Magnetic petrology of equatorial Atlantic sediments: Electron microscopy results and their implications for environmental magnetic interpretation

Abstract

[1] The magnetic microparticle and nanoparticle inventories of marine sediments from equatorial Atlantic sites were investigated by scanning and transmission electron microscopy to classify all present detrital and authigenic magnetic mineral species and to investigate their regional distribution, origin, transport, and preservation. This information is used to establish source-to-sink relations and to constrain environmental magnetic proxy interpretations for this area. . Sediments from two depths corresponding to marine isotope stages 4 and 5.5 were processed. This selection represents glacial and interglacial conditions of sedimentation for the western, central, and eastern equatorial Atlantic and avoids interferences from subsurface and anoxic processes. Crystallographic, elemental, morphological, and granulometric data of more than 2000 magnetic particles were collected by scanning and transmission electron microscopy. On basis of these properties, nine particle classes could be defined: detrital magnetite, titanomagnetite (fragmental and euhedral), titanomagnetite-hemoilmentite intergrowths, silicates with magnetic inclusions, microcrystalline hematite, magnetite spherules, bacterial magnetite, goethite needles, and nanoparticle clusters. Each class can be associated with fluvial, eolian, subaeric, and submarine volcanic, biogenic, or chemogenic sources. Large-scale sedimentation patterns are delineated as well: detrital magnetite is typical of Amazon discharge, fragmental titanomagnetite is a submarine weathering product of mid-ocean ridge basalts, and titanomagnetite-hemoilmenite intergrowths are common magnetic particles in West African dust. This clear regionalization underlines that magnetic petrology is an excellent indicator of source-to-sink relations. Hematite encrustations, magnetic spherules, and nanoparticle clusters were found at all investigated sites, while bacterial magnetite and authigenic hematite were only detected at the more oxic western site. At the eastern site, surface pits and crevices were seen on the crystal faces indicating subtle early diagenetic reductive dissolution. It was observed that paleoclimatic signatures of magnetogranulometric parameters such as the ratio of anhysteretic and isothermal remanent magnetizations can be formed either by mixing of multiple sources with separate, relatively narrow grain size ranges (western site) or by variable sorting of a single source with a broad grain size distribution (eastern site). Hematite, goethite, and possibly ferrihydrite nanoparticles occur in all sediment cores studied and have either high-coercive or superparamagnetic properties depending on their partly ultrafine grain sizes. These two magnetic fractions are generally discussed as separate fractions, but we suggest that they could actually be genetically linked.

Magnetic particle characterization in the Seine river system: Implications for the determination of natural versus anthropogenic input

“…In contrast, fine‐grained goethite is rather a typical product of soil weathering processes [ Jambor and Dutrizac , 1998]. Microcrystalline hematite overgrowths on oxidized magnetite or other, e.g., siliceous grains were described as a conversion product of aged ferrihydrite coatings [ Walker et al , 1981; Freeman , 1986] typical of fluvial material [e.g., Franke et al , 2007b]. The mean grain sizes of samples shown in Figure 3 are typical values of coarse PSD (near MD) magnetite comparable to values reported for fluvial material by Desenfant et al [2004] in the Arc river in southern France.…”

Magnetic and microscopic characterization of magnetite nanoparticles adhered to clay surfaces

“…This anisotropy allows an assemblage to align itself with a magnetizing field in industrial applications and may potentially allow such assemblages to align themselves in the presence of the Earth's magnetic field. Similar assemblages of interacting superparamagnetic grains have been observed in natural settings in previous studies (Lovely et al 1987;Sparks et al 1990;Franke et al 2007). Although these superparamagnetic assemblages would be incapable of retaining a magnetic remanence, it may be possible to recover a recording of the Earth's ancient magnetic field direction in sediments containing a high proportion of such assemblages by measuring a sample's anisotropy of magnetic susceptibility (AMS) tensor.…”

Magnetic mineral inventory of equatorial Atlantic Ocean marine sediments off Senegal-glacial and interglacial contrast

“…The presence of Ti is diagnostic of volcanic spherules (Fredriksson & Martin 1963; Freeman 1986; Franke et al 2007b). The broken spherule observed in our sediments made it possible to measure the internal composition.…”