Low-molecular-weight hydrocarbons in natural hydrothermal fluids have been attributed to abiogenic production by Fischer-Tropsch type (FTT) reactions, although clear evidence for such a process has ...been elusive. Here, we present concentration, and stable and radiocarbon isotope, data from hydrocarbons dissolved in hydrogen-rich fluids venting at the ultramafic-hosted Lost City Hydrothermal Field. A distinct "inverse" trend in the stable carbon and hydrogen isotopic composition of C₁ to C₄ hydrocarbons is compatible with FTT genesis. Radiocarbon evidence rules out seawater bicarbonate as the carbon source for FTT reactions, suggesting that a mantle-derived inorganic carbon source is leached from the host rocks. Our findings illustrate that the abiotic synthesis of hydrocarbons in nature may occur in the presence of ultramafic rocks, water, and moderate amounts of heat.
The Endeavour segment of the Juan de Fuca ridge is host to one of the most vigorous hydrothermal areas found on the global mid-ocean-ridge system, with five separate vent fields located within 15 km ...along the top of the ridge segment. Over the past decade, the largest of these vent fields, the 'Main Endeavour Field', has exhibited a constant spatial gradient in temperature and chloride concentration in its vent fluids, apparently driven by differences in the nature and extent of subsurface phase separation. This stable situation was disturbed on 8 June 1999 by an earthquake swarm. Owing to the nature of the seismic signals and the lack of new lava flows observed in the area during subsequent dives of the Alvin and Jason submersibles (August-September 1999), the event was interpreted to be tectonic in nature. Here we show that chemical data from hydrothermal fluid samples collected in September 1999 and June 2000 strongly suggest that the event was instead volcanic in origin. Volatile data from this event and an earlier one at 9° N on the East Pacific Rise show that such magmatic events can have profound and rapid effects on fluid-mineral equilibria, phase separation, 3He/heat ratios and fluxes of volatiles from submarine hydrothermal systems.
Brothers volcano, of the Kermadec intraoceanic arc, is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world. It has two distinct vent fields, known as ...the NW Caldera and Cone sites, whose geology, permeability, vent fluid compositions, mineralogy, and ore-forming conditions are in stark contrast to each other. The NW Caldera site strikes for ∼600 m in a SW–NE direction with chimneys occurring over a ∼145-m depth interval, between ∼1,690 and 1,545 m. At least 100 dead and active sulfide chimney spires occur in this field and are typically 2–3 m in height, with some reaching 6–7 m. Their ages (at time of sampling) fall broadly into three groups: <4, 23, and 35 years old. The chimneys typically occur near the base of individual fault-controlled benches on the caldera wall, striking in lines orthogonal to the slopes. Rarer are massive sulfide crusts 2–3 m thick. Two main types of chimney predominate: Cu-rich (up to 28.5 wt.% Cu) and, more commonly, Zn-rich (up to 43.8 wt.% Zn). Geochemical results show that Mo, Bi, Co, Se, Sn, and Au (up to 91 ppm) are correlated with the Cu mineralization, whereas Cd, Hg, Sb, Ag, and As are associated with the dominant Zn-rich mineralization. The Cone site comprises the Upper Cone site atop the summit of the recent (main) dacite cone and the Lower Cone site that straddles the summit of an older, smaller, more degraded dacite cone on the NE flank of the main cone. Huge volumes of diffuse venting are seen at the Lower Cone site, in contrast to venting at both the Upper Cone and NW Caldera sites. Individual vents are marked by low-relief (≤0.5 m) mounds comprising predominately native sulfur with bacterial mats. Vent fluids of the NW Caldera field are focused, hot (≤300°C), acidic (pH ≥ 2.8), metal-rich, and gas-poor. Calculated end-member fluids from NW Caldera vents indicate that phase separation has occurred, with Cl values ranging from 93% to 137% of seawater values. By contrast, vent fluids at the Cone site are diffuse, noticeably cooler (≤122°C), more acidic (pH 1.9), metal-poor, and gas-rich. Higher-than-seawater values of SO
4
and Mg in the Cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water/rock interactions. Iron oxide crusts 3 years in age cover the main cone summit and appear to have formed from Fe-rich brines. Evidence for magmatic contributions to the hydrothermal system at Brothers includes: high concentrations of dissolved CO
2
(e.g., 206 mM/kg at the Cone site); high CO
2
/
3
He; negative δD and δ
18
O
H2O
for vent fluids; negative δ
34
S for sulfides (to −4.6‰), sulfur (to −10.2‰), and δ
15
N
2
(to −3.5‰); vent fluid pH values to 1.9; and mineral assemblages common to high-sulfidation systems. Changing physicochemical conditions at the Brothers hydrothermal system, and especially the Cone site, occur over periods of months to hundreds of years, as shown by interlayered Cu + Au- and Zn-rich zones in chimneys, variable fluid and isotopic compositions, similar shifts in
3
He/
4
He values for both Cone and NW Caldera sites, and overprinting of “magmatic” mineral assemblages by water/rock-dominated assemblages. Metals, especially Cu and possibly Au, may be entering the hydrothermal system via the dissolution of metal-rich glasses. They are then transported rapidly up into the system via magmatic volatiles utilizing vertical (∼2.5 km long), narrow (∼300-m diameter) “pipes,” consistent with evidence of vent fluids forming at relatively shallow depths. The NW Caldera and Cone sites are considered to represent stages along a continuum between water/rock- and magmatic/hydrothermal-dominated end-members.
Tritium and helium isotope data provide key information on ocean
circulation, ventilation, and mixing, as well as the rates of biogeochemical
processes and deep-ocean hydrothermal processes. We ...present here global
oceanic datasets of tritium and helium isotope measurements made by numerous
researchers and laboratories over a period exceeding 60 years. The dataset's
DOI is https://doi.org/10.25921/c1sn-9631, and the data are available at
https://www.nodc.noaa.gov/ocads/data/0176626.xml (last access: 15 March
2019) or alternately
http://odv.awi.de/data/ocean/jenkins-tritium-helium-data-compilation/
(last access: 13 March 2019) and includes approximately 60 000 valid tritium
measurements, 63 000 valid helium isotope determinations, 57 000 dissolved
helium concentrations, and 34 000 dissolved neon concentrations. Some
quality control has been applied in that questionable data have been flagged
and clearly compromised data excluded entirely. Appropriate metadata have
been included, including geographic location, date, and sample depth. When
available, we include water temperature, salinity, and dissolved oxygen. Data
quality flags and data originator information (including methodology) are
also included. This paper provides an introduction to the dataset along with
some discussion of its broader qualities and graphics.
Using effective temperature and metallicity derived from SDSS spectra for image60,000 F- and G-type main-sequence stars, we develop polynomial models for estimating these parameters from the SDSS ...image and image colors. These photometric estimates have similar error properties as those determined from SDSS spectra. We apply this method to SDSS photometric data for over 2 million F/G stars and measure the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, which correspond to disk and halo. The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component is spatially invariant, while the median disk metallicity smoothly decreases with distance from the Galactic plane from -0.6 at 500 pc to -0.8 beyond several kiloparsecs. The absence of a correlation between metallicity and kinematics for disk stars is in a conflict with the traditional decomposition in terms of thin and thick disks. We detect coherent substructures in the kinematics-metallicity space, such as the Monoceros stream, which rotates faster than the LSR, and has a median metallicity of image, with an rms scatter of only image0.15 dex. We extrapolate our results to the performance expected from the Large Synoptic Survey Telescope (LSST) and estimate that LSST will obtain metallicity measurements accurate to 0.2 dex or better, with proper-motion measurements accurate to image0.5 mas yr super(-1), for about 200 million F/G dwarf stars within a distance limit of image100 kpc.
During E/V Nautilus NA072 expedition, multibeam sonar surveys located over 800 individual bubble streams rising from the Cascadia Margin between the Strait of Juan de Fuca and Cape Mendocino at ...depths between 104 and 2,073 m. Gas bubbles were collected directly at the seafloor using gastight sampling bottles. These bubbles were consistently composed of over 99% methane with traces of carbon dioxide, oxygen, nitrogen, noble gases, and more rarely higher hydrocarbons. A common previous view was that a biogenic source was responsible for seeps from within the gas hydrate stability zone (upper limit near 500‐m isobath) and a thermogenic source was responsible for seeps from the upper slope and the shelf. Higher hydrocarbons in deep seeps with a biogenic methane signature, as well as the lack of higher hydrocarbons in some shallower seeps with a thermogenic methane signature, show that the origin of the gas cannot simply be attributed to seep location on the margin. Instead, mixing and oxidation processes play an integral role. 3He/4He ratios at Coquille SW point to a contribution of 30% mantle helium, whereas all the other investigated sites are characterized by a crustal helium signature. Hence, the Coquille SW seeps are directly or indirectly connected to the mantle or to very young oceanic crust. The detection of mantle helium in these seeps can thus be used as a tracer for deep‐reaching fracture systems and their changing pathways.
Key Points
Mantle‐derived helium detected in cold methane seeps at the Cascadia Margin can be used as tracer for deep fracture systems
Multiple methane sources as well as mixing and oxidation processes are present at the Cascadia Margin cold seeps
The Cascadia Margin seeps are unequivocally dominated by methane
We study Milky Way kinematics using a sample of 18.8 million main-sequence stars with r < 20 and proper-motion measurements derived from Sloan Digital Sky Survey (SDSS) and POSS astrometry, including ...{approx}170,000 stars with radial-velocity measurements from the SDSS spectroscopic survey. Distances to stars are determined using a photometric-parallax relation, covering a distance range from {approx}100 pc to 10 kpc over a quarter of the sky at high Galactic latitudes (|b|>20{sup 0}). We find that in the region defined by 1 kpc <Z< 5 kpc and 3 kpc <R< 13 kpc, the rotational velocity for disk stars smoothly decreases, and all three components of the velocity dispersion increase, with distance from the Galactic plane. In contrast, the velocity ellipsoid for halo stars is aligned with a spherical coordinate system and appears to be spatially invariant within the probed volume. The velocity distribution of nearby (Z < 1 kpc) K/M stars is complex, and cannot be described by a standard Schwarzschild ellipsoid. For stars in a distance-limited subsample of stars (<100 pc), we detect a multi-modal velocity distribution consistent with that seen by HIPPARCOS. This strong non-Gaussianity significantly affects the measurements of the velocity-ellipsoid tilt and vertex deviation when using the Schwarzschild approximation. We develop and test a simple descriptive model for the overall kinematic behavior that captures these features over most of the probed volume, and can be used to search for substructure in kinematic and metallicity space. We use this model to predict further improvements in kinematic mapping of the Galaxy expected from Gaia and the Large Synoptic Survey Telescope.
Upwelling in the world's strongest current, the Antarctic Circumpolar Current, is thought to be driven by wind stress, surface buoyancy flux, and mixing generated from the interaction between bottom ...currents and rough topography. However, the impact of localized injection of heat by hydrothermal vents where the Antarctic Circumpolar Current interacts with mid‐ocean ridges remains poorly understood. Here a circumpolar compilation of helium and physical measurements are used to show that while geothermal heat is transferred to the ocean over a broad area by conduction, heat transfer by convection dominates near hydrothermal vents. Buoyant hydrothermal plumes decrease stratification above the vent source and increase stratification to the south, altering the local vertical diffusivity and diapycnal upwelling within 500 m of the sea floor by an order of magnitude. Both the helium tracer and stratification signals induced by hydrothermal input are advected by the flow and influence properties downstream.
Plain Language Summary
Oceans soak up over 90% of the energy from global warming and regulate the Earth's climate. Along the ocean floor, more than 630 hydrothermal vents are spewing superhot plumes of water out of cracks in the Earth's crust. At the same time, the ocean floor is being gently warmed by magma under the Earth's crust, known as geothermal heating. But few research studies have measured and compared the effect of both hydrothermal and geothermal heat sources on major ocean currents. In this study, we analyzed over 3 million temperature, salinity and helium data points across the Southern Ocean that houses the world's strongest current, the Antarctic Circumpolar Current. The aim of the study was to determine how hydrothermal heat and geothermal heat affect the already‐turbulent circulation of this current. The study finds that the circulation within a few hundred meters of hydrothermal vents in the Antarctic Circumpolar Current increases by tenfold, compared to circulation around it. The authors show, for the first time, that hydrothermal vents play a major role in ocean currents at a local scale (more than geothermal heat), and this role cannot be ignored, as has previously been done in climate modeling and ocean circulation research.
Key Points
Heat input by hydrothermal vents has a strong local influence on Southern Ocean circulation
Hydrothermal heat reduces stratification over vents and increases it to the south, altering abyssal mixing rates
Both hydrothermal heating and geothermal heating need to be accounted for in studies of abyssal circulation and mixing
Hydrothermal discharge from submarine arc volcanoes is thought to be an important contributor to global hydrothermal budgets, but quantitative flux measurements are scarce. Ahyi Seamount, a shallow ...(<100 m) submarine intraoceanic arc volcano located in the Commonwealth of the Northern Mariana Islands, erupted in May 2014. In May and December 2014, we sampled the hydrothermal plume created by the eruption and estimated chemical fluxes from Ahyi by combining shipboard hull‐mounted Acoustic Doppler Current Profile current vector measurements with continuous and discrete Conductivity, Temperature, and Depth (CTD) data. Towed CTD sections were conducted perpendicular to the mean current direction: a sampling strategy that optimized chemical flux calculations by reducing complexities introduced by temporal variability in the speed and direction of plume dispersion. The Ahyi plume had an elevated optical backscatter signal accompanied by evidence of reduced chemical species and a lowered pH. We found enriched concentrations of H2, 3He, CH4, particulate S, Mn, and Fe, observations consistent with a highly active hydrothermal system. The fluxes of magmatic 3He and Fe from Ahyi were similar to that measured at three slow‐spreading ridge‐crest sites, whereas CH4 and Mn were 100–1,000 times lower. This is the first study to constrain export fluxes of a shallow submarine arc volcano into the euphotic zone. However, our data were collected soon after an eruption and thus may not be fully representative of the longer‐term chemical inputs from Ahyi.
Key Points
This is the first study to constrain chemical export fluxes of a shallow submarine island‐arc volcano
3He and Fe fluxes were similar to that measured at three slow‐spreading ridge‐crest sites; CH4 and Mn were 100‐1,000 lower
An intraoceanic arc volcano can supply a pool of dissolved Fe and other elements into the euphotic zone, potentially enhancing photosynthetic activity
The Australian‐Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid‐ocean ridge system. Here, we report a multiyear effort to locate and characterize hydrothermal activity ...on two first‐order segments of the AAR: KR1 and KR2. To locate vent sites on each segment, we used profiles collected by Miniature Autonomous Plume Recorders on rock corers during R/V Araon cruises in March and December of 2011. Optical and oxidation‐reduction‐potential anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by ∼25 km. Forty profiles on KR1 identified 17 sites, some within a few kilometer of each other. The spatial density of hydrothermal activity along KR1 and KR2 (plume incidence of 0.34) is consistent with the global trend for a spreading rate of ∼70 mm/yr. The densest area of hydrothermal activity, named “Mujin,” occurred along the 20 km‐long inflated section near the segment center of KR1. Continuous plume surveys conducted in January–February of 2013 on R/V Araon found CH4/3He (1 − 15 × 106) and CH4/Mn (0.01–0.5) ratios in the plume samples, consistent with a basaltic‐hosted system and typical of ridges with intermediate spreading rates. Additionally, some of the plume samples exhibited slightly higher ratios of H2/3He and Fe/Mn than others, suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption. The Mujin‐field was populated by Kiwa crabs and seven‐armed Paulasterias starfish previously recorded on the East Scotia Ridge, raising the possibility of circum‐Antarctic biogeographic connections of vent fauna.
Key Points:
Hydrothermal vents were discovered on the Australian‐Antarctic Ridge near 62°S
Plume chemistry is similar to discharge on other intermediate‐rate ridges
Hydrothermal vent fauna, Kiwa crabs, and seven‐armed starfish, were collected
PDF
