Importanceofsubmarinelandslidesfornon-steadystateconditions in pore water systems Ð lower Zaire Congo) deep-sea fan Matthias Zabel * , Horst D. Schulz Fachbereich 5, Geowissenschaften, University of Bremen, P.O. Box 330440, D-28359 Bremen, Germany Received 11 April 2000; accepted 30 March 2001 Most concentration pro®les of sulfate in continental margin sediments show constant or continuously increasing gradients fromthebenthicboundarylayerdowntothedeepsulfatereductionzone.However,averymarkedchangeinthisgradienthas been observed several meters below the surface at many locations, which has been attributed to anoxic sul®de oxidation or to non-local transport mechanisms of pore waters. The subject of this study is to 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, about10mthick,consistsofstrati®edpelagicdepositsrepresentingaperiodofcontinuoussedimentationoverthelast190kyr. It is underlain by a turbidite sequence measuring several meters in thickness, which contains large crystals of authigenic calcium carbonate ikaite: CaCO 3 ´6H 2 O). Ikaite d 13 C values are indicative of a methane carbon contribution to the CO 2 pool. Radiocarbon ages of these minerals, as well as of the adjacent bulk sediments, provide strong evidence that the pelagic sediments have overthrust the lower section as a coherent block. Therefore, the emplacement of a relatively undisturbed sedimentpackageispostulated.Pore-waterpro®lesshowthedepthofthesulfate±methanetransitionzonewithintheturbiditic sediments.Bytheadaptationofasimpletransport-reactionmodel,itisshownthatthechangeinthegeochemicalenvironmental conditions,resultingfromthisslideemplacement,andthedevelopmenttowardsanewsteadystatearefullysuf®cienttoexplain all features related to the pore-water pro®les, particularly, SO 22 4 anddissolvedinorganiccarbonDIC).Themodelshowsthat the downslope transport took place about 300 yr ago. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Non-steady state conditions; Submarine landslides; Anoxic sul®de oxidation; Ikaites; Zaire deep-sea fan One of the most intensely discussed questions in marine geochemistry is whether steady-state condi- tions can be universally assumed for Quaternary deep-sea sediments. Restricted to long periods of constant depositional and geochemical conditions, the simplifying assumption of a steady state may result in reasonable balances of early diagenetic processes. However, many studies on pore waters and their sediments give strong indications that a temporal constancy of diagenetic processes cannot atalltimesbeassumed.Theneglectofthepossibility of non-steady processes could lead to the postulation ofdiageneticprocesses,whicharenotreal,oratleast overrated by the usual single snapshot of a benthic system, given present-day pore-water pro®les. Forexample,thereisthequestionoftheimportance of anoxic sul®de oxidation for the in situ renewal of Marine Geology 176 2001) 87±99 0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII:S0025-322701)00164-5 www.elsevier.nl/locate/margeo * Corresponding author. Fax: 149-421-2184321. E-mail address: mzabel@uni-bremen.de M. Zabel).