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Reductive diagenesis, magnetite dissolution, greigite growth and paleomagnetic smoothing in marine sediments: a new view

Record type: Article

In many anoxic sedimentary environments, the onset of sulfate reduction, and pyritization of detrital iron-bearing minerals, leads to a precipitous decline in magnetic mineral concentration during early diagenesis. The usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution. Detailed rock magnetic and electron microscope analyses of rapidly deposited (~ 7 cm/kyr) latest Pleistocene–Holocene sediments from the continental margins of Oman (22°22.4?N, 60°08.0?E) and northern California (38°24.8?N, 123°58.2?W) demonstrate that pyritization during early diagenesis also leads to the progressive down-core growth of the ferrimagnetic iron sulfide greigite. Greigite growth begins with nucleation of large concentrations of superparamagnetic (SP) nanoparticles at the inferred position of the sulfate–methane transition, which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of SP particles. Looping of hysteresis parameters on a “Day” plot records the dissolution of single domain (SD) (titano-)magnetite and the formation of SP greigite, which then slowly and progressively grows through its SD blocking volume and acquires a stable paleomagnetic signal. This looping trend is also evident in data from several published records (Oregon margin, Korea Strait, Japan Sea, Niger Fan, Argentine margin, and the Ontong–Java Plateau), indicating that these processes may be widespread in reducing environments. Our observations have profound implications for paleomagnetic records from sulfate-reducing environments. The paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10's of kyr, and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10's to 100's of kyr. We therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis.

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Citation

Rowan, Christopher J., Roberts, Andrew P. and Broadbent, Thomas (2009) Reductive diagenesis, magnetite dissolution, greigite growth and paleomagnetic smoothing in marine sediments: a new view Earth and Planetary Science Letters, 277, (1-2), pp. 223-235. (doi:10.1016/j.epsl.2008.10.016).

More information

Published date: 15 January 2009
Keywords: sediment diagenesis, sulfate reduction, magnetite dissolution, greigite, superparamagnetism, hysteresis
Organisations: Ocean and Earth Science

Identifiers

Local EPrints ID: 64793
URI: http://eprints.soton.ac.uk/id/eprint/64793
ISSN: 0012-821X
PURE UUID: 70277284-0c07-4ddb-9d41-0f69148429f2

Catalogue record

Date deposited: 15 Jan 2009
Last modified: 17 Jul 2017 14:11

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Contributors

Author: Christopher J. Rowan
Author: Andrew P. Roberts
Author: Thomas Broadbent

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