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| Titel |
Post-depositional alteration of benthic foraminifera in a methane seep environment |
| VerfasserIn |
Andrea Schneider, Antoine Crémière, Giuliana Panieri, Aivo Lepland, Jochen Knies |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250132797
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| Publikation (Nr.) |
 EGU/EGU2016-13337.pdf |
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| Zusammenfassung |
| Benthic foraminifera tests from the sediment cores taken from the Vestnesa Ridge, one of the
northernmost known marine methane hydrate reservoir, were studied for their visual
appearance, mineral and stable carbon isotopic composition in order to explore their indicator
potential in a methane seep environment.
The Vestnesa Ridge is a sediment drift located in 1200m water depth at 79∘N at Svalbard´
s northwestern continental margin. Observations of gas flares originating from pockmarks
that are aligned along the crest of the ridge show ongoing methane emission. A distinct
sediment layer containing a fossilized assemblage of chemosynthetic bivalves indicates
methane seepage activity at least in the late Pleistocene. We have examined the state of
preservation and geochemical characteristics of foraminifera tests from this bivalve shell
horizon.
Tests of the benthic foraminifera species Cassidulina neoteretis display a variable degree
of post-depositional alteration and formation of diagenetic carbonate overgrowths on calcitic
primary tests. Using binoculars, scanning electron microscope imagery and energy dispersive
x-ray spectroscopy, we distinguish visually and mineralogically different diagenetic phases
on the external and internal test surfaces. Pristine and smooth test surfaces act as
nucleation templates for precipitation of authigenic Mg-calcite crystals causing
complete filling of chambers and encrustation of the external test surfaces. The
presence of Mg-calcite indicates the overgrowth is precipitating in sulfate-poor
sediments.
In addition to benthic foraminifera, we have studied the mineralogical and stable carbon
and oxygen isotope composition of authigenic carbonate nodules found in the bivalve shell
horizon. The mineralogical nature of the carbonates and overgrowths on the foraminifera tests
were found to be identical. The δ13C value of the carbonate nodules is as low as -32.3‰
indicating their methane-derived origin. Authigenic carbonate coated foraminifera tests are
also depleted in their δ13C isotopic composition.
Our findings demonstrate that up to 55% of the δ13C signal in the benthic foraminifera
test record has been created after burial of the tests by overgrowth with isotopically depleted
methane-derived authigenic carbon. Thus, the benthic foraminifera δ13C composition in the
presence of methane is a mixed signal comprising the δ13C value of the pristine foraminifera
test and of the authigenic overgrowth encrusting the test after burial in post-depositional
diagenetic processes.
This research was funded by the Norwegian Research Council through the Centre of
Excellence for Arctic Gas Hydrate, Environment and Climate (CAGE), grant no.
223259. |
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