Comparison of geochemical indices used for the interpretation of palaeoredox conditions in ancient mudstones

Eight geochemical indices used for the interpretation of bottom water palaeo-oxygen concentrations for argillaceous sedimentary rocks have been calculated for a suite of Upper Jurassic mudstones drawn from the Draupne and Heather Formations of the Norwegian North Sea, and the Kimmeridge Clay Formation from onshore England. The eight indices: DOP, C/S, U/Th, authigenic uranium, V/Cr, Ni/Co, Ni/V and (Cu + Mo)Zn have been examined by means of factor analysis, to identify the most reliable. DOP, U/Th, authigenic uranium, V/Cr and Ni/Co form an internally consistent set and are recommended as the most reliable of the indices. Ni/V, C/S and (Cu + Mo)Zn convey little palaeo-oxygenation information and are not regarded as reliable. The parameters identified as reliable (DOP, Ni/Co, V/Cr, U/Th and authigenic uranium), have been calibrated against DOP, for inter-comparison purposes, and to allow their interpretation against DOP-derived depositional conditions.