Importance of submarine landslides for non-steady state conditions in porewater systems - lower Zaire (Congo) deep-sea fan

Most concentration profiles of sulfate in continental margin sediments show constant or continuously increasing gradients from the benthic boundary la yer down to the deep sulfate: reduction zone. However, a very marked change in this gradient has been observed several meters below the surface at man y locations, which has been attributed to anoxic sulfide oxidation or to no n-local transport mechanisms of pore waters. The subject of this study is t o investigate whether this feature could be better explained by non-steady state conditions in the pore-water system. To this end, data are presented from two gravity cores recovered from the Zaire deep-sea fan. The sediments at this location can be subdivided into two sections. The upper layer, abo ut 10 m thick, consists of stratified pelagic deposits representing a perio d of continuous sedimentation over the: last 190 kyr. It is underlain by a turbidite sequence measuring several meters in thickness, which contains la rge crystals of authigenic calcium carbonate (ikaite: CaCO3.6H(2)O). Ikaite delta C-13 values are indicative of a methane carbon contribution to the C O2 pool. Radiocarbon ages of these minerals, as well as of the adjacent bul k sediments, provide strong evidence that the pelagic sediments have overth rust the lower section as a coherent block. Therefore, the emplacement of a relatively undisturbed sediment package is postulated. Pore-water profiles show the depth of the sulfate-methane transition zone within the turbiditi c sediments. By the adaptation of a simple transport-reaction model, it is shown that the change in the geochemical environmental conditions, resultin g from this slide emplacement, and the development towards a new steady sta te are fully sufficient to explain all features related to the pore-water p rofiles, particularly, SO42- and dissolved inorganic carbon (DIC). The mode l shows that the downslope transport took place about 300 yr ago. (C) 2001 Elsevier Science B.V. All rights reserved.