Carbon-sulphur-iron-variations and sulphur isotope patterns of Silurian Graptolite Shales

ABSTRACT Black shale horizons (Graptolithenschiefer) in Silurian and Lower Devonian strata of the Franconian-Thuringian Highlands (Frankenwald-Thüringer Wald) of Germany have high metal concentrations (e.g. U,V,Mo). Organic carbon, sulphur and iron contents, degree of pyritization (DOP) and sulphur isotope ratios from the Lower Graptolite Shales, along with faunal and sedimentological evidence show that the depositional environment was anoxic. Although some data (e.g. C/S plots) do not suggest completely euxinic conditions, the DOP versus C plot indicates a Black Sea-type environment. From the sulphur isotope data the environment fluctuated between'open'and'closed'with respect to sulphate supply, with higher carbon contents reflecting an increased bacterial reduction of the available sulphate. Diagenetic effects were responsible for somewhat unusual plots of C/S ratios, though iron availability may have been influential.