Post-depositional migration of elements during diagenesis in brown clay and turbidite sequences in the North East Atlantic

Two cores from a NE Atlantic pelagic clay area of low accumulation rate each contain a single turbidite with different age, thickness and composition. Diagenesis following introduction of exotic turbidite material into the pelagic clay sequence has resulted in distinctive colour changes. The diagenetic process is thought to be driven by bacterially-mediated organic oxidation, and compositional differences in the turbidites and clays allow examination of the effects on metal concentrations of this process.In one core, a long turbidite section emplaced 330,000 years ago is overlain by clay. Organic oxidation has apparently proceeded from the turbidite top downwards and has maintained a U concentration peak below the oxidation front marked by a colour change. The U source is believed to be U initially associated with the organic matter in the oxidised section of the turbidite. Vanadium and Cu behave similarly, but upward migration of the redox-sensitive metals Mn and Ni is also seen. In the second core an 8 cm turbidite section was emplaced about 170,000 years ago in a clay column. In this case organic consumption now appears complete, but evidence for diagenetic effects is found in a 16 cm compositional alteration ‘halo’ of the underlying pelagic clay. Fe(II) has been enriched and the hydrogenous component of Mn, Co, Ni and Cu removed from the pelagic clay to form the halo.