Vent Fluids Research Articles

Effects of Local Acidification on Benthic Communities at Shallow Hydrothermal Vents of the Aeolian Islands (Southern Tyrrhenian, Mediterranean Sea).

Simple SummaryOcean acidification is causing major changes in marine ecosystems, with varying levels of impact depending on the region and habitat investigated. Here, we report noticeable changes in both meio- and macrobenthic assemblages at shallow hydrothermal vents located in the Mediterranean Sea. In general, the areas impacted by the vent fluids showed decrease in the abundance of several taxa and a shift in community composition, but with a clear biomass reduction evident only for macrofauna. CO2 emissions at shallow hydrothermal vents cause a progressive simplification of community structure and a general biodiversity decline due to the loss of the most sensitive meio- and macrofaunal taxa, which were replaced by the more tolerant groups, such as oligochaetes, or highly mobile species, able to escape from extreme conditions. Our results provide new insight on the tolerance of marine meio- and macrofaunal taxa to the extreme conditions generated by hydrothermal vent emissions in shallow-water ecosystems.The Aeolian Islands (Mediterranean Sea) host a unique hydrothermal system called the “Smoking Land” due to the presence of over 200 volcanic CO2-vents, resulting in water acidification phenomena and the creation of an acidified benthic environment. Here, we report the results of a study conducted at three sites located at ca. 16, 40, and 80 m of depth, and characterized by CO2 emissions to assess the effects of acidification on meio- and macrobenthic assemblages. Acidification caused significant changes in both meio- and macrofaunal assemblages, with a clear decrease in terms of abundance and a shift in community composition. A noticeable reduction in biomass was observed only for macrofauna. The most sensitive meiofaunal taxa were kinorhynchs and turbellarians that disappeared at the CO2 sites, while the abundance of halacarids and ostracods increased, possibly as a result of the larger food availability and the lower predatory pressures by the sensitive meiofaunal and macrofaunal taxa. Sediment acidification also causes the disappearance of more sensitive macrofaunal taxa, such as gastropods, and the increase in tolerant taxa such as oligochaetes. We conclude that the effects of shallow CO2-vents result in the progressive simplification of community structure and biodiversity loss due to the disappearance of the most sensitive meio- and macrofaunal taxa.

Population structure and environmental niches of Rimicaris shrimps from the Mid-Atlantic Ridge

Among the endemic and specialized fauna from hydrothermal vents, Rimicaris shrimps constitute one of the most important and emblematic components of these ecosystems. On the Mid-Atlantic Ridge, 2 species belonging to this genus co-occur: R. exoculata and R. chacei that differ in their morphology, trophic regime and abundance. R. exoculata forms large and dense aggregations on active vent chimney walls in close proximity to vent fluid emissions, whereas R. chacei is much less conspicuous, living mostly in scattered groups or solitary further away from the fluids. However, the recent revision of Rimicaris juvenile stages from the Mid-Atlantic Ridge shows that R. chacei abundance would be higher than expected at these early life stages. Here, we describe and compare the population structure of R. exoculata and R. chacei at the Snake Pit and Trans-Atlantic Geotraverse (TAG) vent fields. We show distinct population demographics between the 2 co-occurring shrimp species with a large post-settlement collapse in R. chacei populations suggesting high juvenile mortality for this species. We describe important spatial segregation patterns between the 2 species and their different life stages. Additionally, our results highlight distinct niches for the earliest juvenile stages of both R. exoculata and R. chacei, compared with all other life stages. Finally, we discuss the potential factors, including predation and competitive interactions, that could explain the differences we observed in the population structure of these 2 species.

Trace Element and Isotope Systematics in Vent Fluids and Sulphides From Maka Volcano, North Eastern Lau Spreading Centre: Insights Into Three-Component Fluid Mixing

Back-arc spreading centres and related volcanic structures are known for their intense hydrothermal activity. The axial volcanic edifice of Maka at the North Eastern Lau Spreading Centre is such an example, where fluids of distinct composition are emitted at the Maka hydrothermal field (HF) and at Maka South in 1,525–1,543 m water depth. At Maka HF black smoker-type fluids are actively discharged at temperatures of 329°C and are characterized by low pH values (2.79–3.03) and a depletion in Mg (5.5 mmol/kg) and SO4 (0.5 mmol/L) relative to seawater. High metal (e.g., Fe up to ∼6 mmol/kg) and rare Earth element (REE) contents in the fluids, are indicative for a rock-buffered hydrothermal system at low water/rock ratios (2–3). At Maka South, venting of white smoke with temperatures up to 301°C occurs at chimneys and flanges. Measured pH values range from 4.53 to 5.42 and Mg (31.0 mmol/kg), SO4 (8.2 mmol/L), Cl (309 mmol/kg), Br (0.50 mmol/kg) and Na (230 mmol/kg) are depleted compared to seawater, whereas metals like Li and Mn are typically enriched together with H2S. We propose a three-component mixing model with respect to the fluid composition at Maka South including seawater, a boiling-induced low-Cl vapour and a black smoker-type fluid similar to that of Maka HF, which is also preserved by the trace element signature of hydrothermal pyrite. At Maka South, high As/Co (>10–100) and Sb/Pb (>0.1) in pyrite are suggested to be related to a boiling-induced element fractionation between vapour (As, Sb) and liquid (Co, Pb). By contrast, lower As/Co (<100) and a tendency to higher Co/Ni values in pyrite from Maka HF likely reflect the black smoker-type fluid. The Se/Ge ratio in pyrite provides evidence for fluid-seawater mixing, where lower values (<10) are the result of a seawater contribution at the seafloor or during fluid upflow. Sulphur and Pb isotopes in hydrothermal sulphides indicate a common metal (loid) source at the two vent sites by host rock leaching in the reaction zone, as also reflected by the REE patterns in the vent fluids.

Activities of 223Ra and 226Ra in Fluids From the Lost City Hydrothermal Field Require Short Fluid Residence Times

AbstractThe residence time of fluids circulating through deep‐sea hydrothermal systems influences the extent of water‐rock reactions and the flux of major and minor elements to the ocean. While the fluid residence times in numerous basaltic and gabbroic systems have been determined, those of the less studied ultramafic systems are currently unknown. Fluids that interact with mantle rocks have fundamentally different chemistries and therefore have unique influences on seawater chemistry. In this first investigation of radium isotopes in a serpentinite‐hosted system, vent fluids were discovered to contain 10–100 times greater activities of 223Ra (half‐life = 11.4 days) than observed in high‐temperature basalt‐hosted systems. The 223Ra activities of 10–109 dpm L−1 produce 223Ra/226Ra activity ratios ranging from 9 to 109. These extremely high 223Ra activities, which are accompanied by low activities of 226Ra, place significant constraints on fluid residence times and the adsorption coefficient of radium between fluid and rock. Our data constrain the nondimensional retardation factor (R) to very low values between 1 and 4, reflecting the extent to which Ra is transported more slowly than fluids due to adsorption and other processes. These results suggest that the residence time of fluids in contact with serpentinite is less than 2 y and perhaps as low as 0.5 y. They are surprisingly similar to those of basalt‐hosted systems. Thus, fluids in hydrothermal systems share similar hydrogeological characteristics despite differences in rock types, underlying porosity, and heat sources, enabling larger‐scale models of hydrothermal biogeochemistry to be developed across systems.

A relict oasis of living deep-sea mussels Bathymodiolus and microbial-mediated seep carbonates at newly-discovered active cold seeps in the Gulf of C\xe1diz, NE Atlantic Ocean

Extensive beds of the deep-sea mussel Bathymodiolus mauritanicus (currently also known as Gigantidas mauritanicus) linked to active cold seeps related to fissure-like activity on Al Gacel mud volcano, Gulf of Cádiz, were filmed and sampled for the first time during the oceanographic expedition SUBVENT-2 aboard R/V Sarmiento de Gamboa. Al Gacel mud volcano is one of up to 80 fluid venting submarine structures (mud volcanoes and mud volcano/diapir complexes) identified in the Gulf of Cádiz as result of explosive venting of hydrocarbon-enriched fluids sourced from deep seated reservoirs. This mud volcano is a cone-shaped edifice, 107 m high, 944 m in diameter constituted by mud breccias and, partially covered by pavements of seep carbonates. Extensive beds of this deep-sea mussel were detected at the northern flank at 810–815 m water depth associated with bacterial mats around intermittent buoyant vertical bubble methane plumes. High methane concentrations were measured in the water column above living mussel beds. Other chemosymbiotic species (Siboglinum sp., Solemya elarraichensis, Isorropodon sp., Thyasira vulcolutre and Lucinoma asapheus) were also found in different parts of Al Gacel mud volcano. Al Gacel mud volcano may currently represent one of the most active mud volcanoes in the Gulf of Cádiz, delivering significant amounts of thermogenic hydrocarbon fluids which contribute to foster the extensive chemosynthesis-based communities detected. This finding is of paramount importance for linking extremophile bivalve populations along the North Atlantic, including cold seeps of the Gulf of México, hydrothermal vents of the Mid-Atlantic Ridge and now, detailed documented at the Gulf of Cádiz.

Do Hydrothermal Shrimp Smell Vents?

Simple SummaryChemical senses play a crucial role in mediating fundamental behaviors in most animals, including habitat selection and navigation. In the darkness of the deep sea, do shrimp endemic to hydrothermal vents use these senses to locate active emissions? Here, we examine the olfactory behaviors of two species of vent shrimp and one coastal species for comparison, to determine whether hydrothermal species have functional olfactory capacities and respond to environmental cues. Among these cues, food odors and vent fluid markers (chemicals, as well as temperature) were tested. Such in vivo experiments on deep-sea fauna are challenging to conduct because the animals sampled at depth may suffer from decompression. We, therefore, used dedicated pressurized equipment, and designed experiments at both deep-sea and atmospheric pressure. Vent shrimp groom their olfactory organs similarly to other crustaceans, yet they bear a dense bacterial cover, raising questions about the role of bacteria in shrimp olfaction. Whilst hydrothermal shrimp have been shown to possess a functional sense of smell, none of our test setups allowed us to demonstrate significant attraction to odors. Both hydrothermal species, however, showed attraction to warm temperature emissions, supporting the hypothesis that temperature is a major cue for orientation in the hydrothermal environment.Deep-sea species endemic to hydrothermal vents face the critical challenge of detecting active sites in a vast environment devoid of sunlight. This certainly requires specific sensory abilities, among which olfaction could be a relevant sensory modality, since chemical compounds in hydrothermal fluids or food odors could potentially serve as orientation cues. The temperature of the vent fluid might also be used for locating vent sites. The objective of this study is to observe the following key behaviors of olfaction in hydrothermal shrimp, which could provide an insight into their olfactory capacities: (1) grooming behavior; (2) attraction to environmental cues (food odors and fluid markers). We designed experiments at both deep-sea and atmospheric pressure to assess the behavior of the vent shrimp Rimicaris exoculata and Mirocaris fortunata, as well as of the coastal species Palaemon elegans and Palaemon serratus for comparison. Here, we show that hydrothermal shrimp groom their sensory appendages similarly to other crustaceans, but this does not clean the dense bacterial biofilm that covers the olfactory structures. These shrimp have previously been shown to possess functional sensory structures, and to detect the environmental olfactory signals tested, but we do not observe significant attraction behavior here. Only temperature, as a signature of vent fluids, clearly attracts vent shrimp and thus is confirmed to be a relevant signal for orientation in their environment.

Characterization of hydrothermal fluids that alter the upper oceanic crust to spilite and epidosite: Fluid inclusion evidence from the Semail (Oman) and Troodos (Cyprus) ophiolites

Pervasive alteration of basaltic oceanic crust by heated seawater at greenschist facies conditions produces two contrasting hydrothermal rocks. “Spilites”, consisting of chlorite + albite + quartz ± actinolite ± epidote, occur typically with regional extents. Locally spilites are metasomatically transformed to “epidosites” consisting of epidote + quartz + titanite + hematite or magnetite. Both alteration types have been proposed as markers of deep hydrothermal upflow in sub-seafloor convection cells, and as sources of the ore metals in basalt-hosted seafloor massive sulfide deposits. Little direct evidence is available for the chemical compositions of these fluids in their states deep in the upflow zones prior to their discharge at the seafloor. To better characterize them we have conducted a field, petrographic and fluid inclusion study of the lavas, sheeted dikes and plagiogranites in the Semail ophiolite, with supporting samples from the Troodos ophiolite. Our results show that both the spilite- and epidosite-forming fluids were single-phase aqueous liquids during the hydrothermal alteration. At some sites their salinity is 3.1–3.2 wt.% NaCleq, which we take to represent the chlorinity of Cenomanian seawater in the Semail realm. At other sites salinities are as low as 2.4 wt.% NaCleq or as high as 5.7 wt.% NaCleq, attributable to liquid–vapor separation and partial remixing deep in the crust along the dew curve of seawater, prior to ascent of the fluids to the sites of fluid inclusion trapping. Hypersaline brines, often accompanied by vapor, are restricted to plagiogranites in both the Semail and Troodos ophiolites and they represent magmatic–hydrothermal fluids that pre-date and are genetically unrelated to the spilite and epidosite alteration. The volcanostratigraphic locations of the samples constrain their maximum depths to 1470–3600 m below seafloor during alteration. The range of possible fluid trapping pressures for all samples is 31–68 MPa. Trapping temperatures vary between sites from 145 to 440 °C for spilite fluids and 255 to 435 °C for epidosite fluids. Quantitative analyses of 12 elements in individual fluid inclusions by LA-ICP-MS define the chemical characters of the two alteration fluids. The Br/Cl ratio in the spilite fluid is the same as in modern seawater and the other elements fit expectations from seawater–basalt experiments at elevated temperature. Accordingly, concentrations of Li, B, Na, Cl, K, Br and Sr in the spilite fluid match those in modern black-smoker vent fluids in basaltic crust. Exceptions are Ca and Fe, which are enriched in the spilite fluid. As these elements may precipitate below or at the seafloor prior to vent sampling, we conclude that the spilite fluids are plausible feeders of basalt-hosted black-smoker vents. The epidosite fluid has broadly similar elemental concentrations to the spilite fluid, but vastly lower Fe, reflecting the highly oxidized state of epidosites. This suggests that epidosite fluids are incapabable of forming basalt-hosted seafloor massive sulfide deposits.

A cold seep triggered by a hot ridge subduction

The Chile Triple Junction, where the hot active spreading centre of the Chile Rise system subducts beneath the South American plate, offers a unique opportunity to understand the influence of the anomalous thermal regime on an otherwise cold continental margin. Integrated analysis of various geophysical and geological datasets, such as bathymetry, heat flow measured directly by thermal probes and calculated from gas hydrate distribution limits, thermal conductivities, and piston cores, have improved the knowledge about the hydrogeological system. In addition, rock dredging has evidenced the volcanism associated with ridge subduction. Here, we argue that the localized high heat flow over the toe of the accretionary prism results from fluid advection promoted by pressure-driven discharge (i.e., dewatering/discharge caused by horizontal compression of accreted sediments) as reported previously. However, by computing the new heat flow values with legacy data in the study area, we raise the assumption that these anomalous heat flow values are also promoted by the eastern flank of the currently subducting Chile Rise. Part of the rift axis is located just below the toe of the wedge, where active deformation and vigorous fluid advection are most intense, enhanced by the proximity of the young volcanic chain. Our results provide valuable information to current and future studies related to hydrothermal circulation, seismicity, volcanism, gas hydrate stability, and fluid venting in this natural laboratory.

High-Ca vent fluids discharged from the Lutao arc volcanic hydrothermal system are associated with albitization and recycling of marine carbonate

The chemical and isotopic characteristics of calcium (Ca) in subduction zones are closely related to the budget of Ca and carbon cycles. Here we investigate the ultra-high Ca concentrations that characterize the hydrothermal fluids discharged from two types of vents, named the Zhudanqu brine vent (ZDQ) and the Huwaichi vapor spring (HWC), in the Lutao hydrothermal system at the north Luzon arc. The Ca concentrations of up to 159 mM and Ca/Cl ratio of up to 0.26 in the ZDQ vent fluids are possibly the highest ever reported for Ca enrichment in global seawater-circulated hydrothermal/geothermal systems. The differences in chemical compositions between the ZDQ and the HWC vent fluids are primary controlled by subcritical phase separation. The brine phase constitutes the ZDQ vent fluids, while the HWC vent fluids represent mixtures of the vapor phase and seawater. Both the vapor and the brine phases exhibit similar δ44/40Ca values (0.72 ± 0.05‰), suggesting no significant Ca isotope fractionation has occurred during phase separation.The hydrothermal endmember before phase separation (the “Lutao endmember”) presents depletions of 213 ± 15 mM of Na, 24.4 ± 0.4 mM of SO42−, and 10.2 mM of K, and enrichment of 130.2 ± 5.5 mM of Ca with respect to the percolated seawater. The total gained Ca is 154.6 ± 5.9 mM with a δ44/40Ca value of 0.67‰ – 0.77‰ (0.72 ± 0.05‰), considering anhydrite precipitation during hydrothermal circulation. The Holocene raised coral reef is unlikely to contribute substantial Ca into the Lutao system. Much of the gained Ca (111.6 ± 7.5 mM) is produced by high-degree albitization of the Lutao host rock, which is promoted by the low water/rock ratio (~ 2), slightly alkaline conditions, and relatively lower temperature of the Lutao system with respect to most mid-ocean ridge hydrothermal systems. Ca derived from this process inherits the Ca isotopes of plagioclase in the Lutao host rocks (δ44/40Ca = 0.82 ± 0.06‰). According to mass and isotopic balances, the recycled marine carbonate is proposed to contribute 43 ± 13.4 mM Ca with a δ44/40Ca value of 0.46−0.63+0.35‰ into the Lutao system. Such isotopically lighter Ca is derived from either pore fluids expulsed from underlying Philippine Sea sediments, or more probably, carbonate-bearing subduction fluids from the subducting South China Sea sediments and slab. The carbonate solubility in the subduction fluids could maintain at 600 mM near the reaction zone. The carbonate-rich fluids were subsequently migrated into the Lutao reaction zone and released an additional 43 ± 13.4 mM Ca via dolomitization. A small amount (~ 9%) addition of carbonate-rich fluids would not significantly change the budgets of Na, Mg, and Cl but could generate substantial Ca enrichment and Ca isotopic variations.

Controls on the formation of porphyry Mo deposits: Insights from porphyry (-skarn) Mo deposits in northeastern China

AbstractPorphyry Mo deposits have traditionally been classified into two major classes, arc-related and Climax-type, based on the tectonic setting and chemistry of associated intrusions. Although there is a consensus that porphyry Mo systems were formed by the optimal coincidence of geological processes operating at different scales, it is unclear what key parameter(s) render systems productive and whether the two classes of porphyry Mo deposits are unique in their mode of formation, or if they share fundamentally similar geological processes. These questions are important as a clearer understanding of the optimum conditions for the formation of porphyry Mo deposits is a prerequisite for more efficient exploration.This contribution presents a detailed assessment of the factors affecting the formation of porphyry Mo deposits through the investigation of barren and mineralized intrusions from the arc-related Songbei-Yangjiazhangzi-Lanjiagou (SYL) ore zone and the Climax-type Hashitu deposit of northeastern China. Our results show that the syn-mineralization intrusions from the SYL ore zone are quite evolved (SiO2 ~ 75 wt%; Na2O+K2O ~ 8.7 wt%) and are characterized by apparent light rare earth element (LREE) enrichments (LaN/YbN = 2.7–33.1) and moderate negative Eu anomalies (Eu/Eu* = 0.4–0.7). They show enriched zircon Hf isotopic compositions [εHf(t) = –11.9 to –4.8], indicating their parental magmas were likely derived from an ancient crustal source. Melt inclusions from the SYL syn-mineralization intrusions contain negligible F and Cl. In contrast, Hashitu syn-mineralization intrusions are characterized by weak LREE enrichments (LaN/YbN = 2.2–6.9) and strong negative Eu anomalies (Eu/Eu* = 0.02–0.10), with SiO2 and Na2O+K2O contents similar to the SYL syn-mineralization intrusions. They show depleted zircon Hf isotopic compositions [εHf(t) = 3.1–5.0], indicating their parental magmas were likely derived from a juvenile crustal source. Melt inclusions from the Hashitu syn-mineralization intrusions contain up to 0.4 wt% F and 0.03 to 0.09 wt% Cl. However, in both cases, the syn-mineralization intrusions are Mo-poor (1–7 ppm Mo), oxidized (above the quartz-fayalite-magnetite buffer), water-saturated (4.4–7.8 wt% H2O), and were emplaced at palaeodepths of 3.3 to 8.3 km. These data imply that magma source composition is not a key factor in the formation of porphyry Mo deposits. In contrast, magma oxygen fugacity, water content, and emplacement depth appear to play fundamental roles in the formation of porphyry Mo deposits of both arc-related and Climax-type.Within individual deposits, no systematic differences between pre- and syn-mineralization intrusions are observed in terms of magma source, fractionation degree, oxygen fugacity, emplacement depth, and volatile and Mo contents. Instead, a crucial apparent difference lies in the geometry of the intrusions, i.e., pre-mineralization intrusions generally occur as flat, ponded bodies, whereas syn-mineralization intrusions commonly develop as small stocks or dikes. Our results, in combination with an examination of other porphyry Mo systems, suggest that the sudden depressurization of magma chambers and subsequent venting of voluminous fluids along focusing structures (such as small stocks or dikes) most likely plays a critical role in the formation of porphyry Mo deposits of both arc-related and Climax-type. The findings of this study indicate that fluid processes in the shallow crust are pivotal for the formation of porphyry Mo deposits and that settings with ideal magmatic-hydrothermal architectures are most likely to develop into productive porphyry Mo systems.

Finite volume modeling of bathymetry and fault-controlled fluid circulation in the Sea of Marmara

Fluid vents in the Sea of Marmara were discovered and investigated by several studies. In this paper, a numerical model is created for the first time to determine the possible transport mechanism behind those fluid emissions at the seafloor. The finite volume method is used for numerical simulations by implementing a commercial finite volume code, ANSYS-Fluent. The thermal and physical rock properties used in our models are taken from previous studies. Bathymetry, fault-controlled fluid flow velocities, and temperature distribution patterns for the Central Basin and Western High in the Sea of Marmara are simulated and presented. Effects of faults, thickness of sediments, and hydrostatic pressure due to the water column thickness on fluid flow are demonstrated. Driving mechanisms of the fluid flow are also discussed. It is found that both seafloor bathymetry and presence of faults can control the location and distribution of fluid emissions at the seafloor.

Experimental study on Fe solubility in vapor-rich hydrothermal fluids at 400–500 °C, 215–510 bar: Implication for Fe mobility in seafloor vent systems

In seafloor hydrothermal systems, vent fluids usually contain elevated dissolved iron (Fe) that is significantly enriched relative to deep ocean seawater. It is commonly thought that Fe is preferentially transported in dense Cl-rich fluids due to the formation of aqueous Fe-Cl complexes. However, Fe enrichment in vapor-rich low-density fluids with low Cl concentrations (<550 mmol/kg) underscores the efficacy of the low-Cl vapor-rich phase to transport Fe in both subaerial and submarine hydrothermal systems. Currently, transport of Fe in low-density vapor-rich fluids is poorly understood due to the lack of high temperature–pressure (T-P) solubility experiments and requisite thermodynamic data. Here, we report new data of Fe solubility from experiments conducted at 400–500 °C, 215–510 bar, targeting fluids with low-density (∼0.1–0.35 g/cm3). The experiments were performed in the KCl-H2O system with hematite-magnetite and K-feldspar-muscovite-quartz as mineral buffering assemblages. Our results show that Fe solubility positively correlates with density and fluid chlorinity, which are affected by temperature and pressure. The equilibrium constants (log Khm) for Fe-buffering reaction Fe3O4(s) + 2HCl(aq) = Fe2O3(s) + FeCl2(aq) + H2O were determined. The new data and the data calculated using Helgeson-Kirkham-Flowers (HKF) equation of state were fit into a density model to extrapolate log Khm for hematite-magnetite Fe buffering reaction over a wide T-P range. The density models for magnetite dissolution reaction and pyrite-pyrrhotite equilibrium were also fit based on HKF to allow redox constraints. We show that calculated Fe solubility are in good agreement with measured values in vapor-rich fluids formed via phase separation in mineral buffered and basalt alteration experiments at elevated T-P. The density model was further applied to model Fe transport in fluids at the Brandon hydrothermal field at East Pacific Rise (EPR) 21°S, with T-P constrained by Si-Cl geothermobarometer. The calculations suggest that the reported Fe concentrations of vent fluids at Brandon reflect phase separation occurring at depth in the seafloor, with T-P up to 450 °C, 400 bar, and redox conditions buffered by pyrite-pyrrhotite-magnetite equilibrium.

Behavior of iron isotopes in hydrothermal systems: Beebe and Von Damm vent fields on the Mid-Cayman ultraslow-spreading ridge

It is now clear that, in some parts of the ocean, inputs of hydrothermal iron (Fe) can make a more significant contribution to the Fe inventory than previously thought. While the Fe isotopic signature of seawater has proved useful for distinguishing between inputs of Fe from atmospheric deposition and seafloor sediments, the Fe isotope signature of hydrothermal vent fluids may change during mixing of vent fluids and seawater. To better constrain the processes leading to these changes, the Fe isotopic compositions (δ56Fe) of dissolved and total dissolvable Fe have been determined in high temperature vent fluids and the buoyant hydrothermal plumes at the Beebe and the Von Damm vent fields, which are located along the ultraslow Mid-Cayman spreading center in the Caribbean Sea.Our results show that the δ56Fe value of dissolved Fe in the earliest stages of buoyant plume formation was lower (as low as −4.08‰) than measured in a high temperature, low-Mg, vent fluid sample (−0.28‰). This indicates that the iron isotopic signature of dissolved Fe is principally controlled by oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides that preferentially incorporate heavy Fe isotopes. In support of this, the δ56Fe value of labile particulate Fe was higher than the δ56Fe value of dissolved Fe. Nevertheless, at Beebe, the δ56Fe value of total dissolvable Fe increased as the proportion of Fe predicted to have been lost from the plume increased, consistent with preferential fall-out of Fe-sulfides that are enriched in light Fe isotopes. The very low δ56Fe values of dissolved Fe in the Beebe buoyant plume are consistent with (i) the high Fe/H2S ratio of the vent fluids, and (ii) high Fe(II) oxidation rates, relative to other vent sites.

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.

Paleoredox conditions, hydrothermal history, and target vectoring in the Macmillan Pass base-metal district, Yukon, Canada: 2 – Pyrite paragenesis and mineral chemistry

ABSTRACTThe MacMillan Pass district in Yukon, Canada, hosts the Tom and Jason clastic sediment-hosted Zn-Pb-Ag-(Ba) deposits. Pyrite-bearing drill core samples were collected from seven drill holes that intersected sulfide mineralization and time-stratigraphically equivalent rocks at varied spatial distances extending up to 3 km away from the deposits to assess the relative timing of pyrite mineralization and the chemistry of pyrite paragenesis. There are four pyrite morphologies: framboids and polyframboids (Py1), subhedral to euhedral inclusion-free crystals (Py2a), silicate inclusion-bearing nodules with serrated edges (Py2b), and euhedral idiomorphic overgrowths on preexisting pyrite morphologies (Py3). These morphological varieties correspond in time from syngenetic to earliest diagenetic growth (Py1), early to late diagenetic growth (Py2a, Py2b), and metamorphic crystallization and/or recrystallization of previous textural varieties (Py3). A representative subset of pyrite grains was analyzed for trace element contents and distributions by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Analyses by LA-ICP-MS reveal that each textural variety of pyrite has a distinct trace element composition that also varies depending on stratigraphic unit. A suite of clastic sediment-hosted sulfide mineralization-related elements was incorporated into Py2 within sulfide mineralized units at greater abundances than that in unmineralized units (e.g., Zn, As, Pb, Tl, Bi). Lead abundances and Pb/Se and As/Mo values in pyrite are the most robust vectoring tools documented. The timing for clastic sediment-hosted Zn-Pb mineralization was syn and/or post late diagenesis (Py2b).A Ba-enriched horizon was identified in rocks and this is interpreted to be the distal time-stratigraphic equivalent unit to Zn-Pb mineralization. The Ba-enriched horizon contains Py2 with anomalous metal (Tl, Co, Mn, Cd, Zn, Sb) contents and abundant macroscopic baryte, and it is interpreted to represent the distal expression of sulfide mineralization-forming hydrothermal activity. Four genetic models for mineralization are reviewed; however, the only model that is consistent with our whole rock and pyrite geochemistry involves venting of buoyant hydrothermal fluid, mixing with ambient seawater, and remaining or sinking into unconsolidated sediments, with lateral migration up to 2–3 km from the vent source.

Archaeal lipid diversity, alteration, and preservation at the Cathedral Hill deep sea hydrothermal vent, Guaymas Basin, Gulf of California, and its implications regarding the deep time preservation paradox

Archaea are among the earliest evolved organisms. Today, they exist in nearly every habitable environment and their highly recalcitrant membrane lipids are both abundant and easily detected in modern sediments. Yet, unlike bacteria and eukaryotes, the lipid biomarker signatures of archaea are almost entirely absent in the ancient rock record (>145 Ma). We present a comprehensive study of archaeal lipids from the Cathedral Hill hydrothermal vent complex in Guaymas Basin, Gulf of California. Here, porewaters reach 155 °C by 21 cm below the sea floor (cmbsf), which enables the near-complete tracking of sedimentary organic matter (SOM) accumulation, diagenetic alteration and thermochemical transformation into petroleum-like hydrocarbons within and beyond the habitable zone of life. Identified intact polar lipids (IPLs) of living archaea included mono- and di-glycosidic archaeols (1G-AR, 2G-AR) and glycerol dialkyl glycerol tetraethers (1G- and 2G-GDGTs). The 1G- and 2G-ARs, potentially derive from methane-oxidizing archaeal groups ANME-1 and ANME-2 and anaerobic thermophilic methanogens, reach sediment depths of ∼ 135 cm and 50 °C temperatures. The 1G-and 2G-GDGT lipids reach a depth of ~180 cm where vent fluid temperatures are ~145 ± 15 °C. Core lipids (CLs) include archaeols (AR) and isoprenoidal and branched GDGTs (iGDGTs, brGDGTs). The core GDGTs (cGDGTs) closest to the vent center have high rates of lipid loss. Up to 95% of all identified archaeal biomarkers, including lipids supplied from the upper water column, are degraded or recycled in the surface sediments and do not reach conditions of late diagenesis and catagenesis. Only ∼ 0.11%, proportional to the lipid bound component in the protokerogen, marks thermochemically cracked biphytane – a prominent archaeal hydrocarbon biomarker. This extreme reduction indicates archaeal lipids do not necessarily become incorporated into kerogen thereby helping to explain the deep time archaeal lipid preservation paradox.

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

ABSTRACTA 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.

Constraints on fluid evolution and growth processes of black smoker chimneys by pyrite geochemistry: A case study of the Tongguan hydrothermal field, South Mid-Atlantic Ridge

Up to now, the Tongguan hydrothermal field is the southernmost-distributed modern submarine massive sulfide discovered on the South Mid-Atlantic Ridge. A well-preserved black smoker chimney which contains three distinct concentric zones was collected from the active vent field. The external outer wall of the chimney contains mainly pyrite with minor sphalerite, chalcopyrite and galena. The sulfides within the internal middle layer consist of predominant pyrite, with lesser amounts of marcasite. The axial conduit is dominated by chalcopyrite, with minor pyrite. To clarify the ore-forming fluid evolution during the black smoker chimney growth, we performed detailed petrographic and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) studies on pyrite grains from the chimney. Seven types of pyrite: framboidal, relict, colloform, dendritic, massive, euhedral and lined pyrites, are identified from exterior to interior of the chimney.As shown by the results of LA-ICPMS, geochemical compositions of pyrite vary across the chimney. Generally, low-temperature elements (Mg, V, Mn, Pb, Zn, Ag and Tl) are enriched in pyrite in the outer wall of the chimney and decrease toward the interior zones. Elements indicative of high temperature conditions, such as Se and Te, show opposite trends and are enriched in pyrite within the inner zones of the chimney. Framboidal, relict, colloform and dendritic pyrites precipitate from low to mid temperature fluids (<250 °C), while massive and euhedral pyrites are products of ore-forming fluids characterized by mid to high temperature (250 to > 335 °C). Relict and lined pyrites have the same chemical signatures, suggesting that they precipitated under similar physicochemical conditions. The observed patterns are resulted from the changes in temperature and fluid chemistry caused by interactions of between seawater and hydrothermal fluids with each other in the chimney.Cobalt concentrations in pyrite from the modern black smoker chimneys located on the mid-ocean ridge apparently vary systematically in response to salinity change of vent fluids. The low Co contents (average 0.32 ppm) in pyrite suggest that the hydrothermal fluids vented from the black smoker chimney are Cl-depleted compared to seawater, most likely caused by phase separation. The systematic variations in geochemistry and morphology of the pyrite across the chimney reflect multiple stages of mineralization. Different degrees of fluid-seawater mixing and phase separation are the main controlling factors during the chimney formation.

Protistan grazing impacts microbial communities and carbon cycling at deep-sea hydrothermal vents

Microbial eukaryotes (or protists) in marine ecosystems are a link between primary producers and all higher trophic levels, and the rate at which heterotrophic protistan grazers consume microbial prey is a key mechanism for carbon transport and recycling in microbial food webs. At deep-sea hydrothermal vents, chemosynthetic bacteria and archaea form the base of a food web that functions in the absence of sunlight, but the role of protistan grazers in these highly productive ecosystems is largely unexplored. Here, we pair grazing experiments with a molecular survey to quantify protistan grazing and to characterize the composition of vent-associated protists in low-temperature diffuse venting fluids from Gorda Ridge in the northeast Pacific Ocean. Results reveal protists exert higher predation pressure at vents compared to the surrounding deep seawater environment and may account for consuming 28 to 62% of the daily stock of prokaryotic biomass within discharging hydrothermal vent fluids. The vent-associated protistan community was more species rich relative to the background deep sea, and patterns in the distribution and co-occurrence of vent microbes provide additional insights into potential predator-prey interactions. Ciliates, followed by dinoflagellates, Syndiniales, rhizaria, and stramenopiles, dominated the vent protistan community and included bacterivorous species, species known to host symbionts, and parasites. Our findings provide an estimate of protistan grazing pressure within hydrothermal vent food webs, highlighting the important role that diverse protistan communities play in deep-sea carbon cycling.

Organic compounds in vent fluids from Yellowstone Lake, Wyoming

Hydrothermal fluids were collected over a three-year interval (2016–2018) from two sublacustrine vent fields in Yellowstone Lake, Wyoming: Stevenson Island Deep Hole (YL16 and YL17 series in 2016 and 2017, respectively) and West Thumb (YL18 series in 2018). The TOC content in SI Deep Hole vent fluids varies from 2.7 mg/L to 6.3 mg/L in YL16 and 5.0 mg/L to 16.2 mg/L in YL17. Compared to YL16, the average TOC value of YL17 has more than doubled. The range of TOC is from 7.3 mg/L to 13.7 mg/L in the West Thumb samples. A positive correlation between TOC and dissolved CO2 concentration is observed in most samples, suggesting their close relationship with sublacustrine heat and mass transfer processes. The organic compounds detected at both vent fields are diverse in molecular structure. In the SI Deep Hole, complex cyclic and unsaturated organic compounds are dominant, with cyclic hydrocarbons having the highest abundance, followed by carboxylic acids, PAHs, and alcohols. At West Thumb vents, propanol is most abundant, followed by alcohols, PAHs, alkanes, and carboxylic acids. This difference in structural complexity and relative quantity of organic compounds may be attributed to the involvement of hydrothermal processes. The vapor-dominated hydrothermal system and extensive fluid-rock interactions in SI Deep Hole facilitate alteration/decomposition of organic precursors. While CO2(aq) is predominant in vent fluids, dissolved CH4 is much less abundant. In SI Deep Hole, the carbon isotopic composition of CH4 in most samples ranges from −20.4‰ to −38.0‰, with values of less than −60‰ in three samples, while CO2 ranges from −4.4‰ to −10.5‰. A temporal variation of δ13C values was obtained for CO2, with the average increasing from −9.2‰ (YL16) to −5.2‰ (YL17). In West Thumb, the δ13C value of CO2 and CH4 fall in a similar range, with an average of −10.5‰ and −19.0‰, respectively. There are multiple sources for observed carbon species: the interaction of vent fluids with subsurface rocks and lake sediments is the predominant process, followed by thermal decomposition of organic matter. Microbial activity on the lake floor may contribute to methane formation, while magmatic degassing is the major process that produces CO2. The connection between vapor-dominated hydrothermal alteration, elevated TOC contents and dissolved CO2 in vent fluids, and enrichment of 13C in CO2 in YL17 in SI Deep Hole suggests an interplay of geological, chemical, and biological processes and their imprints on vent fluids, all of which make significant contribution to the carbon cycle in the Yellowstone Lake ecosystem.