Effectiveness of Fluorescent Viability Assays in Studies of Arctic Cold Seep Foraminifera

Abstract

Highly negative δ13C values in fossil foraminifera from methane cold seeps have been proposed to reflect episodes of methane release from gas hydrate dissociation or free gas reservoirs triggered by climatic changes in the past. Because most studies on live foraminifera are based on the presence of Rose Bengal staining, that colors the cytoplasm of both live and recently dead individuals it remains unclear if, and to what extent live foraminifera incorporate methane-derived carbon during biomineralization, or whether the isotopic signature is mostly affected by authigenic overgrowth. In this paper, modern foraminiferal assemblages from a gas hydrate province Vestnesa Ridge (∼1,200 m water depth, northeastern Fram Strait) and from Storfjordrenna (∼400 m water depth in the western Barents Sea) is presented. By using the fluorescent viability assays CellTrackerTM Green (CTG) CMFDA and CellHunt Green (CHG) together with conventional Rose Bengal, it was possible to examine live and recently dead foraminifera separately. Metabolically active foraminifera were shown to inhabit methane-enriched sediments at both investigated locations. The benthic foraminiferal faunas were dominated by common Arctic species such as Melonis barleeanus, Cassidulina neoteretis, and Nonionellina labradorica. The combined usage of the fluorescence probe and Rose Bengal revealed only minor shifts in species compositions and differences in ratios between live and recently dead foraminifera from Storfjordrenna. There was no clear evidence that methane significantly affected the δ13C signature of the calcite of living specimens.

Influence of methane seepage on isotopic signatures in living deep-sea benthic foraminifera, 79° N

“…4 ), and to other non-seep sites from the Arctic Ocean (from − 0.3‰ to − 1‰) 42 . The δ 13 C values of C. neoteretis from this study are also considerably lower compared to previously published values (− 0.3‰) from the active ‘Lomvi’ pockmark 32 (see above).…”

“…− 2‰) 42 is a clear indication of methane influence. For comparison, previously published δ 13 C values of RB-stained M. barleeanus from the ‘Lomvi’ pockmark did not indicate any influence of methane (− 2.0‰) 32 . In this very active ‘Lomvi’ pockmark, studies have shown that methane transport often occurs via mini-fractures, and it is speculated that the gas can escape without affecting the foraminifera 43 .…”

“…To make our data comparable with other studies on live foraminifera, we deliberately used the most widely used staining method, the Rose Bengal. Since Rose Bengal indicates both live and dead cytoplasm, even weeks to months after the death of an individual 32 , 64 , we nevertheless refer here to Rose Bengal stained foraminifera as ‘live’ specimens, and empty, unstained tests as dead specimens.…”

“…Biological investigations from this pockmark documented the presence of species-rich live macro- and megafaunas 27 , 28 , carbonate outcrops 12 , 29 , and heterogeneous environmental conditions associated with methane release. Also, living benthic foraminifera from ‘Lomvi’ pockmark were previously investigated 30 – 32 ; nevertheless, the isotopic signatures of investigated specimens indicated no influence of methane. For the present study, the neighboring pockmark ‘Lunde’ was selected.…”

Response of benthic foraminifera to environmental successions of cold seeps from Vestnesa Ridge, Svalbard: Implications for interpretations of paleo-seepage environments

“…On the crest of Vestnesa Ridge a series of pockmarks occur (i.e, shallow seabed depressions where methane-rich fluids seep from the seabed), among which the two most active are informally referred to as 'Lomvi' and 'Lunde' (Bünz et al, 2012;Plaza-Faverola et al, 2015). In these pockmarks, TowCam video observations have revealed the presence of cold-seep related structures such as bacterial mats, macro-and megafaunas and methane-derived authigenic carbonate outcrops (Åström et al, 2016;Sztybor and Rasmussen, 2017;Åström et al, 2018;Melaniuk, 2021).…”