MOBILITY AND IMMOBILITY OF REDOX-SENSITIVE ELEMENTS IN DEEP-SEA TURBIDITES DURING SHALLOW BURIAL

During the first few tens of thousands of years after an organic-rich turbidite is emplaced on an abyssal plain. surficial oxidation of the unit occurs by downwards diffusion of bottom water O-2. From work on p iston cores, it is known that a suite of redox-sensitive elements form characteristic relocation peaks in such units around the deepest reac hes (upper few dm) achieved by oxidation (fossil oxic/post-oxic bounda ries). Four individual turbidites now buried at depths of 130-230 m be low seafloor (mbsf), obtained by ODP Leg 157 drilling on the Madeira A byssal Plain (MAP), have been investigated to determine the effects of burial diagenesis on these geochemical signals over 3-7 My. Porewater data indicate that post-oxic conditions persist to around 130 mbsf on the MAP; below which sulphate reduction predominates. All four units have retained high total organic carbon (C-org) contents (similar to 1 .4%) in their lower reaches, despite the fact that bottom waters have previously oxidised this same organic matter in the turbidite tops to similar to 0.2%. The apparent inertness of the turbidite C-org contras ts with the reactivity of organic matter observed in hemipelagic sedim ent from other ODP investigations. In all four cases, low initial CaCO 3 contents have been quantitatively removed along with C-org oxidation , with additional dissolution of carbonate by undersaturated bottom wa ters. Diagenetic fractions of the elements Se, Cd, Sb, TI, and V are p resent in highly-localised peaks and show no evidence of migration. In different combinations, the elements Cu, Co, Ni, and Zn appear to hav e migrated over short (<5 cm) distances in thin (mm) dark-coloured sul phide bands, probably sourced by reduction of a Mn-oxyhydroxide host p hase during early (70 ky) burial. Two examples of local diagenetic pyr itisation (up to 20 wt% FeS2) are observed in the bodies of the two tu rbidites buried to similar to 130 mbsf, confirming that sulphate reduc tion is active at this level. This diagenetic pyrite is associated wit h high concentrations of As, and lesser enrichments of Ni, Zn, Co, Se, Sb, and possibly Mo. Copyright (C) 1998 Elsevier Science Ltd.