Mobility and Immobility of Redox-Sensitive Elements in Deep-Sea Turbidites During Shallow Burial

Abstract

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 O2. From work on piston cores, it is known that a suite of redox-sensitive elements form characteristic relocation peaks in such units around the deepest reaches (upper few dm) achieved by oxidation (fossil oxic/post-oxic boundaries). Four individual turbidites now buried at depths of 130-230 m below seafloor (mbsf), obtained by ODP Leg 157 drilling on the Madeira Abyssal 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 (Corg) contents (∼1.4%) in their lower reaches, despite the fact that bottom waters have previously oxidised this same organic matter in the turbidite tops to ∼0.2%. The apparent inertness of the turbidite Corg contrasts with the reactivity of organic matter observed in hemipelagic sediment from other ODP investigations. In all four cases, low initial CaCO3 contents have been quantitatively removed along with Corg oxidation, with additional dissolution of carbonate by undersaturated bottom waters. Diagenetic fractions of the elements Se, Cd, Sb, Tl, and V are present in highly-localised peaks and show no evidence of migration. In different combinations, the elements Cu, Co, Ni, and Zn appear to have migrated over short (<5 cm) distances in thin (mm) dark-coloured sulphide bands, probably sourced by reduction of a Mn-oxyhydroxide host phase during early (70 ky) burial. Two examples of local diagenetic pyritisation (up to 20 wt% FeS2) are observed in the bodies of the two turbidites buried to ∼130 mbsf, confirming that sulphate reduction is active at this level. This diagenetic pyrite is associated with high concentrations of As, and lesser enrichments of Ni, Zn, Co, Se, Sb, and possibly Mo.