Hydrothermal vents in deep and shallow ocean environments are geochemical conduits that link Earth’s interior with the oceans. These sites of active hydrothermal vents are distributed throughout the global network of ocean ridge spreading centers to ridge flanks and cool off-axis diffuse vent fields in ocean basins and occur in diverse lithological settings – including basalts, ultramafic rocks and sediments – and temperature regimes. These vents are loaded with nutrients from hydrothermal and magmatic activity that drive a vast sub-seafloor biosphere. Particularly near ocean ridge spreading centers magmatism and/or residual mantle heat serve as drivers for abiogenic mineral reactions generating reduced chemical species, which can be utilized by chemolithoautotrophic microbes. Additionally, microbial chemosynthesis within fluids drives near-vent productivity and support animal communities that inhabit these ecosystems. Water-rock-microbe interaction within the oceanic lithosphere considerably affects ocean water chemistry and the chemical composition of the oceanic lithosphere, effectively controlling global element cycles. This session seeks to combine new findings from a multi-disciplinary research community investigating the complex interplays between hydrothermal, magmatic and microbial processes in ocean floor settings, the diversity and extent of the shallow and deep subsurface biosphere, life in extreme environments, or their impact on global geochemical cycles. We also welcome contributions that study ongoing alteration processes and microbial activity in continental crust or oceanic lithosphere exposed on land, or ancient processes preserved in ophiolite sequences, from modern to Archaean systems.
Rock-hosted life through time - Integrating biosignatures of ancient and modern hydrothermal systems
MARUM, University of Bremen, Germany
Recent advances in analytical tools including more sensitive detection techniques have led to the discovery of microbial biosignatures in ultra-low biomass samples such as the oceanic lithosphere. Here, energy fluxes are low and microbial life has adapted to the slow cycling of sparsely available food and nutrient sources along cracks and fissures and the access to Earths chemical energy through water-rock interactions. Nevertheless, our understanding of the habitability of Earths lithosphere and potential connections to the surface world are still in its infancy. Rock-hosted microbes produce unique biosignatures such as diether and tetraether lipids produced by both bacteria and archaea. These lipid biomarkers can be used to trace chemo(litho)trophic life in extant, but also in past ecosystems due to their exceptional preservation as chemical fossils in mineral precipitates. Here, we present lipid data from a diverse set of past and present lithospheric habitats, ranging from the lower ocean crust to active and inactive hydrothermal vents and subsurface mantle rocks to terrestrial ophiolites in order to explore the diversity and abundance of microbes found in these systems. Furthermore, we will discuss the approaches we currently have in place to elucidate microbial metabolisms, microbe-mineral interactions and their potential roles in global geochemical cycles.
The impact of variable Fe concentrations on Fe-binding ligands, dissolved organics and microbial communities in hydrothermal plumes – an experimental study
1Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, Germany; 2Center for Marine Environmental Sciences (MARUM), University of Bremen, Germany; 3Department of Physics & Earth Sciences, Jacobs University Bremen, Germany; 4Geomicrobiology, Department of Marine Biogeochemistry, GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany; 5Molecular Biology of Microbial Consortia, Biocenter Klein Flottbek, University of Hamburg, Germany
Iron (Fe) plays an important role in aquatic environments as an essential, often biolimiting micronutrient but at very high concentrations can potentially be toxic. Consequently, microbes have evolved capabilities to influence Fe bioavailability through production of organic molecules, so called ligands, which can enhance iron bioavailability or be used for detoxification mechanisms. Hydrothermal vents represent a major source of Fe to the oceans and host specialized microbes that are likely capable of influencing Fe speciation through ligands. Through abiotic decomposition of marine dissolved organic matter (DOM) or abiotic synthesis, hydrothermal systems might themselves constitute an additional source of Fe-binding ligands. Iron complexation in these systems is likely crucial in mediating Fe distribution to the water column but the interdependencies are still not well understood. Here we present first insights from experiments that incubated hydrothermal plume microbes in an artificial seawater dilution over a range of different Fe concentrations. The results show how variable Fe levels in conjunction with dissolved organics control Fe-binding ligand systematics and ultimately how this relates to the structure of the microbial community. At lower Fe concentrations the final community structure is more diverse with certain Epsilonproteobacteria as the most dominant group. Overall, ligand concentrations remain relatively low but the diversity of documented Fe-binding DOM formulas is high. In contrast, high Fe incubations are dominated by a group of Gammaproteobacteria and show high ligand concentrations but a very limited diversity of Fe formulas. These findings are further discussed in context of DOM characteristics and ligand stability constants.
Biomineralization processes in low-temperature, shallow-water hydrothermal vent at Tagoro submarine volcano, El Hierro Island (Central East Atlantic)
1Christian-Albrechts-University Kiel, Kiel, Germany; 2Geological Survey of Spain, Madrid, Spain; 3U.S. Geological Survey, Santa Cruz, Ca, United States; 4Portuguese Task Group for the Extension of the Continental Shelf, Paço de Arcos, Portugal
A novel hydrothermal system was discovered at the summit of the underwater Tagoro volcano at 89–120 m depth after the 2011–2012 eruption, characterized by the low-temperature venting of Fe-rich fluids that produced a seafloor draped by extensive Fe-flocculate deposits. The basanite-hornitos are capped by mm- to cm-thick hydrothermally derived Fe-oxyhydroxide sediment and contain micro-cracks and degasification vesicles filled by sulfides (mostly pyrite) and covered by sulfur-oxidizing bacterial mats. Electron microprobe studies on Fe-oxyhydroxide crusts show the presence of various organomineral structures, mainly twisted stalks and sheaths covered by iron-silica deposits, reflecting microbial iron-oxidation from the hydrothermal fluids. Sequencing of 16S rRNA genes also reveals the presence of other microorganisms involved in sulfur and methane cycles. Samples collected from hornito chimneys contain silicified microorganisms coated by Fe-rich precipitates. The rapid silicification may have been indirectly promoted by microorganisms acting as nucleation sites. We suggest that this type of hydrothermal deposits might be more frequent than presently reported to occur in submarine volcanoes. The discovery of this mineralization system and associated microbiota identifies a potential Fe-based chemosynthetic ecosystem, which typically have been studied at spreading centers and arc volcanoes. This underscores the importance of geomicrobiological interactions in shaping mineral deposits on Earth today, and in the geological past. This hydrothermal system provides an excellent laboratory to study the formation and evolution of newly formed hydrothermal deposits and their association with microbiota at an intraplate hot-spot volcanic edifice under low-temperature, shallow-water conditions.
Unexpected high amounts of H2 produced during serpentinization at magma-poor rifted margins
1MARUM – Center for Marine Environmental Sciences, University of Bremen, Germany; 2Department of Geosciences, University of Bremen, Germany; 3Bayerisches Geoinstitut, University of Bayreuth, Germany
At magma-poor rifted margins, serpentinization of lherzolitic mantle rocks releases molecular hydrogen (H2) that supports chemosynthesis-based deep life. Until now, however, H2 fluxes in these systems remain largely unquantified. To help closing this knowledge gap we investigated serpentinization and H2 production using drill core samples from the West Iberia margin (Ocean Drilling Program Leg 103, Hole 637A).
The mostly lherzolitic samples are strongly serpentinized, consist of serpentine with little magnetite, and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg+Fe) < 0.8, and exhibits distinct compositional trends towards a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios range from 0.6–0.92 and 0.58–0.8, respectively. Our data show that more than 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the initial serpentinization of the samples at temperatures of <200°C likely produced about 100–250 mmol H2 per kg rock, which is 2–3 times more than previously estimated. The cold, late-stage weathering of the serpentinites at the seafloor caused additional H2 formation.
Owing to generally lower geothermal gradients, the amounts of H2 produced under conditions close to/within the habitable zone at magma-poor margins are likely larger than those at slow-spreading mid-ocean ridges. These settings may hence be particularly suitable environments for hydrogenotrophic microbial life.
Redox conditions during deserpentinization in western Elba Island, Italy
1Freie Universität Berlin, Germany; 2Universität Bremen, Germany
The observation of oxidized arc melts has led to a discussion about the redox conditions during the dehydration reactions of serpentinites in subduction zones. The discussed range of oxygen fugacities (fO2) between+5 and -2 log units relatively to the QFM buffer allows sulfur to be present either as oxidized or reduced species.
This work investigates the development of the fO2 with serpentines form the western part of the island Elba in Italy. We compared observations of opaque mineral phases and silicates with thermodynamic models. The opaque mineral phases have previously shown to be a good indicator for the redox conditons during the hydration of ultramafic rocks.
The samples have faced different metamorphic grades during the contact metamorphism of the 6.9 Ma Mt. Capanne pluton up to the Amphibole-facies. The peak assemblage shows the paragenesis of prograde grown anthophyllite and olivine. The omnipresence of magnetite between 500 °C and 650 °C indicates an fO2 above the QFM buffer at these temperatures. However, the fO2 does not exceeded the Mt-Hm buffer because hematite has not formed. The maximum fO2 is 2 log units above the QFM buffer and limited due to the ubiquitous presence of pentlandite in the serpentinites. The most abundant paragenesis of pentlandite-magnetite-heazlewoodite and pentlandite-magnetite-pyrrhotite is in equilibrium with 0.01-0.1 mol/kg H2S. Combined with the low sulfur concentrations below 200 ppm in the bulk rock composition a loss of sulfur as a reduced species in the form of H2S is indicated.