| 1. |
- Panieri, G., et al.
(author)
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Diagenetic Mg-calcite overgrowths on foraminiferal tests in the vicinity of methane seeps.
- 2017
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In: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 458, s. 203-212
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Journal article (peer-reviewed)abstract
- Methane is a potent greenhouse gas and some episodes of past global warming appear to coincide with its massive release from seafloor sediments as suggested by carbon isotope records of foraminifera. Here, we present structural, geochemical, and stable carbon isotope data from single foraminiferal calcite tests and authigenic Mg-calcite overgrowths in a sediment core recovered from an area of active methane seepage in western Svalbard at ca. 340 m water depth. The foraminifera are from intervals in the core where conventional bulk foraminiferal δ13C values are as low as −11.3 ‰. Mg/Ca analyses of the foraminiferal tests reveal that even tests for which there is no morphological evidence for secondary authigenic carbonate can contain Mg-rich interlayers with Mg/Ca up to 220 mmol/mol. Transmission electron microscopy (TEM) of the contact point between the biogenic calcite and authigenic Mg-calcite layers shows that the two phases are structurally indistinguishable and they have the same crystallographic orientation. Secondary ion mass spectrometry (SIMS) analyses reveal that the Mg-rich layers are strongly depleted δ13C (δ13C as low as −34.1 ‰). These very low δ13C values indicate that the authigenic Mg-calcite precipitated from pore waters containing methane-derived dissolved inorganic carbon at the depth of the sulfate–methane transition zone (SMTZ). As the depth of the SMTZ can be located several meters below the sediment-seawater interface, interpretation of low foraminiferal δ13C values in ancient sediments in terms of the history of methane seepage at the seafloor must be undertaken with care.
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| 2. |
- Crémière, A., et al.
(author)
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Fluid source and methane-related diagenetic processes recorded in cold seep carbonates from the Alvheim channel, central North Sea
- 2016
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In: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 432, s. 16-33
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Journal article (peer-reviewed)abstract
- Integrated petrography, mineralogy, geochronology and geochemistry of cold seep carbonate crusts and free gas from the Alvheim channel elucidate diagenetic carbonate precipitation and related seepage histories in the central North Sea. Free gas isotope characteristics coupled with carbonate δ13C values as low as − 66‰ VPDB, indicate a predominantly microbial methane source with minor thermogenic contribution. We estimate that ~ 70% of the carbon sequestered into carbonate precipitates was derived from local oxidation of methane. The early stage of crust growth is represented by microcrystalline aragonite and Mg-calcite (10 to 40% mol MgCO3) cementing seafloor sediments consisting of clays, quartz, feldspar, and minor detrital low Mg-calcite and dolomite. Typical association of aragonite cement with coarse-grained detritus may reflect elevated fluid flow and flushing of fine particles prior to cementation close to the seafloor. Middle rare earth element enrichment in early generation microcrystalline cements containing framboidal pyrite indicates diagenetic precipitation within the zone of anaerobic methane oxidation contiguous to iron reduction. The later generation diagenetic phase corresponds to less abundant radial fibrous and botryoidal aragonite which lines cavities developed within the crusts. In contrast to early generation cements, late generation cavity infills have rare earth elements and Y patterns with small negative Ce anomalies similar to seawater, consistent with carbonate precipitation in a more open, seawater dominated system. Aragonite U–Th ages indicate carbonate precipitation between 6.09 and 3.46 kyr BP in the northern part of the channel, whereas in the southern part precipitation occurred between 1.94 and 0.81 kyr BP reflecting regional changes in fluid conduit position.
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