Active diagenetic formation of metal-rich layers in N. E. Atlantic sediments

Abstract

Sediment cores from the Porcupine Abyssal Plain exhibit an indurated layer 0.5–3 cm thick at depths of approximately 50 cm. This is some 15–20 cm below the glacial/Holocene transition as interpreted by radiocarbon dating and the palaeontological criteria of Ruddiman and McIntyre (1981). The layer is forming currently at the oxic/post-oxic boundary in the sediments, as revealed by pore water data: O 2 and NO − 3 are present in solution above the layer, while Fe 2+, Mn 2+, PO 3− 4 and NH + 4 are present in solution below, and all these species show concentration gradients indicating fluxes into the layer. These data are consistent with the hypothesis for the initiation and sustained formation of such layers proposed by Wilson et al. (1986a,b). The elements Mn, Ni, Co, Fe, P, V, Cu, Zn and U are all enriched to varying degrees in the vicinity of the layer. Some differential stratification of these elements in the vertical, consistent with a redox control, is observed at one site with a 0.5 cm layer, with Mn, Ni and Co above, Fe, P, V and Cu in the layer, and U below. At another site the metal-rich layer has higher Fe and P concentrations and is more indurated. Here all enrichments except Co are contained within a single layer sample, 3 cm thick.