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Abstract
In Lower Toarcian marls and marly limestone deposits from the South Iberia Paleomargin (Fuente Vidriera section, Betic Cordillera, southern Spain), siliceous skeletons of radiolarian and spicules of hexactinellid sponges are preserved in pyrite. The original siliceous skeletons (hydrated amorphous silica) were dissolved and totally replaced by pyrite. The radiolarians are locally very well preserved as independent remains or fused to pyrite framboids, in some cases distorted by the pyrite framboid growth. Pyritization of the remains and decay of the organic matter of these organisms by sulphate-reducing bacteria favored the early pyritization at the same time that silica dissolved. Pyrite formation was associated with organic matter decay through sulphate reduction; the amorphous silica skeleton acts either as a nucleation substrate or induces precipitation of pyrite precursors during biogenic silica dissolution. The organic matter may also help to stabilize colloids that are important for framboid precipitation. Pyritization occurred soon after the death of the organism, taking place in the upper sediment column just below the sediment-water interface, at the redox boundary where oxygen-bearing and hydrogen sulphide-bearing waters are in contact. The size of pyritized radiolaria (>40 [mu]m) and the mean size of pyrite framboids (6.3-7.1 [mu]m) are compatible with pyritization within the sediment-water interface under dysoxic conditions. The presence of trace fossils and benthic foraminifera exclude anoxic and euxinic conditions during the early Toarcian in this setting.[PUBLICATION ABSTRACT]