Preservation and diagenetic overprint of geochemicaland geophysical signals in ocean margin sedimentsrelated to depositional dynamics

Abstract

The focus of this work is to study the impact of non-steady state diagenesis on primary geochemical and geophysical properties. We investigated the influence of depositional dynamics on diagenetic processes, by studying sediments recovered in the western Argentine Basin as well as in the northern Cape Basin. The western Argentine Basin is characterized by highly dynamic depositional conditions, and thus represents a suitable sedimentary environment to study non-steady state processes. Geochemical and geophysical investigations were carried out on sediments from selected sites. The main process which drives diagenetic alteration in these sediments is the anaerobic oxidation of methane (AOM). Linear profiles of sulfate pore water concentrations at some of the investigated sites indicate constant low sedimentation rates for a period of time. These sites are characterized by distinct gaps in magnetic susceptibility within the sulfidic zone. The nearly complete loss of magnetic susceptibility is shown to result from the reduction of sulfate by the process of AOM. This process releases H2S into the pore water, which leads to the alteration of iron (oxyhydr)oxides to iron sulfides. In contrast to the western Argentine Basin, the eastern Cape Basin is characterized by relatively constant depositional conditions. Investigations of sediments from this area reveal the occurrence of distinct barium enrichments at the SMT. These authigenic barite fronts are formed by the reaction of upward diffusing barium with interstitial sulfate. At one study site, barite enrichments in sulfate-depleted sediments buried a few meters below the SMT were observed. The occurrence/preservation of these barite enrichments is presumably associated with a decelerated dissolution of barite, which could be explained by high concentrations of dissolved barium in the pore water.