Deformation-enhanced diagenesis and bacterial proliferation in the Nankai accretionary prism
<p>Understanding diagenetic reactions in accreted sediments is critical for establishing the balance of fluid sources and sinks in accretionary prisms, which is in turn important for assessing the fluid pressure field and the ability for faults to host seismic slip. For this reason, we studied...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-09-01
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| Series: | Solid Earth |
| Online Access: | https://se.copernicus.org/articles/12/2067/2021/se-12-2067-2021.pdf |
| Summary: | <p>Understanding diagenetic reactions in accreted sediments
is critical for establishing the balance of fluid sources and sinks in
accretionary prisms, which is in turn important for assessing the fluid
pressure field and the ability for faults to host seismic slip. For this
reason, we studied diagenetic reactions in deformation bands (shear zones
and veins) within deep mud sediments from the Nankai accretionary prism (SW
Japan) drilled at site C0001 during IODP Expedition 315, by means of
microscopic observation, X-ray diffraction, and major- and trace-element
analyses. Deformation bands are not only more compacted than the host
sediment but are also enriched in framboidal pyrite, as observed under
microscopy and confirmed by chalcophile-element enrichments (Fe, S, Cu, As,
Sb, Pb). In tandem, one shear zone sample displays a destabilization of
smectite or illite–smectite mixed layers and a slight crystallization of
illite relative to its sediment matrix, and another sample shows correlated
increases in B and Li in shear zones and veins compared to the host
sediment, both effects suggesting a transformation of smectite into illite
in deformation bands.</p>
<p>The two diagenetic reactions of sulfide precipitation and smectite-to-illite
transformation are explained by a combined action of sulfate-reducing and
methanogen bacteria, which strongly suggests an increased activity of
anaerobic microbial communities localized in deformation bands. This local
bacterial proliferation was possibly enhanced by the liberation of hydrogen
from strained phyllosilicates. We suggest that the proliferation of anoxic
bacteria, boosted by deformation, may contribute to the pore water
freshening observed at depth in accretionary prisms. Deformation-enhanced
metabolic reactions may also explain the illitization observed in major
faults of accretionary prisms. Care is therefore needed before interpreting
illitization, and other diagenetic reactions as well, as evidence of shear
heating, as these might be biogenic instead of thermogenic.</p> |
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| ISSN: | 1869-9510 1869-9529 |