The Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment archives contain a comprehensive record of Earth's history and paleoceanography. Within a few years of core retrieval, most shore-based examinations have been accomplished such that each section of the cores has been assessed with respect to stratigraphy, time scales, proxy data, and correlations among cores. Thus, high-precision subsampling for particular scientific tasks may be performed, which makes the archive cores especially precious. However, some physical or chemical properties of the sediment cores might change during storage in the repository.
In particular, Fe-bearing minerals have been found to react very sensitively to variations of the redox environment. Diverse early diagenetic processes have been documented in this context--for example, reversible sediment color changes related to redox transitions of the structural iron in the clay mineral content of the sediments (Lyle, 1983), as well as alterations of the primary sediment magnetic signal due to authigenic magnetite (Fe3O4) formation (Karlin et al., 1987). More recent studies have revealed that redox transitions of the structural clay iron in sediments imply formation or destruction, respectively, of potent geochemical structures (König et al., 1999), which also represent a new type of proxy for the paleoredox conditions at the ocean floor.
Earlier studies have demonstrated drastic differences in the bulk sediment Fe(II)/Fe(III) ratio as a result of differences in sample handling prior to analysis: air drying of anoxic sediments has resulted in Fe(II) to Fe(III) oxidation of a substantial fraction of the Fe(II) content (e.g., >20% of the total Fe, or ~40% of the initial Fe(II) content [König et al., 1988]). Thus, careful protection of anoxic sediment samples from contact with atmospheric oxygen is imperative with respect to the investigation of parameters that may be modified by Fe redox transitions. In this study, we examine the extent and nature of geochemical alterations that occur in the Fe-bearing mineral content of deep-sea sediments during 6 months of storage in the ODP Bremen Core Repository.