A record of reversed polarity carried by the iron sulphide greigite in British early Pleistocene sediments

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doi: 10.1016/0012-821X(94)90032-9
Authors:Hallam, D. F.; Maher, B. A.
Author Affiliations:Primary:
University of East Anglia, School of Environmental Sciences, Norwich, United Kingdom
Volume Title:Earth and Planetary Science Letters
Source:Earth and Planetary Science Letters, 121(1-2), p.71-80. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X
Publication Date:1994
Note:In English. 17 refs.; illus. incl. chart, strat. col., sketch map
Summary:Palaeomagnetic measurements were made on samples extracted from a short sequence of early Pleistocene estuarine clays, now exposed in a sea cliff near Sheringham on the north Norfolk coast, UK. On the basis of earlier palynological work, these clays had been ascribed a Pastonian (late Tiglian) age. The clays show marked changes in colour, from reddish-brown at the top of the unit, to blue-grey in the middle, and grey-brown at the base. The palaeomagnetic data vary in close association with these colour changes. The top and basal brown clays show scattered normal directions of low intensity, while the middle blue clays show strongly clustered reversed directions, of much higher intensities. Some samples taken from the boundary between the middle blue clays and upper red clays show upon demagnetisation a normal overprint on a stable reversed polarity. Using high-gradient magnetic extraction, magnetic concentrates have been obtained from the strongly magnetic middle blue clays. The presence of iron sulphide minerals in these concentrates was identified using energy-dispersive X-ray analysis during scanning electron microscopy. More specifically, X-ray diffraction identifies greigite as the only detectable ferrimagnetic mineral in the magnetic concentrates. Rock magnetic measurements show clear qualitative differences in the magnetic mineralogies of the three clay subunits, but absolute identification of the magnetic mineralogy of the weakly magnetic upper and basal brown clays has not yet been possible. We interpret the sequence as a primary reversed polarity record. This record is carried by the iron sulphide greigite as a chemical remanence acquired during "syn"-depositional reduction of iron via the decomposition of organic material in these anoxic tidal clays. Subsequently, the upper and basal subunits of the clay have been oxidised by permeation of groundwater from the adjacent coarse-grained sediments. Most of the griegite in the oxidised margins of the clay has been altered as a result, to a new, less efficient magnetic recording material which thus carries a later, scattered, low-intensity, normal overprint.
Subjects:Alternating field demagnetization; Cenozoic; Chemical remanent magnetization; Clastic sediments; Clay; Demagnetization; Greigite; Isothermal remanent magnetization; Lower Pleistocene; Magnetic inclination; Magnetic intensity; Magnetic susceptibility; Magnetization; Natural remanent magnetization; Paleomagnetism; Pleistocene; Quaternary; Remanent magnetization; Reversals; Sampling; Sediments; Sulfides; X-ray data; X-ray diffraction data; East Anglia; England; Europe; Great Britain; Norfolk England; United Kingdom; Western Europe; Sheringham England; Skelding Hill; Tiglian; West Runton England
Abstract Numbers:94M/3526
Record ID:1994020706
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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