『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