical and mineralogical study

Abstract

Three 1-m cores of recent sediments from the Lagoon of Venice were sampled to obtain the vertical distribution of Fe, Zn, Cu, Pb and Cd, and to study authigenic iron sulphides from the geochemical and mineralogical viewpoints. Zn, Cu, Pb and Cd occur in superficial sediments in concentrations many times higher than those of their natural background; the existence of ZnCd and CuPb couples were revealed by their very similar depth profiles. The latter couple is also closely correlated with sulphide-sulphur. Iron is characterized by quite a different behaviour, showing a sharp increase at a depth of 35 cm. From a geochemical viewpoint, authigenic sulphides were distinguished into pyrite and Acid Volatile Sulphides (greigite and mackinawite). Their distribution is characterized by high concentrations of AVS in the first 25 cm of sediments, dramatically decreasing with depth. Pyrite, present in lower amounts, shows the opposite trend, although in one core both species were present in comparable amounts. Mineralogical study of the heavy mineral fraction revealed that the iron sulphides are composed of pyrite (organised both in the classic framboidal growth and in unordered micro-crystal swarms) and iron “mixed sulphide” aggregates, with variable optical patterns and colloform, botroidal or massive growth. No heavy metals were detected in the crystalline lattice of the sulphides. Down to 25 cm all types of sulphides occur, and with increasing depth pyrite clearly prevails. This behaviour may have various causes, e.g. chemico-physical control by specific microniches, temporal and spatial variations favouring the formation of different types, evolution of iron mixed sulphides into more stable forms.