A New Insight Into Seawater-Basalt Exchange Reactions Based on Combined δ18O—Δ′17O—87Sr/86Sr Values of Hydrothermal Fluids From the Axial Seamount Volcano, Pacific Ocean

The δ18O values of submarine vent fluids are controlled by seawater-basalt exchange reactions, temperature of exchange, and to a lesser extent, by phase separation. These variations are translated into the δ18O values of submarine hydrothermal fluids between ca. 0 and + 4‰, a range defined by pristine seawater and equilibrium with basalt. Triple oxygen isotope systematics of submarine fluids remains underexplored. Knowing how δ17O and δ18O change simultaneously during seawater-basalt reaction has a potential to improve i) our understanding of sub-seafloor processes and ii) the rock-based reconstructions of ancient seawater. In this paper, we introduce the first combined δ17O-δ18O-87Sr/86Sr dataset measured in fluids collected from several high-temperature smoker- and anhydrite-type vent sites at the Axial Seamount volcano in the eastern Pacific Ocean. This dataset is supplemented by measurements of major, trace element concentrations and pH indicating that the fluids have reacted extensively with basalt. The salinities of these fluids range between 30 and 110% of seawater indicating that phase separation is an important process, potentially affecting their δ18O. The 87Sr/86Sr endmember values range between 0.7033 and 0.7039. The zero-Mg endmember δ18O values span from -0.9 to + 0.8‰, accompanied by the Δ′17O0.528 values ranging from around 0 to −0.04‰. However, the trajectory at individual site varies. The endmember values of fluids from focused vents exhibit moderate isotope shifts in δ′18O up to +0.8‰, and the shifts in Δ′17O are small, about −0.01‰. The diffuse anhydrite-type vent sites produce fluids that are significantly more scattered in δ′18O—Δ′17O space and cannot be explained by simple isothermal seawater-basalt reactions. To explain the observed variations and to provide constraints on more evolved fluids, we compute triple O isotope compositions of fluids using equilibrium calculations of seawater-basalt reaction, including a non-isothermal reaction that exemplifies complex alteration of oceanic crust. Using a Monte-Carlo simulation of the dual-porosity model, we show a range of possible simultaneous triple O and Sr isotope shifts experienced by seawater upon reaction with basalt. We show the possible variability of fluid values, and the causal effects that would normally be undetected with conventional δ18O measurements.

Draft Genome Sequence of Desulfurobacterium sp. Strain AV08, a Thermophilic Chemolithoautotroph Isolated from a Deep-Sea Hydrothermal Vent

A thermophilic chemolithoautotrophic bacterium was isolated from vent fluids at Axial Seamount, an active deep-sea volcano in the northeast Pacific Ocean. We present the draft genome sequence of Desulfurobacterium sp. strain AV08.

Axial Seamount: A Wired Submarine Volcano Observatory in the NE Pacific

<p>Axial Seamount is the most active submarine volcano in the NE Pacific Ocean, and is monitored by instruments connected to a cabled observatory (the US Ocean Observatories Initiative Cabled Array), supplemented by autonomous battery-powered instruments on the seafloor at ~1500 m depth.  Axial Seamount is a basaltic hot spot volcano superimposed on the Juan de Fuca spreading ridge, giving it a robust and apparently continuous magma supply.  It has had three effusive eruptions in the last 23 years in 1998, 2011, and 2015.  Deformation measurements have been conducted at Axial Seamount since the late 1980’s with bottom pressure recorders (BPRs) that can detect vertical movements of the seafloor with a resolution of ~1 cm.  This monitoring has produced a long-term time-series including co-eruption rapid deflation events of 2.5-3.2 meters, separated by continuous gradual inter-eruption inflation at rates that have varied between 15-80 cm/yr.  The overall pattern appears to be inflation-predictable, with eruptions repeatedly triggered at or near a critical level of inflation.  Using this pattern, the 2015 eruption was successfully forecast within a one-year time window, 7 months in advance.  As of January 2021, Axial Seamount has re-inflated ~2.1 m (~83%) of the 2.54 m it deflated during the 2015 eruption, but the rate of inflation has been decreasing since then.  Our current eruption forecast window is between 2022-2026, based on the latest rate of inflation.  Modeling of the seafloor deformation data along with other recent results from ocean bottom seismometers and multichannel seismic surveys inform our interpretation of the subsurface structure of the volcano and the geometry and depth of the shallow magma storage system.</p>

Seafloor incubation experiment with deep-sea hydrothermal vent fluid reveals effect of pressure and lag time on autotrophic microbial communities

Depressurization and sample processing delays may impact the outcome of shipboard microbial incubations of samples collected from the deep sea. To address this knowledge gap, we developed an ROV-powered incubator instrument to carry out and compare results from in situ and shipboard RNA Stable Isotope Probing (RNA-SIP) experiments to identify the key chemolithoautotrophic microbes and metabolisms in diffuse, low-temperature venting fluids from Axial Seamount. All the incubations showed microbial uptake of labelled bicarbonate primarily by thermophilic autotrophic Epsilonbacteraeota that oxidized hydrogen coupled with nitrate reduction. However, the in situ seafloor incubations showed higher abundances of transcripts annotated for aerobic processes suggesting that oxygen was lost from the hydrothermal fluid samples prior to shipboard analysis. Furthermore, transcripts for thermal stress proteins such as heat shock chaperones and proteases were significantly more abundant in the shipboard incubations suggesting that hydrostatic pressure ameliorated thermal stress in the metabolically active microbes in the seafloor incubations. Together, results indicate that while the autotrophic microbial communities in the shipboard and seafloor experiments behaved similarly, there were distinct differences that provide new insight into the activities of natural microbial assemblages under near-native conditions in the ocean.