Pyritization: a palaeoenvironmental and redox proxy reevaluated

Abstract

Pyrite as a proxy for the redox states of bottom waters was evaluated in the context of modern salt marsh sediments. Two sites, 40 m apart, with very different physicochemical properties were sampled seasonally. The sites differed from each other in (1) the amount of tidal flushing, (2) vegetation, (3) redox condition below the sediment–water interface, (4) carbon content and (5) solid phase iron oxyhydroxide and iron sulfide content of the sediment. The calculated degree of pyritization (DOP) varies between 0.44 and 1.0, depending on the site and the season. Different DOP values were obtained for the same samples when different techniques were applied for extraction of reactive iron. Framboidal pyrite was dominant; however, other textural forms of pyrite were also observed. The size distribution of the individual pyrite crystals and the framboids was fairly narrow; with the majority of them having diameters <10 μm. Despite deposition taking place in aerobic waters (fully oxygenated waters) at the sampled sites, both the DOP and pyrite textures indicated dysoxic (waters with low oxygen concentrations) to euxinic (waters with no oxygen present, and H 2S may be continually or intermittently present) bottom waters. These results indicate that caution must be applied when using DOP and pyrite morphologies as proxies for interpreting the redox conditions of different depositional environments.