Rock magnetic signatures in diagenetically altered sediments from the Niger deep‐sea fan

Abstract

[1] Diagenesis has extensively affected the magnetic mineral inventory of organic-rich late Quaternary sediments in the Niger deep-sea fan. Changes in concentration, grain size, and coercivity document modifications of the primary magnetic mineral assemblages at two horizons. The first front, the modern iron redox boundary, is characterized by a drastic decline in magnetic mineral content, coarsening of the grain size spectrum, and reduction in coercivity. Beneath a second front, the transition from the suboxic to the sulfidic anoxic domain, a further but less pronounced decrease in concentration and bulk grain size occurs. Finer grains and higher coercive magnetic constituents substantially increase in the anoxic environment. Low-and high-temperature experiments were performed on bulk sediments and on extracts which have also been examined by X-ray diffraction. Thermomagnetic analyses proved ferrimagnetic titanomagnetites of terrigenous provenance as the principal primary magnetic mineral components. Their broad range of titanium contents reflects the volcanogenic traits of the Niger River drainage areas. Diagenetic alteration is not only a grain size selective process but also critically depends on titanomagnetite composition. Low-titanium compounds are less resistant to diagenetic dissolution. Intermediate titanium content titanomagnetite thus persists as the predominant magnetic mineral fraction in the sulfidic anoxic sediments. At the Fe redox boundary, precipitation of authigenic, possibly bacterial, magnetite is documented. The presence of hydrogen sulfide in the pore water suggests a formation of secondary magnetic iron sulfides in the anoxic domain. Grain size-specific data argue for a gradual development of a superparamagnetic and single-domain iron sulfide phase in this milieu, most likely greigite.Citation: Dillon, M., and U. Bleil (2006), Rock magnetic signatures in diagenetically altered sediments from the Niger deep-sea fan,

Diagenetic Fate of Biogenic Soft and Hard Magnetite in Chemically Stratified Sedimentary Environments of Mamanguá Ría, Brazil

“…The anticlockwise loop of hysteresis ratios fits a model where reductive diagenesis leads to selective dissolution of the finest (biogenic) magnetic particles within an initial detrital + biogenic magnetic mineral assemblage, which results in a relatively coarser residual assemblage, while reaction of diagenetically released iron with sulfide in the sulfidic diagenetic zone produces authigenic SP/SD greigite that causes the trend to loop back to higher M rs / M s values. This trend has been associated with progressive reductive diagenesis of shallow organic‐rich sediments around the world (Roberts, ; Rowan et al, ), including the Oman, northern California, Oregon, and Argentine margins, Korea Strait, Ontong‐Java Plateau, Niger Fan, and Japan Sea (Dillon & Bleil, ; Garming et al, ; Karlin, ; Liu et al, ; Rowan et al, ; Tarduno, , ; Yamazaki et al, ). The magnetization of sediments in the sulfidic zone in which greigite is proposed to grow is weak (Roberts, Zhao, et al, ), so greigite has not always been detected in this zone.…”

Discrimination of biogenic and detrital magnetite through a double Verwey transition temperature

“…The SD FORC distribution for samples from zone 3 (Figures k and l) is likely related to preservation of fine‐grained magnetite within sulfidic sediments. Preservation of fine‐grained magnetite (when large detrital grains are dissolved) would also produce a down‐core coercivity increase [e.g., Dillon and Bleil , ] and could cause the observed looping of hysteresis parameters in a Day plot [e.g., Yamazaki et al ., ; Rowan et al ., ]. But why did large magnetite grains dissolve, while some fine‐grained magnetite particles were preserved in sulfidic diagenetic environments?…”

Environmental magnetism: Principles and applications

“…As sedimentation continues, this zonation migrates upward so that sediments through which the anoxic zone has passed are eventually characterized by widespread dissolution of detrital magnetic minerals and occurrence of SP and SD greigite. This is a common situation in shallow marine settings [ Rey et al , 2000, 2005; Emiroglu et al , 2004; Liu et al , 2004; Kawamura et al , 2007; Zheng et al , 2010; Mohamed et al , 2011; Roberts et al , 2011a] and in deeper marginal marine sediments characterized by high accumulation rates [e.g., Leslie et al , 1990; Tric et al , 1991; Roberts and Turner , 1993; Florindo and Sagnotti , 1995; Horng et al , 1998; Robinson et al , 2000; Jiang et al , 2001; Roberts and Weaver , 2005; Dillon and Bleil , 2006; Vasiliev et al , 2008; Rowan et al , 2009; Roberts et al , 2011a], and is responsible for the widespread lack of genuine depositional paleomagnetic signals often reported in diagenetically reduced marine sediments.…”