Low δ¹³C in tests of live epibenthic and endobenthic foraminifera at a site of active methane seepage

Abstract

To investigate the use of benthic foraminifera as a means to document ancient methane release, we determined the stable isotopic composition of tests of live (Rose Bengal stained) and dead specimens of epibenthic Fontbotia wuellerstorfi , preferentially used in paleoceanographic reconstructions, and of endobenthic high‐latitude Cassidulina neoteretis and Cassidulina reniforme from a cold methane‐venting seep off northern Norway. We collected foraminiferal tests from three push cores and nine multiple cores obtained with a remotely operated vehicle and a video‐guided multiple corer, respectively. All sampled sites except one control site are situated at the Håkon Mosby mud volcano (HMMV) on the Barents Sea continental slope in 1250 m water depth. At the HMMV in areas densely populated by pogonophoran tube worms, δ 13 C values of cytoplasm‐containing epibenthic F. wuellerstorfi are by up to 4.4‰ lower than at control site, thus representing the lowest values hitherto reported for this species. Live C. neoteretis and C. reniforme reach δ 13 C values of −7.5 and −5.5‰ Vienna Pee Dee Belemnite (VPDB), respectively, whereas δ 13 C values of their empty tests are higher by 4‰ and 3‰. However, δ 13 C values of empty tests are never lower than those of stained specimens, although they are still lower than empty tests from the control site. This indicates that authigenic calcite precipitates at or below the sediment surface are not significantly influencing the stable isotopic composition of foraminiferal shells. The comparatively high δ 13 C results rather from upward convection of pore water and fluid mud during active methane venting phases at these sites. These processes mingle tests just recently calcified with older ones secreted at intermittent times of less or no methane discharge. Since cytoplasm‐containing specimens of suspension feeder F. wuellerstorfi are almost exclusively found attached to pogonophores, which protrude up to 3 cm above the sediment, and δ 13 C values of bottom‐water‐dissolved inorganic carbon (DIC) are not significantly depleted, we conclude that low test δ 13 C values of F. wuellerstorfi are the result of incorporation of heavily 13 C‐depleted methanotrophic biomass that these specimens feed on rather than because of low bottom water δ 13 C DIC . Alternatively, the pogonophores, which are rooted at depth in the upper sediment column, may serve as a conduit for depleted δ 13 C DIC that ultimately influences the calcification process of F. wuellerstorfi attached to the pogonophoran tube well above the sediment/water interface. The lowest δ 13 C of live specimens of the endobenthic C. neoteretis and C. reniforme are within the range of pore water δ 13 C DIC values, which exceed those that could be due to organic matter decomposition, and thus, in fact, document active methane release in the sediment.