Processes and Signals of Nonsteady-State Diagenesis in Deep-Sea Sediments and their Pore Waters

Abstract

Nonsteady-state conditions — induced by changes in the fluxes of electron donors and acceptors and environmental conditions — are shown to have been and to be still widespread in sediments of the equatorial and South Atlantic Ocean. Typical diagenetic phenomena initiated under such nonsteady-state conditions comprise the fixation and downward progression of redox boundaries and reaction fronts. Intervals most severely altered by diagenetic overprint often occur cyclically within the sedimentary record and are mostly associated with full glacial/interglacial transitions. The extent of post-depositional oxidation of organic carbon as well as the dissolution and reprecipitation of minerals across these glacial terminations was shown to depend on the overall sedimentation rate and the magnitude of change encountered in the various depositional and geochemical factors. A sedimentation rate of about 2 cm/kyr was confirmed to be the critical value below which no significant amounts of non-refractory organic carbon are preserved. The influence of climatically induced variations in environmental conditions is not restricted to the geochemical boundaries in the vicinity of the sediment surface (e.g. oxic/post-oxic and Fe redox boundary) but well extends into much deeper sediment sections — namely into the zone of anaerobic oxidation of methane (AOM). In this way, processes within the zone of AOM can produce a further profound diagenetic alteration of the sediment composition up to hundreds of thousands of years after initial deposition and thus a significantly delayed chemical log-in. The long-term utility of all primary and secondary signals — also those formed and initially preserved across the oxic/post-oxic and Fe redox boundaries — is ultimately controlled by the geochemical processes within and below the sulfate/methane transition (SMT). While dissolution of authigenic and productivity-related barite takes place in sulfate-depleted sediment sections, iron sulfides as well as sulfurized organic matter and associated trace elements have a high potential to survive burial below the SMT. Nonsteady-state diagenesis can be triggered not only by changes in conditions at the sediment/water interface like TOC input, sedimentation rate or O2 content of bottom water but also by processes in the underlying sediment — namely the formation and/or liberation of methane. Apart from the distinct alteration of the solid-phase composition, variations in the upward flux of methane also have a considerable impact on the shape of sulfate pore water profiles. Modelling the effects of such variations in methane flux on sulfate profiles has illustrated that considering possible nonsteady-state situations in the sediment/pore water system is of utmost importance for the interpretation of pore water data.