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| Titel |
Using noble gases in the pore water of ocean sediments to characterize CH4 seepage off the coast of New Zealand |
| VerfasserIn |
Yama Tomonaga, Matthias S. Brennwald, Rolf Kipfer |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250074231
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| Zusammenfassung |
| Newly developed analytical techniques to determine the abundances of noble gases in
sediment pore water [1, 5] allow noble-gas concentrations and isotope ratios to be measured
easily and routinely in unconsolidated lacustrine sediments [6, 7]. We applied these
techniques for the first time to ocean sediments to investigate an active cold methane seepage
system located in the South Pacific off the coast of New Zealand using 3He-4He ratios
determined in the sediment pore water.
Our results [8] show that more 3He-rich fluids are released in the vicinity of the
Pacific-Australian subduction zone than at the forearc stations located closer to the New
Zealand coast. However, the 3He-4He isotope signature in the sediment column indicates
that only a minor part of the He emanating from deeper strata originates from a (depleted)
mantle source. Hence, most He in the pore water is produced locally by the radioactive decay
of U and Th in the sediment minerals or in the underlying crustal rocks. Such an occurrence
of isotopically heavy crustal He also suggests that the source of the largest fraction
of methane is a near-surface geochemical reservoir. This finding is in line with a
previous δ13C study in the water column which concluded that the emanating methane
is most likely of biological origin and is formed in the upper few meters of the
sediment column [2]. The prevalence of isotopically heavy He agrees well with the
outcome of other previous studies on island arc systems [3, 4] which indicate that the
forearc regions are characterized by crustal He emission, whereas the volcanic
arc region is characterized by the presence of mantle He associated with rising
magma.
References
[1]   Brennwald, M. S., Hofer, M., Peeters, F., Aeschbach-Hertig, W., Strassmann,
K., Kipfer, R., and Imboden, D. M. (2003). Analysis of dissolved noble gases in
the pore water of lacustrine sediments. Limnol. Oceanogr.: Methods 1, 51–62.
[2]Â Â Â Faure, K., Greinert, J., Schneider von Deimling, J., McGinnis, D., Kipfer,
R., Linke, P. (2010). Methane seepage along the Hikurangi Margin of New
Zealand: Geochemical and physical data from the water column, sea surface and
atmosphere. Mar. Geol. 272, 170–188.
[3]Â Â Â Sano, Y., Nakajima, J. (2008). Geographical distribution of 3He-4He ratios
and seismic tomography in Japan. Geochem. J. 42, 51–60.
[4]Â Â Â Sano, Y., Wakita, H., Giggenbach, W., 1987. Island arc tectonics of New
Zealand manifested in helium isotope ratios. Geochim. Cosmochim. Acta 51 (7),
1855–1860.
[5]   Tomonaga, Y., Brennwald, M. S., Kipfer, R. (2011). An improved method
for the analysis of dissolved noble gases in the pore water of unconsolidated
sediments. Limnol. Oceanogr.: Methods 9, 42–49.
[6]   Tomonaga, Y., Brennwald, M. S., Kipfer, R. (2011). Spatial distribution and
flux of terrigenic He dissolved in the sediment pore water of Lake Van (Turkey).
Geochim. Cosmochim. Acta 75Â (10), 2848–2864.
[7]   Tomonaga, Y., Blättler, R., Brennwald, M. S., Kipfer, R. (2012). Interpreting
noble-gas concentrations as proxies for salinity and temperature in the world’s
largest soda lake (Lake Van, Turkey). J. Asian Earth Sci., 59, 99–107.
[8]   Tomonaga, Y., Brennwald, M. S., Kipfer, R. Using noble gases in ocean
sediments to characterize active methane seepage off the coast of New Zealand.
Mar. Geol., submitted. |
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