Abstract
Foraminifera in sediments exposed to gas-hydrate dissociation are not expected to have cellular adaptations that facilitate inhabitation of chemosynthesis-based ecosystems because, to date, there are no known endemic seep foraminifera. To establish if foraminifera inhabit sediments impacted by gas-hydrate dissociation, we examined the cellular ultrastructure of Melonis barleeanus (Williamson, 1858) from the Vestnesa gas hydrate province (Arctic Ocean, west of Svalbard at ~79 °N; ~1200-m depth; n = 4). From sediments with gas hydrate indicators, living M. barleeanus had unusual pore plugs composed of a thick, fibrous meshwork; mitochondria were concentrated at the cell periphery, under pore plugs. While there was no evidence of endosymbioses with prokaryotes, most M. barleeanus specimens were associated with what appear to be Type I methanotrophic bacteria. One foraminifer had a particularly large bolus of these microbes concentrated near its aperture. This is the first documented instance of bona fide living M. barleeanus in gas-hydrate sediments and first documentation of a foraminifer living in close association with putative methanotrophs. Our observations have implications to paleoclimate records utilizing this foundational foraminiferal species.
Highlights
Methane hydrates are plausible energy sources, but they are natural hazards because methane decomposition adds carbon to the oceans and atmosphere, contributing to climate change
The timing and duration of past hydrate dissociation events can be interpreted by analyzing benthic foraminiferal tests from seep areas for their δ13C signature[5]
Benthic foraminiferal tests found adjacent to active methane seeps commonly have depleted δ13C values[5,6,7,8]
Summary
The Vestnesa Ridge is a NW-SE trending, ~100-km long, 1–2-km thick contourite located in the Arctic Ocean, west of Svalbard, at ~79°N (Fig. 1a,b). Core 15-2-893MC, hereafter referred to as core 893MC, was collected in an active seep site with focused flow in the Lomvi pockmark characterized by filamentous sulfide-oxidizing bacteria, outcropping carbonate crusts (Fig. 1f), and indurated hydrate. In core 893MC, there was a clear decrease in the isotopic values of the dissolved inorganic carbon with increasing sediment depth (i.e., δ13CDIC = −3.34‰ at 0–1 cm; −14.25‰ at 1–2 cm; −20.15‰ at 2–3 cm) This vertical pore water δ13CDIC gradient indicates that the location of 893MC was geochemically active at the time of collection and that methane oxidation was occurring close to the sediment-water interface. Both our visual observations of the seafloor and geochemical data confirm that the core 893MC collection locality was an active methane emission site when sampled
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