Publication Date:
2021-05-12
Description:
The subsurface evolution of shallow-sea hydrothermal fluids is a function ofmany factors including fluid–mineral
equilibria, phase separation, magmatic inputs, and mineral precipitation, all of which influence discharging fluid
chemistry and consequently associated seafloor microbial communities. Shallow-sea vent systems, however, are
understudied in this regard. In order to investigate subsurface processes in a shallow-sea hydrothermal vent, and
determine how these physical and chemical parameters influence the metabolic potential of the microbial communities,
three shallow-sea hydrothermal vents associated with Panarea Island (Italy) were characterized. Vent
fluids, pore fluids and gases at the three sites were sampled and analyzed for major and minor elements, redoxsensitive
compounds, free gas compositions, and strontiumisotopes. The corresponding data were used to 1) describe
the subsurface geochemical evolution of the fluids and 2) to evaluate the catabolic potential of 61 inorganic
redox reactions for in situ microbial communities. Generally, the vent fluids can be hot (up to 135 °C), acidic
(pH 1.9–5.7), and sulfidic (up to 2.5 mM H2S). Three distinct types of hydrothermal fluids were identified,
each with higher temperatures and lower pH,Mg and SO4, relative to seawater. Type 1 was consistently more saline
than Type 2, and both were more saline than seawater. Type 3 fluids were similar to or slightly depleted in
mostmajor ions relative to seawater. End-member calculations of conservative elements indicate that Type 1 and
Type 2 fluids are derived from two different sources, most likely 1) a deeper, higher salinity reservoir and 2) a
shallower, lower salinity reservoir, respectively, in a layered hydrothermal system. The deeper reservoir records
some of the highest end-member Cl concentrations to date, and developed as a result of recirculation of brine
fluids with long term loss of steam and volatiles due to past phase separation. No strong evidence for ongoing
phase separation is observed. Type 3 fluids are suggested to be mostly influenced by degassing of volatiles and
subsequently dissolution of CO2, H2S, and other gases into the aqueous phase. Gibbs energies (ΔGr) of redox reactions
that couple potential terminal electron acceptors (O2, NO3
−, MnIV, FeIII, SO4 2−, S0, CO2) with potential electron
donors (H2, NH4
+, Fe2+, Mn2+, H2S, CH4) were evaluated at in situ temperatures and compositions for each
site and by fluid type.When Gibbs energies of reaction are normalized per kilogram of hydrothermal fluid, sulfur
oxidation reactions are the most exergonic, while the oxidation of Fe2+, NH4
+, CH4, and Mn2+ is moderately energy
yielding. The energetic calculations indicate that the most robust microbial communities in the Panarea hot
springs combineH2S fromdeepwater–rock–gas interactions with O2 that is entrained via seawater mixing to fuel
their activities, regardless of site location or fluid type.
Description:
Published
Description:
21-45
Description:
4A. Clima e Oceani
Description:
JCR Journal
Description:
restricted
Keywords:
submarine hydrothermal systems
;
subsurface processes
;
thermodynamics
;
04. Solid Earth::04.04. Geology::04.04.12. Fluid Geochemistry
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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