Cold-seep ostracods from the western Svalbard margin : direct palaeo-indicator for methane seepage ?

Despite their high abundance and diversity, microfossil taxa adapted to a particular chemosynthetic environment have rarely been studied and are therefore poorly known. Here we report on an ostracod species, Rosaliella svalbardensis gen. et sp. nov., from a cold methane seep site at the western Svalbard margin, Fram Strait. The new species shows a distinct morphology, different from other eucytherurine ostracod genera. It has a marked similarity to Xylocythere, an ostracod genus known from chemosynthetic environments of wood falls and hydrothermal vents. Rosaliella svalbardensis is probably an endemic species or genus linked to methane seeps. We speculate that the surface ornamentation of pore clusters, secondary reticulation, and pit clusters may be related to ectosymbiosis with chemoautotrophic bacteria. This new discovery of specialized microfossil taxa is important because they can be used as an indicator species for past and present seep environments (http: //zoobank.org/urn:lsid:zoobank.org:pub:6075FF30-29D5-4DAB-9141-AE722CD3A69B).


Introduction
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 δ 13 C 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 de-tection 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 palaeomethane seepage.
Published by Copernicus Publications on behalf of The Micropalaeontological Society.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.

Materials and methods
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), andHorne et al. (2002).(Bate, 1972).Subcentral muscle scars composed of one boomerang-shaped frontal scar and a vertical row of four elongate adductor scars.
Especially the type species of both genera (i.e.Rosaliella svalbardensis gen.et sp.nov.and Xylocythere turnerae Maddocks and Steineck, 1987) have substantial similarity, for example, in the general patterns of primary reticulation and pore conuli distribution.However, Xylocythere species have a ventrolateral ridge and a spine on their posterior end, a more rectangular outline, and a less inflated shell (Maddocks and Steineck, 1987;Steineck et al., 1990).In contrast, Rosaliella does not have any ridge or spine, and has an oval outline and more inflated shell.In internal view, Xylocythere species have enlarged (tooth-like) anterior and posterior ends of median hinge bar in LV (Maddocks and Steineck, 1987;Steineck et al., 1990), but Rosaliella lacks such a tooth-like structure at each end of the median hinge bar in LV.Because these differences are substantial, we erect Rosaliella gen.nov.as an independent genus from Xylocythere.The type species Rosaliella svalbardensis is also similar to Laocoonella commensalis (de Vos, 1953) in surface ornamentation (de Vos, 1953;de Vos and Stock, 1956).But Rosaliella svalbardensis is much larger than Laocoonella commensalis.In addition, hingement of Lao-coonella is more similar to that of Xylocythere (in lacking denticulation at least in an end of median hinge bar; see de Vos, 1953;Maddocks and Steineck, 1987), rather than that of Rosaliella.Holotype.Adult female RV, USNM 696652 (Seep2) (Fig. 2f-j).
These results indicate that Rosaliella svalbardensis is associated with methane seepage and probably endemic to the methane seep environment.More specifically, the habitat of this species is probably related to the presence of bacte-rial mats.Its high abundance in an active seep site suggests that this species can be a good indicator of not only presence/absence but also of the strength of release of methane.Furthermore, Rosaliella svalbardensis has relatively large and well-calcified valves and a morphology (Figs.2-3).Thus, this species can be used as a direct palaeo-indicator for methane seepage allowing reconstructions of long-term changes in seepage activity.It is likely that fossil valves of this species will be discovered from long sediment cores from methane seep sites.
This species can be a useful indicator of palaeo-methane release.
2. The hypothesis that pore clusters, secondary reticulation, and pit clusters are related to ectosymbiosis of chemoautotrophic bacteria merits further investigation.
3. Macroevolution of chemosynthetic taxa in seep, vent, and organic fall habitats remains poorly understood (Smith et al., 2015).Thus, discovery of specialized taxa for the chemosynthetic environments is important especially in microfossil taxa that have abundant and excellent fossil records in deep-sea sediments and that are widely used in palaeoceanographic research.

Figure 1 .
Figure 1.Locality maps.(a) Map of Svalbard and the research area marked by red dot.(b) Details of research area showing positions of the multicore stations.Pockmark bathymetry data are from Bünz et al. (2012).

Table 1 .
Sample and locality information.Derivation of name.In honour of Rosalie F. Maddocks (University of Houston, USA) for her work on ostracods from chemosynthetic environments.