It is important to understand causes behind changes in the activity of release of methane in the geological past because methane is a ∼ 25 times more powerful greenhouse gas than carbon dioxide, and it constitutes an important factor in regional and global climate change (Nisbet and Chappellaz, 2009; Consolaro et al., 2015; Hopcroft et al., 2017). Reconstructions of deep-sea seep activities in the geological past have often been based on δ13C values measured in foraminiferal shells, but the signals are often caused by secondary mineralization of diagenetic carbonate, making inferences about timing of seepage events difficult (Uchida et al., 2008; Consolaro et al., 2015; Sztybor and Rasmussen, 2017a). So far, apart from certain macrofossils (e.g. vesicomyid bivalves), very few other indicator species for the detection of past methane seepage in sedimentary records have been described (e.g. Sen Gupta et al., 1997; Bernhard et al., 2001). Because of their large size and low abundance compared to microfossils, quantitative studies of deep-sea macrofaunas are difficult.

Methane hydrate provinces are widely distributed in the Arctic Ocean (Biastoch et al., 2011). The stability of methane hydrate is known to be sensitive to climate change (Berndt et al., 2014). In turn, methane seepage may have contributed to rapid climate change (Nisbet and Chappellaz, 2009; Dickens, 2011). Release of methane creates a unique chemosynthetic ecosystem (Van Dover et al., 2003), and thus there may be unique microfossil communities or endemic microfossil species providing unequivocal indications for palaeo-methane seepage.

Ostracoda are small crustaceans that have bivalve-like calcified shells. They are diverse: > 20 000 living species are estimated, and, among them, ∼ 8000 species have been described (Horne et al., 2002; Rodriguez-Lazaro and Ruiz-Muñoz, 2012). Most species are sensitive to changes in various environmental factors (e.g. temperature, salinity, oxygen, organic matter supply) (Horne et al., 2002; Schellenberg, 2007; Yasuhara and Cronin, 2008; Mesquita-Joanes et al., 2012). Their calcified shells are abundantly preserved in marine sediments (Yasuhara et al., 2017). Thus, ostracods are a widely used microfossil group in the reconstruction of various palaeoceanographic and palaeoclimatological changes. They have been successfully applied in the reconstruction of past sea-level, temperature, salinity, and other environmental changes (Frenzel and Boomer, 2005; Yasuhara and Seto, 2006; Iwatani et al., 2012; Cronin, 2015). However, methane seep ostracods remain poorly investigated, and any ostracod species, genera, or faunas endemic or specific in methane seep environments have not been known until now (Karanovic and Brandão, 2015).

Here we report on deep-sea ostracods from Vestnesa Ridge in the eastern Fram Strait, carefully collected from an active pockmark generated by strong and persistent release of methane from the seafloor (e.g. Bünz et al., 2012; Sztybor and Rasmussen, 2017a, b). We discovered Rosaliella svalbardensis gen. et sp. nov., and this species or genus is likely endemic to methane seepage environments; thus, their well-calcified microfossil shells can be useful indicators of methane release in the past and present.

The samples were collected with a video-guided multicorer (MUC) during the RV Poseidon cruise 419 to Vestnesa Ridge in August 2011 (Fig. 1). In total three sites within the cold-seep pockmark (MUC 7, 9, 12) and one control station outside the pockmark (MUC 11) were sampled (Fig. 1; Table 1). The cores of MUC 7 and MUC 12 were taken in bacterial mats, while MUC 9 was retrieved from a field of (chemosynthetic) tubeworms. Each of the cores was subsampled on board. The cores were cut into 1 cm thick slices, preserved in alcohol stained with rose bengal, and kept cool until further processing. For this study, the core top 1 cm slices were used. In the laboratory the samples were wet-sieved (0.063, 0.1, and 1 mm) and then dried. We used the > 100 µm size fraction for ostracod analysis. This sieve size allows one to obtain adult and late-stage juvenile specimens of most species. All ostracod specimens in a sample were picked, mounted on microfossil slide, and identified to species level. The number of specimens refers to valves.

Uncoated ostracod specimens were digitally imaged with a Hitachi S-3400N variable pressure scanning electron microscope (SEM) in low-vacuum mode, at the Electron Microscope Unit, The University of Hong Kong. Figured specimens are deposited in the National Museum of Natural History (Washington, DC, USA; catalogue numbers USNM 696651–696672). M. Yasuhara's personal catalog number (Seep1–15, 17–23) is indicated in parentheses. For the higher classification scheme, we mainly refer to the World Ostracoda Database (Brandão et al., 2017), Whatley et al. (1993), and Horne et al. (2002).

Subclass Podocopa Sars, 1866

Order Podocopida Sars, 1866

Suborder Cytherocopina Baird, 1850

Superfamily Cytheroidea Baird, 1850

Family Cytheruridae Müller, 1894

Genus Rosaliella gen. nov.

Rosaliella svalbardensis gen. et sp. nov.(Figs. 2–3)

Rosaliella svalbardensis gen. et sp. nov. is described. This species can be a useful indicator of palaeo-methane release.