Rowan,C.J., Roberts,A.P. and Broadbent,T.(2009): Reductive diagenesis, magnetite dissolution, greigite growth and paleomagnetic smoothing in marine sediments: A new view. Earth and Planetary Science Letters, 277, 223-235.

『海洋堆積物における還元続成作用と磁鉄鉱溶解とグレイジャイト成長と古磁気スムーズ化:新しい考え』


Abstract
 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.

Keywords: sediment diagenesis; sulfate reduction; magnetite dissolution; greigite; superparamagnetism; hysteresis』

1. Introduction
2. Materials and methods
 2.1. Oman margin: core CD143-55705
 2.2. Northern California margin: core L13-81-G138
3. Results
 3.1. Oman margin core CD143-55705
  3.1.1. Zone 1 (0-2 m)
  3.1.2. Zone 2 (2-4.65 m)
  3.1.3. Zone 3 (4.65-10.8 m)
 3.2. Northern California margin core L13-81-G138
4. Discussion
 4.1. Interpretation of diagenetic trends
 4.2. SP enhancement during reductive diagenesis
 4.3. Evidence for SMT migration within the sediment column
 4.4. Timescales of magnetic iron sulfide growth
 4.5. Similar down-core patterns in globally distributed records
 4.6. Are the above processes ubiquitous in reducing sediments?
5. Conclusions and implications for the magnetic record in anoxic marine sediments
Acknowledgements
Appendix A. Supplementary data
References


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