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| Titel |
Aragonite precipitation induced by anaerobic oxidation of methane in shallow-water seeps, Tyrrhenian Sea, Italy |
| VerfasserIn |
Johanna Wiedling, Hanna Kuhfuß, Christian Lott, Michael E. Böttcher, Anna Lichtschlag, Gunter Wegener, Christian Deusner, Wolfgang Bach, Miriam Weber |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094083
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| Publikation (Nr.) |
 EGU/EGU2014-9431.pdf |
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| Zusammenfassung |
| In the shallow-water organic-poor silicate sands off the West coast of Elba, Italy, we found
aragonite precipitates within a radius of 10 cm to methane seeps in 20 - 40 cm sediment
depth. The shallow seep site was mapped by SCUBA diving and in an area of 100 m2 nine
gas emission spots were observed. The gas emission, containing 73 Vol. % methane, was
measured to be 0.72 L m-2 d-1. Findings of anaerobic methane oxidizing archea (ANME 1,
2, 2a, 2b) and sulphate reducing bacteria (SRB) as well as in vitro rate measurements of
anaerobic oxidation of methane (AOM) with a maximum of 67 ± 7 nmol CH4 cm-3
d-1 led to the hypothesis that carbonate precipitation is coupled to these microbial
processes.
Porewater analysis showed elevated concentrations of dissolved inorganic carbon (DIC)
(up to 15.5 mmol L-1) and hydrogen sulfide (up to 6.6 mmol L-1). The presence of
bicarbonate and the ambient temperature (14 – 25 °C) facilitate the precipitation of
needle-shaped aragonite. Oxygen isotope compositions of the mineral are consistent with
the ambient temperatures and may indicate a recent diagenetic formation of this
mineral.
Although precipitation should not be preserved in these sandy permeable sediments,
influenced by seasonality, wave action, and fluid flow, we found up to 10-50 cm3 irregular
pieces of cemented sand grains, very often encrusting dead seagrass rhizomes.
Commonly known carbonate structures, especially from the deep sea, are chimneys,
mounds, hardgrounds and nodules. These structures are well known from seep and vent sites,
usually showing the same range of stable carbon isotope fractionation as the escaping
methane.
The permeable sediment at the Elba site possibly allows the gas to frequently change its
pathway to the sediment surface and thus precipitation can occure at several spots and more
irregular than in the reported sites. Preservation of precipitates, however, requires
sufficient authigenic aragonite to be formed before fluid dynamics changed the flow
path.
The Elba aragonites, showed a carbon isotope signature of -14.9ovs. VPDB,
mirroring the isotopic signature of the pore-water DIC at this sediment depth. Similar
δ13C-compositions of -15.3owere obtained for the discharging methane, giving room for
discussion about the origin of the gas.
We suppose that AOM is the main driver for aragonite precipitation in the permeable
sands at the shallow-water seeps because of (1) very low organic carbon contents (0.5 mg/g)
in the sediment, (2) 13C enrichment in the methane gas, (3) elevated DIC concentrations in
the pore-water, and (4) AOM in vitro activity. Thus, aragonite precipitates of the seep site
near Elba may represent a unique system to study ongoing abiogenic seep carbonate
formation at shallow depth as a modern analogue for seep carbonates occurring in the
geological record. |
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