HR: 1340h
AN: PP23B-1437    [Abstracts]
TI: Methane Venting Documented by Lowest Hitherto Reported $\delta$$^{13}$C of Live Epibenthic Foraminifer {\it Fontbotia wuellerstorfi}
AU: * Mackensen, A
EM: amackensen@awi-bremerhaven.de
AF: Alfred Wegener Institute, Columbusstrasse, Bremerhaven, 27568 Germany
AU: Wollenburg, J
EM: jwollenburg@awi-bremerhaven.de
AF: Alfred Wegener Institute, Columbusstrasse, Bremerhaven, 27568 Germany
AU: Licari, L
EM: llicari@awi-bremerhaven.de
AF: Alfred Wegener Institute, Columbusstrasse, Bremerhaven, 27568 Germany
AB: To investigate the use of benthic foraminifera as a means to assess modern and document ancient methane release, we determined the stable isotopic composition of tests of several live and dead species. Foraminiferal shells were collected from three push-core samples obtained with a ROV and three multiple corer samples. All samples are from the Haakon Mosby Mud Volcano (HMMV), a cold methane-venting seep at the Barents Sea continental slope in 1250 m water depth. We compared stable isotope ratios of these samples with those from a close-by reference station being not influenced by methane seepage but otherwise by the same overall environmental conditions. In areas densely populated by pogonophors, $\delta$$^{13}$C values of live {\it F. wuellerstorfi} are by 4.4$\permil$ lower than at the reference station and thus the lowest values hitherto reported for this species. Also {\it Cassidulina neoteretis}, a common shallow infaunal high Arctic species, reaches $\delta$$^{13}$C values of -7.5$\permil$ VPDB, which is by 6.2$\permil$ lower than at reference station. Surprisingly, $\delta$$^{13}$C of empty {\it F. wuellerstorfi} and {\it C. neoteretis}, tests are by 1 to 2$\permil$ and by about 4$\permil$, respectively, higher than those of live specimens, although still significantly lower than empty tests from the reference station. This documents that overgrowth at or below the sediment surface is not influencing the stable isotopic composition of these shells. It rather demonstrates that disturbance of sediments due to upward convection of porewater and fluid mud during active methane venting phases at this site mingles tests just recently calcified with older ones secreted at intermittent times of less or no methane discharge. Live epibenthic and shallow endobenthic foraminiferal $\delta$$^{13}$C values at HMMV exceed those that could be due to organic matter decomposition and thus in fact document active methane release. However, empty test $\delta$$^{13}$C values do not differ significantly from those measured in high productivity non-seep environments, but might be distinguished by their greater carbon isotopic variability.
DE: 9315 Arctic region
DE: 4267 Paleoceanography
DE: 3030 Micropaleontology
DE: 1050 Marine geochemistry (4835, 4850)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting