32. OXIDATION FRONTS IN MADEIRA ABYSSAL PLAIN TURBIDITES: PERSISTENCE OF EARLY DIAGENETIC TRACE-ELEMENT ENRICHMENTS DURING BURIAL, SITE 950 ¹

John Thomson,² Ian Jarvis,³ Darryl R.H. Green,² and Damon Green ³

ABSTRACT

The emplacement of organic-rich turbidite units on abyssal plains is commonly followed by the early diagenetic redistribution of redox-sensitive elements. This occurs only at the summit of the turbidite unit, where it is caused by the downward diffusion of seawater oxygen, which continues for as long as each turbidite is in diffusive contact with bottom waters. The duration of this process is generally a few tens of thousands of years on the Madeira Abyssal Plain (MAP), where turbidite emplacement occurs at frequent intervals. The known sequence of redox-sensitive element redistribution peaks around oxidation fronts in turbidites, derived from studies of conventional piston core material sampling the upper few tens of meters of Quaternary sediments on the MAP, is compared here with that in a single turbidite recovered at Site 950, and now buried to 230 mbsf. This unit was deposited during the late Miocene (~7.6 Ma) and has experienced consecutively post-oxic and sulfate-reducing conditions during burial. The organic carbon (C_org ) profile and C_org /S ratio are remarkably similar to those in more recent examples. The labile (nondetrital) fractions of many redox-sensitive elements remain concentrated around a sharp color change from dark green (below) to greenish gray (above), which marks the limit of penetration of oxygen when the turbidite top was exposed on the seafloor. The elements Cd, Sb, Se, Tl, and V are all still found in peaks immediately below the original oxic/postoxic boundary although, as in the younger turbidites, the peak maxima are not exactly coincident. Cobalt, Cu, Ni, and Zn have apparently moved upward in chromatographic-like fronts, which are seen as thin dark-purple or gray bands in the core. Labile As, S, and U, which were selectively removed from sediment above the color change by the bacterially driven redox processes that accompanied the oxic decomposition of C_org in the turbidite, have retained their characteristic stepped-concentration pro-files. The successive passage through ~130 m of post-oxic and ~100 m of sulfate-reducing conditions (each lasting ~3.8 m.y.) has had remarkably little effect on trace-metal distributions.

¹ Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998. Proc. ODP, Sci. Results, 157: College Station, TX (Ocean Drilling Program).
² Challenger Division for Seafloor Processes, Southampton Oceanography Centre, European Way, Empress Dock, Southampton SO14 3ZH, United Kingdom.
³ School of Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey KT1 2EE, United Kingdom. Correspondence author: I.Jarvis@kingston.ac.uk