Stable isotopic composition of bottom water and benthic foraminifera from Hakon Mosby Mud Vulcano@en

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, d13C values of cytoplasm-containing epibenthic F. wuellerstorfi are by up to 4.4 per mil lower than at control site, thus representing the lowest values hitherto reported for this species. Live C. neoteretis and C. reniforme reach d13C values of -7.5 and -5.5 per mil Vienna Pee Dee Belemnite (VPDB), respectively, whereas d13C values of their empty tests are higher by 4 per mil and 3 per mil. However, d13C 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 d13C 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 d13C values of bottom-water-dissolved inorganic carbon (DIC) are not significantly depleted, we conclude that low test d13C values of F. wuellerstorfi are the result of incorporation of heavily 13C-depleted methanotrophic biomass that these specimens feed on rather than because of low bottom water d13CDIC. […]为探究底栖有孔虫（benthic foraminifera）在记录古甲烷释放事件中的应用价值，我们针对两类目标类群开展了壳体稳定同位素组成测定：一类是优先用于古海洋重建（paleoceanographic reconstructions）的底上生活（epibenthic）有孔虫*Fontbotia wuellerstorfi*，分别采集其经玫瑰红染色（Rose Bengal stained）的活个体壳体与死壳体；另一类是采自挪威北部冷甲烷渗漏冷泉（methane-venting seep）区的高纬度底内生活（endobenthic）有孔虫*Cassidulina neoteretis*与*Cassidulina reniforme*。我们分别通过遥控无人潜水器（remotely operated vehicle, ROV）与视频导向多管采样器（video-guided multiple corer），从3个活塞岩心（push core）与9个多管岩心（multiple core）中获取了有孔虫壳体。除1个对照站点外，所有采样站位均位于巴伦支海大陆坡（Barents Sea continental slope）水深1250米处的哈康莫斯比泥火山（Håkon Mosby mud volcano, HMMV）。在HMMV区域内须腕动物管栖蠕虫（pogonophoran tube worms）密集分布的站位，含细胞质的*F. wuellerstorfi*壳体的δ¹³C值较对照站点最高低4.4‰，为该物种迄今已报道的最低δ¹³C值。活个体*C. neoteretis*与*C. reniforme*的δ¹³C值（以维也纳佩迪贝莱姆尼特标准Vienna Pee Dee Belemnite, VPDB计）分别可达-7.5‰与-5.5‰，而其空壳体（empty tests）的δ¹³C值则分别高出4‰与3‰。不过，空壳体的δ¹³C值从未低于染色个体，但仍较对照站点的空壳体更低。这表明沉积物表面或沉积物内部的自生方解石（authigenic calcite）并未显著影响有孔虫壳体的稳定同位素组成。较高的δ¹³C值实则源于活跃甲烷渗漏阶段孔隙水（pore water）与流体泥（fluid mud）的向上对流作用：该过程将新近钙化的壳体与甲烷排放较弱或停滞时期分泌的老壳体混合在了一起。由于悬浮食性（suspension feeder）的*F. wuellerstorfi*含细胞质个体几乎全部附着于须腕动物体上（该类生物可伸出沉积物表面达3厘米），且底层水溶解无机碳（bottom-water-dissolved inorganic carbon, DIC）的δ¹³C值并未显著降低，因此我们认为，*F. wuellerstorfi*壳体的低δ¹³C值并非源于底层水低δ¹³C_DIC，而是由于其摄食了富含¹³C亏损的甲烷氧化生物量（methanotrophic biomass），并将其同位素信号纳入壳体形成所致。[…]