Ultramafic hydrothermal systems on the Rainbow abyssal hill: a wide variety of active and fossil chemosynthetic habitats and communities

Hydrothermal circulation at ultramafic-hosted sites supports a large variety of high- and low-temperature hydrothermal vents and associated ecosystems. Along the Mid-Atlantic Ridge (MAR), different types of habitats for chemosynthesis-based organisms have been identified in a serpentinization context, e.g. the high temperature vents at Rainbow and Logatchev, and the low temperature, off-axis Lost City vents. Each displays a certain degree of isolation and endemic taxa. Much remains to be understood about the temporal dynamics and biogeography of these communities over geological time scales.
During the MOMARDREAM_08 cruise, numerous dead bivalve shells and associated carbonates were dredged from close to the active Rainbow vent field (36°N). This material indicates past hydrothermal activity on top of a heavily sedimented ultramafic structure, 2.5 km east of the Rainbow field (Clamstone site) at ~25 kyr, and on the slope of the same structure, 1.2 km north-east to Rainbow field (Ghost City site) at ~110 kyr.
The Clamstone site is characterized by abundant shells of the vesicomyid bivalve genus Phreagena (previously unknown from the MAR) distributed over a large area and associated with rarer specimens of the bivalve Thyasira. The shell isotopic signatures of the burrowing species Thyasira suggest an exposure to 13C-depleted sediment pore water DIC, resulting from methane oxidation. Conversely, Phreagena shells are enriched in 13C, more consistent with a higher contribution of seawater derived DIC at the sediment-water interface.
At the older Ghost City site authigenic and biogenic carbonates contain benthic molluscs, including abundant Bathymodiolus aff. azoricus specimens, and rarer examples of Phreagena and Thyasira, pectinids, protobranchs, and the gastropods Paralepetopsis ferrugivora, Helicrenion sp., Pseudosetia sp., Anatoma sp., and Lurifax sp. Other benthic fauna include isopods, ostracods, foraminifera, bryozoans and echinoderms. Some of these taxa are shared with other MAR vent sites. The low δ13C values in the Ghost City Bathymodiolus shells also suggest a contribution of depleted DIC from oxidized methane. The Ghost City carbonates are morphologically similar to those described at Lost City, lacking the metal sulfides which characterize high temperature vent deposits, and contain mineralogical features that were likely associated with methane and hydrogen rich alkaline fluids. These lines of evidence suggest that low-temperature serpentinization metal-depleted and alkaline fluids similar to Lost City fuelled the Ghost City high biomass community.
The Ghost City and Clamstone faunas show chemosynthetic communities have been present on the MAR for at least 110,000 years, with abundances and diversities different from the living vent communities at Rainbow, Lost City or Logatchev.
Ultramafic hosted hydrothermal circulation offers a range of electron donors, such as methane, sulfide, and possibly hydrogen, to autotrophic symbionts in a wide variety of different habitats both on sediment cover and mineral hard substrates. Our results indicate that high biomass assemblages of diverse chemosynthetic taxa, from both vent and seep genera, can be sustained in these various ultramafic habitats including those associated with alkaline fluid, not necessarily associated with ridge axis. Owing to their expected widespread spatial distribution and their geochemical diversity, such low temperature serpentine-hosted habitats might have played, therefore, a major role in the ability of chemosynthetic fauna to disperse over ocean basin scales.

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UPMC Univ Paris 06, LECOB UMR 8222, Benthic Ecogeochemistry Laboratory, OOB, 66650 Banyuls-sur-mer, France
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Crispin T.S.
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School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
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de Rafelis
UPMC Univ Paris 06, ISTeP UMR 7193, Biomineralization and Sedimentary environments Laboratory, 75252 Paris cedex 05, France
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Le Bris
UPMC Univ Paris 06, LECOB UMR 8222, Benthic Ecogeochemistry Laboratory, OOB, 66650 Banyuls-sur mer, France
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