Magnetotactic bacteria and dysaerobic marine environments: In situ magnetometry

Over the past 10 years, numerous studies have clearly established that in situ production of biogenic magnetite, mostly by magnetotactic bacteria, can be an important source of remanent magnetization in sediments and sedimentary rocks. These magnetite producing bacteria are widely distributed geographically and are found in both freshwater and marine sediments. However, these bacteria only produce magnetite crystals in poorly oxygenated environments (3 to 5% oxygen saturation = microaerophilic conditions = dysaerobic environments). The objective of this thesis is to explore the relationship between the production of biogenic magnetite and dysaerobic environments from a theoretical (Part I) and applied (Part II) perspective.;In Part I, an earlier dysaerobic biofacies model is revised and redefined based on recent literature and the hypothesis that poorly oxygenated (dysaerobic) sediments can be characterized by their inventory of biogenic magnetite is tested. For this purpose a multidisciplinary study was undertaken on surface sediments collected from 7 stations located along an oxygen gradient (from oxic to anoxic) across the Oxygen Minimum Zone off Peru. This study involved the measurement of total magnetic susceptibility, frequency dependent susceptibility, geochemical and crystal morphology (TEM image analysis). These results show that shallow dysaerobic surface sediments have a distinctive magnetic inventory compared to aerobic and anaerobic biofacies and that these magnetic characteristics are accounted for by in situ bacterial production of magnetite. Biogenic magnetic signature of sediment is proposed as a new attribute of dysaerobic (poorly oxygenated) marine facies. Speculations on the use of biogenic magnetite as an ecological and paleoecological indicator of oxygen tensions in near surface sediment pore waters are given.;In Part II, a theoretical concept is proposed linking sediment's biogenic magnetic signature, redox potential discontinuity horizon and dysaerobic (microaerophilic) habitats within the sediment column. Supporting empirical evidence is presented and a new magnetic sensing device (patent applied for) was developed for in situ vertical magnetic measurements (scanning) of sediment columns. A laboratory prototype was built and tested. These findings suggest that the vertical position of biogenic magnetic signature in sediment columns can be used to estimate the biochemical and biological conditions of the seafloor. The results are discussed in terms of the potential use of in situ magnetometry as a new remote sensing technique for status and trend monitoring of enrichment of coastal sediments with reactive organic matter.