Shallow-water Hydrothermal Systems Research Articles

Environmental inventory of mercury (Hg) for the marine shallow water hydrothermal system at Panarea, Italy

Hydrothermal fluids, hydrothermal gases, porewater and seawater were collected from the La Calcara (LC) hydrothermal area (n = 34). Additional samples were taken at Bottaro North (n = 2), Black Point (n = 3) and Panarea Harbor (n = 3). Total Hg (THg) porewater concentrations ranged from 300 to 6200 pM, while dissolved Hgdiss concentrations were generally lower by two to three orders of magnitude. Hydrothermal fluids had concentrations up to 26,000 pM. Mono- and dimethyl Hg (MeHg) were below detection. Total Hg in the gases (Hggas) ranged from 0.9 to 1899 nmol/m3.The THg concentration correlated positively with temperature (r = 0.86), while Hgdiss did not show any correlation (r = −0.02). The origin of Hg in the study area was interpreted to be hydrothermal. However, a clear trend was absent since ambient temperature porewaters had Hg concentrations as high as those porewaters with elevated temperatures. This was considered to be due to a combination of chemical reactions and complex flow patterns beneath the hydrothermal system. The concentration of Hg in the hydrothermal gases ranged from 0.9 to 1899 nmol/m3. This large range was ascribed to subsurface reactions with H2S and CH4 combined with the presence or absence of a sediment cover.Above the LC area, Hg concentrations were higher than those in Mediterranean seawater, indicating that the hydrothermal discharge adds Hg to the Panarea coastal ocean and its ecosystem. This effect is local regarding seawater chemistry and global Hg budgets since with distance Hg concentrations returned to values as expected for Mediterranean seawater. However, since most, if not all, marine food chains originate in coastal water, hydrothermal Hg may bioaccumulate in a given food chain. With >70 known marine shallow-water hydrothermal systems, this may be a worldwide phenomenon that warrants further study.

Lithium isotope systematics and water/rock interactions in a shallow-water hydrothermal system at Milos Island, Greece

The active venting fluids of Milos Island, located within the southern Aegean Sea, belong to a shallow-water hydrothermal system (< 200 m depth) that shows chemical compositions and evolution processes comparable to those of mid-ocean ridges (MOR). In this study, we analyze Li and δ7Li in 69 vent water samples, grouped into two types based on their salt content. The low-Cl end-member (EM) Cave fluids show a relatively high Li content (0.39–0.54 mM) with MORB-like δ7Li (∼4.5 ‰, MORB = 3.7 ‰) compared to that of seawater, and the high-Cl brine fluids contain remarkably high Li (6.14–10.6 mM) and variable δ7Li (1.4–8.7 ‰). The latter fluids may have derived from metamorphic basement modified by seawater interactions at ∼300 °C. A scenario using a steady-state dissolution/precipitation model can generate consistent Li and δ7Li patterns, where linear correlations of Cl and Li suggest phase separation occurred after water/rock interaction at depth. On the contrary, no significant δ7Li variation in most Milos fluids suggests limited isotopic fractionation occurred during phase separation. More importantly, the detected Li enrichment in the high-Cl fluids implies a large Li flux, ∼3.4 × 107 mol/yr, to the ocean from the Milos system. Assuming that 10% of the world's shallow-water systems discovered to date have similar Li outputs to those of Milos, this Li flux would represent ∼1.8% of MOR hydrothermal fluxes which is on the order of ∼13 × 109 mol/yr. These results emphasize the importance of Li flux derived from shallow-water hydrothermal systems, which should not be excluded from the calculation of the marine Li budget and its impact on the global silicate weathering cycles.

Novel Insights on the Bacterial and Archaeal Diversity of the Panarea Shallow-Water Hydrothermal Vent Field.

Current knowledge of the microbial diversity of shallow-water hydrothermal vents is still limited. Recent evidence suggests that these peculiar and heterogeneous systems might host highly diversified microbial assemblages with novel or poorly characterized lineages. In the present work, we used 16S rRNA gene metabarcoding to provide novel insights into the diversity of the bacterial and archaeal assemblages in seawater and sediments of three shallow-water hydrothermal systems of Panarea Island (Tyrrhenian Sea). The three areas were characterized by hot, cold, or intermediate temperatures and related venting activities. Microbial biodiversity in seawater largely differed from the benthic one, both in α-diversity (i.e., richness of amplicon sequence variants-ASVs) and in prokaryotic assemblage composition. Furthermore, at the class level, the pelagic prokaryotic assemblages were very similar among sites, whereas the benthic microbial assemblages differed markedly, reflecting the distinct features of the hydrothermal activities at the three sites we investigated. Our results show that ongoing high-temperature emissions can influence prokaryotic α-diversity at the seafloor, increasing turnover (β-)diversity, and that the intermediate-temperature-venting spot that experienced a violent gas explosion 20 years ago now displays the highest benthic prokaryotic diversity. Overall, our results suggest that hydrothermal vent dynamics around Panarea Island can contribute to an increase in the local heterogeneity of physical-chemical conditions, especially at the seafloor, in turn boosting the overall microbial (γ-)diversity of this peculiar hydrothermal system.

Shallow-water hydrothermalism at Milos (Greece): Nature, distribution, heat fluxes and impact on ecosystems

Submarine hydrothermal activity is responsible for heat and chemical exchanges through the seafloor. Shallow-water hydrothermal systems (SWHS), while identified around the globe, are often studied in a way that is less comprehensive than their deep-ocean counterparts (e.g., along ridges), where systematic optical and acoustic mapping is more prevalent and coupled to in situ observations and sampling. Using aerial drones, an AUV, and temperature measurements at 10–40 cm subseafloor, we investigated in 2019 one of the most extensive SWHS known to date, in Paleochori and nearby Spathi and Agia Kyriaki Bays (south of Milos, Greece). Hydrothermal venting, found from the shore to water depths of almost 500 m, shows emissions of gases and high-temperature fluids, often associated with bacterial mats and/or hydrothermal mineral precipitates. This study provides extensive drone mapping coupled with local AUV surveys for seafloor characterization and ground-truthing from the shore to ~20 m water depth. Seafloor photomosaics also provide a detailed context to samples, measurements and observations carried in situ. We interpret the photomosaics to define distinct seafloor types, linked to this hydrothermal activity. White hydrothermal patches (WHPs) often show a clear polygonal organization, together with outflow areas that are both more dispersed and distributed. Polygonal patterns likely result from fluid convection in a sandy porous medium heated from below. These WHPs display elevated subseafloor temperatures, typically >50°C, with maximum values of ~75°C. Photomosaics also display textures of biological origin, including seagrass and bioturbation patterns. Widespread bioturbation by burrowing shrimps is often associated with WHPs, bounding them, but also occurs on sandy seafloor away from hydrothermal patterns. Subseafloor temperatures at these bioturbated areas are of ~30–40°C, and are thus transitional between hot WHPs and sedimented seafloor unaffected by hydrothermal activity (~24°C). In addition to linking subseafloor temperature data and interpreted seafloor photomosaics, our results provide a comprehensive general overview of this SWHS, of the organization of its hydrothermal outflow through the seafloor, and of the underlying subseafloor fluid circulation. This paper also gives the first perspectives on the heat fluxes of the system, and constitutes a background for other studies on the nature and distribution of microbial communities, controlled by this hydrothermal activity.

Mercury in the hydrothermal fluids and gases in Paleochori Bay, Milos, Greece

Seafloor hydrothermal activity may constitute a considerable mercury (Hg) source to the oceans, but the flux from marine shallow-water hydrothermal systems (MSWHS) remains poorly constrained to date. To study the presence of Hg in MSWHS in Paleochori Bay (Milos Island, Greece), sea surface, bottom, pore fluid and hydrothermal gas samples were collected in June of 2017, October of 2018 and July of 2020, and analyzed for Cl, Br, SO4, As, Ca, Fe, K, Mg, Mn, Na, Si, Sr, H2S, unfiltered total Hg (THg), and filtered Hg (Hgdiss). Specific sites were selected for the analysis of volatile elemental Hg (Hgo), dimethylmercury (DMHg), monomethylmercury (MMHg), and Hg in the gas phase (Hggas). Concentrations of THg observed in samples collected from the sea surface were elevated compared to surface samples taken outside Paleochori Bay. The highest surface water concentrations (~10 to 15 pM) were measured in samples collected directly above shallow-water hydrothermal discharge areas. Pore fluids outside Paleochori Bay were significantly lower in THg (0.8 to 8.6 pM) than those taken inside (17.4 to 1511 pM). Porewaters collected from areas with visible gaseous or fluid emission were highly variable but generally elevated in THg concentrations (185 to 5066 pM). Concentrations within gases ranged from 0.7 to 2791 nmol/m3. The vast majority of samples with highly elevated THg (> 100 pM) had low Na/K ratios (< 15), indicative of rapidly rising fluid. Concentrations of Hg0, DMHg, and MMHg were below detection limits in all samples.Bottom substrate type (e.g., rocky vs. sediment covered) likely affected infiltration rates of oxygenated seawater below the sediment-water interface, thereby affecting Hg speciation and removal by precipitation. Flux rates from porewaters compared to those with gaseous emission were high (12.56 to 1088 mol THg/year and 0.37 to 1.85 mol THg/year). Sites with slow gaseous emission rates are hypothesized to have extended subsurface reaction times, resulting in lower Hg concentrations emitted to bottom waters. However, increasing rates of gas emission did not necessarily indicate higher Hg concentrations. The scavenging of Hg in the sediments and advective flux out of Paleochori Bay likely prevent significant accumulations of THg in the water column of Paleochori Bay. The total atmospheric flux from Paleochori Bay using average flux calculations over the entire surface area would contribute 6 mmol Hg/year to the atmosphere.We hypothesize that Hg concentrations within the pore fluids of Paleochori Bay reflect a balance between mixing and precipitation in the subsurface. A three-component mixing system of vapor, brine and seawater determines THg concentrations; however, precipitation due to sulfur cycling, changes in redox conditions and temperature, all play a crucial role in removing Hg from emitted fluids and gases.

Recent Developments of Exploration and Detection of Shallow-Water Hydrothermal Systems

A hydrothermal vent system is one of the most unique marine environments on Earth. The cycling hydrothermal fluid hosts favorable conditions for unique life forms and novel mineralization mechanisms, which have attracted the interests of researchers in fields of biological, chemical and geological studies. Shallow-water hydrothermal vents located in coastal areas are suitable for hydrothermal studies due to their close relationship with human activities. This paper presents a summary of the developments in exploration and detection methods for shallow-water hydrothermal systems. Mapping and measuring approaches of vents, together with newly developed equipment, including sensors, measuring systems and water samplers, are included. These techniques provide scientists with improved accuracy, efficiency or even extended data types while studying shallow-water hydrothermal systems. Further development of these techniques may provide new potential for hydrothermal studies and relevant studies in fields of geology, origins of life and astrobiology.

Terrestrial shallow water hydrothermal outflow characterized from out of space

We investigate the potential of satellite imagery to map and monitor the activity of shallow-water hydrothermal systems, which are often found around volcanic islands. For this study, we used publicly available data and proprietary WorldView-2 satellites images, with spectral bands that can penetrate up to water depths of 30 m. Shallow water hydrothermal sites are visible on satellite imagery, primarily with publicly available data, demonstrating the potential of satellite imagery to study and monitor shallow water hydrothermal activity. We focus our work on volcanic islands, showing intense near-shore, shallow-water hydrothermal activity, and distinct styles of hydrothermal venting. Satellite imagery constrains regional outflow geometry and the temporal variability or stability of these systems. Milos Island shows hydrothermal outflow associated with reflective mineral precipitates and/or bacterial mats, which are stable over time (2010–2014). These outflows locally define polygonal patterns likely associated with hydrothermal convection in porous media. In Kueishantao Island individual hydrothermal plumes charged with particles are visible at the sea surface, and display great variability in intensity and distribution of plume sources (2002–2019). Worldwide we have identified ~15 shallow water hydrothermal sites with satellite imagery, that are similar to either the Milos system (e.g., Vulcano and Panarea, Italy), or the Kueishantao system (numerous sites in Pacific volcanic islands). This study demonstrates that satellite imagery can be used to map and monitor different types of shallow-water hydrothermal systems, at regional scale, and monitor their evolution. Satellite data provide not only regional and temporal information on these systems, unavailable to date, but also the regional context for follow-up in situ field data and observations (e.g., instrumental monitoring, sampling, observations and mapping with divers or AUVs) to understand both the nature and dynamics of these systems, and ultimately the associated fluxes.

Bacterial Diversity and Biogeochemistry of Two Marine Shallow-Water Hydrothermal Systems off Dominica (Lesser Antilles).

Shallow-water hydrothermal systems represent extreme environments with unique biogeochemistry and high biological productivity, at which autotrophic microorganisms use both light and chemical energy for the production of biomass. Microbial communities of these ecosystems are metabolically diverse and possess the capacity to transform a large range of chemical compounds. Yet, little is known about their diversity or factors shaping their structure or how they compare to coastal sediments not impacted by hydrothermalism. To this end, we have used automated ribosomal intergenic spacer analysis (ARISA) and high-throughput Illumina sequencing combined with porewater geochemical analysis to investigate microbial communities along geochemical gradients in two shallow-water hydrothermal systems off the island of Dominica (Lesser Antilles). At both sites, venting of hydrothermal fluids substantially altered the porewater geochemistry by enriching it with silica, iron and dissolved inorganic carbon, resulting in island-like habitats with distinct biogeochemistry. The magnitude of fluid flow and difference in sediment grain size, which impedes mixing of the fluids with seawater, were correlated with the observed differences in the porewater geochemistry between the two sites. Concomitantly, individual sites harbored microbial communities with a significantly different community structure. These differences could be statistically linked to variations in the porewater geochemistry and the hydrothermal fluids. The two shallow-water hydrothermal systems of Dominica harbored bacterial communities with high taxonomical and metabolic diversity, predominated by heterotrophic microorganisms associated with the Gammaproteobacterial genera Pseudomonas and Pseudoalteromonas, indicating the importance of heterotrophic processes. Overall, this study shows that shallow-water hydrothermal systems contribute substantially to the biogeochemical heterogeneity and bacterial diversity of coastal sediments.

Relative Importance of Chemoautotrophy for Primary Production in a Light Exposed Marine Shallow Hydrothermal System.

The unique geochemistry of marine shallow-water hydrothermal systems promotes the establishment of diverse microbial communities with a range of metabolic pathways. In contrast to deep-sea vents, shallow-water vents not only support chemosynthesis, but also phototrophic primary production due to the availability of light. However, comprehensive studies targeting the predominant biogeochemical processes are rare, and consequently a holistic understanding of the functioning of these ecosystems is currently lacking. To this end, we combined stable isotope probing of lipid biomarkers with an analysis of the bacterial communities to investigate if chemoautotrophy, in parallel to photoautotrophy, plays an important role in autotrophic carbon fixation and to identify the key players. The study was carried out at a marine shallow-water hydrothermal system located at 5 m water depth off Dominica Island (Lesser Antilles), characterized by up to 55°C warm hydrothermal fluids that contain high amounts of dissolved Fe2+. Analysis of the bacterial diversity revealed Anaerolineae of the Chloroflexi as the most abundant bacterial class. Furthermore, the presence of key players involved in iron cycling generally known from deep-sea hydrothermal vents (e.g., Zetaproteobacteria and Geothermobacter), supported the importance of iron-driven redox processes in this hydrothermal system. Uptake of 13C-bicarbonate into bacterial fatty acids under light and dark conditions revealed active photo- and chemoautotrophic communities, with chemoautotrophy accounting for up to 65% of the observed autotrophic carbon fixation. Relatively increased 13C-incorporation in the dark allowed the classification of aiC15:0, C15:0, and iC16:0 as potential lipid biomarkers for bacterial chemoautotrophy in this ecosystem. Highest total 13C-incorporation into fatty acids took place at the sediment surface, but chemosynthesis was found to be active down to 8 cm sediment depth. In conclusion, this study highlights the relative importance of chemoautotrophy compared to photoautotrophy in a shallow-water hydrothermal system, emphasizing chemosynthesis as a prominent process for biomass production in marine coastal environments influenced by hydrothermalism.

Geochemical characteristics, speciation and size-fractionation of iron (Fe) in two marine shallow-water hydrothermal systems, Dominica, Lesser Antilles

Marine shallow-water hydrothermal systems have so far largely been neglected with respect to trace metal fluxes and possible stabilizing complexation processes, even though they emit their fluids directly into the photic zone. The impact of stabilized dissolved metal input by shallow vents into surface waters as well as the effect on the transport and bioavailability of bioactive trace metals within the area of highest primary production rates in the world oceans is therefore, at present, mostly unknown.In this study, we investigated the concentration, size fraction distribution (colloidal and soluble) as well as redox speciation and labile concentrations of the limiting micronutrient iron (Fe) at two marine shallow-water hydrothermal systems (Champagne Hot Springs and Soufriere) off the coast of Dominica, Lesser Antilles Island Arc, Caribbean.Geochemical characterization of the two different vent sites showed that both are affected by meteoric and seawater influence, with a stronger meteoric influence at Soufriere than at Champagne Hot Springs. Measurements of soluble and labile Fe were performed using a modified competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLE-AdCSV) with salicylaldoxime (SA) as the artificial ligand. Our results show that focused fluids discharging at the seafloor, as well as hydrothermal pore fluids are, despite a calculated theoretical oxidation half-life of only 6.4min, highly enriched in Fe(II), indicating a strong complexation of Fe(II), strong enough to prevent Fe(II) from oxidation and precipitation. Since these fluids show enriched dissolved organic carbon (DOC) concentrations, and very low fractions of chemically labile Fe, complexation may occur by organic carbon, which was recently suggested to also be a factor in stabilizing particulate Fe(II) in deep-sea hydrothermal non-buoyant plumes.Our results indicate that shallow-water hydrothermalism off the coast of Dominica releases high concentrations of stabilized, bioavailable Fe(II) into the photic zone, which influences the biogeochemical cycle of Fe in surface waters. Considering the abundance of marine shallow-water hydrothermal systems in many regions, such processes may even play a role in the global oceanic dFe cycle.

Shallow-water hydrothermal vents in the Azores (Portugal)

The impact of global warming has been a major issue in recent years and will continue increasing in the future. Knowledge about the effects of ocean acidification on marine organisms and communities is crucial to efficient management. Island envi- ronments are particularly sensitive to externally induced changes and highly dependent on their coastal areas. This study summarises the published information on shallow-water hydrothermal vents of the Azores. These environments were reported to exhibit high metal concentration and acidified seawater due to the diffusion of acidic volcanic gases (mainly CO 2 ) and a considerable temperature range. In some vents a water input with lower salinity was reported. These conditions result in a depletion of some of the species but can also enhance a diversity gradient between the unique shallow marine hy- drothermal ecosystems and the surrounding common coastal marine environment, potentiating the co-existence of a high vari- ety of metabolisms and so biodiversity. Metal content on species from vent areas was reported to be associated with volcanic activity and signs of organism's chronic stress seemed to result in modifications on their morphometry and internal composi- tion. Species able to survive at vent conditions are indicated as potential sentinels for studying the effects of increasing tem- perature and acidification on marine organisms and as bioindicators of metal accumulation studies at the Azores. Further information on CO 2 flux, metals concentration in the sediments and seawater and on the geochemistry of fluids from active shallow-water hydrothermal systems is needed. Also, research on the productivity of shallow- water vent areas at the Azores and on food chains and interactions between trophic levels at these environments is recommended as it will contribute to a better knowledge of metal bioavailability, accumulation and biomagnifica- tion. This research should be complemented by investigations directed to the venting periodicity and episodicity and metal deposits resulting from hydrothermalism. This would increase the value of the Azorean vents as natural laboratories to the implementation of multidisciplinary research aimed at contributing to predict and/or to infer about ocean acidification effects on marine organisms and communities.

Rare earth elements in hydrothermal fluids from Kueishantao, off northeastern Taiwan: Indicators of shallow-water, sub-seafloor hydrothermal processes

The rare earth element (REE) geochemistry of hydrothermal vent systems has been investigated intensively, but few studies have been carried out on marine shallow-water hydrothermal systems like that at Kueishantao. Here we present novel data sets of REE in hydrothermal fluids from Kueishantao, off northeastern Taiwan. The total REE (I REE) pound concentrations of yellowish fluids are similar to those of whitish fluids, 813-1212 ng/L, and are significantly higher than that of ambient seawater. The yellowish fluids have chondrite-normalized REE (REEN) distribution patterns with slight convex-downward curvatures at Eu; and the REE patterns of the whitish fluids are smooth at Eu, which is related to the lower temperature and more oxidizing conditions. The Kueishantao hydrothermal fluids are slightly enriched in light REE (La-Nd) relative to the heavy REE (Gd-Lu). The behaviors of REE in both yellowish and whitish fluids are affected by the short time of water-rock interaction. The REE distributions in the yellowish fluids are also affected by very low pH (2.81 and 2.29), boiling of the fluid and precipitation of native sulfur. In the whitish fluids, adsorption by small particles and formation of REE-chloride complexes has played a role in the distribution of REE.

Comparative study of sampling methods and in situ and laboratory analysis for shallow-water submarine hydrothermal systems

A primary aim for sampling of submarine hydrothermal vents is to minimize inclusion of ambient seawater. Here, we compare the results of three different sampling methods (air displacement, two-valve bottle, and syringes) for shallow submarine systems. Mixing of hydrothermal fluid with seawater is unavoidable; however, calculations based on linear mixing models allow estimation of the hydrothermal fluid end-member composition. The results show that sampling with a two-valve bottle and syringes are the best options because they allow collection of samples with a large proportion of hydrothermal fluid. Additionally, we compare the results of in situ and laboratory analyses of the fluid samples, and demonstrate that determination of chemical composition in situ is the best option for some components, as re-equilibration affects some component concentrations (i.e. bicarbonate). Conversely, silica determination in situ usually underestimates the concentration in the fluid, as it does not account for polymeric silica. Other components can be measured either in the field or in the laboratory.

Distribution, speciation and bioavailability of arsenic in a shallow-water submarine hydrothermal system, Tutum Bay, Ambitle Island, PNG

Shallow-water hydrothermal vent systems can introduce large amounts of potentially toxic elements, such as arsenic (As), into coastal marine environments. The first step in understanding and describing the potential impact of these elements throughout hydrothermally influenced coastal ecosystems is to determine the element's distribution and speciation, which in turn influences the availability of the toxin for biological uptake. Shallow submarine hot springs near Ambitle Island, Papua New Guinea, are discharging as much as 1.5 kg per day of arsenic directly into a coral-reef ecosystem. We have investigated the bioavailability of the As throughout Tutum Bay by studying vent fluid, seawater, pore water, precipitates, and sediments. In addition to measuring As abundance, As speciation (As(III), As(V), and the methylated species DMA and MMA) was determined in various waters. The As concentration for discrete mineral phases in vent precipitates and sediments was determined by sequentially extracting arsenic from the easily extractable, carbonate, Fe-oxyhydroxide or hydrous ferric oxide (HFO), and residual fractions, each of which have a different bioavailability. Diffuse venting seems to play a critical role on the distribution of As throughout Tutum Bay surface sediments, which have a mean As concentration of 527 ppm while excluding the vent precipitates (range = 1483 to 52 ppm). Up to 54 ppm As were extracted from the easily extractable fraction of surface sediments (mean = 19.7 ppm), using a K 2HPO 4/KH 2PO 4 buffer at pH = 7.2. Arsenic from this fraction is considered to be the most available for biological processes, and therefore the most dangerous for biota. However, sequential extraction shows that 98.6% of the As in vent precipitates, and a mean of 93.3% in surface sediments (range = 88.2% to 96.3%), is coprecipitated with the hydrous ferric oxide (HFO) fraction. Thus, the bulk of the As being discharged into Tutum Bay is scavenged by the HFO, and should remain stable unless the physicochemical conditions surrounding the oxides change. In surface seawaters of Tutum bay, we found as much as four times the average seawater concentration of As (8.4 μg/L compared to ∼2 μg/L). The abundance of As in seawater just above the sediment/water interface is near normal, although As(III) in both surface and bottom seawater throughout Tutum Bay is substantially enriched compared to average seawater. Hydrothermal venting therefore provides bioavailable As by two major pathways throughout Tutum Bay: 1) easily-exchangeable As from hydrothermally influenced sediments to as far away as 200 m from focused venting, and 2) in surface seawaters, which may allow for biological uptake by phytoplankton and transfer up the food web.

Stable and radiogenic isotopes as tracers for the origin, mixing and subsurface history of fluids in submarine shallow-water hydrothermal systems

The shallow-water hydrothermal system in Tutum Bay on the west side of Ambitle Island, Papua New Guinea provides us with an exceptional opportunity to study isotope systematics in a near shore setting. Compared to seawater, the hydrothermal fluids in Tutum Bay have lower values for δD, δ 18O, δ 13C, and 87Sr and higher values for 3H, δ 34S(SO 4) and δ 18O(SO 4). The δ 18O and δD records for vents 1 and 4 indicate that fluid compositions remained stable over an extended period. Interpretation of isotope data clearly demonstrates the predominantly meteoric origin of Tutum Bay hydrothermal fluids, despite their location in a marine environment. δ 18O and δD values are identical to mean average annual precipitation in eastern Papua New Guinea. The hypothesis that these fluids are a simple product of mixing between seawater and onshore hydrothermal fluids from the Waramung (W-1) and Kapkai (W-2) thermal areas has been rejected, because the observed δ 37Cl, 3H, δ 34S(SO 4) and δ 18O(SO 4) values cannot be explained by a simple mixing model. The application of δ 18O(SO 4) and δ 13C thermometers in combination with 3H values corroborates the three-step model of Pichler et al. [Pichler, T., Veizer, J., Hall, G.E.M., 1999. The chemical composition of shallow-water hydrothermal fluids in Tutum Bay, Ambitle Island, Papua New Guinea and their effect on ambient seawater. Marine Chemistry 64 (3) 229–252], where (1) phase separation in the deep reservoir beneath Ambitle Island produces a high temperature vapor that rises upward and subsequently reacts with cooler ground water to form a low pH, CO 2-rich water of approximately 150–160 °C, (2) caused by the steep topography, this CO 2-rich fluid moves laterally towards the margin of the hydrothermal system where it mixes with the marginal upflow of the deep reservoir fluid. This produces a dilute chloride water of approximately 165 °C, and (3) possibly the entrainment of minor amounts of ground or seawater during its final ascent.